Issue with overpressure protection - How to evaluate maximum system pressure?

[OscarGr]

Dear all,

Here comes a rather strange question, at least to my impression!

I am currently working for a customer in Sweden, they want to implement ASME III 7000 for their nuclear applications when it comes to overpressure protection (this question is not specific to ASME III I guess, the same procedure probably holds for the other ASME-standards as well). I am myself a beginner when it comes to the methods of ASME for overpressure protection but their impression/interpretation about how you evaluate the total pressure at relieving conditions to compare with the max 10 % above Design Pressure rule is to me unfamiliar. So I would hope for your thoughts about this and if you have any experience with evaluating the 10 % of Design Pressure or other similar ASME rule for overpressure protection.

What they do is basically this:
Lets assume the following:
They have a system with a Design Pressure of 100 psig. On this system they have a relief valve with a set pressure of say 100 psig. This valve has a certified ackumulation of 10 % above Pset and a spring adjustment tolerance of 3 % of the set pressure. Furthermore lets assume that they made a calculation of inlet line pressure losses and found it to be 3 psi. Furthermore from the relief valve there is a hydrostatic pressure difference in their system from the relief valve to the lowest point in a tank which amounts to 5 psi. Then they make the following calculation:

Total pressure in lowest point of the tank = Set pressure + Ackumulation + Spring adjustment tolerance + Inlet pressure line loss + Hydrostatic pressure difference = 100 psig + 10 psi + 3 psi + 3 psi + 5 psi = 121 psi

This pressure is then way above the design pressure + 10 % = 110 psig. Therefore they would not allow this installation.
Any thoughts and comments about this method is highly appreciated! Is this the intent of the ASME code? Is this common practice?

Yours truly,
Oscar

 

[racookpe1978]

Look at your first "assumption".

Now, re-read the specifications - you MUST learn to do that if you are going to be doing ANY part of the nuclear work ANYWHERE.

Now, starting from those Code specifications for a 100 psig operating pressure of your fluid at your plant, what is the maximum surge pressure? (include instrument tolerances also)
What is the required relief setting?
What is the required hydrostatic test pressure?

If the "receiving tank" maximum pressure is expected to be 5 psig (??) and the backpressure during relief valve operation is 3 psig, that does not change the relief valve setting requirement. It will likely change the FLOW expected during worse case conditions during relief valve operation (so you'd face a requirement to make the relief line larger, or the relief tank vent better), but those backpressures do NOT change the requirement to protect the original pipe being relieved.

Design pressure is NOT "relief pressure" ... It can't be.

 

[OscarGr]

Thank you for your answer however I have been working in the nuclear industry in Europe for many years, but not using the ASME code while its mostly used in the US. Also I am fully aware that Design Pressure and Relieving pressure are completely different. Furthermore in the example I gave the operating pressure is way below the set pressure of the valve and the design pressure.

Also I know how to calculate the mass or energy input to the system in question and determine the relief requirement. I am merely trying to understand if it, for the given set pressure, acumulation, inlet lines losses, hydrostatic pressure, tolerances etc. given above, is the intent of the ASME III article 7000 code to make an algebraic summation of the pressure contributions to the maximum pressure at relieving conditions and compare to the 10% design rule given in article 7000.

 

[waross]

If the lower part of the tank has a hydro-static difference of 5 PSI does that not result in a greater pressure at the lowest point than at the relief valve.
Then in order to protect the lowest point of the tank, don't you subtract the hydrostatic pressure rather than adding it?
eg. 95 PSI at the PSV = 100 PSI or design pressure at the lowest point of the tank.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[OscarGr]

Yes waross, when determining the set pressure for the valve I would subtract the hydrostatic pressure as necessary to avoid obtaining a too high pressure in the lowest point of the tank. In the same way when evaluating the maximum pressure one would add the hydrostatic pressure to pressure at the SRV to obtain the highest pressure in the system.