What to do if the strength test pressure is higher than the nominal pressure of the pipeline?
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Sometimes, in such cases, the calculated stress capacity of the pipe is higher than the nominal pressure rating chosen for it. Is this design choice reasonable? For example, if the operating pressure of a steam pipeline is 1.0 MPa, the design pressure is set at 1.1 MPa, and the hydrostatic test pressure is 1.65 MPa; meanwhile, the engineering pressure class of the pipeline is designated as 16 kg class. Clearly, the strength test pressure is higher than the nominal pressure of the pipeline. Is such a selection reasonable?
It is reasonable, but using a hydrostatic test pressure of 1.65 MPa is not appropriate; the effects of the design temperature and the test temperature need to be taken into account. For specific conversion details, refer to Article 89(3) of the Safety Supervision Regulations for Pressure Pipelines – Industrial Pipelines.
There’s no major issue with this pressure. A pressure test at 1.5 times the nominal value is the standard practice; in fact, there are also correction factors for temperature and pressure. Seamless steel pipes are subjected to pressure testing upon leaving the factory, and I believe the pressure applied is around 18 MPa. You should check this information – reading books on pressure pipelines will help you understand it better.
The rule here is that this formula should be used for conversion only when the design temperature is higher than the test temperature. According to the regulations, the hydrostatic test pressure must not be lower than 1.5 times the design pressure; 1.65 is used here, which fully meets the specification requirements
Well, I’ve read some books on the subject as well. My main doubt is: in such cases, it seems incorrect to use a pressure rating of 16 kilograms for pipe flanges and similar components. It’s not that I’m worried about what will happen to the pipes themselves
Reasonably, a hydrostatic test involves instantaneous pressure
Fittings and valves also undergo strength testing, usually at 1.5 times the standard value. The valve with a pressure of 1.6 MPa allows for short-term pressurization to 2.4 MPa
Take a look at the technical requirements for the manufacturing of these pipe fittings; the pressure test upon leaving the factory is also 1.5 times the operating pressure. In other words, for valve pipe fittings with a pressure of 1.6 MPa, the pressure test should be conducted at 2.4 MPa
I’m also wondering about this: when determining the pipe grade, should the hydrostatic test pressure be taken into account? The nominal pressure of the pipe grade must be high enough to cover that test pressure Please give me some advice!