The holding period depends on the time needed for inspecting the system thoroughly. Minimum is 10 minutes. Especially the gas tests calls for 100% weld and mechanical joints being examined.
Also we have to ensure the limits of test pressure defined by individual components also to be taken care not to damage the components for eg. the soft seating or lining or to replace them with pipe spools.
To my info the repeated call for Hydrostatic test on the same system, the pressure need not be changed from the code to its design pressure as most of the case if any major leaks are identified, the test is performed again at the same pressure and not to reduced pressure. It is independent of the holding time. Holding time risk are purely defined by inspecting time and the in-line components and not of the piping components. It is a usual practice to remove all in-line components which has reduced testing pressure conditions.
The maximum test pressure is the pressure at which the pipe reaches its yield stress. In most of the cases the problems can be tracked back to the original thickness calc of the piping system where in somecases you may find the thickness not sufficient to take the test pressures but will suit the design pressure. When you design the piping class for rating condition you may have some margins to accommodate the test pressure but not always especially in the larger size range above 24". This makes the site testing team to reduce the test pressure adjusting to actual pipes purchased and being used and of the elevation impact of the system. Also piping connected with vessels pose a diff. situation and defining the test pressures requires a different judgement as the approach differs from company to company.
Most important thing is to question the need of hydrotest for a system to process dept. who defines the requirement.
Regards,
Kannan.
| Aarpee <ramprasad.chari@gmail.com> Sent by: materials-welding@googlegroups.com 24/10/2008 08:29
|
|
As per B31.3 Hydro test is carried out at 1.5X Design Pressure.Holding
time for Piping spools is normally 30 minutes but at times 4-6 hours
for Gas applications.Can this be held for 24 hours?What is the
potential risk?It is normally indicated that after Hydro inspection
for 10-30 minutes at 1.5X Design Pressure,if you intend doing further
inspection then same can be done at a pressure not more than the
design pressure.Opinion of the group is being sought thro this mail
Regards
Aarpee
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