Closed loop pneumatically actuated valve operation 3

[dmertz]

A potential problem in one of our quick close valves was brought to my attention.

During normal operation, the actuator to the main valve is held closed by an 80 psi instrument air line. Upon loss of air, the main valve is meant to close quickly by way of a quick exhaust valve (QEV).

Please see the linked figures:
1.

http://files.engineering.com/getfil...e2b4d3fb52&file=Normal_operation_of_valve.JPG

2.

http://files.engineering.com/getfil...b9c5&file=Loss_of_air_condition_for_valve.JPG

3. Further description of the QEV operation can be found here:

http://files.engineering.com/getfile.aspx?folder=6e71a13a-3b50-4c0f-b8d2-4cc10ca738c5&file=QEV.JPG).

The potential problem seems to arise on cycling the main valve. There appears to be no vent to atmosphere, thus when the main valve loses air supply, the QEV vents air back into the actuator, equalizing the pressure and increasing the closing speed of the main valve (spring return closes the main valve).

When the main valve is opened again, that air remains in the line. Thus when the actuator closes, it has to overcome a greater pressure in the line than it did initially. For example, equalizing pressure occurs at 50 psi, then 50 psi must be overcome to close the actuator. On the next cycle, however, the air will still be in the exhaust line and the equalizing pressure when the air supply is removed will be higher, say 60 psi. Thus as the valve is cycled, it appears that it will eventually reach a state where the air supply pressure will not overcome the pressure in the exhaust line of the actuator.

If someone could explain how this closed loop system is supposed to work, I would appreciate it. I am not very experienced in closed-loop valve systems so I may be missing something. Thank you.

 

[terje61]

Hello Dmertz,

To me it seems that you have an on/off valve that has a spring to close pneumatic actuator.

In closed position the actuator has no air pressure. When you feed 80 psi instrument air to the quick exhaust valve, probably with a solenoid valve, it will direct the air flow to the actuator and the valve will open.
The air on the other side of the flapper is pushed out.

At this moment you have 80 psi on both sides of the quick exhaust membrane.

The moment you switch the solenoid valve the supply line will go to 0 psi and the quick exhaust will trip. All instrument air in the actuator is relieved at the quick exhaust valve. The springs will force the actuator to close completely. The back side of the flapper will suck in fresh air from the outside.

I do not see how you can vent air back into the actuator while closing the main valve.

Gr.
Terje

 

[JLSeagull]

You may have a closed loop purge in the actuator air system. This is very common offshore to avoid salt laden air from entering the actuator. Often the exhaust includes a pilot valve with a check valve to the atmosphere. The solenoid valve is not connected to the actuator, instead activates the pilot valve. In one case the pilot valve applies instrument air to the actuator. In the trip case the pilot valve blocks the air supply and bleeds the actuator. The bleed port is also connected to the vent side of the pilot valve so that the instrument air bleed fills the cylinder instead of salt sea air. Perhaps you can find a typical example at the engineering toolkit site or via a search.

 

[gerhardl]

I have had a quick look at your schematics. Maybe I am missing something, but why the closed loop? Other driving gas than air to be controlled?

Your actuator seems to be a quarter turn (or perhaps a bit more than 90 deg) - vane-type actuator, and I assume from drawing that the spring is outside the two air chambers in the actuator. (For reasoning below the spring placement does not actually matter)

The actuator is in principle constructed as a double-acting (two chamber) actuator, with assistant (fail-safe) spring force.

The force from air in the return chamber could be 0 (spring force alone closing), but the chamber has at least to be vented to air for in - and out (or to closed loop with considerably lower pressure than the operating in-pressure in the in-chamber) - all to avoid problems.

Normally this is solved for emergency closing valves with a 5/2 direct acting universal solenoid valve (or 2 pices 3/2 way) with high capacity and perhaps SIL- certification. (Or standard pilot operated solenoid valves if suitable, but might block if samll pressure differences between in and out)

If this does not give short enough closing time, the pipe of the in-chamber has to be fitted with additional standard quick-exhaust valve for return.

This is exactly like your solution, but without the closed loop, causing you the problems.

If the closed loop is essential for gas control: split the loop, use a solenoid solution as above with separate return and pressure ports, and arrange a separate low pressure vessel with low-pressure control for return side to guarantee the necessary operating pressure differences.

Please comment, perhaps additional suggestions might be given.

 

[dmertz]

Thank you all for the great comments.

Terje: You are correct as to the operation of the valve. There is indeed a solenoid valve. It is a 3 way pilot operated quick exhaust type (Open to vent / Closed to air supply in de-energized state). The bleed from the quick exhaust does indeed go back into the actuator. This is supposed to increase the speed of closing the main valve.

