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Solenoid operated Vented Relief valve

Solenoid operated Vented Relief valve

Solenoid operated Vented Relief valve

(OP)
Hi, I am working on a system which comprises of a 65 litres/min Constant pressure axial piston pump (set at 175 Bar), together with a 110v AC Solenoid operated Vented Relief valve in the system set at 200 Bar, being utilised to discharge "walking trailers" via associated pipework, hoses and Quick Release couplings. The problem being that occasionally the system is started with the Quick Release couplings dis-connected so that the pump is "dead headed" at 175 Bar when the Vented Relief valve is energised. The pulsation in the pipework upon energising or de-energising leaves a lot to be desired?
Does anyone know of a manufacturer of a Time/Speed controlled solenoid vented relief valve to prevent/reduce the pulsation?, or even suggest an alternate method of control?
Thanks.

RE: Solenoid operated Vented Relief valve

I'd change the pump to a pressure-compensated type.  It will destroke at a set pressure, say 175 bar, and not pulsate the plumbing.

Ted

RE: Solenoid operated Vented Relief valve

(OP)
Hi Ted,
My explanation of the system may not have been correct/clear initially.
The system has a pressure compensated pump set at 175 Bar, the problem being the "hydraulic hammer" created in the system when the solenoid operated vented relief valve set at 200 Bar energises or de-energises, with the Quick release couplings dis-connected (dead headed).
I am looking for some method to control the speed of achieving or relieving the 175 Bar system pressure (pump compensator setting)by the use of a proportional control solenoid or something similar. Any suggestions?   

RE: Solenoid operated Vented Relief valve

There is no 'slack' in the system, it will be incredibly responsive to any changes in pressure, particularly when the main spool opens or closes.

You could add a large accumulator 54 LTR+

Or

Grind the edge off the spool in the VRV. It'll soften the system a little, but will also add a little heat.

It's never a good idea the load/unload a pump against such a small volume of oil.

RE: Solenoid operated Vented Relief valve

There is a simple little trick you can do to a vented relief valve which "softens" its action – but you need access to the relief valve's "vent" port. Depending on the make/model of the relief valve this might be a port on the subplate and/or in the body of the relief valve itself or maybe even accessible from the relief valve pilot head. You might be able to get access to the correct part of the circuit by slipping a test-point adaptor slice under the DCV (if it is a CETOP 3 valve). A connection to this part of the relief valve's inner workings can be used to give a remote control to a lower pressure than the one set locally at the valve – so the port might be labelled X or V.

Inside the vented relief valve there will be a communication, via a small "control orifice", between the top and bottom of the main stage poppet. The "vent" port gives access to the top of the main stage poppet. The solenoid valve (when de-energised) connects the top of this poppet to tank so the poppet just lifts and the pump is unloaded (full displacement = full flow, but the "bypass" pressure will be just a few bar).
 
When the solenoid valve is energised then the vent port connection to tank is blocked off, the chamber above the relief valve main stage poppet becomes pressurised (via the control orifice) and the relief valve closes. The pump is still delivering oil and, when the relief valve closes (takes about 10 milliseconds), the pressure will rise very rapidly because of the small volume of oil between the pump and the relief valve. The pump has to de-stroke as soon as the pressure reaches 175 bar but it can't do it instantaneously, and all the time the pump is still delivering oil then the pressure will be rising above the setting of the pump compensator. You probably find the pressure goes high enough to cause the relief valve to open again (200 bar). Eventually the pump de-strokes far enough to match the flow demands of the circuit, the relief valve will close and the pressure will stabilise (175 bar).

This is the trick – connect a small twin wire braided hose (SAE100R2AT or EN853:2SN, say 3/8" bore and about two metres long) to the relief valve vent port and just plug the other end of the hose. The effect of this hose is to slow down the closing of the relief valve just enough to take the destructive shocks out of the system.

What you will have done is introduce a relatively large, but squashy, volume to the top side of the relief valve main stage poppet. A hose like this will typically have a maximum working pressure of 330 bar so will be quite OK in your 175 bar system. The volume of this hose will be about 140 cc. A twin wire braided hose will typically reduce the bulk modulus of the oil by a factor of four so instead of the compressibility of the oil being about 1/2% per 70 bar it will be more like 2% per 70 bar. To raise the pressure in this hose to 175 bar you have to put in about 7 cc (175/70 x 0.02 x 140).

When you energise the solenoid to switch the relief valve to maximum pressure, the effective setting of the relief valve can't rise to maximum until the hose has been charged up to full pressure as well. The hose is charged up by flow through the relief valve control orifice (typically 0.8 mm diameter) and the pressure drop across that orifice will be the same as the bypass pressure of the relief valve – the spring bias of the main stage poppet (typically 3 bar). A 3 bar pressure drop across a 0.8 mm orifice gives a flow of about 560 cc/min and this means the required 7 cc will be delivered in about 0.75 seconds. If you make the new hose longer or fatter this time period goes up, if you make it shorter or thinner it goes down. (You could use an accumulator but then you have issues with pre-charge pressures and pressure ratios and PSSR and certification etc.)

In practice, when the solenoid valve is first energised the effective setting of the relief valve takes about 0.75 seconds to reach 175 bar (at which point the pump will start to de-stroke). The pump will continue to deliver oil while it is de-stroking and the pressure will continue to rise at the same rate (about 0.23 bar per millisecond). If the maximum pump flow is 65 L/min then my guess is that the pump displacement is about 45 cc/rev and a pump of this size will typically need 20 milliseconds to de-stroke. So by the time the pump has de-stroked the relief valve setting has only had chance to reach 180 bar.

When you come to de-energise the solenoid, the new hose will very quickly discharge its 7cc compressibility volume through the solenoid valve and the system will unload just as before.

There is no risk of over-pressurising the system with this technique, because the 200 bar maximum setting of the relief valve is still present, but very rapid pressure rises will cause the relief valve to open earlier than would otherwise be the case.

It is important that you bleed the air out of the little hose when you install it. Pull the connector off the relief valve solenoid and start the system – the pressure will remain very low because the relief valve will still be vented. Bleed the air out of the hose by slackening off the plug you put in its free end. Stop the system, refit the solenoid connector and see if there is any improvement. The difference should be marked – if you want to quantify it then get hold of a digital pressure gauge that can record peak pressures and try the start-up procedure both with and without the hose modification.

You could go down the route of proportional pressure relief valves with up/down ramp time settings built into the amplifier, but this little trick is cheap enough to try (especially if you have a spare hose lying around that you can just experiment with) and if you don't like it you've lost nothing.

DOL
 

RE: Solenoid operated Vented Relief valve

I think that no matter what tricks are done with the control lines, as soon as the main poppet/spool closes, the full displacement of the pump will pushed into a very small space, the whole system will 'jump'. Likewise, when the valve is vented and allowed to open, the pump will decompress very quickly and 'jump' again.

Choking the control lines will add some latency to the system and stop the hunting or 'hydraulic hammer', but it will still 'jump' as the pump come on and off load.

