Stress Analysis of PSV connected Piping Systems Using Caesar II

What is PSV?

PSV or pressure safety valves are a type of valve and are very common to any process industry. To protect any equipment from overpressure PSV systems are used in lines. When the pressure inside the system/equipment exceeds a pre-determined level (normally Set Pressure), they are activated automatically and release the pressure by popping up and bring the equipment pressure to a safe operating level.

Two types of PSVs are extensively used in process industries:

  • Open discharge PSV
  • Closed discharge PSV

Are PSV Connected Systems Critical?

Due to an uncertain event if the pressure of any equipment becomes higher than the set pressure of the installed PSV s then they pop up and reduce the system pressure. During popping up activity the PSVs exert huge reaction force over the system. During the analysis of PSV connected stress systems, we have to consider this reaction force. This is the main reason that PSV connected systems become stress critical. The following write up will try to explain the methods used during the analysis of such systems using Caesar II.

Required Documents for Stress Analysis:

The following documents are required while inputting in Caesar II.

  • Stress isometrics.
  • P&ID and line list.
  • PSV datasheet with reaction force and PSV weights.
  • Equipment GA and datasheet if the equipment is part of the stress system.

PSV Reaction Force Application Philosophy:

Before we start the actual analysis we should first know the reaction force. Normal practice is to obtain the reaction force from the PSV vendor or manufacturer. However if during the preliminary stage of analysis data is not available then the reaction force for open discharge PSVs can be calculated using the below-mentioned formula (from API RP 520). But later it must be corrected for forces received from the vendor.  

Open Discharge PSV
Fig. 1: Typical Open Discharge PSV Connection

Reaction force at the point of discharge in lbf, F=[(W/366)* √{K*T/(K+1)*M}]+A*P

  • Here, W=flow of any gas or vapor in lbm/hr      
  • K=ratio of specific heats (Cp/Cv) at the outlet condition  
  • T=temperature at the outlet in Degree R
  • M=molecular weight of the process fluid          
  • A=area of the outlet at the point of discharge in inch^2          
  • P=Static pressure within the outlet at the point of discharge in psig.          
  • Cp and Cv=Specific heat at the constant pressure and at constant volume respectively.    

For closed discharge PSV systems, there is no specific method to calculate the reaction force. Complex time history analysis can be used to exactly calculate the reaction force for closed discharge PSV systems.    

The reaction force application philosophy for open discharge PSV (PSV output discharges into the atmosphere) connected systems is the same throughout the process industries. But for closed discharge, PSV connected system the force application philosophy varies from organization to organization. Some organization applies the reaction force for closed discharge PSVs but some organizations do not consider it. So user to follow the company-specific project guidelines is such cases.

Where to apply the reaction force?

The following figure (Fig. 2) shows the points where the reaction force is required to be applied for open discharge PSVs.

open discharge PSV
Fig.2: Reaction force Application point for open discharge PSV connected systems

Fig. 3 shows the application point (If required) of reaction forces for closed discharge PSV connected systems.  

closed discharge psv
  Fig.3: Reaction force Application point for Closed discharge PSV connected systems

Caesar II Load Cases for PSV Connected Systems:

PSV forces are considered as occasional forces. So occasional Stress due to PSV reaction force has to be calculated and to be limited within 1.33 times Sh (As per code ASME B 31.3). Here Sh=Basic allowable stress at hot condition. Based on company practice PSV reaction force is added either with a design temperature case or with operating temperature case. Also, some organizations have the practice of making One PSV popping up and others stand by load cases. Accordingly, make the load cases as shown below:  

reaction force
  Fig.4: Caesar II methodology to enter the reaction force
load cases
    Fig.5: Caesar II simple load cases for analysis PSV connected system

The following load cases assume two temperatures (T1=Operating temperature, T2=design temperature) along with Wind and Seismic load cases:  