JLSeagull: This is exactly the system we are trying to use. In our case, however, the worry is that the solenoid pilot valve does not bleed off the air inside the actuator. It seems that the quick exhaust valve closes to the pilot valve and re-routes the air to the actuator before much air can bleed off. We found this system (

http://documents.rotork.com/docs/AC...s/SPECIAL APPLICATIONS/Closed Loop/F603E.pdf)

which is similar to ours. However, the system shown in this PDF has a spring-loaded check valve to bleed off air on one side of the actuator. Our current system does not have this ability. We were thinking of adding it. Thoughts?

gerhardl: We are using a closed loop to avoid ingestion of hazardous/corroding gases into the system. The valve is used in a natural gas environment. The spring action for the actuator is indeed outside the actuator--on top of it really. The actuator is quarter turn and constructed for use as you say. I appreciate your suggestions and will look into different valve arrangements. However, the most simple solutions seems to be to add a spring-loaded check valve as shown in the PDF linked above. Could you comment on that?

Thanks again.

 

[JLSeagull]

I don't like the needle valves shown on the Rotork detail. These are easily left nearly closed, thus the valve not functional. Control the air flow rate with the tubing size.

Look at the Versa VSP-3501 series normally closed pilot relay valve. The VSP-3301 should be OK for valves up to about NPS 3. Use a check valve to the atmosphere that is as large or larger than the tubing to minimize the pressure drop.

 

[dmertz]

JLSeagull: Our current design does not have any needle valves. Thanks for the suggestions on the pilot valves.

I spoke with the designer of this system and they gave me the following information: On the fitting connecting the quick exhaust valve to the actuator, there is a hole drilled venting to atmosphere to prevent the air lock condition that we were worried about. So that solves the problem.

I thought there was an environmental issue with outside gas being ingested, but apparently the only reason for using the "closed loop" was to get an air assist to the actuator when closing the main valve.

Thanks again for all the help.

 

[PeterIgg]

Maybe I missed something, but if the closed loop (or more correctly, I feel, a non-breathing) system is used to assist the spring then the actuator is simply undersized and the best solution is either to increase the spring (and pressumably the air pressure too) or upsize the whole actuator. As I understand it the ONLY purpose of the non-breathing system is to keep the ambient air out of the cylinders, using the 'clean' instrument air to fill the void left by the operation of the actuator. You certainly need to include a check valve to release the pressure to avoid the system locking up when re-energising, as you have discovered and as per the Rotork circuit.

If you want to assist the spring I would suggest using a 5/2 SOV in lieu of the 3/2 you have and use pneumatic supply pressure to assist the spring stroke. The down side of this is that it relies on the presence of the air system that may not be available in an emergency - a local vessel may assist in overcoming this but now we are in 'sledge hammer to crack a wallnut' scenario. You need to be careful, though, not to over stress the stem and drive train if you go this route, you will probably need to incorporate a regulator to reduce the supply pressure to the 'spring' side.

 

[dmertz]

Thanks for the input, PeterIgg.

On this particular valve we require a closing time of <100ms. The air assist is not for the valve to function but rather for it to meet this quick close requirement. Adding a pressure vessel as you said does not seem to me the best option since this is a fail-to-close on loss of air.

 

[gerhardl]

I agree somewhat with some of the latest comments anyway, even if this is failclosing with spring at loss of air:

a) Spring should be checked to give enough force at all situations. If not, change to stronger spring within allowable construction limits.

b) If not fast enough closing : Check for all possible return air restrictions, both quick venting valve, solenoid valve, return pipeline dimension and port restrictions at actuator itself etc.

 

[PeterIgg]

dmerz,

I am getting a better feel for your problem. Is it still existing or have you solved it? Either way I would be interested in an update.

What is the size of the valve? Swept volume of the actuator? Basically it seems to me that it is really an air flow problem - how to get sufficient volumes of air in and out. I still think that the 5/2 SOV may assist in the solution as you will be preventing the partial vacuum from forming as the spring decompresses (I am assuming a piston/spring actuator as per standard Bettis/Rotork construction), pushing the piston. Have you tried upsizing the air connections? Not that I was achieving the speeds you are searching for, but I recall changing 1/2" lines to 3/4" lines on one job made a surprisingly big difference.

 

[dmertz]

PeterIgg: The solution that was taken by the valve supplier was to bore a small hole in the fitting on the inlet side of the actuator. In answer to your question about speeds, the fittings in the exit line from the actuator were bored larger to allow more airflow and thus faster closing speed. The valve is 2" and the actuator is Kinetrol Model 089-120. I can calculate the swept volume if you are still interested, otherwise I would consider the issue closed.

Thanks for your help.