Which SRV capacity should be used for piping calculation?

[Ximon]

Hi,

I am having an issue trying to follow the API standards. We are getting different opinions on whether the piping for a API 526 valve designed according to API 520 should be calculated for the "certified capacity" or if it should be calculated for the "certified capacity" divided by 0.9. Instructions for this is very clear in ASME and ISO 4126, but not as clear as you could expect in API 520 pt 2.

In API 520 pt 2 (2008), 4.2.2, it says that "The pressure loss should be calculated with using the rated capacity of the pressure-relief valve."

Does this mean that you should use the actual measured capacity for the relief valve OR the "certified capacity" ( = 0.9 x measured capacity, according to code)?

All the best

Simon

 

[don1980]

Rated capacity is the capacity which is determined by use of the 0.9 multiplier. So, "certified capacity" and rated capacity are the same thing. This is the value that is used to calculate the piping pressure loss.

 

[CMA010]

Note that according ISO 4126 the actual capacity should be used.

 

[Ximon]

Thanks so far

For using ASME valves as API valves the condition is:
(A * Kd)_API_rated < (A*Kd)_ASME_certified = (0.9*A*Kd)_ASME_actual

I have seen examples where the ASME actual is not only 10 %, but 15% higher than API.

What are the implications on the pressure drop? Well, pressure drop is proportioanl to Flow^2 and flow is proportional to Area. This means that the ACTUAL pressure drop would be 1.1^2 = 1.21 times higher if the area is 10% bigger and 1.15^2=1.32 times higher if the area is 15% bigger.

If the API rated inlet pressure drop has been calculated to 3.0%, the actual pressure drop could be 4.0%. Is the contingency in the "max 3% inlet pressure drop" condition good enough to handle this?


CMA010: The problem described above is covered by ISO 4126 in a clear and understandable way. But I high to use API at my company.

// Simon