How to prevent a valve from sticking

Members of the chemical-processing industry address the question posed by a reader of Chemical Processing magazine: What is causing the valves to stick? Possible causes offered include undersize actuators and fluctuations in the plant air supply.

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Proces Puzzler from the July 2004 CHEMICAL PROCESSING magazine

 

In one of our units, we are processing a mixture of heated hydrogen and light hydrocarbons. In one section of the plant, there are a lot of valves with air-operated actuators. Once in a while, one of these valves will stick, shutting down the process unit. The valves, which open and close about every two-and-a-half minutes, have 20 seconds to reach the positioner before going into alarm. Almost all of the valves in this section of the plant have been stuck at one time or another. We generally have to force the valve open with a large "cheater bar." What is causing the valves to stick?

 

Ensure seal is rated for hot-hydrogen service
Here are a couple of things to explore:

1. How good is the initial seal for hot hydrogen service? A sizeable investment may be needed to upgrade the stem seal.

2. Can the actuator/stem be enclosed in some sort of housing to keep the dust and grit away from the oiled stem when the valve is open? If so, keep in mind that the "cheaters" may have damaged the stems and the valves may need new parts before an effective seal is even possible.

3. Consider an actuated valve that does not containa rising stem but is already contained inside the seal and has no exposed surfaces.
Kirk Johnson, technical associate
ISP Technologies, Texas City, Texas

Actuators are undersized

If it's possible to regularly blow some sort of air through the system every week or so, that may solve your problem.
Jonathan Newsome, maintenance planner
Georgia Pacific Resins, Conway, N.C.
There are two things that concern me. The first is the definition of "light hydrocarbons." C12+ unsaturated hydrocarbons would be suspect candidates, and they could easily be traced by a gas chromatograph or mass spectrometer analysis at any competent laboratory. The second is the moisture and salt (by salt I mean almost any inorganic) content of the stream. It is truly amazing what will pit a valve, given the right circumstances.
Tom Richard
ChemVantage Consultants LLC, St. Louis
The first thing to do is to check the instruction manual and study the construction and operation of the valves. Next, contact the manufacturer. They know more about their product than anyone else. On your side, you may want to check your air supply for cleanliness (water and other material that can block the airlines) and leakages.
Rey Flores, operations engineer
PPG Industries Inc., Berea, Ky.
It is possible that the control system/valve feedback is not well calibrated. I have seen this before. Another possibility is that the actuator safety/trip switches are set incorrectly, which causes a closure that is too tight. If this were the case, however, electric actuators would overheat. If you have an air-driven system, there is probably no chance of overheating.
W.P. Stewart, P.E., president
Stewart Technology Associates, Houston
1. First check to see whether there are fluctuations in the plant air supply. Next time a valve sticks, check to see what the plant air pressure is reading.

Determine the composition

Contact the supplier

Calibrate the control system

Check the air supply

2. Ensure that all flow-control valves and check valves on the actuators are operating correctly.
Joe Turnbow, maintenance supervisor
Quality Films Inc., Schoolcraft, Mich.
A few things to consider:

Don't overtighten the packing

1. Since you mentioned that valves around this area of the plant have gotten stuck, evaluate your torquing procedure as it relates to tightening packing in valves. You might be overtightening the packing, causing the valve to freeze up. Also, if the packing is too tight and isn't compatible with the fluids passing through the valve, this could contribute to sticking problems. I would evaluate the compatibility of the packing and the fluids. Make sure operating conditions do not exceed recommended temperatures and pressures for the packing.

2. Check your operating conditions, especially pressure drop. Make sure there is no moisture present in the hydrogen and light hydrocarbon stream. If you are experiencing the Joules-Thompson effect, and moisture is present, you could be forming hydrates inside the valve over time, causing it to stick. Make sure the mixture is dry as a bone.
Eric M. Roy, principal engineer
Westlake Group, Sulphur, La.

Use higher-pressure air

From what I can picture, these valves are relatively slow-opening and your actuator could be limited in strength. In several situations we have used higher actuator supply pressure (say, 80 psig) to ensure valve function. It also is possible to get a stronger actuator.

Randy Miller, senior staff engineer
Syngenta Crop Protection Inc., Cold Creek, Ala.

Install PTFE seats and packing

Your valves probably have a seat composition or stem-packing composition that is degrading in the constant presence of hot hydrogen. When this happens, these components swell and eventually bind the valve. Try installing one or two valves that use polytetrafluoroethylene (PTFE) or glass-filled PTFE material for the seats and stem packing, or refit some existing valves with these.
Lisa Miller, process design engineer
Icosa Co. Inc., Greensburg, Pa.



Diane Dierking is senior editor for Chemical Processing magazine. E-mail her at ddierking@putman.net.

I think you have one of two problems. Either there is some type of residue being left on the valve seat from the process or the actuators are too small. There are other possible problems, but these are the most likely and the first two that I'd check.

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