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Thursday, February 12, 2009

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Conducting overpressure scenario analysis, derivation of relief load, follow by pressure safety valve (PSV) sizing and finally selection of PSV are common activities in oil and gas project. Nowadays, API Std 521 is commonly used for sizing and hence the required API effective discharge area is the result of the calculation. PSV with effective discharge area larger than required API effective discharge area shall be selected. However, many PSVs' certified area are presented in ASME area.

How to relate API effective discharge area and ASME area ?

This is a very common question and problem face by many young engineers. Sometime engineers may wonder why a PSV with ASME "D orifice" is sufficient for a relief scenario required API "E orifice". Reason for the difference between API and ASME is dated back to 1962 when ASME Section VIII Code was changed to derate all certified relieving capacities by 10%. PSV manufacturers have decided to increase PSV flow area by 10%, instead of derating their capacity by 10%. Nevertheless the API "orifice" still remain unchanged.

A PSV with ASME flow area (AASME), ASME discharge coefficient (KASME) and API discharge coefficient (KAPI),

Corrected API effective discharge area, AAPI = KASME x AASME / KAPI

Example,
A calculated effective area based on KAPI = 0.973, AAPI = 0.12 inch2. A API "D orifice" with 0.11 inch2 is insufficient. A "E orifice" with 0.196 inch2 is required.

Let check the National Board published data, a ASME "D orifice" will have KASME = 0.859 and flow area of AASME = 0.15 inch2, the equivalent API effective discharge area would be

AAPI = KASME x AASME / KAPI
AAPI = 0.859 x 0.15 / 0.973
AAPI = 0.132 inch2

The ASME "D orifice" is having API equivalent effective discharge area of 0.132 inch2 is higher than required effective discharge area of 0.12 inch2. Thus an ASME "D orifice"is still sufficient.

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posted by Webworm, 11:48 AM

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