2 replies to this topic

[Alfreedo]

API 520 Part 1 4.1.1 says:

 

5.4.1.1 Relieving pressure, shown as P1 in the various sizing equations, is the inlet pressure of the

pressure-relief device at relieving conditions. The relieving pressure is the total of set pressure plus

overpressure. The examples cited in this section for the determination of relieving pressure refer to PRVs,

however, they are also applicable to nonreclosing pressure-relief devices (see Figure 15 and Figure 18 for

pressure level relationships for these types of devices). The effects of inlet pressure drop on specification

of relieving pressure for PRV sizing can be neglected if the inlet pressure drop does not exceed 3 % of

set pressure.

 

 

When I check my inlet losses, I am at 3.08% at rated flow.  The PSV has been in service for years and has a history of relieving a few times each year (another story) and has had no history of chattering or chattering damage so the company's policy is to allow the exceedance of the 3% rule.  The API 520 paragraph shown above says that you can't ignore losses when determining relieving pressure at the face of the inlet flange of the PSV when you exceed the 3% inlet losses. 

 

To me, this is to ensure that you have the required capacity which means I would only have to add the inlet losses when those losses exceed 3% at the required flow, not at the rated flow.   In other words, if the losses are only 1.5% of set pressure at required flow, I think I can still use 110% of set pressure as my relieving pressure at the face of the PSV inlet flange for sizing the PSV even though the losses slightly exceed 3% at rated flow.

 

Am I correct in my assumption.

 

 

[Bobby Strain]

You recognize that the API RP does not stipulate 3.0%.

 

Bobby

[latexman]

Have you read the 7th Edition (Oct. 2020) API 520 Part II 7.3 in it's entirety?  If not, you probably should.