Hydraulics resistance question.

[itsmoked]

Hallo.
EE here. Out of my element.

I am updating and old system.
I have built all the electrical controls (plc, yada, yada) for a new hydraulics system. The system runs five double acting rams. The rams are evenly spaced at 100 thru 180 feet from the pump/control package. This is down 1/2 inch hoses.
The rams are 2" bore, 1" rods with 13.62" strokes.
A stroke is supposed to take 27 seconds.

Now my problem is this:
The existing system terminates a cycle(opening or closing) by a limit switch. Some of the limit switches have failed. Some always stay made, some never make. I was requested to "make do" with a pressure switch. In other words when the ram dead-heads the pressure will climb rapidly to the pump's relief pressure. I expected to have a pressure switch in the pump circuit that could provide this input and there is one. It makes on rising pressure at 2,200psi. Another switch makes on falling pressure of 1800psi.
There is also a 1/2 gallon accumulator precharged with 1200psi Nitrogen.
The pump is a 0.67GPM at 2,000psi.


I asked what the low pressure switch was for. I was told by the guy who provided the hydraulics, "The pump may come on and if it charges the accumulator up to 2,200psi while a cycle is running then you'll want to turn off the pump. When the pressure in the accumulator drops to 1,800psi you'd want to turn the pump back on. Cycling until the limit switch actuates" ... Doh!

Now I recognize that if there is a large enough restriction between the accumulator and the rams then the pump could well end up providing more fluid then the ram/system can accept and you would end up charging the accumulator to the 2,200psi point BEFORE the stroke is complete. But if this cycling mode occurs I don't see how I can use the pressure to terminate a cycle!?!?

Looking at the info I have provided is it possible to estimate whether I can expect cycling?

Near as I can figure the worst case possibility is retraction.

2 inch bore = 3.14sqin
1 inch ram = 0.78sqin
Area = 3.14 - 0.78 = 2.34sqin
Volume = 2.34sqin x 13.6in = 32in³
32in³ = 0.138gallon

0.138gal / 27sec/60sec/min = 0.30GPM

Looks like I could have a problem??

Any help greatly appreciated.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MikeHalloran]

You can expect the system to express odd and unanticipated behaviors. Hydraulic systems typically can generate enough force to turn unanticipated behaviors into dire consequences.

You might want to investigate why new limit switches have already failed, and ask why Mr. "make do" feels qualified to redesign the system on the fly, and how much responsibility he will or can accept for the modified system, and why it's not possible to just replace the switches.

Best advice: Declare it's outside of your expertise, and walk away.



Mike Halloran
Pembroke Pines, FL, USA

 

[Yorkman]

I'm with Mike unless there is some type of pressure bypass/relief incorporated into the system, I'd be real concerned with the response time of a pressure swith Vs. a position type limit switch. With a stroke time of 27 secs. I'd be concerned in trying to nudge the ram up that last 1/2" - 1/4" and counting on the pressure switch to hit the mark with out damaging a ram or personell

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[quark]

I agree with the above posts that pressure control can be troublesome.

Do all the rams act at once or in a sequential manner? The buffer vessel gives you better flow control (though this is not required for a PD pump) and also enhances the pump life. You should first look into why the precharge is much less than the minimum system pressure requirement. If you have to provide minimum 1800psi then filling up the receiver from 1200psi to 1800psi is a wastage.

I would break up, if it is must to do, the pressure control into two parts. One control fills the buffer vessel. Second, actuating the rams with respect to the accumulator pressure. If all the rams act at once, the I will chose a point at the end of the supply header and cycle ends when the header pressure equals accumulator pressure. This will minimize problems related to restrictions either partial or full but leakage in the rams will kill the entire logic.

 

[itsmoked]

MikeHalloran; The limit switches are associated with water gates in a public drinking water reservoir. They are in murky water in poorly designed protection that renders them impossible to service. They will be serviced someday when the reservoir is drained.

Walking away from a multi month project is not an option.

Yorkman; This thing is coated with relief valves, check valves, and bypasses. Safety is not a problem. There are relief valves for the relief valves.

Hello Quark; The old system is currently being run by starting the pump opening the valve and listening for the pump sound of a dead head. I don't believe there is a problem with extension/retraction damage.

Only one ram is allowed to be in motion at any one time. No multiple ram motion.

