Compressed Air Ball Valves - Will Not Close

[Charlesan]

Our customer has 3" Reduced Bore Carbon Steel Ball Valves installed on a compressed air line,used as isolation valves,have been in the open position for long periods,need to isolate the system for work on other parts of the plant but cant close the valves,seats are Virgin PTFE,compressed air is 6 Barg with a lot of moisture,any ideas

 

[JLSeagull]

Some ball valves include lubrication connections on the body. If so equipped, try your luck with the recommended lubricant.

 

[JimCasey]

Compressed air usually has significant amounts of water in it. When you compress the air, you compress the humidity, too, and it condenses.

You say the valve body is carbon steel. The BALL may also be carbon steel. In that case it has probably rusted enough to make it impossible to close. OR the body has rusted and the rust has squeezed the (Virgin PTFE) seats against the ball so hard that they coldflowed around the edges of the port, locking the ball in position.

One of the big focuses in the instrumentaion industry lately has been Partial Stroke Testing for Emergency Shutdown Valves. On some regular basis the valves are cycled enough to know if they are going to be able to close when it REALLY matters.

 

[Charlesan]

Thanks for that,the ball is stainless steel,agree with your point on rust impaction on the seat,we have also advised the partial stroking of valves as part of the service plan,will let you know what the valve looks like when we get the valve open

Thanks

Charlesan

 

[BValve]

3inch with 6 Bar service pressure ball valves are commonly floating ball type design with stainless steel trim. JimCasey assumption (dust)sounds to be your problem.
Unfortunately, no flushing port on floating type ball valves.
Partial stroke devices remain expensive for a 3 x 2 ANSI 150 rated ball valve. I suggest you to change them to trunnion mounted with Stainless trim plus a SS overlay or ENP plating in seat housing. It is more expensive but you will save money on controls and partial stroke device. Double block and bleed Trunnion mounted ball with drain port plug will also , as a routine, allow you to drain any liquid entrapped in valve body either in open or closed position.

 

[BillBirch]

Solution
1. Get four to six metre scaffold tube and slip oner the operating lever of the valve.

2. Get largest employee to hang on the end of the tube in the direction of valve operation - it will open!

 

[JimCasey]

BillBirch:
You know that's going to happen but it is probable that the stem will break or twist, or the slot in the ball will be deformed. Somebody will get a booboo (or worse) and the lawyers will become involved.

 

[jistre]

This is interesting, Charlesan. Keep us posted on how you deal with it because this is one of those operations questions that might prove very helpful to know the answer to.

Of course, my first instinct without seeing your P&IDs is to forget about getting the valve to operate and just go to the next block valve upstream of the offending valve, block that whole section off, replace the offending valve, reopen the blocked section, then use the new valve to block off for the planned maintenance. Of course, I don't know your plant's air supply layout, and this may not be possible.

I'm interested to find out what you see when you open the valve up. If the air gets hot enough because of the compression, then creep of the virgin PTFE is a distinct possibility. If you end up changing the valve out, you may want to consider using a "less creepy" seat material over the virgin PTFE.

 

[Charlesan]

Thanks jistre,you make a good point about PTFE and "cold flow,coupled with the dirt impaction on the seat and into the cavity should prove to be the problem,going back to PTFE a friend has told me they always used "Acetal" seats on this service for this very reason,I think this is some form of Nylon material, we are also looking at installing three piece swing out ball valves on this job,so at least we can get at the seats in a hurry if we need to in the future,will let you know how things progress

 

[JimCasey]

Find your local Worcester distributor and ask for a copy of Technical Paper 12D: Failure analysis of Ball Valves. It has lots of gory pictures of destroyed seats and other internal valve parts, and reasons that they failed. Then when you pull your valves apart you'll be better equipped to recognize the way to avoid repeating the problem.

 

[gerhardl]

Looking at all above answers, all most common failure causes seems to be covered. Two points come to my mind though:

1. 'Cant be closed' as in 'stuck in 100% open positions, not able to move at all' or 'able to move seemingly to closed or almost closed, or partly, and then stuck leaking'.

Or 'air actuator not having sufficient torque?'
All valves behaving equally? In case not: Difference of placement in system?

Reason for questions: The probability of the cause(s) will vary according to the detailed description of the failure.

2. After the information already given in your description: I would guess at a combination of water and dirt, perhaps in combination with traces of oil, rust, (or even particles that 'shouldn't be there (winding tape, cutting spoons, excess dirt) dried on ball backside when open, gluing the ball to seats and also maybe the stem to stem sealings.

The clue to all is that with a good quality of ball valve, and a proper constructed and drained and filtered air system, stuck ball valves should not occur at all.

Recommendation: Recheck the complete air supply system. All indicates that investment in better drainage / air drying and filtering points will repay in less wear and maintanance.

Drawing of seats is less common under the given circumstances. Cause for drawing of seats is most often either poor or wrongly selected valve quality or (most probably again) excess dirt and smudge in system, hindering proper closure or opening, giving uneven force distribution on the seats.

Hair thin openings (almost, but not total closure of the ball valve) can give supercritical speed, seat drawing and cavitation damage. This is more often seen at higer pressures and temperatures, and are more typical for ballvalves in steam systems.

And as mentioned: A good quality 3-piece ballvalve in all SS with standard teflon seats installed would also probably repay. At normal conditions floating seats would be OK. Check and move valves at regular periodes (once a year or half year should be more than sufficient if the air system is good).

Standard procedure in our workshop, in activity with air instruments since 1927: If anything doesn't work, take it carefully apart, clean it, dont do anything but put it carefully back together again and see if it works then.

This advice has worked for the last 80 years, and are still as good, even if no one ever will admit that their air system is anything but top quality.

Good luck!

 

[htownvalve]

Is this the Technical Paper you're wanting?

 

[DrJim]

Charlesan,
One or both of two areas can be holding your valve open. One, the stem may be locked from turning due to rust (oxdiation if you like that word better). Two, ball may be locked from turning for whatever reason. It could be that rust has formed on the back side of the seats and caused the seats to be pushed into the ball.

First check to see if the stem can be turned back and forth just a bit. If so, chances are that the stem is ok. If so, it is probably a bound up ball - which I would expect. A steady application of torque, and lots of it, should turn the ball, if it is going to turn. Stem is probably 316 ss also. Chances are, it will twist if not break in shear due to the torque. The stem slot in the ball is usually not a problem, the stem part the goes in the slot may shear with only minor deformation to the slot in the ball. Depending on the design, it is hard to say where the stem will fail.

Use good judgement and avoid doing something stupid along the way.

Over time, PTFE seats will find the uneven spots in the ball and form around them which, in effect, glues the ball to the seats. Also, the seats are typically crushed between the body and ball during assembly in this type of valve. Rust just increases the crush and ball seat binding.

In this application, it is hard to envision chemical deposition of minerals to the ball to bind the valve. Also, dust and dirt are probably not a problem.

For a replacement valve, consider a full stainless floating ball valve. You can go to a trunnion with maybe a better chances of turning, but which is more costly and has a higher chance of leaking when closed along with other potential problems not assoicated with floating ball valves.

Wishes for a successful solution. Let us know how the problem is resolved.