Oldhydroman gets a star for the thoughtful and considered contribution...without being patronising.

RE: Solenoid operated Vented Relief valve

Hi Hydromech and thanks for the star (I presume that's good? I'm new here).

The point of the mod I described earlier is that the relief valve can't actually close until the pump has come off stroke. It's just as if you started with the relief valve at a low setting and then slowly turned the knob  to increase the setting up to the required value. As you reach 175 bar the pump will start to de-stroke but will still able to push its output flow across the relief valve. There is no sudden closing and rapid reopening of the relief valve involved.

I've done this trick several times in the past with good effect. But, thinking about it a little more deeply now, there is another issue which comes into play when you do this with a pressure compensated pump. A typical pressure compensated pump will have a controller which exhibits a small pressure rise as the pump adjusts itself between full displacement and zero displacement. So if you set the pump for 175 bar when on full delivery the pressure will rise to about 180 bar when on zero delivery. Or, more probably, the pump is set to 175 bar when dead-headed and will actually start to de-stroke from full displacement to zero only when the outlet pressure first rises above 170 bar.

So, in this application and with this modification, when the relief valve is slowly increasing its effective setting from 170 bar to 175 bar (while the little hose is being charged up) the pump displacement will track it and reduce its displacement at the same rate (full displacement at 170 bar, zero displacement at 175 bar). The slower the relief valve goes between these two values the slower the pump displacement reduces.

The effect is the same as using a proportional relief valve with a ramp setting in the amplifier. You gradually ramp up the setting of the relief valve rather than give it the "all-or-nothing" approach of a bang-bang solenoid valve. Unfortunately with this 'additional-volume-in-the-control-line' trick there is little you can do about the decompression shock - if you start to put in throttle valves to increase the time taken to decompress the little hose then this also increases the bypass pressure.

Don't get me wrong - I'm not a Luddite who spurns electronics whenever possible. I appreciate that you can make a perfectly acceptable slow ramp up and ramp down of pressure using a simple proportional relief valve pilot head and a basic plug-top amplifier. It is worth remembering, however, that there is more than one way to skin a cat. And maybe this is a roughish environment where an electronically controlled valve would be considered unsuitable...  

There is another approach which can be useful to adopt if you have a compressed air supply available. That is to use a pneumatically operated relief valve pilot head as a remote control for the [bigger] two stage system pressure relief valve. A valve such as SUN hydraulics RBAR-AWN has a 50:1 ratio between the pneumatic pressure you apply to its pilot port and the resulting pressure setting the hydraulic part of the valve.

So to set the big relief valve to 200 bar you need to apply just 4 bar air pressure to the head of the new pilot valve you've just connected to the big valve's remote control port. Now with compressed air being the way it is [compressible] it is very easy to construct a pneumatic circuit which takes a decent amount of time to reach the required pressure. A small pneumatic pressure regulator, a small 3/2 NC pneumatic solenoid valve, a length of nylon tubing and a few jets are all you need.

Install the solenoid valve with a decent length of tubing between its service port and the new relief valve pilot head. Connect the solenoid valve inlet port to the outlet of the pressure regulator. The solenoid valve exhaust port just goes to atmosphere (you might want to install a silencer or diffuser to be polite).

If you put a jet or throttle valve on the solenoid valve service port then you can slow down the rate at which the inside of the nylon tube pressurises (when you energise the solenoid) and de-pressurises (when you de-energise the solenoid). But you only have the one adjustment: set it so the pressurisation rate is good then see if the de-pressurisation rate is still acceptable. If you want individual control of the two rates then you need two jets or two throttles, one on the inlet port of the solenoid valve (controls pressurisation rate) and one on the exhaust port (controls de-pressurisation rate).

I've also done this before and it works well, but its only really feasible if you already have the air supply handy.

DOL

 

RE: Solenoid operated Vented Relief valve

(OP)
Hi all, Thanks for the replies - plenty of options to think about!!!!!.
Would it be practical to introduce a solenoid operated proportional contol valve to control a vent line signal connected to tank?
The existing Solenoid Operated Vented Relief valve is an "Integrated Hydraulics" 7VR150 valve.

RE: Solenoid operated Vented Relief valve

Can you put a lockout at the quick-disconnect couplers so that the relief valve solenoid cannot be actuated if the couplers are disconnected?  Does someone have to manually connect and disconnect the couplers?
If the system can be operated while an operator is near the couplers, put the on-off switch there.

Ted

RE: Solenoid operated Vented Relief valve

Hi pob786

Warning – another very long post:

Yes, you can do what you're asking with that relief valve. The 7VR150 valve body has a small port (stamped "3") which probably has as 1/4" NPT plug in it right now. This is the "vent" port. If you connect this port to tank the relief valve will unload. If you put a small relief valve between this port and tank then you can dial up a setting somewhere between zero bar (or very nearly) and the setting of the 1VR100 cartridge currently screwed into the original valve body (200 bar). Putting a proportional relief valve on this vent port allows you to dial in the setting electrically - and using an amplifier with a ramping function allows the setting to ramp up and down at a rate you can set yourself.

Choice of valve: it is important that the proportional relief valve you add is a small, direct operated one that is specifically made for use as a pilot control valve. A bigger valve will not respond properly with such a tiny flow rate going through it (the pilot flow will be about 0.5 L/min) and a bigger valve might have so much internal leakage that it begins to interfere with the operation of 1VR100 valve. I don't work for a hydraulic valve manufacturer or supplier so I have no axe to grind, but I do like the Sun valves (good honest-to-goodness products in my opinion) or the Wandfluh valves (if you like the feel of precision Swiss engineering). Integrated hydraulics (now part of Eaton) also have a suitable valve and there may be political reasons why you wouldn't want to mix manufacturers but there is absolutely no technical reason to avoid it.

Also be aware that a proportional relief valve with no current applied to its solenoid will not go down to a pressure as low as the basic unloaded relief valve. And don't forget that the bypass setting of the 1VR100 valve will add to the setting you apply to the vent port. For example, if you chose a Sun RBAP-XAN (assuming nitrile seals are OK for you) then that valve is capable of setting a pressure range of between 20 and  210 bar – but that is the setting of the little relief valve itself. So when the little relief valve is sitting at 20 bar your big relief valve will be setting a system pressure of about 23 bar. If you dial up the full 210 bar on the little valve then your big valve would theoretically be set at 213 bar but will actually be set at just 200 bar because that is the setting of the manually operated pilot stage in the end of the original 1VR100 cartridge. The remote valve can dial up a lower setting than the original but not a higher setting. If you choose a proportional relief valve with a maximum setting which is too high then the minimum setting problem gets worse and the resolution of control gets worse as well (a proportional relief valve with a maximum setting of 350 bar has a typical minimum setting of 35 bar).

