Every once in a while, I come across a process design that leads to code issues down the road. This usually occurs with small pressure vessels (e.g., less than 5 cubic ft) that fall outside the scope of ASME Section VIII and also fall outside the scope of being a “confined space” (e.g., too small to bodily enter). So the business did not provide a means to isolate the vessel for maintenance or “pressure/leak testing” of the attached piping, which is covered by B31.3. In this article, I will explain the “77% option” that a business can use to pressure-test its piping.
345.4.3 Hydrostatic Test of Piping With Vessels as a System
(a) Where the test pressure of piping attached to a vessel is the same as or less than the test pressure for the vessel, the piping may be tested with the vessel at the piping test pressure.
(b) Where the test pressure of the piping exceeds the vessel test pressure, and it is not considered practicable to isolate the piping from the vessel, the piping and the vessel may be tested together at the vessel test pressure, provided the owner approves, and the vessel test pressure is not less than 77% of the piping test pressure calculated in accordance with para. 345.4.2(b).
There is a very good reason why 345.4.3 is called “Hydrostatic Test of Piping With Vessels as a System” and why there is NOT a section titled “Pneumatic Test of Piping With Vessels as a System“.
Granted, these smaller pressure vessels are so small that ASME does NOT officially recognize them as “pressure vessels” (e.g., they are less than 5 cubic ft in volume). However, that does NOT diminish the risks associated with pneumatically pressuring these vessels. As we know, the safest method of pressure testing is hydrostatic testing, but when the business has a strong argument against using it for its process piping, we are left with the pneumatic method for pressure and leak testing.
If the test pressure of the piping is HIGHER than the test pressure of the vessel, we are permitted to drop the test pressure of the piping DOWN to the vessel’s test pressure, provided the following condition is met:
The test pressure of the vessel MUST BE ATLEAST 77% of the piping’s required test pressure.
If the vessel’s allowable test pressure is GREATER THAN or EQUAL TO 77% of the piping’s calculated test pressure, the entire system (piping + vessel) can be tested together at the vessel’s test pressure.
But what if the Vessel’s MAWP is Too Low? (Less than 77% of the piping required test pressure))
If the pressure vessel’s MAWP is so low that its allowable pneumatic test pressure falls BELOW 77% of the required piping test pressure, you CANNOT test them together at the lower pressure. Doing so would under-test the piping by an excessive margin. In this scenario, you have two (2) options:
#1 – Isolation (Standard Practice): blind off or isolate the piping system from the pressure vessel. The piping is then pneumatically tested at its full required pressure, and the vessel is tested separately according to its own design code (e.g., ASME Section VIII).
#2 Component Replacement/Separate Certification: If isolation is physically impossible, the piping system design must be re-evaluated, or the vessel must be disconnected during the pressure testing event.
Important Safety Reminders for Pneumatic Testing
Because pneumatic testing stores significantly more energy than hydrostatic testing, ASME B31.3 imposes strict safety mandates:
Pneumatic Test Pressure Limit: Per ASME B31.3 Paragraph 345.5.4, the pneumatic test pressure must be 110% of the piping’s design pressure (unlike hydrostatic, which is 150%).
Vessel Test Pressure Limit: If testing the vessel concurrently under ASME Section VIII Div 1, the standard pneumatic test pressure for the vessel is 1.1 times the MAWP multiplied by the temperature correction factor.
Pressure Relief: A pressure relief device MUST BE PROVIDED during a pneumatic test to ensure the system pressure does not exceed the test pressure by more than the lesser of 10% or 50 psi.