Practical Process Engineering

Inlet is class 600 and outlet is class 150. So,I select 3 K 4 . It means inlet flange is 3", orifice size designation is K and outlet flange is 4".

I missed one thing. API-526 helps us determine the flange sizes based on material of construction, design pressure, design temperatures and outlet pressure (bp). Suppose: Material is CS Design pressure= 50 barg Design temperature= 100 C Backpressure= 10 barg

The only thing that diffrs is overpressure. Maximum allowable overpressure can’t exceed maximum allowble accumulation in the pressure vessel. Here: Maximum allowable accumulation is 1.1 * MAWP= 1.1 * 50= 55 barg Therefore, overpressure is: 55-45 = 10 bar 10/45 * 100= 22.22% The relieving pressure remains the same. All parametrs are the same. So, Orifice K is still good.

Example/ Solve previous example when the set pressure is 45 barg.

I select orifice K with effective discharge area of 1.838 in2

Solution/ Relief load= 30000 kg/h A "single non-fire safety valve" set at MAWP means it has maximum allowable overpressure 10% Set pressure= 50 * 100= 5000 Kpa gauge Relieving pressure= (1.1 * 5000)+ 101.325= 5601.325 Kpa absolute Relieving temperature= 100 +273.15= 373.15 K (Please note that relieving pressure and temperature must be in absolute pressure). bp= 10 barg= 1000 Kpag= 1101.325 Kpa absolute I opened a new case in Aspen HYSYS using Peng-Robinson as the EOS to obtain some basic information: Molecular weight= 22 Kg/Kmole Z= 0.9018 Ideal Cp/Cv= K= 1.209 Critical pressure ratio = [(2/ K+1)^ (K/K-1)]= 0.5627 In our example the real pressure ration is= 1101.325/5601.325= 0.196 So the flow is critical because actual absolute pressure ratio is less than critical pressure ratio. Backpressure is higher than 10%, but less than 50%. Balanced PSV can be the first choice (we have no other information regarding pressure drop in inlet piping) Kd= 0.975 (for early sizing per API recommendation) Kc= 1 as no rupture disk is installed upstream of the safety valve Kb for this installation is 1. C coefficent= 0.0257 per tabulated data in API-520 Part 1 A= [30000/(0.0257* 0.975*5601.325*1*1)] * sqrt (373.15*0.9018/22)= 835.95 mm2= 1.295 in2 Now, we refer to API-526, table 1 to select the proper orifice size.

Example/ Size safety valve for a following conditions. Fluid: a mixture of: CH4: 55 %mole C2H6:25 %mole C2H4:10 %mole C3H8: 5 %mole H2:3% mole CO2:0.5 %mole H2O:0.5 %mole N2:1 %mole Single non-fire safety valve Set pressure @ 50 barg= MAWP Relief load: Constant at 30000 Kg/h Relieving temperature: constant at 100 C bp: total bp @ 10 barg

It seems safety valve sizing is straightforward. Not always! If temperature is not constant, if composition changes time by time, if fluid phase may change and finally, if relief load may vary during the relieving due to any reason, you’d better calculate effective discharge area for all possible scenarios and find the maximium area. This is the challenge and why dynamic simulation is employed sometimes.

Tabulated values of C in API-520. A graphical method is, also, offered by API.

In SI units

In USC units

There is still one more coefficient in the denaminator: "C". It is function of K: Ideal gas specific ratio. You can calculate it directly using Ideal gas K obtained from a simulation software like Aspen HYSYS.

Use this graph to obtain Kb for subcritical flow of liquid

Use this graph to obtain Kb for critical flow of gas/vapor

A: Effective discahege area( in2 /mm2) W: Mass relief load (Kg/h or Lbm/h) T: Relieving temperature. Should be absolute temperature. (K or R) P1: Relieaving pressure. Should be absolute. Relieving pressure= set pressure+ allowable overpressure. (Kpa absolute or psia) Z: Compressibility factor (0.8< Z<1.1) M:Molecular waight (Kg/Kmole or Lbm/Lbmole) Kd: Effective coefficient of discharge. (=0.975) API recommends 0.975 for priliminary sizing. Manufacturers must be consulted to obtain real amount of this coefficient. This is why vendor sizing result differs from our early sizing. Software developed by vendors, themselves, have this coefficint and you no longer need to contact them. Kb: Capacity correction factor. We know why we need this correction factor for ballanced PSV. Two graphs were posted already to obtain this coefficient for gas/vapor and liquid services). However Kb=1 for conventional and pilot-operated PSV's. Please note that here we are only talking about critical flows. But, the flow through a pilot operated safety valve is isubcritical in most cases as the total bp exceeds 50%. If you install a pilot-operated PSV on a critical service, you can use above formulae with Kb=1. Please consult with the manufacturer to obtain right Kb when a balanced PSV is installed on subcritical service. Kc: 1 if no rupture disk installed upstream of a safety valve and 0.9 if installed. V: Volumetric flow rate (scfm at 14.7 psia and 60 F or Nm3/min at 101.325 Kpa and 0 C) Gv: Specific gravity of gas/vapor