I have a flowmeter with a built in switch that can be set to any flow between 0.05 and 1.0GPM. It is between the pump and the accumulator. The directional valves are fed by the accumulator and the pump essentially in parallel. Should I instead try to make this a flow sense system instead of pressure?

pump => flowmeter/switch => accumulator
||
=> directional valves => rams

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[maytag]

You don't have a lot of options here-using pressure or flow for position is starting out behind the 8 ball. Normally you would use a temposonic or a resolver for cylinder position. In your case I think I would use pressure switches or transducers in the A and B lines going to the cylinders and all this is going to tell you is that you have met X resistance. Being you have a PLC, I would put a timer in it where if after X seconds the pressure switch/transducer doesn't make the system would shut down(i.e. protection in case of a blown hose or pipe) Good luck, Maytag

 

[budt]

itsmoked;

Can you put the Flow Meter with the Switch on it on the discharge of the lowest pressure Relief Valve?

All pump flow should be going to the cylinder that is cycling until it reaches the end of stroke at which point the relief valve will start bypassing and send a signal from the Flow Meter switch.

You may have to have a timer to delay pump stop to assure the pressure at the cylinder has equalized with the pressure at the pump.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[MikeHalloran]

1. Ah. The schematic is more complicated than anyone here conjectured. Which adds more possible behaviors. You need somebody like Bud there, in person.

2. Just because the gates are under water doesn't mean the switches have to be under water.







Mike Halloran
Pembroke Pines, FL, USA

 

[maytag]

You might add that the system is bare bones cheap-if you had a pressure compensated or load sensing pump with an adequate motor that you could let it run indefinitely with the valve picked up or have cylinders with some type feedback. You generally get what you pay for. Maytag

 

[itsmoked]

maytag; Yeah I hear you but have no options with cylinder monitoring. The divers insist they can't do anything. And I have incorporated an absolute 'system' timeout. Ram motion should take 27 seconds. If 40 seconds pass without flow dropping to zero then I declare a system error. There is also a Tank Level switch for a low oil system error. If the fluid temperature exceeds 130F we get a system error.

There is pressure limiting on each channel, rams will not get more than 1,500psi delivered to them. There are flow limiting valves on each line of each cylinder to limit flow for ram velocity control.

budt; The only relief valves are on the pump heads and really should not be allowed to operate. (tough on them)
The pump has an in-head bypass and should not run in bypass for more than 5secs. Bypass is set to 2,200psi.

MikeHalloran; I can't believe they didn't do something above water.... sheesh.

maytag; I wish I'd had VFDs on the motors and just run them at the desired flow. They could have ditched half the plumbing.

Today I got them to change the system. Mind you this is all built and we are commissioning it.


The schematic is now:
[red]
||=> 2000psi pressure switch
||
||=> 1600psi pressure switch
||
pump =>||=> accumulator
||
||flow-gauge w/switch => directional valves => rams
[/red]

I will now attempt to run the system in the following manner:

1) turn on pump
2) turn on desired valve
3) turn off pump if pressure reaches 2000psi
5) turn on pump if pressure drops to 1600psi
6) consider cycle ended if cylinder is dead headed via stopped (zero)flow.

Stay tuned.




Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[quark]

This seems to be a better setup than the previous one and there is no reason as to why it shouldn't work. Just make sure that the flowmeter is after the accumulator tap off (unlike your first schematic).

I would still prefer a DP switch across accumulator and ram supply header as a secondary control.

I would be comfortable if the pump, accumulator and ram header were in series to get the maximum advantage of accumulator.

Good luck,

 

[itsmoked]

Hi quark,

The flowswitch is between the acc and the valve block.

DP switch?

There is some sort of restrictor to the acc that has a parallel unrestricted path back.

Thanks.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[quark]

Suppose, there is a temporary restriction after flow switch and before ram and still you have a chance that cycle will end. A differential pressure switch put across accumulator and ram inlet will ensure whether zero flow is due to restriction in the midway or due to full traverse of ram (this is indicated by zero dp).

But in your case pump parallelly suplies to both accumulator and ram. If you can offset this (here zero dp doesn't mean zero flow), then dp gives you good assurance. That is why I said that pump, accumulator and ram should have been in series.

No idea but if you can describe about the restrictor!