Practical Process Engineering

Hit Add PSV button in Home ribbon and place the mouce cursor on the Feed Gas line. Once you see a white square, click on it to add the PSV

Here in tha last tab "Orifice Manager", you see the sizes of API-526 standard orifices. You can change these default numbers to vendor specific areas and also, you can enter specifc flange sizes and ratings.

Next tab is to enter parametrs we use to calculate relief load and PSV sizing for low pressure and atmospheric tanks per API-2000.

Choose if the line sizing must be done based on require flow or rated flow. I already explained how they differ. You need to enter pipe roughness and equivalent length (including pipe and fittings). As a default, equivalent length is 50 m = 164 ft. To control AIV (Acoustic Indiced Vibration) , velocity at outlet pipe should be controlled. As a rule of thumb, Mach number should be around 0.5 right at the outlet, but it can increase up to 0.9 at pipe outlet (connecting point of tail pipe and subheader or flare header). You can change this default numbers. Also, pressure drop in inlet pipe should be controlled not to exceed 3% of the set pressure in order to stop chattering. This number is adjustable. There is a table for you to limit maximum allowable bp for different types of safety valves.

In the tab "Line Sizing" you enter line sizing criteria and basic information.

For reieving temperature, if you select "Reference", the software adopts operating temperature as the relieving temperature. But, you have this option to enter a different temperature. You can ignore this section and enter desired temperature later during the sizing. Latent heat is a significant parameters in fire case sizing. You can select "Calculate latent heat" and let the software do it for you. The software is able to use fractional vaporization method. So, enter two numbers for the initial and final vaporization percentage. If you select "Incluse sensible heat in latent heat calculation", software considers it for latent heat calculation. Beaware that, this leads to safety valve oversizing. You see a number 116.3 KJ/Kg that is equal to 50 Btu/Lbm. API-521 recommends to use this number in area near the critical point when latent heat minimizes. If you do not select other options, software use this default number as the latent heat. However, this idea has been criticized by some researchers. I will talk more in details about latent heat calculation later.

"Scenarios" tab allows you to enter relieving temperature or enter important parameters for external pool fire or tube rupture. Also. for control valve failure scenario, you can chosse a suitable sizing method among PSV plus, ANSI/ISA, Universal Gas Sizing and vendor specific.

As a rule of thumb, if required flow is less than 25~30% of rated flow, safety valve is oversized and succeptible to chattering. You can choose "Warn when require flow is less than" and enetr a percentage to see if the safety valve is oversized compare with what vendor offers you or not. The default is 30%. You can change %overpressure here. The defaults are 10%, 16% and 21%. Enter default amount of superimposed bp's. As a default, initial built-up backpressure is 10% of the set pressure. You can change it to whatever you need. If you do not change it here, you have this option to enter it later during sizing. The last thing is heat capacity ratio. API recommends to use ideal gas ratio. But, Aspen gives you two options: Ideal and Semi-ideal (uses real Cp).

You can change sizing settubgs here. It has several tabs. Let's start with "General Setup".

This is safey analysis window. Let's start with preferences. Click on the preferences manager

This flowsheet showsup. Select "Safety Analysis" from the window right bottom of the navigation pane. I highlight it.

#PSV_Sizing #Aspen_HYSYS #Aspen_Plus I found following file in Drive C of my laptop. Open the file, click on "Natural Gas Purification" subflowsheet and press"Sub-flowsheet environemnt".

There are still many things to be discussed including gas/vapor sizing for Z out of 0.8~1.1 and two-phase fluid. However, I postpone it to later and would like to solve a few example in Aspen HYSYS/ Aspen Plus.

