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Actual Orifice Areas For Different Relief Valve Manufacturers Based On Api 526 Orifice Designations

Actual Orifice Areas For Different Relief Valve Manufacturers Based On Api 526 Orifice Designations Dear All,

API 526 provides provides orifice designations such as D, E, F..... etc. for relief valves with corresponding orifice cross-sectional areas. However, most manufacturers of relief valves provide the same API 526 designations with different areas (higher than as given in API 526) using a de-rated coefficient of discharge for the orifice. They call it the "actual orifice area" or "actual discharge area" which when de-rated with their own coefficient of discharge becomes equal to the API 526 orifice area.

In practice, if you are calculating the flow rate for a relief valve and specifically for the purpose of inlet / outlet line sizing and for fulfilling the inlet pressure drop criteria of 3% of set pressure and outlet line pressure drop of 10% of set pressure (conventional relief valve) then you must use the "actual orifice area" in the API 520 Part 1 formulas for flow rate for a given orifice as per the specific manufacturer.

Today's blog entry provides an excel table for the various reputed relief valve manufacturers across the globe whose "actual orifice area" is higher than the area as given in API 526 corresponding to the particular alphabetical orifice designation. This table should be useful if somebody selects a specific model of a given manufacturer such as "Farris" or "Leser". Not all manufacturers or models are covered but some more famous manufacturers and their common models are covered for the benefit of the members and readers of "Cheresources" and they can use it to determine the actual flow rates using formulas of API 520 Part 1.

It is important to note that this excel workbook does not include orifice areas as given in ASME Section VIII and covers only API 526.

Members and readers are welcome to provide their comments on the blog entry and the attached excel workbook.


Attached File  Actual_Orifice_Areas_API526_Type_PSVs_Vendor_Data.xlsx (12.36KB)
downloads: 2514

Bobby Strain
Sep 03 2017 01:54 PM

There is also something called a "redbook" that lists actual areas as required by ASME. If you look at the design requirements for API and ASME, you will note the requirements are a bit different.



Having read your post, I need to make some clarifications;


1. API-526 Lists Effective orifice areas as listed also in API-520 (sizing).

Effective orifice areas should only be selected when using API-520 Effective coefficient of discharge (eg.0.975 for vapour).


The purpose of this, is to bring all manufacturers on a level playing field at enquiry stage. Ie., they all calculate the same area. This why API-RP-520 is a Recommended Practice and a user standard.


2. Actual orifice areas, are exactly that, as measured. They vary between manufacturers (as you have stated) since each valve design has a differing Actual coefficient of discharge derived from type test. The average actual coefficient of discharge is then derated by a 0.9 factor to give Kdr. Hence in sizing for actual area and capacity only ACTUAL area and Kdr can be used together. Never mix API effective with ASME actual figures.


The ASME calculation method is the same as the API-520 method except API only states effective areas and coefficients.


Effective areas are only approx 10% less that actual (not uniform due to varying test coefficients. A larger actual area is compensating for a low coefficient).


The excel sheet in the main specifies actual areas (in my opinion, should also list corresponding actual coefficient from "red book NB-18" against the state of fluid (liq/vap). Heading should be corrected. Note also for Anderson Greenwood Crosby "J series" could cover various valve styles all with varying coefficients same caution applies to other makers styles). Best to always refer to manufacturers catalogue and/or NB-18.


Hope that clarifies.

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