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Meter-Out Flow Control Set Higher than Would-be Normal Flow

Meter-Out Flow Control Set Higher than Would-be Normal Flow

Meter-Out Flow Control Set Higher than Would-be Normal Flow

(OP)
Please see the simplified hydraulic circuit below. Many components not shown for simplicity. For now, this is just a thinking exercise and not an actual situation.

Cylinder retracting. Gear pump putting out 10 GPM pump flow to rod end. 10 GPM is its absolute maximum flowrate at motor absolute maximum RPM. 1.5:1 cylinder ratio. That means flow from cap end would normally be 15 GPM. But pressure-compensated flow control is set to 16 GPM. Overrunning load acting on cylinder attempting to push fluid out of the cap end as cylinder retracts. What happens? I think flow from cap end would be 16 GPM. Pump is not able to fill up the rod end fast enough, which would require it putting out 16 GPM / 1.5 = 10.67 GPM. Pressure at rod end would drop to 0. Pressure at cap end is load-induced pressure. Cylinder retracts at a speed equivalent to 16 GPM. Am I thinking about this correctly?

This is a way to control the speed of the cylinder as it retracts with the overrunning load, while avoiding a pressure intensification situation. Any downsides? If the pump can't fill the rod end fast enough, is that a problem?

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

With less than 0 gage pressure in the rod end, the rod seal may allow air to enter the system.

Ted

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Air in via rod seal or will vacuum will occur on rod end (if pushing force on rod is big enough). And vacuum will make the rod side cavitate. But if you have a work port relief on the rod side of the directional valve, it should be combined with anti-cavitation check valve. That way the rod side will get additional flow from the return gallery in the directional valve. Actually the same oil as just left the capped side.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Your system may work if you add a scheck valve between tank and orifice off cylinder. But we never do such a circuit. Today we have cunterbalance valves, an efficient and inexpensive system.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

(OP)
As I said, it's just a thinking exercise.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Would tank atm pressure over drive the pump/motor? Just a random thought.

Ted

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Could you have two flow controls?

One set at <10 as shown and the other at >6 but feeding back into the gear pump inlet line with a check valve in place to only allow flow in one direction?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

(OP)
OP here. To those who say negative pressure and cavitation will result - isn't the fluid in the rod end of the cylinder at atmospheric pressure? The fluid is flowing into the vacant rod end and almost able to keep up, just not at any meaningful hydraulic pressure. Why wouldn't the pressure in the rod end be 0 psig, or 14.7 psia? And the pump discharge pressure is still above atmospheric because of pressure drops from pump to cylinder.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

(OP)

Quote:

Could you have two flow controls?

One set at <10 as shown and the other at >6 but feeding back into the gear pump inlet line with a check valve in place to only allow flow in one direction?

I'm not sure I understand fully. You mean two flow controls in series back-to-back?

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

"Almost able to keep up" - Yes, but not able means there is a volume deficit between the cylinder movement and the oil flow in. That needs to be accommodated somehow to preserve mass - result the piston pulls a vacuum.

If mass flow required by the volume created in the piston space isn't filled with oil (almost keeping up) then what is it filled with?

Does that make sense?

NO I mean add a horizontal line between your two vertical lines between the cylinder and the controller and insert a flow controller in there with a NRV (flow left to righ). Make that 16 gpm one 10 gpm and make this new one 6 gpm.

The in effect the balance of the oil (6gpm) is simply being transferred from one side of the piston to the other.

So two flow controllers in parallel, but feeding different end points.

See my terrible mark up below.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Quote (HydraulicsGuy)

To those who say negative pressure and cavitation will result - isn't the fluid in the rod end of the cylinder at atmospheric pressure?
As you drew the opening statement schematic noting indicates that the rod side is directly connected to the atmospheric pressure. You are obviously using e positive displacement pump which disconnects (theoretically no internal leakage) the rod side from the atmosphere.

As I said in a previous comment atmospheric ventilation can be done with an anti-cavitation check valve

Quote (akkamaan)

it should be combined with anti-cavitation check valve



Quote (HydraulicsGuy)

And the pump discharge pressure is still above atmospheric because of pressure drops from the pump to cylinder.
Nope, if cylinder is retracting faster than the pump can fill on the rod side the pump pressure will be negative vs atmospheric pressure.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Quote (LittleInch)

See my terrible mark up below.
AS long as we understand what you mean the "mark up" is OK. It's called "effort" cook

But what do you do with your additional "flow orifice" when you extend the piston?

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

There should be a no return valve added to allow flow from left to right only. I ran out of space...

The issue I see about the nrv as shown is that it leaves the return uncontrolled.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

(OP)

Quote:

if cylinder is retracting faster than the pump can fill on the rod side the pump pressure will be negative vs atmospheric pressure.

I'll buy the "vacuum pressure in the rod end of the cylinder" argument for a second. Let's say the rod end is at -5 psi gauge, or 9.7 psi absolute. As I said, some components not shown for simplicity. If there are components and plumbing to the tune of 200 psi worth of pressure drops between pump and cylinder, then pump discharge pressure would be 9.7 psi + 200 psi = 209.7 psi, well above atmospheric pressure. Is that not right?

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

(OP)

Quote:

If mass flow required by the volume created in the piston space isn't filled with oil (almost keeping up) then what is it filled with?

