Metal seated ball Vvlve leak 5

[zorans]

I have experienced following problems with 4" #1500 ball valves - metal seated, top entry, double-block-and-bleed.


4" #1500 Ball valve is with seats that are pushed by springs against the ball. In the middle of ball valve, there is 1/2" hole drilled to ball cavity in which we connected drain ball valve 1/2"#1500.

Here is the scenario...

1. With closed 4" ball valve was pressurized it from one side with 100 bar of natural gas.

2. With 1/2" drain ball valve connected to ball cavity we checked seat leakage by opening the valve and no leak was detected - which means metal seat is tight on that side

3. Using 3/4" bypass line around 4" ball valve we pressurized same ball valve from the other side on the same pressure - and still no leakage

4. 1/2" Drain valve was closed and 4" ball valve opened/closed - 3-4 times, letting gas go inside the valve and valve cavity

5. We tried to open 1/2" drain ball valve again to release the pressure from valve cavity, but after just small opening it was leaking and not stopping
for 5-6 mins

6. We closed back the 1/2" valve and tried to release the pressure after the 4" ball valve on 1" relief pipe down the line to atmosphere but the pressure remained 100 bar - like the ball valve is open

This all very strange to me.

I think it might have something with springs and metal sealing when the pressure enters the ball cavity -
maybe that pressure is causing springs and seats not to function well.

Hope for some help on this...

 

[tr1ntx]

4. 1/2" Drain valve was closed and 4" ball valve opened/closed - 3-4 times, letting gas go inside the valve and valve cavity

5. We tried to open 1/2" drain ball valve again to release the pressure from valve cavity, but after just small opening it was leaking and not stoppingfor 5-6 mins

When you tried Step 5, was the 4" valve open or closed?

Yes, it sounds like the seats might be stuck from relieving cavity pressure. You said the 1/2" drain valve continued leaking with a small opening, you may need to open it fully and try to "slam" the upstream seat on to the ball.

Who is the manufacturer?

 

[zorans]

When you tried Step 5, was the 4" valve open or closed? - It was closed...

Yes we tried to open it fully, but it was leaking so much.

Manufacturer is from China with API6D certificate.

What we tried in test lab is to leave 1/2" drain valve opened all time and the pressure was applied from the side of the closed ball valve.

Then open and close the valve quickly. As drain valve was opened, pressure was not able to accumulate in cavity. In this scenario valve worked well.

 

[JLSeagull]

Although the description sounds out of spec, API STD 598 permits leakage in metal-seated valves. Thd leakage rate depends upon the valve size.

 

[zorans]

I think that iso5208 is allowing 45 bubbles per second for dn100, but what we saw is very high flow from 1" relief pipe.

 

[tr1ntx]

"When you tried Step 5, was the 4" valve open or closed? - It was closed..."

Have you tried draining the cavity with the 4" valve in the open position? It probably won't change the situation, but sometimes not having the valve "pressure-locked" will let the seat free up and move against the ball. I would still call it failed though, even if that does cause it to seal. The seat should not become jammed, as it seems to be doing.

"Manufacturer is from China" . . . well, there you have it.

 

[zorans]

Actually we tried to drain cavity in closed position - I will try today in opened.

What is a bit frustrated is that our engineer was in China for witness testing, after that when valves arrived we tested at our facility, than valves were delivered to site and tested before the installation, than the whole installation was test and except minor problems we did not have any serious one.

The seat testing was done with water 1.1 times working pressure and 1.5 times working pressure for body testing as API6D requires. Manufacturer has API6D certificate.

At the end valves are not functioning ...

If the problem is China - why is API issuing certificates to them?

I have attached seat drawing, maybe you can see something that it is missing.

 

[JLSeagull]

The forgings and castings for most manufacturers are made in China, India or Brasil. The machining and assembly locations may differ among the manufacturers. Most clients are increasing their QA requirements largely due to these issues. Some clients are very satisfied with the shop performance in China.

 

[tr1ntx]

So, this valve has been tested previously, with natural gas, and worked fine, but now at start-up it's doing this. Is that the case?

If that is the situation, it makes me wonder if there is sticky condensate or pipeline trash that has gotten in and made the seat stick in its bore. Welding rod stubs and weld slag cause tons of problems.

If the valve has not been tested with natural gas previously, only with water, I will opine that the different flow forces of the natural gas do not overcome a design or manufacturing problem which causes the seat to bind.