Now you've got your new proportional relief valve (and its valve body if you're going for a cartridge execution) you need to connect the inlet of the new valve to the vent port of the old valve. For the sake of hydraulic pressure stability try to keep the two valves as close together as you can – you could even consider using a male-male fitting to mount the new valve body straight onto the vent port of the old valve body. Connect the outlet of the new valve either to the return line (via a tee fitted to the outlet port of the old valve) or back to tank all on its own. If you do have a spare drain port on the lid of the tank then you could use that but beware:

1)    The spare port might not have a drop tube fitted inside the tank (and the flow straight onto the surface of the oil in the tank will promote the entrainment of air and the formation of foam). Even though the flow is tiny and only present when the proportional relief valve is the one setting the pressure; following best practice is usually worthwhile.
2)    The silt which can gather at the bottom of the tank usually takes up a deposit pattern that corresponds to the flow coming back from the various drop tubes and return filters that are throwing the oil back. There will be no silt under the big flow tubes and plenty of silt under the zero flow [spare] tubes. If you suddenly bring a spare drop tube into play then the first time there is any flow down it you could disturb the existing pattern and bring forth a whole barrage of silt which will damage the pump and maybe swamp the filter – putting it on bypass and then allowing the silt everywhere in your system. Probably not an issue with a 0.5 L/min pilot flow but it's always worth remembering.
3)    If the drain port does have a drop tube but has not previously been brought into play then there is a risk that the moist cloud of air above the oil surface has also been present inside the drop tube and there may be some rust on its inner surface. Disturbing this rust will [slightly] increase the contamination level of your fluid.

If you decide it's not worth the hassle and you connect the outlet of the proportional relief valve to the outlet of the existing relief valve then any back pressure on the existing outlet line will add to the pressure setting you've just dialled up. Not really a problem though because you would just be using the proportional valve to give you the ramping function up to the original setting of the relief valve.

Now for the amplifier: once again there is absolutely no reason to use an amplifier from the same manufacturer as the valve – especially on something as straightforward as a small, direct operated proportional relief valve. The amplifier will almost certainly need a 24 V DC supply (although 12V DC versions are available). You might not have this 24V DC supply available (since the original unloading solenoid was 110V AC) so you will need a small power supply somewhere in your electrical system. A switch-mode power supply is probably best because they are now so cheap it isn't worth bothering with anything else. Something like part number 282-473 from RS components would be fine – use a couple of fuses on the 110V AC power to it but you don't really need any fuse or MCB protection on the output side because the  power supply has it all built in.

You could use a DIN rail or chassis mount proportional valve amplifier, if you wanted it to be in your electrical enclosure, and then wire out to the proportional solenoid using some screened cable. The solenoid isn't sensitive to electrical interference but screened cable is usually recommended because the solenoid puts out quite a lot of interference that might affect other things nearby. If you don't use screened cable then....well, let's face it,  you wouldn't be the first. Make sure you use a solenoid plug with no extra gizmo's in it (no diodes, varistors, surge suppressors, LED's etc.) because these can interfere with the circuits in the amplifier. Unless, of course, your particular choice of amplifier is one that requires you to fit a diode into the solenoid plug but if this is the case it will say so clearly in the wiring instructions.

Alternatively you could use a "plug top" amplifier – this fits straight onto the proportional solenoid and is a handy choice provided the environment is right for it (no extremes of temperature, no huge ingress of water, no massive vibration, no walkie-talkies or cell phones within 1 metre etc.). Again go for screened cable if you can – but then some manufacturers pre-wire these with 2m or 5m leads and they never seem to use screened cable for that, so don't worry about it.

A typical plug top amplifier will have the 0V and 24V supply connections, a 10V output voltage connection (for you to connect your own potentiometer – not needed here) and an input connection. Giving it 0V on the input connection, or leaving this connection open, will cause the amplifier to output its minimum current (Imin – which you set on one of the amplifier adjustments). Giving it 10V on the input connection will cause the amplifier to output its maximum current (Imax – also set on one of the adjustments). There may be a dither frequency adjustment (not critical in this application as long as you don't set it too high or too low) and then there will be the all-important ramp adjustment. A simple amplifier will only have one adjustment and the value you set applies for ramping up and ramping down. A more sophisticated amplifier will have separate adjustments for ramp up and ramp down.

Typical control scheme: wire the amplifier's 10V output terminal back into the input terminal via a pair of volt-free contacts on whatever switch or relay you were previously using to energise the 110V AC solenoid. (By the way this solenoid, if retained, still needs to be energised.) Temporarily disconnect the input signal to amplifier but leave its 24V supply on, start the pump, and energise the original loading solenoid (if retained). Gradually increase the Imin current adjustment until the pressure just begins to rise. Set the time ramp adjustment to maximum (warning some of these amplifiers have the setting the other way round to what you would expect – some turn anti-clockwise to increase the time period). Reconnect the 10V input signal and then adjust the amplifier's Imax setting until the pressure goes up to the value you want – you might have to temporarily increase the setting of the pump pressure compensator so that you can get the 200 bar. Then re-adjust the time ramp setting so that when the signal is applied you get the rate of increase you're looking for. Remember to reset the pump compensator to 175 bar.

Note that if you want a gentle de-pressurisation you have to take away the amplifier input signal to make it ramp down but leave the original loading solenoid energised (if retained) and leave the pump running. If your existing motor control scheme just drops out the loading solenoid at the same time as stopping the pump then you will need some sort of timer to keep the pump running (and the solenoid energised) long enough to allow the amplifier to ramp the pressure down. But don't interfere with any emergency stop circuits that might be present – when the emergency stop circuit is opened just stop the pump and drop out the loading solenoid immediately. If you want to bring the proportional valve to minimum pressure immediately then cut the 24V supply to the amplifier (but some don't like this) or utilise the blocking/enable connection that some amplifiers have (such as the Wandfluh P02 amplifier). Knocking off the 110V AC feed to the 24V power supply won't work because the power supply has loads of internal capacitors and the 24V output won't go off immediately.

Phew! Are you sure you still want to do it?  

DOL
 

RE: Solenoid operated Vented Relief valve

Maybe I am missing something here.

You have a pressure compensated, variable pump set at 175 bar.It goes to a closed centered system, either valves centered or quick couplers disconnected.

In parallel is a 200 bar vented relief valve. Is this on the main outlet flow of the pump, or only on the controller of the pump? i.e. is it venting the controller to destroke pump to 0, or is it venting the entire pump flow to tank and making it a fixed max displacement pump? Many, or most, variable piston pumps don't like running at maximum flow and no load pressure, so I assume you are venting the controller only.

Is it energized to 200 bar (I think is what you mean), or energized to vent to about 0?

Why the need for the vented relief valve at all? Is it an E stop feature?   

Or is it just to decompress the circuit to allow coupling the connectors to the trailer?   There are couplers that can be made up under pressure, but they usually require cycling the pump pressure side (after making the connection) to open the internal poppets. (Usually used on an implement connected to a power unit with the control valves on the power unit, where the cylinder side may have pressure in the lines, but the same concept works with pressure in the body or source side.)

Where is the control valving for the circuit? Is it on the trailer, after the connection in discussion? Or is there any control on the pump or tractor side?

