3 replies to this topic

[Roark]

Hello

I am working in an LNG project, and have been tasked with marking up the ball valves in the project with the appropriate orientation of the cavity vent. As you know, the "cavity vent" is a small hole drilled in the ball of the valve so the LNG trapped inside the ball can be relieved and will not cause overpressure if/when it warms up and vapourises.

My concern is about the orientation of such vent. In some references I have read that it must point to the "upstream" (or "high pressure") side of the valve, but in other cases I have read the opposite.

My common sense says that it must point to the direction where the potential for overpressure is lower (e.g. if the section of pipe upstream the valve is protected by a TRV, it must point upstream; if the TRV is located downstream, it must point downstream), but need some "support" (either technical or moral ) to back this up.

If you have any experience of this and could provide some feedback/reference to support this, it would be greatly appreciated.

Regards

Edited by Roark, 06 July 2011 - 03:32 AM.

[Art Montemayor]

Roark:

About 50 years ago, when ball valves started to be applied in the chemical processing industries, I came to the problem of traping saturated liquid within the cavity of the ball valve and not having a relief for it when it subsequently vaporized. I started to instruct my valve supplier to drill a small (approximately 1/8" diameter) hole that communicated to the UPSTREAM side. Other engineers in industry at the same time probably also found that this was a quick and acceptable manner of getting out of the potential problem. I don't know if they also specified the upstream side to be vented, but my reason was because I reasoned that if I did it this way, I would have a more positive pressure on the ball and this would allow it to seat better and ensure a secure, no-leak situation.

As I progressed through my career I noticed that indeed, just about every ball valve that I found vented in other installations also had the upstream side vented. I accepted this as a confirmation that most other engineers agreed with my reasoning. If anyone has a different reasoning or explanation for which direction the vent hole should be directed, I would be interested in knowing their version.

[Roark]

I don't know if they also specified the upstream side to be vented, but my reason was because I reasoned that if I did it this way, I would have a more positive pressure on the ball and this would allow it to seat better and ensure a secure, no-leak situation.

Art

I am sorry, I did not fully understand your explanation. Could you please ellaborate a bit more on why would there be "a more positive pressure on the ball" when venting towards the upstream side of the valve?

Please have a look at the attached picture. It shows a section of an LNG line that bypasses the HP pumps (hence the S1A/S9A spec break upream of V4T027. This valve (V4T027) has its cavity vent orientated towards upstream, as you suggested. Vent valve VV4T018 also has its vent orientated upstream (otherwise the vapourisation of the LNG will pressurise the blind downstream of the valve). However, V4T028 has its cavity vent towards the downstream pipe. I think this is because it must vent the interior of the ball towards the side that will have a lowest potential for overpressure. In this case, this side is the downstream, as it is connected to the discharge header of the HP pumps (which as you know flow to the vapourisers and the sendout line).

Also, I thought (perhaps naively) that pointing the vent to the low pressure side you will be "promoting" the vapourisation, or making it easier, but I guess this does not really make much sense.

Once again, thanks for your thorough and knowledgeable explanations.

Regards

Attached Files

•  cavityvent.JPG   13.01KB   122 downloads

[Queiroz]

Roark,

I can identify two main reasons for placing the hole UPSTREAM.

1. As you wisely said, cavity vent is a small hole drilled in the ball of the valve so the liquid trapped inside the ball can be relieved and will not cause overpressure if/when it warms up and vapourises. You want to make sure that this process ocurrs as smoothly as possible, so the orifice should be placed where the pressure differential is lower, i.e., upstream

2. The most common sealing method is the one in which the seating load is regulated by the fluid pressure acting on the ball http://www.pdhonline... 108 to 119.pdf. If the venting hole was to be placed downstream, you would have a continous leak on this valve. This is because when the fluid pushes the valve to the downstream seat it creates a space between the upstream seat and the ball. The fluid is then allowed to enter the valve cavity and escape through the venting hole. You can see an explainatory drawing here http://www.flowserve...rols/SB 441.pdf.

Best regards,
João Alegre Queiroz