partially close ball valve for receiving pig

[Jinnie]

I have a trouuble with pigging operation. Let me introduce you first. see the picture attached for your reference. And if you know pigging operation well you will understand myquestion easily. While receiving a pig, if the bypass line at receiver station is not large enough and I could not fully close mainline valve at receiver. So I decided to partially close mainline valve with fully open bypass line. The objective of partially close mainline valve is to make almost flow go through bypass line and the pig can reach pig trap.

My question is mainline valve is ball valve how damage on that ball valve?

 

[DanTh3Man]

Without careful selection of the valve and design of the downstream piping you're going to have a problem. Operating a ball valve in the partially open position will direct the flow to either the valve body, the flange, or the downstream piping wall instead of down the bore of the line. This can in turn cause significant erosion and lead to a loss of containment.

If you want to do this on a regular basis I would recommend selecting a ball valve that will survive this (consult with a valve supplier) and you should have a chromium-carbide overlay spool that can be easily replaced immediately downstream of the valve.

 

[Danlap]

No one could provide a definitive answer to your question.
Damage means literally (after pigging) is worst condition compared to before. Question: How are/were the leak rate and ball condition 'before PIG-ing'? Before and after result should be comparative and have reference.
General idea of PIG is to clean the pipeline. Subjected substance to be cleaned is it polymerization (of medium), sand/dirt or other deemed abrasive medium?
And how long is the duration this PIGing process (how long the exposure of this valve at full DeltaP)? soft/metal seated, single piston or double piston effect, Size, pressure, velocity and partial opening of the valve, among many others parameter?

It can take a split second to damage the ball/seat if there is any hard particle e.g. sand inside pipeline. Hence afterwards valve will leak.
But it might also take some quite of time to damage partially open ball valve.
Some theories behind this:
- Flow induced vibration. Aim for ρv²< 10,000 kg/ms² for liquid and multiphase or less than 50,000kg/ms² for gas. Above this vale susceptibility to failure is medium to high. Again, in conjunction with time exposure. Those values are more for weeks if not months in service, not sure whether it is comparable with 1-3 days of PIGing activities.
- Velocity (through) obturator (ball, seat, etc) shall be less than 8 or 12 m/s

It is like asking the question. is it safe to operate up to 500degC for carbon steel valve for 1 day only, I have concern over thermal expansion. Is my PTFE ball valve safe during half day offset up to 260 degC, will it still be leak tight?
All of those answers (before the actual event) are mostly subjective. If all disciplines agree that risk is ALARP, then it is ALARP.

Regards,
MR

https://nosuchvalve.com

All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

http://www.eng-tips.com/faqs.cfm?&rat1=2&fid=376

 

[Jinnie]

Thank you for your best answer. this is gas pipeline with some debris and liquid hydrocabon. when pigging all those substance could come into valve directly.

Ive known well this is quite bad for ball valve to do like this. Normally I do this way only 10-15 min per time and 3-4 times per year. I don't have bypass line across mainline valve so I could not inspect aspect of valve and seat.

Due to this procedure have been performing more than 10 years, I supposed this valve can not be 100% seal anymore. Anyway lucky that this valve is not a shutdown valve. this process still have SDV behind this valve.

Thanks and regards
Jennie

 

[LittleInch]

Well this does beg a few questions.

When you say bypass line is not large enough what do you mean exactly?

I know sometimes people have put in very small lines for reasons I don't understand, but even then your pressure drop shouldn't be big enough to change flow too much?

What sort of velocity are you doing?

The difficulty with this operation is how much do you close it? Not enough and the pig won't move once it gets to the trap. too much and there's so little bypass it's not worth doing. There are no real singlas or flow or pressure indication so how do you control it?

I don't believe in this partial valve closure business for pigging for the reasons shown in this thread.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jinnie]

Dear LittleInch

I supposed to mean overall gas connot go through this bypass due to the pressure downstream extreamly decrease after the mainline valve is closed. I didn't say about te velocity because it's sure that the velocity go higher than 120 ft/s.

Not every of my pipeline is in this case. Only old one which has old design that 1/3 size of mainline is enough for total flow rate when pigging.

Normally when pig is coming to receiver (2-3 hrs before estimated time) I will partially close (maybe 50-60%) and the pig is going to pig trap easily. But except ILI pig. it stuck at the main tee. Then I will slowly close the mainline valve until fully close the ILI pig can go into pig trap.

Jennie

 

[JChanMe]

Ball valves are not designed for partial operation!! Not even the metal seated, it is intended for fully open/closed position. For soft seated valves, the seats will be damaged easily.

James Chan