PRD sizing using API 520-Part 1 1

[VL0]

Hi all,

When using equations (9) and (10) from the API 520 Part 1 (October 2020) for the sizing of PRD (sizing for critical flow) I don't get the same results.
Indeed, when using equation (9) I get a required discharge area almost twice as much as the value when using equation (10). Is there a reason for that?

It is stated in 5.6.3.1. that: "Each of the equations may be used to calculate the required discharge area, A, to achieve a required flow rate through a PRD."
But how can I know which equations to use when they don't give the same results? What equation shall I use in the future?

Thank you for your help.
VL

 

[The Obturator]

Equation 9 is mass flow and 10 is volumetric flow. Have you converted the flow correctly? Please detail your example of the base flow used.

Note also there is a separate safety relief valve forum, but we can carry on here in this case.


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

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

 

[VL0]

Thank you for your reply.
In my case, see the data used for the calculation:
For both equations:
C=0.026
Kd=1
P1=1100 kPa
Kb=1
Kc=1
T=153.15 K
Z=1
M= 16 g/mol

For equation 9, the mass flow used is 454 kg/h (W)
For equation 10, the volumetric flow used is 5.86 m3/min (V)

 

[The Obturator]

If 454 kg/h is the known and correct mass flow, the equivalent volumetric flow, where M = 16, should be 10.58 nm3/min, so yes, you would get a difference of about twice as much. Check you conversion.


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

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