If this is just to decompress to hook up lines, I would add a small piloted line venting the control, with either a small orifice in line or a soft shift valve line as Ted suggested. Those just damp the motion of the control spool with fluid and are very simple and reliable if you don't need variable or proportional control.   Have that valve normally closed, energize to open the valve to vent the control through the orifice. That happens quickly, before the operator can move to connect the couplers so proportional control would not be necessary.

kcj


 

RE: Solenoid operated Vented Relief valve

Ah - now there's a thought.

All that stuff I was spouting before was based on the assumption that the unloading relief valve was being used to keep the pump unloaded during start-up of the electric motor.

Chances are this power unit has something like a 22kW motor and the solenoid DCV which is venting the relief valve is de-energised while the motor is running up to speed (maybe via a star-delta starter - which makes the motor weak while connected in star). Then a few seconds after starting the motor (or when the delta contactor kicks in) the solenoid is energised and the relief valve switches to its 200 bar setting.

Maybe pob786 could confirm but I just assumed that was the case because the 7VR150 relief valve is good for 100 L/min and wouldn't be the sort of animal you would use to fiddle with the tiny flows involved with the pump controller.

Having the vented relief valve set at 200 bar for a pump set at 175 bar would seem about the right sort of separation of settings to stop them interfering with each other. The relief valve acts as protection against something seizing in the pump displacement control mechanism and can also lop off the pressure peaks when the pump just can't de-stroke fast enough - especially when the downstream volume is small because someone forgot to attach the couplers to the trailer.

Personally I wouldn't worry about running a variable displacement pump at full flow and low pressure (it will be about 5 bar when the 1VR100 relief valve is unloaded at 65 L/min). The high flow rate through the pump carries the heat away and the little bit of pressure stops the pistons from flailing around. Neither would I worry about running the pump at full pressure and zero displacement for long periods - unless it is one of those bent axis pumps with no case drain line, you know, the ones where the leakage flow is effectively recirculated back into the suction side.

If the pump has a load-sensing controller that is defeated in the factory to turn it into a simple pressure compensated unit then there may be a possibility of resurrecting that function to give a two stage pressure compensation setting. Or maybe the pump already has an option for a remote pressure control which can be brought into play. With this facility you could temporarily dial up a very low setting for the pump pressure compensator on start-up, say 20 bar. With a low setting like this the pump would begin to de-stroke that much earlier when the solenoid valve is closed. Once the pump has reached minimum displacement you can switch it to the 175 bar setting and avoid the huge pressure pulse you get when the pump tries to get all the way back to minimum displacement ... because it will already be close to minimum displacement.

You might even get away with holding the loading solenoid energised during the starting of the electric motor because the power requirement to compensate at 20 bar is so low. The pump would start off at full displacement, find that the system pressure had increased to 20 bar during the first few revolutions of the motor and then back itself off to zero displacement before you'd even got up to full speed. Then once running, you could select the 175 bar setting, the pump would start to come on stroke and not get very far out of neutral before starting to come back again.

So, pob786 (sorry, that seems so impersonal) a circuit diagram would be a useful thing to take the discussion further. Could you clarify the problem as well - is the problem one of huge pressure surges if you start the motor-pump set with the trailer not coupled up? Or is it an issue of needing to de-pressurise the [running] system so the hoses can be coupled and decoupled?

If you want to simply soften the switching action of the existing relief valve over 150-300 milliseconds then the little hose trick or the soft-shift solenoid valve attached to the vent port of your relief valve would probably do for you. If you wanted to gradually ramp up the pressure consistently and over a period of up to 10 seconds or so then I think you're right to look towards a proportional valve solution.

DOL  

RE: Solenoid operated Vented Relief valve

With no mention of a directional valve, there is no way to know that the system is closed-center.
I believe there should be a directional valve but none is mentioned.  It could be a closed-center motor spool valve that would when 'off' block pump flow and connect both quick couplers to tank for easy connect and disconnect, whether the pump is running or not.  Using a solenoid relief valve to turn the system on and off is akin to using a circuit breaker as a switch.  Not my personal favorite way to actuate a system on and off.

Ted

RE: Solenoid operated Vented Relief valve

Until the original poster returns with more info we are just in speculation.
But it has been a very interesting trail of information learning and to file away for future use.  Good posts.

RE: Solenoid operated Vented Relief valve

(OP)
Hi all, Please find attached schematic diagram. The system comprising of a 30Kw motor, a Bondioli & Pavesi HPA4 45cc axial piston "pressure control" pump (65 ltres/min)with compensator set at 175 Bar, an Integrated Hydraulics 7VR150 vented relief valve (set at 200 Bar) and a VRB 04 Flow control valve, with no Direction control valve in the system.
As suggested by "Oldhydroman" the vented relief valve type fitted in the pump supply is to enable the motor/pump to attain speed before being energised (10 seconds)at which time the powerpack will supply the required flow/pressure dependant on type of material being discharged (various bio-fuels).
The system being :- When a "Walking Floor Trailer arrives, he reverses upto a discharge hopper ready to discharge and connects his Trailer to the powerpack via hoses and Quick release couplings. The powerpack is contolled from a "control room", together with associated equipment, and several criteria has to be met to enable the system to function in "Automatic mode" i.e. Trailer in position relative to "Receiving hopper" (proximity switch), "hopper" not full (level probe)and several more "interlock systems" beyond the "hopper".
The Truck operator is required to connect the Quick release couplings, initially, operating an "emergency stop button" which stops the power pack only, and upon completing connection of Quick release couplings re-setting the "emergency stop button", at which time the power pack will re-start and commence to discharge the Trailer.
If the "discharge " is too quick, causing the "level probe" in the hopper to operate, the vented relief valve solenoid is de-energised stopping discharge of material from the trailer until the hopper level probe "state" energises the vented relief  solenoid, and the trailer continues to discharge.
As stated previously the concern is the sudden "shock loading" to associated pipework upon the vented relief valve energising/de-energising!!!!
 Again, Thank you for your interest/replies to hopefully resolve the issues.
    

RE: Solenoid operated Vented Relief valve

Hi Gents

I agree with Ted, turning the hydraulic power on and off via the unloading relief valve isn't elegant, maybe a little utilitarian ... but certainly not elegant. Thanks for your further explanation; I think you're nearly at a full solution.

As far as I see it these are your wishes and constraints:

1)You want a reduced rate of rise or fall of pressure when the HPU is started or stopped (even when the hoses haven't been connected to the trailer).

2)You want a reduced rate of rise or fall of pressure when the pump is loaded or unloaded (with the electric motor still turning) – this happens when the wider control system signals that the discharge must resume or pause.

3)When the motor is turning, the hoses are connected and the pump is unloaded (because the wider control system signals the discharge must pause) then the pressure must drop low enough to stop the action of the walking floor mechanism no matter how little is left in the trailer.

4)The HPU must not be running when the driver is connecting and disconnecting the hoses (it's a bit rough on the electric motor all this starting and stopping but is it for safety reasons?).

