Hydrotest pressures in piping

[ArunavaSinha]

I have three queries related to Hydro testing:
(i) Why piping hydro test is done at pressures higher than design pressure?
(ii) Why as per ASME Section III NB, NC, ND, hydro test pressure = 1.25 times of design pressure whereas as per ANSI B 31.1 (Power piping code), Hydro test pressure = 1.5 times of design pressure

Thanks in advance

 

[bimr]

1. It is in related to allowable stress. The B31.3 allowable stress values at design temperature for piping materials shall not exceed two-thirds of corresponding yield strength.

It is obvious, as per above with 1.5 times of design pressure value for hydrotest there wouldn't be any possibility that the hydrotest pressure to exceed the yield strength.

ASME B31.3 (process piping code) specifies hydrotest pressure as follows:

P = Design Pressure X 1.5 X (Stress Value of the metal at test temperature / Stress Value of the metal at design temperature)

2. The test pressure must never exceed a pressure that would cause yielding, or the maximum allowable test pressure of some component exposed to the test. In the case of ASME B31, section 137.1.4, and the Boiler and Pressure Vessel Codes, the maximum test pressure must not exceed 90 percent of yield for any component exposed to the test. The test pressure is needed to demonstrate that the system can safely withstand the rated pressure.

http://www.wermac.org/others/ndt_pressure_testing.html

 

[zdas04]

ArunavaSinha,
You might want to think about this logically. If the design pressure is synonymous with Maximum Allowable Working Pressure (MAWP), then a test pressure lower than design pressure would not have demonstrated that the pipe can sustain MAWP without yielding. Some number greater than design pressure is required to demonstrate that operating at MAWP is safe.

As to the amount over design pressure, the code numbers include the minimum test pressure and often don't have a maximum test pressure, they do not specify the required test pressure. If the BPVC says 125% and the piping code says 150% (it doesn't, but pretend it does) then if you want to test to 200% you have satisfied both codes (as long as you don't exceed the yield limitations). When I say that the piping code doesn't specify a test pressure I mean that the code has various conditions. I use B31.8 mostly, so I'll refer to it. If you look at table 841.322(f) there are two prescribed pressure test columns. For Class 1 Div 1 (low population density, water test) you have to test to at least 1.25 maximum operating pressure (not necessarily MAWP), but there is no maximum. For Class 4 (high rises on ROW) the test has to be at least 1.4 times MOP and no maximum (in some conditions there is a design pressure maximum, but no maximum in other conditions). The point of that discussion is to say that the code specifies minimum performance, rarely required performance.

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