Good way to think about it. I'm with you. The answer is air, and it's at less than atmospheric pressure because it's being pulled in (flowing) across the rod seals, as hydtools and akkamaan noted originally. The fact that the air is flowing means there's a pressure drop, meaning it's at less than atmospheric pressure once it gets inside the cylinder. It's good to interact with other engineers about these things and think them through.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Sounds like you've got there.

One issue here is that your 10gpm is being fed by a gear pump which regardless of the pressure downstream will only let 10pm through the pump. Other pumps might let more through if the pressure downstream went negative to inlet pressure into the pump ( probably atmospheric but then we don't have details.

But follow the mass flow. It cannot be created or destroyed!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Why connect the rod end to any hydraulic input? The prevailing load is enough to move the piston to retract all the time. Make the setup single acting.

Ted

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Quote (HydraulicsGuy)

The answer is air
Is "air" and "vacuum" the same thing?
If we theoretically do not let any air in on the rod side while expanding the volume on the rod side there is no air. It must be a vacuum.
That vacuum will be in form of uncompressed oil, extracted gas "bubbles" from the oil, or just "nothing". The process that leads to "cavitation".

So I think it is better to give it the term "vacuum" than calling it "air"...

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Yes but when is a "vacuum" not a "vacuum"? - When it is more than 0 psia. With the gases and oil vapour it would be, as hydraulics guy says, "air but at a pressure lower than atmospheric. The only question is how much lower? Could be 100mbara, could be 900 mbara, but it's still "air".

Semantics and different to what most people understand as "vacuum", but otherwise you need to define quite closely what you mean by "vacuum"

Even a search on definition has both of those - "a space entirely devoid of matter" and "a condition well below atmospheric pressure"

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

If you start with a 100% sealed container filled with 10cui of atmospheric pressure air and expand the volume we will get "vacuumed" air (with its normal contents of the gases of O, N, CO2, etc) we will get "air" under a certain level of vacuum.
BUT
If we start with a container filled with 10cui liquid at atmospheric pressure and expand the volume we will get "liquid" under a certain level of vacuum.
AND
If we start with a container filled with "nothing" at atmospheric pressure ie the volume 0 (zero) and expand the volume we will get a vacuum with "nothing" in it (if you don't extract molecules from the casing material.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

(OP)
I think I have a pretty good feel for the situation. What I've learned is that putting a flow control that allows more than pump flow x cylinder ratio to flow from the cylinder is never a good way to go, due to potential for air to get into the system, or for cavitation to occur once that end of the cylinder is pressurized again.

I had seen an existing hydraulic circuit where there was a flow control positioned like I have in the original sketch, which would have led to a pressure intensification situation if set below 15 GPM. I was wondering why the designer did this. So I thought, 'maybe the plan was to set the flow control to more than 15 GPM to avoid the pressure intensification'. So I began to ponder what would happen if the flow control were set to greater than 15 GPM. Now I know what could happen (air ingress, cavitation once re-pressurized). The moral of this story is this I think: "avoid having a pressure-compensated flow control for meter-out when the external load on the cylinder is overrunning". If set too low, you get pressure intensification. If set too high, you get air ingress and cavitation.

Thanks to all who responded, thanks for the discussion.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

How do you get pressure intensification when the piston is retracting? The system was capable of providing pressure required to extend against the same prevailing load that is returning the piston. You may have flow differences, extend verses retract. As the piston retracts the cap end pressure must be less than that pressure required to extend with the same load. Otherwise the load will not cause the piston to retract without rodend pressure.

Ted

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

If you have a gravity-overhauling load - e.g. turn your diagram 90 degrees anticlockwise and have the cylinder carry some sort of fixed load - then there is a special valve you can use on the bottom of the cylinder to avoid the situation that you are fretting about. That valve is called ... a "counterbalance valve".

Functionally, it's like a pressure relief valve with a check-valve allowing free flow in the other direction.

The idea is that you set the pressure on this counterbalance valve high enough so that the pressure on the top of the cylinder never drops below zero based on the pressure underneath the piston supporting the load.

They're not particularly fussy about that pressure setting matching the instantaneous load. As long as the setting is "more than enough", it will work. Obviously an excessively high setting of the counterbalance valve will make the machine stop working.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

(OP)

Quote:

How do you get pressure intensification when the piston is retracting?

If flow control is set to 14 GPM out of cylinder cap end, pump flow into cylinder rod end will be less than 10 GPM on a 1.5:1 cylinder. That means some pump flow is dumping over relief valve. That means pump and rod end pressure go to relief valve setting. With a load pushing against the cylinder, the load and the rod end pressure together create intensified pressure on the cap end.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

(OP)

Quote:

If you have a gravity-overhauling load - e.g. turn your diagram 90 degrees anticlockwise and have the cylinder carry some sort of fixed load - then there is a special valve you can use on the bottom of the cylinder to avoid the situation that you are fretting about. That valve is called ... a "counterbalance valve".

Functionally, it's like a pressure relief valve with a check-valve allowing free flow in the other direction.

The idea is that you set the pressure on this counterbalance valve high enough so that the pressure on the top of the cylinder never drops below zero based on the pressure underneath the piston supporting the load.

They're not particularly fussy about that pressure setting matching the instantaneous load. As long as the setting is "more than enough", it will work. Obviously an excessively high setting of the counterbalance valve will make the machine stop working.

Please read my post above yours. I’m not asking how to make this work.

RE: Meter-Out Flow Control Set Higher than Would-be Normal Flow

Ok, I understand you thinking. I was thinking in terms of past experience designing a pressure intensifier where pressure on the large area creates higher, intensified pressure on the smaller area.

Ted

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