5)When the HPU isn't running, the pressure in the hoses to the trailer must drop low enough for the couplings to be connected or disconnected easily and without excessive spillage of oil.

And these, I believe, are the problems:

A)There are several ways of reducing the rate of rise of pressure when you want to load up the pump – either because the motor run-up time is complete or because the wider control system signals that discharge can resume. There are also several ways of reducing the rate of fall of pressure when you want to unload the pump to pause the discharge. But, if you also want to reduce the rate of decay of pressure when you stop the whole HPU then you will need to make some significant changes to the way the electric motor is controlled.

B)If you use a proportional relief valve or a soft switching pilot valve to control the rate of pressure change then the action of that new valve has to be co-ordinated with the existing 110V AC solenoid operated loading valve.
 
C)If you decide to dispense with the action of the existing solenoid operated loading valve then you need to find a way of taking it out of circuit. (You don't have to solve both B and C; it's one or the other for these two problems.)

D)A proportional relief valve, when given no input signal, might not be able to set the pressure low enough to pause the discharge when the trailer hasn't got much left in it (typical minimum setting = 20 bar)

Sorry mate, I know all you asked was could you put a proportional valve into your system and look how many words have been typed in order to say "yes".

If the gist of the matter, as summarised above, is about right then, for what it's worth, here's my opinion.

On problem A: the UK Health and Safety Executive or OHSA or whoever it is that causes your heart to sink when they turn up unannounced (and that's the beauty of this forum – where exactly in the world are you?) anyway, 'they' might take a dim view of using the action of "resetting" an emergency stop button to actually "start" the HPU. It would be better if the driver pressed a green button to signal that it was time to start the HPU. Imagine the driver facing you in court because something went wrong and he says "It is reasonable for me to expect the HPU to remain off until I press some sort of 'start' button!".

The powers that be might also take a poor view on the use of an "emergency stop" button to perform the "normal stop" that is part of your normal operating procedure. The guts of the problem are that when you want to have a controlled rate of fall of pressure because you want to stop the HPU, then you do actually have to let the electric motor run a little bit longer after you've pressed the stop button. This is so that the relief valve can gradually reduce its setting and get to the low pressure condition while the pump is still delivering and the motor still turning. Then once the pressure has got really low you can turn off the electric motor and let it, and the pump, coast to a stop. If you turn off the electric motor before the relief valve is at a low setting then the motor and the pump will stop dead and that doesn't do them any good at all.

You need some sort of delay-off timer in the motor start circuit so the motor is turned on as soon as you press the green button but only stops a few moments AFTER you've pressed the red button. But, and this is the killer, you just can't tolerate any delay in turning off the motor if it's a real emergency. In a nutshell, I think you need:

- A recessed green button for starting the HPU. (Start the motor, let it run up to speed then ramp up the pressure.)
 
- A flush red button for the "normal stop" of the HPU. (Ramp down the pressure and only then stop the motor.)
 
- A big, fat, bright red, pull-to-reset or twist-to-release, mushroom headed, can't-miss-it, can't confuse it with anything else, "emergency stop" button which will turn off the power to the electric motor and also instantly unload the pump. And no one cares if the pump didn't like it because it was an emergency.

On problem B: if you go for the soft switching solenoid valve then to load the pump you need to energise both new and old solenoids together. To unload the pump you need to de-energise the new solenoid first but delay the de-energisation of the old solenoid until the new soft switching valve has completed the soft unloading of the pump. You might be able to tie the old solenoid in with the motor contactor (so it comes on and off with the running of the motor) and operate the new solenoid via the original control circuit (so it comes on 10 seconds after starting the motor, goes on and off when the wider control system dictates and also goes off as soon as you press the "normal stop" button – remember the motor needs to run on a little when there is a "normal stop"). When you do an emergency stop the pump will unload immediately because of the de-energisation of the old solenoid.

With the proportional valve solution you will see a small jump in pressure from ~3 to ~20 bar when the original solenoid is energised and then the pressure will gradually rise to the full value as the proportional valve is ramped up. If you tie in the original solenoid with the motor contactor then you will be starting the pump against a 20 bar (or so) load. This will probably be OK because your electric motor is very generously sized. When you do the "normal stop" you would ramp down to about 20 bar and then unload the pump completely just as you turn off the motor. When you do an emergency stop the pump will unload immediately because of the de-energisation of the old solenoid.

On problem C: if you just wanted to get rid of the original solenoid valve (or its action) you could:

- Wire it up so it is permanently energised [except the HPU will go wrong when the red-hot-for-years-but-essentially-useless solenoid eventually stops working]

- Screw something mechanical into the valve's manual override feature and force it into a "switched" position. Then don't bother energising it [it's not pretty but it works]

- Change it for a normally closed solenoid valve (the other option available on the 7VR150 relief valve) but never wire it up [seems a waste]

- Change it for an A879 blank cavity plug [if you can get one from Integrated Hydraulics – I think their range has been consolidated into a "best-off" selection now that they've been taken over by Eaton]

- Change it for a 2CN20 needle valve (fits in the same A879 cavity) and wind the needle valve fully closed [but this leaves an adjuster showing for someone to fiddle with]

- Change it for a 3CA20 check valve (fits in the same A879 cavity)

- Change the whole valve body or fit a new relief valve that doesn't have this solenoid loading valve feature, or better still fit a new relief valve in a body that can also accommodate the new soft start valve or proportional relief valve.

With no original solenoid valve to do the rapid de-pressurisation in an emergency you could chop the 24V DC supply to the proportional solenoid valve (so the ramp down function is curtailed) – but, as I said before, some amplifiers don't like that too much and it shortens their service life. If you only had the soft start valve available to do the unloading then you can't change its opening rate and you would have to rely on the motor coming to a dead stop (but with the soft start option we are only talking about 300 milliseconds delay).

And finally, problem D:  just how low does the pressure have to be before the hydraulic supply can no longer move any part of the walking floor of an empty trailer? Maybe you could experiment; there's a chance that this might not be a problem at all. But if it is then don't get rid of the old loading solenoid valve and don't tie it in to the motor circuit but sequence its operation with the proportional valve (ramp down as far as you can and then drop the last bit of pressure by de-energising the old solenoid).

There are ways of fitting a sequence valve to isolate the trailer connection hose from the supply until the pump pressure reaches some critical value. But then you would also have to incorporate some sort of venting circuit for the hoses and it all gets a bit ugly.

And that's me completely exhausted on the subject (sorry for the length of this post AGAIN. I just get carried away - reminder to self: get a life!)

Good luck - let me know how you get on.

