Inlet losses ISO 4126 1

[Chachacha1000]

Following API, you use a rated flow to work out inlet losses to PSV; ie rated flow = required flow times de-rating factor (which is = actual orifice A / required A), required being = design (from relief load Calc).

Following ISO 4126, you have to use instead “actual flow”/0.9. Actual flow is defined as capacity calculated with certified discharge coefficient (ie needs vendor data).

Which approach is more conservative to work out inlet losses & why? I think ISO is as long as you can assume that the actual flow (certified) is the same as the rated flow (also certified)? What if the rated flow is not certified?

Thanks!

 

[The Obturator]

Can you reference the specific API/ISO-4126 specification/paragraph you are referring to?

There won't be much difference between the 2 methods.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[shvet]

See applicable pressure vessel code or practice.

М-6.а ... the flow characteristics of the upstream system shall be such that the cumulative total of all nonrecoverable inlet losses shall not exceed 3% of the valve set pressure. The inlet pressure losses will be based on the valve nameplate capacity corrected for the characteristics of the flowing fluid.

The "3% rule" (ASME BPVC, Appendix M) is currently accepted as the criterion for the upper limit on inlet losses to safety relief valves. This rule requires that the nonrecoverable (friction) losses be less than 3% of the set pressure when the valve is operating at nameplate capacity, corrected for the properties of the given fluid (see §2.5.2 for the wording of the ASME Code; also see §2.2.2 of API 520-11). This flow capacity is the "relieving capacity" of the valve at 10% overpressure. Some designers and valve manufacturers follow the more conservative practice of using the best estimate flow rate at 10% overpressure for the loss calculation. This flow is about 10% higher than the relieving capacity (see §2.6). Note that standards for countries other than the USA may specify this higher flow basis (British Standard 6759, for example).

9.4.1 ... The limiting pressure drop is that calculated for the maximum (rated) capacity, based on selected orifice size, of the valve installed for the contingency that results in the maximum frictional pressure drop, considering the effects of temperature and molecular weight or specific gravity. Rated capacity shall be used except for cases involving either fire load relief, or relief of fluids that are 100% liquid at the pressure relief valve inlet (regardless of whether or not flashing occurs across the PR valve and regardless of whether or not the PR valve is liquid capacity certified). In these two cases, the design relief load shall be used. Note that the contingency that sets the PR valve size is not always that which results in the maximum inlet line pressure drop. Therefore, the sizing contingency and any other valid contingency that results in a required relief area equal to or greater than 75% of the relief area required for the sizing contingency should be checked.

2.6 The pressure drop shall not exceed 3% of the set pressure. This shall be based on the relief valve capacity.

Pressure drop between the protected equipment and the pressure relief valve should not exceed 3% of the set pressure. The pressure drop should be calculated using the rated capacity of the pressure relief valve.

9.0 Depending on the actual relief valve capacity, the pressure drop of the inlet piping and fittings shall not exceed 3% of the valve set pressure ... Exceptions to this requirements are only allowed in the case of a pilot-operated valve with a suitably arranged remote pilot connection close to the source of overpressure.

1243.1 The evaluation of the non-recoverable (i.e., frictional only) inlet line pressure loss is performed using standard equations for fluid flow through pipe and fittings. Each pipe segment or fitting is represented in the calculation by a flow resistance coefficient or an equivalent length of pipe. As specified in the excerpt from Section VIII Appendix M quoted above, the flow rate is assumed to be the full nameplate (i.e., rated) capacity of the valve.

10.6.4.b For new installations, to prevent relief valve chatter and damage, total nonrecoverable pressure loss between equipment or pipeline protected (including pipe entrance loss) and a conventional pressure relief valve inlet shall not exceed 3% of set pressure of the valve for flow corresponding to installed valve area (i.e., rated capacity of the valve)

6.2.1 ... The total pressure drop between the vessel and the relief valve, including the pressure drop due to entrance, contraction, fittings, etc... should be kept below 3% of the pressure relief valve set pressure (in gauge units). The pressure loss should be calculated using the rated capacity of the pressure relief valve (API 520, Part II, § 2.2.2). This pressure drop limitation is related to the pressure relief valve blowdown characteristic (6-7 %), and is imposed to prevent valve chattering. This may be a critical factor for set pressure below 50 psig. Some clients may require that it be calculated on the basis of the actual capacity of the relief valve : rated capacity is generally 10 % less than actual. The 10 % margin is required by ASME Code, section VIII.

2.1 The upstream and downstream line shall be sized based on the rated capacity of the PSV. Piping to pressure relief valve inlet shall be as short as possible. The upstream line shall be sized so that the pressure loss is below 3% of valve set pressure to avoid valve chattering.

 

[georgeverghese]

Pouring more oil on this vexing issue, certified discharge coeff is probably only at full - rated flow, while the required area is typically calculated at some notional value of K assumed by the designer at the selection phase of design. And at this required flow, the actual K is not the same as the one used for rated flow.