API 520 Part I 10th Edition Ballot Item 2.5 This ballot covers the following item: Formula for viscosity correction factor for two‐phase flow in Annex C of Part I
D. Moncalvo
04‐2015 ‐ Comment by D. Moncalvo, very complex 11/2015 ‐ Agenda Item in Long Beach, reference as an example green book for viscosities > 100 cp. D. Moncalvo suggested adding one sentence that says below 100 cP you can use Kv equal to 1, and above that 520 Part I should refer the user to the relevant sources, one example being the CCPS Guidelines for Pressure Relief and Effluent Handling Systems. PAH to incorporate into red‐line 9/2106 ‐ Sent to API for Ballot 11/2016 ‐ Question raised about whether new equation is source of or curve fit for old figure. Is curve invalid for Re < 80 (and equation to be used instead), or is equation invalid for Re < 80 (and figure should be used)? P. Henry to follow up with Davide to determine source and limitations for new equation. If equations are invalid at low Re numbers, consider adding second equation for low ranges. 05/2017 ‐ Agenda Item in Las Vegas, D. Moncalvo led this discussion. This was a balloted item, and a question was brought up as to how accurate was the equation below a Reynolds number of 80? A response was made from Professor Darby. A motion was made to close this action item w/ no objections. Ready to Ballot
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Resubmitting previous ballot:
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2015‐02
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API 520 Part I 10th Edition Ballot Item 1.8
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This ballot covers the following item:
2015‐02
Formula for viscosity correction factor for two‐ phase flow in Annex C of Part I
D, Moncalvo
04‐2015 ‐ Comment by D. Moncalvo, very complex 11/2015 ‐ Agenda Item in Long Beach, reference as an example green book for viscosities > 100 cp. D. Moncalvo suggested adding one sentence that says below 100 cP you can use Kv equal to 1, and above that 520 Part I should refer the user to the relevant sources, one example being the CCPS Guidelines for Pressure Relief and Effluent Handling Systems. PAH to incorporate into red‐line
Davide Moncalvo provided the following changes to API 520 Part II paragraph 5.8.1 and Annex C related
to the viscosity correction factor.
5.8 Sizing for Liquid Relief: PRVs Requiring Capacity Certification 5.8.1
General
5.8.1.1 The ASME Code requires that capacity certification be obtained for PRVs designed for liquid service. The procedure for obtaining capacity certification includes testing to determine the rated coefficient of discharge for the liquid PRVs at 10 % overpressure. 5.8.1.2 The sizing equations for pressure-relief devices in liquid service provided in this section assume that the liquid is incompressible (i.e. the density of the liquid does not change as the pressure decreases from the relieving pressure to the total backpressure).
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5.8.1.3 Valves in liquid service that are designed in accordance with the ASME Code, which require a capacity certification may be initially sized using Equation (28) or Equation (29). In USC units: Gl P1 P2
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Q 38 K d K w K c K v
11.78 Q K dK w K c K v
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In SI units: A
(28)
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Gl P1 P2
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where
(29)
is the required effective discharge area, in.2 (mm2);
Q
is the flow rate, U.S. gal/min (L/min);
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Kd is the rated coefficient of discharge that should be obtained from the valve manufacturer;
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for preliminary sizing, an effective discharge coefficient can be used as follows: — 0.65, when a PRV is installed with or without a rupture disk in combination; — 0.62, when a PRV is not installed and sizing is for a rupture disk in accordance with 5.11.1.2.2; Kw is the correction factor due to backpressure; if the backpressure is atmospheric, use a value for Kw of 1.0. Balanced bellows valves in backpressure service will require the correction factor determined from Figure 31. Conventional and pilot-operated valves require no special correction (see 5.3); Kc is the combination correction factor for installations with a rupture disk upstream of the PRV (see 5.11.2); use the following values for the combination correction factor: — 1.0, when a rupture disk is not installed;
— 0.9, when a rupture disk is installed in combination with a PRV and the combination does not have a certified value; Kv is the correction factor due to viscosity Where the liquid has a viscosity of 100 cP (0.1 Pa s) or less, the viscosity correction factor can be set to one. The factor can be determined from Figure 37 or from Equation (30). This equation is applicable for ReL ≥ 80. Below that threshold engineering judgement should be used;
170 Kv 1 ReL
0.5
(30)
;where is the specific gravity of the liquid at the flowing temperature referred to water at standard conditions;
P1 is the upstream relieving pressure, psig (kPag); this is the set pressure plus allowable overpressure;
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P2 is the total backpressure, psig (kPag).
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Annex C Modification to C.2
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C.2.1.1.7 Once the value for the mass flux has been determined, the required orifice area can be calculated using Equation (C.9) or Equation (C.10). In USC units:
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0.04W K dK b K c K vG
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(C.10)
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where
277.8 W K dK b K c K vG
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In SI units: A
(C.9)
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A
A
is the required effective discharge area, in.2 (mm2);
W
is the mass flow rate, lb/h (kg/h);
Kd is the discharge coefficient. For a preliminary sizing estimation, a discharge coefficient of 0.85 can be used for a two-phase mixture or saturated liquid entering the PRV inlet. For the case of a liquid entering the PRV inlet, a discharge coefficient equal to 0.65 is consistent with the single-phase method in Equation (28) and Equation (29). Note that a value of 0.65 may result in a conservative valve size for liquids that are only slightly subcooled. The user may select other methods for determining a discharge coefficient [8] [9] [10]; Kb is the backpressure correction factor for vapor that should be obtained from the valve manufacturer. For a preliminary sizing estimation, use Figure 30. The backpressure correction factor applies to balanced-bellows valves only;
Kc is the combination correction factor for installations with a rupture disk upstream of the PRV (see 5.11.2); —
the combination correction factor is 1.0, when a rupture disk is not installed;
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the combination correction factor is 0.9, when a rupture disk is installed in combination with a PRV and the combination does not have a certified value;
Kv is the viscosity correction factor. For two-phase flows where the liquid has a viscosity of 100 cP (0.1 Pa s) or less, the viscosity correction factor can be set to one, see ISO 4126-10 [17]. Above that threshold engineering judgment must be used. Add to References:
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[17] ISO 4126-10. Safety devices for protection against excessive pressure. Sizing of safety valves for gas/liquid two-phase flow, 2010