DOL
 

RE: Solenoid operated Vented Relief valve

(OP)
Hi all, especially "oldhydroman",
Thinking about the options available I was considering one more, and "sounding you out" regarding it.
If the pump controller was replaced to make the pump a "pressure/flow compensated pump" with a "standby pressure" of 20/25 Bar. The "LS signal" to the pump supplied from the vent port on the solenoid operated relief valve (7VR150) with a 1mm "orifice" for example, in the line supplying a "CETOP 3" type four way, two position solenoid operated valve, which in turn would  supply the "LS" signal port on the pump.
When the CETOP 3 valve is de-energised, the input signal at "P" port is connected to tank at "B" port and the pump "LS Signal" at "A" port is connected to tank at "T" port.
With the vented relief valve energised, this would give a pressure in the system of 20/25 Bar ( dependant on orifice sizing).
When the CETOP 3 valve is energised "P" port is connected to "A" port which in turn supplies signal to "LS" port on the pump, with "B" port connected to tank at "T" port.
With the vented relief valve energised, this would give a pressure in the system of whatever is required to discharge material upto the compensator setting of 175 Bar.
A time delay between energising/de-energising the vented relief valve and CETOP 3 valve could be introduced, except for when "emergency stop button" is operated, but would this be necessary because as soon as the CETOP 3 valve is de-energised, the pump would de-stroke to minimum flow?
What are your thoughts on this regarding practicality?
Thanks.
I am there solely to commission the power pack, the electrics/ hardware are supplied  and installed by a third party, and like yourselves, I do not particularly agree with their logic/control method of the systems.


 
   

RE: Solenoid operated Vented Relief valve

Hi "pob786" and everybody else.

I sort of mentioned a few days ago the possibility of changing the pump controller but didn't flesh out the approach at that time. I've sketched the circuit of what you're proposing and I don't think I've ever seen anyone connect the vent line of a relief valve to the load sensing port of a pump – has anyone else any experience of doing that? I struggle to see what you would achieve by linking together the two controls and I would also worry that the respective actions of the two components would interfere with each other.

If we call the original relief valve loading solenoid S1 and the new CETOP 3 valve solenoid S2 – this is what you would get for all the different tunes you could play:

(Case 1) S1 = off and S2 = off:
The relief valve would be vented (twice over actually) and the pump LS port would also be connected to tank. The pump would be trying to compensate at 20/25 bar but wouldn't be able to reach this pressure because of the vented relief valve so you would have the pump on full flow with the pressure around 3 bar.

(Case 2) S1 = on and S2 = off:
You think you've loaded up the relief valve but you haven't because the CETOP3 valve is still connecting P-B (P is connected to the vent port and B is connected to tank). However, if you have a 1.0 mm jet in this line then the back pressure from the flow coming out of the vent port (via the 0.8 mm control orifice) will cause the relief valve "bypass" pressure to rise by about 1.5 bar.  The pump would be trying to compensate at 20/25 bar but wouldn't be able to reach this pressure because of the vented relief valve so you would have the pump on full flow with the pressure around 4.5 bar.

(Case 3) S1 = off and S2 = on
Exactly the same as case 1: the relief valve is unloaded by its own solenoid valve (S1) so you can't get any pressure signal to the pump LS port. The pump would be trying to compensate at 20/25 bar but wouldn't be able to reach this pressure because of the vented relief valve so you would have the pump on full flow with the pressure around 3 bar.

(Case 4) S1 = on and S2 = on
The relief valve is loaded up and the LS port of the pump is receiving the same pressure signal as it has on its outlet. This disables the flow compensator in the new displacement controller so the pressure rises to 175 bar (don't forget you have a flow control valve in the circuit as well).

It might help the analysis of the situation if you consider the reaction of the pump when you de-energise the CETOP 3 valve. It won't automatically "de-stroke to minimum flow" but what it will do is behave as if its pressure compensator setting had just been reduced to 20 bar (ish). If the circuit resistance is so low that the pump can push out some oil and still not raise the pressure to 20 bar, then the pump will stay on full displacement. If the circuit resistance is higher then the pump outlet pressure will be higher, and if we reach the giddy heights of 20 bar then, and only then, will the pump de-stroke.
 
You propose tying together in a single CETOP 3 DCV the dumping of the relief valve vent connection and the dumping of the pump LS connection. What will happen is that when you de-energise the CETOP 3 valve solenoid you drop the pump compensator setting down to 20 bar but you also drop the relief valve setting down to 4.5 bar with the result that the pump will still be on full displacement.

If you want to go down the route of turning the pump into a "load sensing" version (pressure/flow compensated) then you are on the right track for a quieter, more efficient, cooler and longer living system. But I wouldn't try to confuse the hydraulic controls by picking up your load sensing signal from the vent port of the relief valve – you'd be much better off picking up a load sensing signal from just downstream of your flow control valve (VRB04).

[Incidentally, before going too far down this route you need to confirm exactly what this flow control valve is. If it is a barrel type flow control valve from MCT then it isn't pressure compensated (your circuit diagram was sort of indicating that it was a pressure compensated valve). Don't worry about the mistake in the diagram – you actually need the valve to be a simple throttle valve.]

When you pick up your load sensing signal from just downstream of this flow control valve and lead it to the LS (load sensing) port of the new pump controller (actually I think the pump manufacturer marks this port as "X") then the pump will still behave as if was a "pressure compensated pump" BUT with someone sitting on it and constantly/instantaneously re-adjusting the compensation pressure setting so that it was always about 20 bar higher than the pressure of the hydraulic supply being delivered to the trailer (this is just another way of thinking about what the pressure/flow pump displacement controller is actually doing).

This load sensing signal can be fed to the pump via the CETOP 3 valve you propose or a simpler cartridge style 3/2 valve (the flows are tiny so a small valve would be appropriate). The purpose of the solenoid valve would be to let the pump X port receive the load sensing signal when the solenoid was energised but to let the pump X port be dumped to tank when the solenoid was de-energised. Let's call this solenoid S3 and analyse the situation further:

(Case 5) S1 = off and S3 = off
The relief valve would be vented and the pump X port would also be connected to tank. The pump would be trying to compensate at ~20 bar but wouldn't be able to reach this pressure because of the vented relief valve so you would have the pump on full flow with the pressure around 3 bar.

(Case 6) S1 = on and S3 = off
Imagine you were in the condition of case 5 and then you become case 6, i.e. you just energised S1. The relief valve setting rises to 200 bar. The pump is still on full stroke – the pressure rises very quickly but as soon as it reaches 20 bar the pump starts to de-stroke. The pressure at the delivery QRC to the trailer would be 20 bar. Is this low enough to prevent the walking floor from moving when the trailer is nearly empty?

(Case 7) S1 = off and S3 = on
The pump would be trying to compensate at 23 bar but will still on full stroke and push all of its flow across the relief valve at ~3 bar.

(Case 8) S1 = on and S3 = on
The relief valve is running at a 200 bar setting so the pump won't deliver any flow across it. The pump compensator setting will become 20 bar higher than the load pressure. The maximum flow from the pump to the trailer will be whatever this 20 bar pressure drop causes across the flow control valve. If the pump pressure ever rises to 175 bar the pump will start to de-stroke.

Your comment about not needing to de-energise the loading valve (S1) is quite valid and I've seen many systems that just use the dumping of the pump LS signal as the means of keeping the power low during start-up.

