Practical Process Engineering

I stop at this poit. You can review what I posted. I will be back later today to solve a few examples.

U: Viscosity is in Saybolt Universal Seconds mu: Absolute viscosity in centipoise at relieving temperature

Note: API-520 recommends an itterative method for viscous liquids (Saybolt Universal Seconds>= 100). Here I summarize it in flowchart style: 1- Calculate orifice area assuming Kv=1. 2- Refer to API-526 and select proper effective discharge area. I call this area A1. 3- Back calculate from A1 to Reynolds number using formulae in next picture. 4- Using Re you calculate in step 3, obtain a new Kv from previous picture (graphically or manual calculation) 5- Calculate a new area based the new Kv and select proper effective discharge are from API-526. I call it A2. 6- If A2>A1, go back to step 3 and start calculation for a new Reynolds number and a new Kv and new A2 using one size larger than A1 you selected in step 2.

Kv for liquid PSV sizing

This graph is only valid for balanced safety valves on liquid services.

Q: Required capacity: GPM or LPM Kd: Effective discharge coefficient; 0.65 recommended by API for priliminary sizing Kw: Backpressure correction factor; 1 for atmospheric backpressure. 1 for conventional and pilot-operated valves. For ballanced bellows valves on liquid services, use the picture I upload it a few days ago. I forward it again. Kc: Combination correction factor; 1 for installation without rupture disk and 0.9 for installation with rupture disk Kv: Viscosity correction factor. Calculate it or obtain it graphically. G1: Specific gravity P1: Gauge Relieving pressure; Kpag or psig P2: Absolute total bp; Kpag or psig

#Liquid_PSV_Sizing

Superheat correction factor when T<= 1200 F. Please refer to formulae for gas/vapor in critical flow if T>1200 F.

How to calculate KN for steam PSV sizing

A: Effective discharge area; mm2 or in2 W: Required relief load; Kg/h or Lbm/hr P1: Absolute relieving pressure Kd: Effective discharge cofficient; 0.975 Kb: 1 for conventional and pilot-operated PSV's and obtain it graphically for ballanced PSV's Kc: 1 when no rupture disk is installed upstream the safety valve and 0.9 weh rupture disk is installed KN: Correction factor for Napier equation. See next picture. K SH: Superheat correction factor; 1 for saturated steam, for superheated steam with T <= 1200 F, see following table and for T> 1200 F, Forget about this section and use formulae for gas/vapor in critical flow.

#Steam_PSV_sizing in critical flow

PB is totla bp. DO NOT USE THIS GRAPH FOR BALLANCED SAFETY VALVES.

#Alternative_Sizing_Method It is only for conventional and pilot-operated safety valves in subcritical flow. Use critical flow formulae, but obtain Kb from following graph. Remember, Kb was 1 for for pilot-operated and conventional safety valves under critical flow. But, here we use other values than 1 because the flow regime is subcritical.

Graphical method to obtain F1. Please note that K is ideal gas heat capacity ratio.

A: Effective discharge area; mm2 or in2 W: Required relief load; Kg/h or Lbm/h P1: Absolute relieving pressure P2: Absolute total bp T:Absolute relieving temperatre Z: Calculated compressibility factor (should be between 0.8 and 1.1. If no calculated Z is available, use Z=1 per API recommendation) M:Molecular weight Kd: Effective discharge coefficient (0.975 for PSV on gas/vapor service) Kc: Combination correction factor; With rupture disk= 0.9, without rupture disk=1 V: Required volumetric flowrate: SCFM at 14.7 psia and 60 F or NM3/min at 101.324 Kpa and 0 C Gv: Specific gravity F2: Coefficient of subcritical flow. You can compute it using a formula in API-520 or obtain it graphically.

Subcritical flow- Gas and Vapor

#Safety_Valve_Sizing #Subcritical_flow If the ratio of backpressure to relieving pressure (absolute values) exceeds the critical pressure ratio, the flow is critical. So, high bp systems are succeptible for such flow regime. Pilot-operated PSV's are mainly installed on such applications. But, conventional safety valves with special adjusments may be installed on these applications. Ballanced PSV's are mainly sized using appropriate formulae for critical flow. However, If they are installed on a subcritical flow application, you can't use API-520 recommende Kb values and you manufacturer should be consulted. Here are the equations for critical flows:

Review following website for flange pressure ratings: http://www.wermac.org/flanges/flanges_pressure-temperature-ratings_astm_asme.html