ASME Section VIII Division 1, UG-99 specifies the hydrostatic test conditions for pressure vessels and heat exchangers.
Maximum Hydrostatic Test Pressure
An upper limit for the test pressure is not mentioned in ASME Section VIII Div 1. But it says that ASME authorized inspector has the right to reject the vessel if there is any visible permanent deformation. (UG 99(d)). To be on safer side, many industries try to limit the stress due to hydro test pressure less than 90% of the yield stress.
Minimum Hydrostatic Test Pressure
Option 1
ASME Section VIII Div 1, UG-99 (b)
Minimum Hydrostatic Test Pressure = 1.3 X MAWP x LSR
MAWP is Maximum allowable working pressure. It is always equal to or more than the design pressure. For example, you need just 5mm wall thickness for withstanding the design pressure of 10bar, and the vessel is actually made with a wall thickness of 8mm, the MAWP is more than 10bar. It may be 15bar for instance. Nevertheless, the design engineer has the right to write the MAWP as any number above design pressure and below the actual MAWP (15bar here) on name plate and U1 form. He can even say the MAWP= Design pressure, which is 10bar. The end note 36 marked in the paragraph (UG-99b) allows the MAWP to be equated to design pressure.
LSR is lowest stress ratio. It is a conversion factor used to convert the ASME material stress value at the test temperature to the Stress value at the design temperature. It is a kind of temperature correction factor. The purpose of the hydrostatic test is to simulate the real design stress on the vessel walls. So, this is the only way practically possible in a shop.
Option 2
Another method to find out the minimum test pressure is using the MAP, Maximum Allowable Pressure at test temperature.
UG-99(c)
Minimum test pressure= (1.3 X MAP) – Hydro static head
This method is less popular compared to UG-99(b)
Test temperature
The Code recommends that the metal temperature during hydrostatic test be maintained at least 30°F (17°C) above the minimum design metal temperature, but need not exceed 120°F (48°C), to minimize the risk of brittle fracture.
Minimum Hold time
ASME section VIII don't say anything about the test pressure hold time. Normally it is determined by the quality procedure of the shop or by ASME authorized inspector. 30 minutes to 60 minutes is generally followed. When it comes to Shell and Tube heat exchangers, TEMA (Tubular exchanger Manufactures Association) specifies minimum test duration as 30 minutes.