But getting back to your first post – how does this change of control help the pressure surges you were getting? Well imagine the system had been reconfigured as:
- A standard (not venting) relief valve set to 200 bar and connected to the pump outlet.
- A pressure/flow compensator on the pump with the pressure compensator part set to 175 bar and the flow compensator part set to 20 bar.
- A load sensing signal coming from a point where the supply feeds into the trailer circuit.
- A solenoid valve on the pump LS signal which, when energised, feeds the signal to the pump X port and, when de-energised, dumps the pump X port to tank. This solenoid is S3.

Step 1 - To start up the HPU...
What you do: de-energise S3, allow the motor to run up to speed, then energise S3 to bring the pump pressure up to whatever it needs to be (but with a limit of 175 bar).

What actually happens: the instant before the motor is started the pump will be on maximum displacement because of the bias spring in the swash plate angle shifting mechanism. The pump starts to deliver oil as soon as the motor rotation starts but the flow rate will be low because the shaft speed is still low. Within a few revolutions the pressure will have risen to 20 bar. The pump now starts to reduce its displacement even though it's probably only doing a few hundred rpm and the flow rate is still relatively low (the pressure might rise a little above 20 bar while the pump is trying to de-stroke). By the time the motor reaches full speed the pump is already on minimum displacement. The power needed by the motor during its run up is minimal because the pressure was always low – and so was the flow.

Step 2 - To pause the discharge...
What you do: de-energise S3.
 
What actually happens: the pump pressure compensator is effectively set to 20 bar when the solenoid is de-energised. At the instant of de-energising S3 the system pressure would have been much higher than 20 bar so the pump will immediately de-stroke. You [probably] can't drive the walking floor with a measly 20 bar so the cylinders are actually stalled, the pump can't push out any oil without the pressure exceeding 20 bar so the pump stays at minimum displacement (or ever so slightly off minimum just to accommodate circuit and internal leakages).

Step 3 - To resume the discharge...
What you do: energise S3.
 
What actually happens: at the instant of energising S3 the load sensing signal was just 20 bar (the previous compensation pressure of the pump) but the pump wasn't receiving this 20 bar signal on its X port which was why the system pressure was 20 bar. Now the solenoid is energised the pump gets the 20 bar signal so sets its output to 40 bar – and that 40 bar signal gets through to the pump so the new compensation pressure is now 60 bar and so on (although the full effect is more continuous than the stepwise explanation I've just given).  The pressure gradually ramps up until it gets high enough for the walking floor mechanism to start to take some flow.

Step 4 – To stop the HPU normally...
What you do: de-energise S3, wait a few seconds for any residual pressure to decay and then stop the motor.

What actually happens: follow the description above for pausing the discharge, but when the pump is at zero displacement and the pressure is just 20 bar then stop the motor. For a fraction of a second (just as the motor is running down) the pump might start to come on stroke because its output flow is no longer keeping up with internal leakage and the pressure will drop below 20 bar. If the pump does come on stroke it will hardly produce any flow before the pressure comes back up and the only power input to the hydraulic system will be coming from the momentum of the motor innards. This might bring the motor to a stop a little sooner than would otherwise be the case but you wouldn't notice it and there's no harm to be done.

Step 5 – To stop the HPU in an emergency...
What you do: stop the motor and de-energise S3 at the same time.

What actually happens: if the pump was at a working pressure at the instant of the emergency stop then the pump will begin to de-stroke as soon as the solenoid is de-energised (this reduces the flow output) and at the same time the shaft speed starts to drop (this also reduces the flow output). But there is no electricity driving the motor and if the pump is still trying to push oil into the circuit then the power needed to do this has to come from the deceleration of the motor shaft so the motor will come to a sudden stop.

OK then, what was wrong with the current design and why is this proposal much better?

The particular flow requirement of the walking floor mechanisms probably means the pump is only on part displacement, say 50%. In the current scheme the pump is on 100% displacement when unloaded (because it is pushing all its flow across the vented relief valve). The pump has to get from 100% displacement to 50% displacement when brought back onto load. It can't move its bits at an infinite speed so, for a fraction of a second, while it's still trying to get to the right position, the displacement of the pump is too big for the flow requirements of the system. The pump is pushing more oil into the system than it needs and the pressure will rise. If the system volume is very small then any tiny mismatch of flow equates to a huge change of pressure. And what you get is a huge pressure pulse which shortens the life of the components (including the hoses) and promotes leaks.

Right at the start of this thread the question was about curing the horrible thing that happens when the system is brought onto load with the trailer hoses disconnected – well, with the hoses disconnected the system volume will be small (that makes the system exaggerate the pressure rise when there is an excess of flow).  And the system's flow requirement will be nil (hoses are disconnected) so the pump has to get from 100% displacement to 0% displacement. This takes longer than getting from 100% to just 50% which means the excess flow will be much worse and the system is already super-sensitive to excess flow and the result is... massive, massive pressure peaks.

Why is the new scheme better? When the pump is unloaded it is on ZERO displacement. When the pump comes onto load it has to get from 0% to 50 % displacement. It can't do this instantly so, for a fraction of a second, while it is trying to get to the right position, the flow will be too LOW and so will be the pressure  - and you don't get the massive pressure peaks.

And finally, is there a booby prize for the longest post?

DOL
 

RE: Solenoid operated Vented Relief valve

pob786
quote "I am there solely to commission the power pack, the electrics/ hardware are supplied  and installed by a third party, and like yourselves, I do not particularly agree with their logic/control method of the systems."

So why are you trying to redeign the system?  I would send a report to the system designer/installer telling them to add a directional control valve, such as I mentioned earlier.  The power unit can be turned 'on' at the start of the work shift and remain running.  When no discharge is being done the pump will run at minumum displacement, minimum power.  The directional valve can have a metering spool to ease the start of the truck discharge.  The truck operator can operate the directional valve to permit connect/disconnect of the couplers and operate the discharge cycle.  He does not have to stop the hydraulic pump to connect/disconnect the couplers.  The 'Emergency Stop' should be used only for that.

Starting and stopping the hydraulic system should not be the normal mode of operation.  Start and stop the hydraulic flow, not the pump.

Ted

RE: Solenoid operated Vented Relief valve

I kind of agree about turning off the flow not the HPU but, for reasons of "safety" (and to pamper to the ill-informed and the risk-averse members of the board), some organisations do require the HPU to be turned off completely before connecting and disconnecting the QRC's. It's not nice for the pump or for the electric motor to be turned on and off too often. For the electric motor in particular there are real issues regarding the maximum permissible number of "starts per hour" (might be as low as 6 but it depends on if its a DOL, star-delta or soft start). At the end of the day though, the pump and the electric motor do not have a God given right to be treated with respect. If the end user has some sort of operational policy which shortens the service life of the equipment then that's just fine ... because it is his equipment to do with as he pleases.

I suppose the supplier of the HPU might want to say to his customer something along the lines of "If you are going to treat the equipment that badly then don't expect any warranty cover" but it's a tough economic climate and the supplier might just have to say nothing and hope for the best.