Example/ Size a relief valve for liquid service per foollowing conditions: Fluid: Crude oil Viscousity= 500 cP Set pressure= 600 psig Single relief valve with 10% overpressure bp= variable with maximum 160 psig Required relief load: 800 gpm API= 37 Solution/ Gauge relieving pressure= 600 psig Gauge total bp= 160 psig Safety valve is balanced PSV as the bp exceeds 10%. Specific gravity= 141.5/(API + 131.5)= 141.5/ (37+131.5)= 0.84 Viscous fluid ===> USE ITERATION METHOD Q= 800 gpm Kd= 0.65 for priliminary sizing per API recommendation Kw= bp correction factor. Using graphical method for presssure ratio 160/600=0.267, Kw is 0.925 Kc=1 Start iteration with Kv=1; First iteration: A= [800/38*0.65*0.925*1*1]* [sqrt (0.84/(600-160))]= 1.53 in2 ====> Effective discharge area per API-526 table is 1.838 in2 for orifice K Re= (800*2800*0.84)/(500*sqrt(1.838))= 2776 ===> Kv= 0.947 A=1.53/ 0.947= 1.615 in ===> Standard orifice size per API-526 table= 1.838 in2 for orifice K No more iteration is required. The final selection is 3 K 4.

Example/ (A good example including two notes) Select and size PSV for following conditions. Fluid= Air W= 5000 SCFM T= 100 F Set pressure= 20 psig Single non-fire PSV Solution/ Note1: The most important thing here is set pressure less than 30 psig. So, the maximum allowable acculmulation is only 3 psi per API-520. So, the overpressure is 3 psi not 1.1*20= 2 psi Relieving pressure= 20+3+14.7= 37.7 psia bp= 3 psig + 14.7= 17.7 psia Relieving temperatre= 560 R M=29 K=1.4 Z=0.99 Critical pressure ratio= 0.53 Actual pressure ratio= 17.7/37.7= 0.47 < 0.53 ===> Critical Flow C= 356 Kd= 0.975 Kc=1 Note 2: Balnaced PSV is selected. For set pressure < 50 psig, Kb should be obtained from the manufacturer. I consider it 1 here. A= [5000*sqrt (560*0.99*29)]/(6.32*356*0.975*37.7*1*1)= 7.474 in2 6 Q 8 is selected.

Example/ Select and do priliminary sizing a safety valve for following condotions: Fluid= Ammonia and Air (30/70 mole%) Require relief load= 200,000 Lb/h Set pressure= 200 psig= MAWP Overpresssure= 10% Relieving temperature= 90 C Total bp= 120 psig RD is installed upstream of the safety valve. Solution/ Absolute relieving pressure= ( 200*1.1)+ 14.7= 234.7 psia Absoulute total bp= 120+14.7= 134.7 psia Relieving temperature= 654 R Using HYSYS and Peng-Robinson EOS; Molecular weight= 25.37 Ideal K= 1.358 Z=0.98 Critical pressure ratio= 0.535 Real pressure ratio= r= 134.7/234.7=0.57 The flow is subcritical and pilot-operated PSV or conventional with specific adjestments could be used. F2= Coefficient of subcritical flow= 0.728 Kd=0.975 Kc=0.9 because RD is instaled u/s of the safety valve A= [200,000/ (735*0.728*0.975*0.9)] * sqrt [(654*0.98)/(25.37*234.7*(234.7-134.7))= 13.97 in2 The effective orifice area per API-526 table is R with 16 in2 area; 6R8

Solution/ Relieving pressure= (240*1.1)+14.7= 278.7 psia Relieving temperature 700+ 460= 1160 R PSV is conventional (if not specified clearly in the problem statement) with atmospheric constant bp Kd= 0.995 (If not specified in the problem statement, use 0.975 recommended by API) Kb=1 Kc=1 KN= 1 for relieving pressure less than 1500 psia K SH= 0.85 (per tabulated data in API-520) W= 88500 Lbm/h A= 88500/(51.5*287.7*0.995*1*1*1*0.85) = 7.0624 in2 I select orifice Q with 11.05 in2 effective discharge area and inlet size 6" and outlet size 8" as the inlet calss is 300 and outlet is 150 pounds; 6Q8

Example