2 replies to this topic

[Bill B]

We recently sized a relief valve for steam service. The vendor (Crosby) rep recommended an HSJ conventional valve (2 1/2 K 4). I did some quick calcs and critical flow is computed, even with discharge to atmosphere; with a small bit of tailpipe and an elbow, the dP is much greater than 10%.
Two questions:

1. Does it make sense for a manufacturer to design a valve, such that there is choked flow at the discharge?

2. I was told, but cannot confirm, that the dP must be limited to 3% for an ASME Code Section I valve. It that is the case, I don't believe we could achieve this without a very large RV outlet flange.

I suspect I am missing something here. Any thoughts?

[fallah]

We recently sized a relief valve for steam service. The vendor (Crosby) rep recommended an HSJ conventional valve (2 1/2 K 4). I did some quick calcs and critical flow is computed, even with discharge to atmosphere; with a small bit of tailpipe and an elbow, the dP is much greater than 10%.
Two questions:

1. Does it make sense for a manufacturer to design a valve, such that there is choked flow at the discharge?

2. I was told, but cannot confirm, that the dP must be limited to 3% for an ASME Code Section I valve. It that is the case, I don't believe we could achieve this without a very large RV outlet flange.

I suspect I am missing something here. Any thoughts?

What is the set pressure value?

Are you calculated DP based on set pressure?

Please clarify for better assistance.

[JoeWong]

1. Does it make sense for a manufacturer to design a valve, such that there is choked flow at the discharge?

When you said manufacturer, is it PSV manufacturer ? If so, the discharge pipe is not determined by PSV manufacturer. It is determined by the engineering contractor or package supplier.

2. I was told, but cannot confirm, that the dP must be limited to 3% for an ASME Code Section I valve. It that is the case, I don't believe we could achieve this without a very large RV outlet flange.

I suspect I am missing something here. Any thoughts?

As i understood from ASME Section I,
Para. PG-67.2 defines the 6% Maximum Boiler Overpressure requirement.
Para. PG-67.3, One or more Safety Valves on the boiler shall be set at or below MAWP. If additional valves are used the highest pressure setting shall not exceed MAWP by more than 3%.
Para. PG-72.1, Safety Valves shall be designed ... to attain full lift at a pressure no greater than 3% above their set pressure.

Example, a boiler with 2 PSV with MAWP of 100 barg.
PSV1 set at 100 barg with accumulated overpressure not exceeding 103 barg.
PSV2 may set at 103 barg with accumulated overpressure not exceeding 106 barg.

For the second part of query, i believe we need to go in detail. Basic information is required for further checking. If you wish to get more advice from member in this forum, provide your basic input data and your results.