There's not many bells and whistles on this HPU, for example, I can't see a cooler, a dual level float switch, any over temperature thermostats, a filling QRC, a tank drain valve, a limit switch on the suction isolator etc. Would it be fair to say that the HPU has been built to a price because of budget constraints? That might be why there is no DCV.

Don't get me wrong, "Horses for courses" is what I say, but, speaking personally, I often have to shrug my shoulders and hand over a system design which is more "Flybe economy class" than "Singapore Airlines business class". Is it possible, however, that this system is just too basic and that it will cost the end user much more in the long run?

In the link below (if it works) is a scheme I came across once to give a simple co-ordination between the unloading relief valve and the pilot operated DCV. I'm not suggesting it for the solution here because it is more suitable for a fixed displacement pump (but, pob786, if you like it then feel free to adopt it). I've drawn the DCV symbol out with all it's detail because that allows you to see the unusual choice of pilot valve. Also note that the pilot supply to the DCV (port X) comes from the vent port of the relief valve.

When you energise either one of the solenoids just the bypass pressure is applied to the DCV main stage spool. That's not enough to get it to shift but the vent line is no longer connected to tank and the system pressure will now begin to rise. The spool shifts a bit (but not too fast because the pilot line flow is limited to a 3 bar drop across a 0.8 mm jet). As the spool shifts the pressure begins to rise, and as the pressure rises the spool shifts more, and so on ... until the DCV spool is fully shifted; and only then can the supply pressure rise to maximum.

When both the solenoids are off the T port of the DCV connects to tank with absolutely no flow going through that line. If you had QRC's on the connections to the actuator then, when the DCV is in neutral, there will be no pressure on either of the couplings, not even any back pressure from a flow to tank. This would allow the QRC's to be connected or disconnected easily and with minimal leakage. You might even be able to convince a Luddite that it would be quite safe to operate the system like this.

DOL
 

RE: Solenoid operated Vented Relief valve

Need to ask OP if forward and reverse flow are required or not.  If not, then only two of the three positions of the directional control valve would be used.

Ted

RE: Solenoid operated Vented Relief valve

Absolutely, but I put the circuit up the way I did as an example of bi-directional flow for anyone else that might be interested. I took the view that if the reader could fathom the meaning of the diagram I attached then they would also see the ways in which it could be re-configured for the exact requirements of whichever machine was being powered.

In this application I suppose it all comes down to a choice of how you want to unload the pressure compensated pump. You could have full flow at low pressure (vent the relief valve) or full pressure at [nominally] zero flow (de-energise a DCV). When you are operating the walking floor mechanism then the pump will probably need to be on part displacement. If your unloading scheme previously had the pump on full flow then it has to reduce its displacement to get to the right position and this is where the original pressure peak came from. If your unloading scheme previously had the pump on zero flow then the displacement has to increase to get to the right position and you will get a dip in pressure (and maybe a small overshoot just as the pump homes in on the exact position).

Of course, whichever option you choose then you get the opposite effect when you unload the pump. There will be a pressure peak as you try to force the pump back to zero displacement and a pressure drop as you let it go to full displacement (which isn't a problem because you let the pressure drop anyway by venting the relief valve). The pressure peak isn't so much of a problem either if you have a DCV because the pulse will be contained within the HPU and not experienced by the larger external system. A soft switching valve would be nice in this application.

Ted, for the benefit of others who may be following this thread, do you think there would be an interest in exploring some of the ways that a two stage DCV can have its switching time increased? Or should this thread now be allowed to taper off?

DOL

RE: Solenoid operated Vented Relief valve

I'm letting this thread go.  Suggestions have been offered.

Ted

RE: Solenoid operated Vented Relief valve

I guess I cannot just let it go.  Pressure pulses result from rapid changes in velocity, flow.  When the directional control valve is actuated and no couplers are connected there will be a slight variation in pressure since the piping to the couplers is full of fluid, velocity is zero and pump flow is zero.  There will be a small surge as the previously unpressurized part of the system is pressurized and component expansion occurs.  No harm, no banging.  De-energize the directional control valve and make the coupling connections.  There will be a sudden pressure drop as the pressurized part of the system to the coupler connects to the return to tank line.

If the couplers are connected when the directional control valve is actuated, fluid piping is full and flow is zero there will be a small surge as the previously unpressurized part of the system and acutators is pressurized.  Starting the load from rest is a zero-velocity event, flow will increase as the load is accelerated up to the point where the flow control takes over control of flow.

Now I'm finished.  Very informative replies, DOL

Ted

RE: Solenoid operated Vented Relief valve

The bane of engineers, cannot stop thinking about a problem.

pob786, try what may be an easy test.  Change the sequence of the relief valve and pump.  Energize the relief valve before starting the pump.  If the couplers are not connected, the pump will run several revolutions as it starts and reach the 175bar pressure compensation setting, then continue to run up to speed at near-zero torque load as the swashplate will be nearly flat.  This will be a low load startup rather than coming to full flow before the relief is energized after the pump start.  The pressure rise will be that of the pump reaching 175bar pressure which will be slower that slamming the relief valve shut and suddenly stopping the bypass flow.  There should be no hammering and banging.  The shutdown sequence would be: turn off the pump then de-energize the relief valve to drop system pressure.  No new components, just the system as is.

Ted

RE: Solenoid operated Vented Relief valve

Ted,

I stopped contributing long ago...

There is only so much help that can be offered on this forum. People can only take the advice and use it or ignore it.

There can be a tendency for people to "show what they know", which can obscure the real solution to the problem.

This forum can only offer advice, if people want a solution, they might have to pay for it!!!

Adrian  

RE: Solenoid operated Vented Relief valve

But the advice should be the best advice that's going. For what it's worth, my advice would be that you tread very carefully when attempting to start an electric motor with a pressure compensated piston pump where the relief valve has not been vented.

The pump will be on full displacement right from the beginning. It won't start to de-stroke until the pressure rises to the setting of the compensator. At that instant both the pressure and the displacement will be at a maximum which means the required drive torque will also be at a maximum.

If the motor is being started with a star-delta starter then there could be insufficient torque output from the motor for it to be able to complete it's run up and it could stall, or else creep along at low speed. The current will remain at 3-4 times full load current for the remainder of the star-running period. If just creeping round then the motor might even come to a stop during the time delay between the star and delta contactor energisation. Finally, the delta contactor will come in and you will perform a DOL start on a fully loaded motor (expect 8-9 times full load current).

When you turn off the motor without venting the relief valve then the rotating parts of the pump and motor will come to a sudden stop and the excessive stress in the flexible shaft coupling might shorten its life considerably.

Some people worry about the effect on the pump when starting and stopping on full pressure ... but I take a philosophical view on this. If it was a hydraulic motor with the same rotary group then the application might require it to start and stop on full pressure and you wouldn't be concerned about it.

Sorry gents, I was going to consider this thread well and truly talked out but I felt I needed to throw in that particular caution.

Over and out.

DOL

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