Sizing of relief valves in process piping

[metaltribe]

For thermal expansion case in the Liquid piping, size of (NPS 3/4 x NPS 1) is commonly used as per API STD 521 para 5.14.2.
If there is reason to believe that this size is not adequate, calculated procedure must be applied.

What is the duty? Gas/liquid?

Relief valves are sized in accordance with the procedure in API 520 Part 1, but you don't normally see relief valves on process piping, they are normally on vessels. You do, however, see thermal relief valves on piping to guard against thermal expansion. In this case, it is usual to just install the smallest relief valve which is 1D2 if I recall correctly.

[vivek68]

Thermal relief valve generally considers

1. expansion of trapped liquid due to solar radiation/ambient temp change
2. No vapourisation during heating

It is liquid drop expulsion to relieve the pressure arising due to the expansion of trapped liquid in a pipeline section with closed condition . size 3/4 " x 1" is generally acceptable . even if u do calculation u will find size covering the flow rate calculated. This size has been standardised across industry.