Hydraulic valve-actuation

[zdas04]

I've used pneumatic actuators on control valves my whole career. Periodically, I've tried to recover the exhaust gas (using field natural gas which has an intrinsic value at low pressure instead of air which doesn't) with pretty crummy results--getting a pneumatic control valve to go fully shut with residual pressure on the diaphragm is a challenge that I have not been able to overcome.

I'm currently working on a project where I have pressurized oil (on a flooded screw compressor) available. I've used this oil to drive unloader-valve actuators with gvery good results. Now I'm thinking that there is some real benefit to using the oil to actuate the control valves. My problem is that I can't just waste the oil at the end of the control cycle, I have to recover it. I can dump it into the suction side of the compressor (about 40-100 psig lower pressure than the supply pressure) and have it flow into the process back to the oil accumulator, but the exhaust pressure will still be non-zero.

I'm wondering if setting up a hydraulic actuator for a non-zero rest pressure is more reliable than doing the same for a pneumatic actuator?

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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[hydtools]

Does the flooded screw compressor draw its oil from a non-pressurized reservoir? If so, return oil to it.

Ted

 

[MikeHalloran]

>>I'm wondering if setting up a hydraulic actuator for a non-zero rest pressure is more reliable than doing the same for a pneumatic actuator? <<

No.

Both rely on a pressure difference, and will function normally if sufficient difference is available, but a pressurized reservoir or return sink does not count toward the difference.


Mike Halloran
Pembroke Pines, FL, USA

 

[zdas04]

hydtools,
If the reservoir had been at atmospheric pressure, the question would have been trivial. The process takes oil out of the high pressure reservoir and injects it into the low pressure side of the oil-flooded screw compressor, the compressor boosts the pressure back up to the pressure of the oil reservoir. I want to use some of the oil in this process to do work before I dump it into the compressor.

Mike,
I'm not sure I understand your post. With pneumatic equipment (which also relies on a differential pressure) the process starts getting "mushy" if the exhaust pressure is non-zero and it can take considerable time for a control valve to go fully shut even when there is adequate dP. My best guess is that you get pressure oscillations in the exhaust piping because of the springiness of the compressible gas. I just know that I've had serious operational difficulties when I've tried to capture the vented gas from the end devices.

What I'm wondering is if I use oil to drive the control valve if the valve will experience the slow, inconsistent evolution toward closed that I see in pneumatics.

David

 

[MikeHalloran]

I was trying to say that hydraulic valves don't like backpressure any more than pneumatic ones do, but they will misbehave more smoothly.

In both cases, you are effectively running both systems with a supercharged return reservoir. Except for considerations of body strength and such, the pressure of the HP supply relative to atmosphere doesn't enter into the picture; only the difference between the HP and LP source/sink.

I.e., you can't get something for nothing. The full HP pressure is only available if you exhaust the working fluid to atmosphere.


Or, you _could_ get gas-driven reciprocating hydraulic intensifier pumps, e.g. from Haskel, to operate between any reasonable pair of gas pressures on the gas side and produce arbitrarily high hydraulic pressures on the liquid side. That's probably more complexity than you want to add, but it can be done.



Mike Halloran
Pembroke Pines, FL, USA

 

[zdas04]

I'm just trying to use energy I was throwing away across a choke to do a little bit of work very intermittently. One of the valves is a level control dump valve. It dumps 5-6 times per day for 10-20 seconds each time. The valve requires 10-15 psid and I have somewhere between 40 and 100 psid. The numbers work out on paper, but they worked out on paper for pneumatics too. When I open the "vent", a spring should slam the valve shut. In pneumatics the "slamming" action is mushy and I tend to blow the vessel dry and blow some gas into the water system. That is what I am trying to avoid by looking at hydraulics.

David

 

[hydtools]

You are saying you have a spring-return valve actuator, a single-acting actuator. Then in either case the spring must return against whatever pressure is on the actuation side whether it is pneumatic or hydraulic. Whether or not the spring 'slams' the valve shut depends on the spring force being much greater than the exhausted actuation side pressure.

Ted

 

[zdas04]

That's what I thought too in pneumatics, but some sort of noise or feedback in the exhaust piping complicated the process dramatically. Not sure if oil will have the same problem.

David