Formulae for gas/vapor in critical flow

API-520 assumes the reliving process is isentropic. For gas/vapor services, it assumes compressibility factor is between 0.8 and 1.1. If no calculated Z is available, API-520 recommends to use Z=1 We will discuss other conditions that do not meet above assumtions and, also, two-phase sizing in future. We talked about critical and subcritical flows. API presents equations for following conditions: Gas/vapor in critical flow Gas/vapor in subcritical flow (Pilot-operated PSV and conventional PSV with specific adjustments) Alternative formulae for subcritical flow using critical flow formulae Steam Liquids

#Safety_Valve_Sizing I start this section by introducing some new terms and reviewing sizing procedure. To start the safety valve sizing, we need some basic information from the procees, PFD , relieving fluid data, temperature, possible overpressure scenarios, etc. We have to know the relief load as well. It is very easy to calculate or specify it in some cases (a PSV installed on compressore discharge shall relieve whole discharge flow. No more calculation is required), and it would be very difficult to predict it in other cases (a PSV installed on a distillation tower that distills a wide boiling range mixture. It may need rigorous dynamic simulation). The relieving process happens at fixed temperature for a fixed moleceualr weight fluid sometimes. On the other hand, temperature fluctuates or relieving fluid may experience compositional change (like distillation towers) in some other cases. Therefore, you would better size the PSV for various cases to determine largest orifice area; GOVERNING CASE. Using Formulae in API-520 part 1, you start priliminary sizing. This sizing determines "effective discharge area". This effectice discharge area is calculated based on "required flow". So, required flow is the relief load you already calculated or specified per the best information you got from PFD, HAZOP review, operational experience, detailed calculations, etc. Effective discharge area is not the final area. It is valid for early sizing. Then you refer to API-526 to choose the standard size orifice. API-526 lists standard orifice areas and you select an orifice with larger size but close to effective discharge area you already calculated. For example, you calculated the effective discharge area as 4". The closest orifice size but greater than 4" is orifice N with 4.34" effective area. Now you are ready to fill out the datasheet and place the purchase order. The vendors may have the same orifice area in stock or offer something larger to you. They send the PSV to you along with a new datasheet. What you see on this new datasheet might be totally different as what you ordered. The vendors do not use API-526 to manufacture their safety valves. I discuss later why thier orifice size differs from us. So, the orifice size they offer is larger than the effective area you already calculated. this new orifice area is called "Actual Discharge Area". They, also, back calculate the flow as well. This new flow is called "Rated Flow". You have to make sure the rated flow in not much larger the required flow and also the actual discharge area is not very larger than the effective discharge area. Chattering and premature failure is the direct result of oversized valves. Summary: Sizing procedure: 1- Specify the possible overpressure scenarios 2- Calculate the relief load for every single scenario. 3- Calculate the effective discharge area. If molecular weight, fluid phase and composition are constant, the largest relief load determines the governing case; the largest effective area. Otherwise, you would better to calculate the effective discharge area for all conditions and find the largest effective orifice area. 4- Fill out the datasheet and place the purchase order. 5-,Vendor sends back the PSV and new datasheet. Make sure actual discharge area and rated flow do not differ greately. Some vendors have their own specific softwares. You can use them and choose the right size directly witout using API formulae. However, I review the API formulae and solve some examples.

The last thing I mention here about pilot-operated safety valves is another important application for them is when the total bp is higher than 50% of the set pressure. The bp is enough high not to allow spring-loaded types work properly, whle pilot-operated valve operation mechanism is independent of the bp. Please note that when the bp exceeds 50% criterion, the flow regime may change to subcritical. In PSV sizing section we will review equations for both critical and subcritical regimes. I close this introductory section and start more attractive topic : PSV sizing.