Use of NPT threaded valves Subsea

[RangerBill]

A client is looking for our definitive guidance regarding the suitability of fitting an NPT threaded 1" vent valve assembly into a blind flange at the end of a subsea hydrocarbon spool. The flange is duplex the valve 316. The spool is located under a wellhead flowbase.

Failure of the NPT thread where the nipple on the valve screws into the blind flange would result in a loss of containment requiring diver intervention to isolate/replace. The main line isolation valves adjacent are normally open and manual. The well would have to be shut-in and the flowline isolated at a remote manifold to fully isolate. The whole piping and valve spool with which this assembly is fitted is ‘dumped’ under a wellhead structure and protected with flexible concrete mats.

I personally wouldn't touch an NPT threaded valve into a blind flange with a bargepole, but a 'cheap' client is determined to get away with it. I think its too mechanically vulnerable from damage and a ¼ turn away from being unscrewed. We ourselves proposed a flanged double block and bleed assembly integral with the blind, though it is being avoided by the 'cheap' client on cost.

Can anyone remember a section in any relevant piping design code, which covers a scenario such as this?

 

[dahoopastress]

Section 2.7 of API 14E states that:
"All piping 1 1/2 inch or less in diameter should be socket welded for:
a) Hydrocarbon service above ANSI600
b) Hydrocarbon service above 200 deg F
c) Hydrocarbon service subject to vibration
d) Glycol service"

However, it goes on
"Occasionally, it may not be possible to observe the guidelines given above, particularly when connecting to equipment. In this case the connection may be threaded......"

Not sure if a topsides piping code is really relevant.....

Why don't you seal weld it?