Valves for Offshore oil application 1

[bmoorthy]

Understand that the Top sides of the Offshore platform piping systems generally under go a sensitive leak test prior to commissioning.

Some operators insist this has to be done using helium traser. Some ask for testing at 1.1 times the line pressure and some ask for 0.9 time the line pressure and some at operating pressre. But the testing is undertakne. only the pressure varies.

This is the undertaken by Shell, Exon, BP and other operators.

Questions:
1) Out of 100 valves taken at random (Assuming that the manufacturer has fully tested to API/ANSI B16.34 et al) produced to general specification (not specifically designed for fugitive levels), how many valve may leak when subjected to sensitive leak test (From Stem packing/Gland packing area), what has been the past record in this aspect?

2)Is it that "it is not worth specifying in the general valve specification that the valves have to meet the final sensitive leak test requirement or at least specify a TYPE test kind of requirement, since the number of valves that might really pose problem and cost of repair/Replacement during the final commissioning and during operation are much less as compared to procuring the valves that would be tested for Helium presssure test"?

3) During the hazop studies, is this a point a standard agenda or is this an unknown risk?

4) Is there a international standard that has published the thresold leakage rate that is tolerable during the operations for a given fluid?

5) How is the acceptance criteria for Fugitive leakage rate established? Is that the threshold leakage rate in unit space divided by number of leakage paths minus some factor of safety?

 

[JLSeagull]

Review ISA 93.00.01, Standard Method for the Evaluation of External Leakage of Manual and Automated On-Off Valves. This may be useful to discuss the testing and the acceptance criteria.

HAZOP is among several process hazard analysis techniques. HAZOP evaluates low or high flow, pressure temperature, level and such operating properties. HAZOP does not address topics like valve leakage or area classification issues such as devices not rated for the area.

I recommend specifying in the general valve specification that the valves have to meet the sensitive leak test requirement at the factory using the client acceptance criteria. However, after the fabrication yard lets the valve rust, covers the stem with "black beauty", leaves nasty hydrostatic test fluid in the line when the module ships offshore, etc. all bets are off.

The cost of repair, replacement or return to the beach during commissioning or startup is very high. Purchase any special leak testing for the valves at the manufacture's shop or a third party facility among the acceptance criteria.

My experience reflects very poor seat leakage performance during startup. The rusty hydrostatic fluid remaining in lines is a likely factor. My guess is that over 70% of the valves on a large offshore platform had seat leakage issues. I don't think that the on-off valves were much of a fugitive leakage problem at least compared to the rising stem control valves.