High temperatures from flow control valve on fluid-dump arm of circuit?

[downranger]

Hi guys,

I am busy diagnosing the cause of high temperature within a hydraulic circuit which has not been problematic in the past. The circuit runs a low-displacement, high force application. The fluid heats up well past 220F after a couple hours. We suspect that the problem may be the amount of fluid travelling from the dump valves to the reservoir, since lots of fluid is being dumped (can feel and hear) and the piping becomes very hot to the touch.

There is a flow control valve which is between the dump-valves in the manifold and the reservoir. This valve is 7/8 of a turn open (full range is 8 turns). I suspect that the pressure drop over this partially open valve is the source of the heat. What is the appropriate setting for this valve? The circuit path goes pump -> filter -> directional cartridge valve -> Port in parallel with dump valve.

Is there any reason for this flow control valve to be partially open/closed? If I open it more, there will be less sudden pressure drop, and thus less heat generation. If I close it, all fluid should travel through the relief valve to the reservoir causing the pump to heat up more. Any thoughts?

 

[hydtools]

A circuit schematic would be helpful as would a description of the system operation, i.e. what is its purpose.

Ted

 

[downranger]

[URL unfurl="true"]http://imgur.com/gallery/0yXDt8o/new[/url]

Thanks hydtools. I have attached a schematic above. The flow control valve I am referencing is labelled as 31. The accumulator dump-valves are labelled as 30. Note that only Port 1 (P1) is in use. The system runs a mid-sized fatigue testing machine. Fatigue tests are going to be up to 60hrs in duration. The heat exchanger is sized for a system which is twice as large, so it is not a capacity problem in that respect.

 

[MikeHalloran]

The dumped flow needs to return to the tank through the/another heat exchanger.

Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

"The system has not been problematic in the past.". What changed, operation cycles? fluid viscosity? a component?
Was some servicing performed?

Ted

 

[downranger]

hydtools,

The only thing that we believe could have been altered is the state of the aforementioned flow control valve (several students have access). All other aspects have been verified as unchanged. Operational cycles will be changing, but we are still in the setup/calibration stage and have not run an untried cycle yet. No servicing has been performed - the unit is old (installed in '91), but has only 1500 hours of operation, which in my limited experience with hydraulic systems seems low.

Are you able to explain the function of the flow control valve in the accumulator dump line? Is it normal for the accumulator(s) to dump fluid back to the reservoir when there is no actuation occurring? I believe that if I close off this dump line the fluid will return back to the reservoir through the relief valve, which means the motor power is turned to heat at the relief valve as opposed to in the dump line. The two options seem to yield the same outcome.

MikeHalloran,

I agree that a heat exchanger on the return flow would be ideal, but the system has always functioned correctly without it up until the last time we used it several years ago. Could age be a factor in the heat problem?

Thank you for your responses

 

[HPost]

Has the logic changed at all?

Valve 31 only sees flow if valves 30 are not closed. If we assume that valve 27 have opened, but valves 30 are not closed, as they should and may be did before the problem occurred, the full pump flow will go over 31.

I expect that even at 7/8ths open, the valve is still restrictive and so is generating a high pressure drop. With such a high pressure differential over valve 31, the heat generation will be very high.

I would suggest checking to make sure that the valves 30 are all closing.

Valve 31 is there to provide some level of resistance in the dump line to keep the pressure up. It's never a good idea to let the pressure go to low as it can affect the function. It takes longer to build pressure and that can often have consequences on the function. That said, perpetual flow over the valve will make the system hot.

I see that there is water to oil heat exchanger on main return to the tank. You have got water flowing...right? It may be the design intent to have the heat exchanger pull out any heat that is added by valve 31.

If you have no water flow, or if the heat exchanger is clogged, as they do get in hot weather, your water flow could have dropped off and that will make your system hot. Oil flow over valve 31 will just keep adding heat energy to the oil and with nothing to take it out, you'll cook the system.

Check your water flow and check the operation of the valves marked 30.

Cheers

Adrian

 

[hydtools]

Good point, Hpost. Make sure the heat exchanger is working properly. I would guess the oil is already hot before going through the flow control. You will not suddenly feel high temperature at the outlet of the flow control. The heat exchanger control temp is indicated as 120F. Is the sensor working properly?

Ted

 

[HPost]

There is a thermostat control on the water supply to the heat exchanger. The drawing says it should open at 120F...you say you are getting 220F. I'll make a bet that the valve's not opening. Its not drawn with any control logic, so I'll take guess that its a thermocouple or wax stat of some sort.

Either there is no or low water supply, or the water inlet to the heat exchanger is not opening.

 

[downranger]

HPost/hydtools,

Thank you for your continued input. The heat exchanger does turn on, and water flow is audible. I think disassembling the heat exchanger and checking for obstructions is a good course of action. Both the of the water input and output lines of the heat exchanger become very cold, suggesting there is water flow in and out. I am now thinking that there may be an obstruction of the oil flow through the exchanger. While I'm at it, I will check the sensor too.

With respect to the valves marked "30" - under what condition should these valves be opening? Based on my understanding of accumulators, the dump valves should only open when the system is turned off so that the accumulator can automatically depressurize. Would one expect these valves to be open when the pump on, but no flow is being drawn from the "P1"?

 

[HPost]

Valve 30 should close to generate pressure at P1 - P3. If one or more don't close, there will be constant flow across the orifice.

Your water in and out should not be cold. Water in yes, but if there is oil flow and it's as hot as you say, the flow out should be warm.

 

[Lantraxco]

Need more data. The pump is pressure compensated at 3,000 psi, Electrically controlled as the sheet shows SV1 and "Low Compensator 500 psi" You should never see more than 3,000 psi in the system as the pump will destroke at that pressure. With the exception of spikes or malfunctions the system relief should never open under any configuration of valves or loads.

I would assume that if the #30 valves are "accumulator dump valves" they are a safety feature used to empty the accumulators on system shutdown to depressurize the system. Valve #31 is simply there to dump the oil in a controlled manner rather than shock the system and the reservoir with a huge rush of released flow.

If only circuit P1 is in use, then only valves SV3 and SV9 should be changing states, for flow to occur through the variable flow control 31, a linked pair of these valves have to be open in series. Valve SV9 is either malfunctioning or is being activated improperly if you have constant flow through valve 31.

 

[73lafuite]

1. Hello,
2. I think the distributors 30 serve only to drain the accumulators when the hydraulic unit is stopped by security. Thus the coils of these valves must be supplied continuously during operation of the power unit. The flow valve 31 prevents an excessive drain flow accumulator to prevent shock bladder against the anti extrusion valve of the accumulator.
3. Be aware that when oil flow does not produce mechanical energy in a cylinder or an engine, each pressure drop of 100bar produces a temperature rise of 5.5 °C fluid. Whether in a flow valve or a pressure valve.
4. A simple way to find out where the abnormal internal leakage is to start the power unit the morning when the oil is cold and not to move with the machine. Let it run continuously and with hand or a laser thermometer watch what pipes are hot. You can determine which valve is problematic.
Good luck.