Nitrogen Preservation on Pig Receiver 2

[Kene1]

We are carrying out Nitrogen preservation on a pig receiver at a pressure of 0.5 bar as specified in the procedure. A leak test carried out on this facility at 15 bar could not detect any leak but yet yet we kept on experiencing drop in pressure (15 to 6 bar in a week). I will appreciate advice from this forum on the next line of action as all the mechanical joints and barrel door have been properly checked. The pig receiver is designed to hold a pressure of 80 bar.

 

[LittleInch]

This level of pressure loss over that length of time is actually a really small amount of gas / leakage. If the joints are sound and no bubbles, then this looks like a valve very slowly passing (probably one or bubbles a second).

Ditto the door seals are very rarely completely and absolutely bubble tight. Your relief valve may be passing every so slightly.

If you really want to seal this vessel absolutely and this seems a bit unecesasy, then you will need to examine the leak tightness of each valve.

IMHO, there are probably so many possible fugitive leak paths that I would question whether it is required.


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

 

[Kene1]

Thanks LittleInch for the contribution, we have actually isolated the pig receiver from other inplant piping and as such a valve passing gas may not have any effect. The pipeline and the end facilities are being preserved for a minimum of 3 years since the Central Processing Facility is still under construction.

 

[mbt22]

How big is the system, and how was leak tested initially? I'm curious what volume leak rate this actually represents. 0.3% pressure drop an hour average, the ~0.5% initially at 15 bar is probably within a leak test past criteria. If it's passed a leak test at 80 bar, that might be more challenging.

If you need to track it down, helium in the nitrogen with a suitable detector is great for tracking down small leaks.

Matt

 

[zdas04]

For that kind of time, I'd put a rupture disk under any PSV and insert skillet blinds on the valves. If the closure leaks a couple of bubbles a day you will eventually reach a point where the dP across it is too low to further leak and you'll be fine.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Kene1]

The leak test is being carried out using the soaping/bubble method. All the flanged connections were properly taped and leak checked with a soap solution. We also had a situation where a leak on an instrument (corrosion) was detected through a 'hissing sound'. The volume is about 5 m3.
zdas04, the pig receiver have adequately isolated using a blind so we dont have an issue of gas passing.

 

[Kene1]

Just on an update on the Nitrogen preservation activity. We are currently planning to release the Nitrogen and check the barrel door and if possible replace the door seal if it is weak.

 

[bimr]

Definitely have a leak somewhere.

For leak detection you should buy a gas detector.

http://www.lacotech.com/leaktesting/handheld-leak-detectors.aspx

A nitrogen/helium mix provides the following benefits:

1. improved detection. It can detect much smaller leaks due to the size of the test gas molecules. Helium molecules are far smaller than nitrogen molecules and so are better able to pick up small leaks.

2. safe and easy to use, inert and non-flammable. The leak detection mix is compatible with all standard pressure testing equipment (including nitrogen regulators) and is suitable for leak testing any size of system.

3. cost effective.

There are also firms that will do this test for you.

http://www.heliumleakdetection.net/Helium-Leak-Testing/helium-leak-testing-services.html

 

[Kene1]

While still waiting for the arrival of the barrel door seal so that we can commence replacement, we notice that the pressure drop stopped at 4.5bar. This will be monitored for another 4 days and if no change occurs we will therefore assume stabilization and thereafter open the vent and preserve at 0.6 bar.