Hydrostatic Weld Testers

[MDAust]

Hi All,
we have been using Flange Weld Testers and Internal Weld Testers (see Inpipe Products - one of a number of equipment suppliers)to allow hydrostatic testing of individual welds. This technique allows the testing of individual welds rather than testing the whole piping spool or system - which could be extensive.
My question is in regard to the hoop stress applied to the pipe by the device sealing mechanism (a torqued bolt activating a mechanical seal)- how do you limit the possibility of exceeding the maximum allowable pipe hoop stress. I have been asked to advise on a 1500# unit with a test pressure of 35MPag. Traditionally we have only used them on 150# and 300# systems.
I can only guess the sure way would be to conduct a test and instrument the pipe with strain gauges and torque the unit until seal failure. Have not seen any formal calculations or test reports on the stresses.
Hope somebody else has had experience with these devices.
Thank you, Mark

 

[rconner]

These would of course be great questions to the inventors, designers and promoters of such devices. That being said, it would appear most of the loading you are talking about would be quite localized and circumferential in nature, might well be tolerated by at least strong pipes, and sizeable flanges might also strengthen the pipe somewhat to withstand loads in at least this direction where the loading is applied. I am a however little just a little uneasy by the prefacing verbiage "rather than testing the whole piping spool or system - which could be extensive", so the only other thing I will mention is testing only seams of the best pipeline does not test the system nor joint structurally and for multiple other reasons is not an equal to e.g. a well-conducted hydrostatic test of an installed pipeline "system" (to many things can happen in its life cycle).

 

[MDAust]

Hi rconner,
sorry, I should have mentioned these devices are only used for Brownfield work. All piping would have been had NDT and hydrotest previously.
Thank you, Mark

 

[racookpe1978]

You're right to question, but the answers have to come from the specifics of each testing company's specific geometry.

For example, assume an internal flange is used to block the test fluid from going down the pipe. That unbalanced horizontal force can be calculated, right? (Area of inside of pipe x differential pressure).

That horizontal force can only be resisted by the "friction" of the plug to the wall of the pipe (and must resist the overturning forces of any differences between differences in friction between any two worse-case places around the plug's circumference.

So, each company has to maximize wall-to-plug friction, and maintain a working minimum area of plug-wall-to-pipe-wall that creates enough force between plug and pipe. Bigger this area of friction, lower the expansion force against the pipe's wall is required, more effective the pipe plug. But, the bigger (longer) the plug, the more expensive it is to make and the harder it is to get into place.