 

[zorans]

Valves were only tested with water. During startup with natural gas, problem is started. Actualy, once high pressure fluid enters valve cavity, problem starts. If the pressure is 50 bar, valve is working ok. If the pressure is 100 bar seats are like stuck and closed valve is leaking very much.
One thing we will try - to use stronger springs, but I am not sure that it can be solution.

 

[tr1ntx]

It worked at 50 bar. Then you went up to 100 bar and it leaked. Did you go back down to 50 bar after that? If so, did it work or still leaked?

I'm curious, is that a pig receiving valve?

 

[zorans]

It worked again on 50 bar.

 

[zorans]

No it is just regular ball valve on gas storage facility.

 

[zorans]

We now tried the other same size/rate valve.

From one side it leaked on 15 bar. We than tried on 10 it was good. Again on 15 and it leaks.

After that we tried on the other side same valve, and it was good on 10, 20, 30, 40,50 bar and on 65 it leaked. Than tried again on 50 and it was good. Again on 65 and leaked again.

It looks like on some pressure seats are moving away from ball and can not come back to seal, like it is bind.

Now, why is from one side holding only 15 and from the other it is holding 50 bar is strange.

 

[tr1ntx]

I'm beginning to think this is just a seat problem. Like you, I at first thought the seat was getting in a bind and not returning to contact after the body cavity was pressurized. But when you said that it worked at 50, then leaked at 100, then when you went back down to 50 bar it worked, that makes me wonder how the seat would have come unbound. Now you told us another identical valve leaks at 15 bar on one side and 65 on the other. This situation sounds like the seats won't seal consistently for gas, irregardless of cavity pressure.

What operating temperature was specified when the valves were chosen? I assume it was high because of the metal-seated arrangment. Are you at that operating temperature when testing?

 

[zorans]

Temperature is up to 40 deg C. Valves are metal seated due to very polluted gas as it is for underground storage facility and for high pressure - 275 bar operated pressure.

We tested with nitrogen from bottle, so pollution is not the case for our our testing.

 

[tr1ntx]

OK, it is not low operating temperature causing a problem. It is not pipeline pollution. The fact that the second valve won't reseal at 15 bar on one side doesn't seem as if it would be caused by the seat binding when pushed back into its bore by cavity pressure. On the other seats which worked at 50 bar, but not above, the binding seemed to be a likely scenario.

If you take the valve apart to change springs, do you have a way to precisely measure the ball diameter? I'm curious to see what the ball-to-seat mating shows.

 

[gerhardl]

Leaking from one side at lower pressure. Possibel cause: difference in geometry of forces from this side: less exposed area on seat sealings for pressure of seat sealings against seat and (area x pressure) (or) difference in other mechanical forces if pressurized from this side?

OK at 50, not OK at higher, OK at fifty again: deflection or non permanent (automatically reversibel within limits) deformation of mechanical parts of seat sealings? Caused by reasons as above?

Solution? Stronger sealing material or strengthening of seat sealings by altered seat-sealing geometry?

 

[zorans]

@tr1ntx

We always first test seat just with pressure from one side in ball closed position and without pressure in valve cavity and on air 6 bar and water 250 bar, seats are tight - no leak.

The problem starts once pressure enters valve cavity.

As ball is in one piece with stem we measured ball diameter change on the place where seats are and ball is accurate to 0.00 millimeter - looks like perfect. But stem has 0.05 mm error in coaxial sense. It is above 0.03mm which manufacturer designed.

My opinion is that the purpose of springs is only to cover these errors in geometry.

@gerhardl

Do you think that with stronger springs this can be fixed?


We just tested third valve. From one side it leaked at 15 bar, from the other working ok at 90 bar. At moment, we have no option to try above 90 bar. Maybe tomorrow.

 

[tr1ntx]

OK, now with more information, we see that it is not a seat seal problem.

In addition to covering small errors in geometry, the springs provide the initial sealing force so that there is not leakage as pressure begins to rise from 0.

Stem-to-ball axial alignment: 0.03mm (design) versus 0.05 (measured), that is a large percentage variation, but in terms of the actual distance, I wouldn't think 0.02mm would be a large enough error to cause leakage. Like you said, the springs should make up for that. But I don't know the spring rate or how much deflection they have.

Stronger springs should help, although at only 15 bar, the seats shouldn't be binding in the first place. Stronger springs might make it work, but they might just mask the real problem.

Then again, perhaps the springs are designed too weak in the first place and are thus incorrect for the valve assembly.