Load CaseStress TypeDescription
L1 WW+HPHYDHydrostatic Case
L2                W+T1+P1                           OPE Operating temperature case
L3W+T2+P1                            OPE Design temperature case
L4 W+T1+P1+F1                     OPE Operating temp+PSV reaction ( PSV 1 popping up)
L5W+T1+P1+F2                     OPE Operating temp+PSV reaction ( PSV 2 popping up)
L6 W+T1+P1+WIN1 OPE Operating temp+Wind from North
L7 W+T1+P1+WIN2 OPE Operating temp+Wind from South
L8W+T1+P1+WIN3OPEOperating temp+Wind from East
L9W+T1+P1+WIN4OPEOperating temp+Wind from West
L10W+T1+P1+U1OPEOperating temp+Seismic from North
L11W+T1+P1-U1OPEOperating temp+Seismic from South
L12W+T1+P1+U2OPEOperating temp+Seismic from East
L13W+T1+P1-U2OPEOperating temp+Seismic from West
L14W+P1SUSSustained case
L15L4-L2OCCPure PSV Reaction
L16L5-L2OCCPure PSV Reaction
L17L6-L2OCCPure Wind
L18L7-L2OCCPure Wind
L19L8-L2OCCPure Wind
L20L9-L2OCCPure Wind
L21L10-L2OCCPure Seismic
L22L11-L2OCCPure Seismic
L23L12-L2OCCPure Seismic
L24L13-L2OCCPure Seismic
L25L15+L14 OCCPure Occasional+Sustained
L26L16+L14 OCCPure Occasional+Sustained
L27L17+L14 OCCPure Occasional+Sustained
L28L18+L14 OCCPure Occasional+Sustained
L29L19+L14 OCCPure Occasional+Sustained
L30L20+L14 OCCPure Occasional+Sustained
L31L21+L14 OCCPure Occasional+Sustained
L32L22+L14 OCCPure Occasional+Sustained
L33L23+L14OCCPure Occasional+Sustained
L34L24+L14OCCPure Occasional+Sustained
L35L3-L14EXPPure Expansion
L36L2-L14EXPPure Expansion

Output Study:

  • Check Code stresses for load cases L1, L14 and L25 to L36. It is better to keep stresses for L1 and L14 below 60% and for the rest within 80%.
  • Check forces for load cases from L1 to L14.

Better Engineering Practices

  • It is a better practice to use 3-way restraints in both inlet and outlet piping of PSV connected systems if feasible (As shown in figure 2 and 3 above).  However if not possible then try to provide 3-way restraint in outlet only by layout modification.
  • In normal operating case Safety valve inlet line temperature will be operating temperature up to inlet of safety valve and Safety valve outlet line will be in ambient temperature up to the header.
  • Sometimes a Dynamic Load Factor (DLF) of  2 is used for calculating PSV reaction force.
  • If any stress failure or abnormal routing changes are required, then certain local area from header can be used average temperature for 2 meters or 5D which is higher (Safety valve outlet joining at header junction point) and also shall be taken process engineer’s approval.
  • If required stress engineer shall provide R.F pad for the trunnion type support.
  • If connection of PSV closed system is emerging from the header with 45˚ put SIF for this tapping. If required tapping point of the outlet line and outlet header shall be reinforced to reduce SIF.
  • In case of any safety valve assembly is placed on top platform of any vessel, Support can be taken either from top platform or support can be arranged from the top portion of vessel taking clip from the vessel. In both the cases the load and locations of support or clip equipment vendor must be informed through the mechanical group along with the clip information.
  • Do not provide spring below safety valve inlet line

Few more references for you

Modelling Relief Valve (Pressure Safety Valve) Thrust force
Routing Of Flare And Relief Valve Piping: An article
Various types of pressure-relieving devices required for individual protection of pressure vessels in process plants


References:

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

24 thoughts on “Stress Analysis of PSV connected Piping Systems Using Caesar II

    1. Sir, Please indicate direction of reaction force in the bend for Closed PSV System. Also, what is the significance of inputting this reaction force on both vector1 and vector2 in the Caesar II spreadsheet. The ideas you shared are very helpful specially for us who are beginners of this field.

    2. Dear Sir, Can you explain the below point with a diagram and other way

      •In case of any safety valve assembly is placed on top platform of any vessel, Support can be taken either from top platform or support can be arranged from the top portion of vessel taking clip from the vessel. In both the cases the load and locations of support or clip equipment vendor must be informed though the mechanical group along with the clip information

  1. Sir, Please indicate direction of reaction force at the bend for Closed PSV System. What is the significance of inputting this force on both vector 1 and vector 2 in the caesar2 spreadsheet. Your blogs is very helpful specially for us who are new on this field. Thanks a lot!

  2. Sir, Please indicate direction of reaction force at the bend for PSV Closed System. Also, what is the significance of inputting this reaction force on both Vector1 and Vector2 in the Caesar II spreadsheet. Thank you for the ideas you shared to us, they are all very useful specially to us who are new in this field.

    1. Andrew Corilla,
      I feel the sign convention matches with that of CAESAR II convention. the direction of PSV outlet piping determines the direction (sign) of the force acting on the first elbow. It will be in the direction from PSV outlet towards first elbow, with CII sign convention (positive or negative)
      I feel that entering force values in Vector 1 and Vector 2 at the same node is not correct. It has been done only for explanation, I feel by Mr. Dey. Vector 1 corresponds to F1 and should be entered for PSV-1 pop up case at elbow-1 node.
      Vector 2 corresponds to F2 and should be entered for PSV-2 pop up case at elbow-2 node.
      Hope this helps. Thanks.

  3. Please Tell how to Enter Reaction forces in classic Piping Input. How to Resolve the thrusts. May have to enter a 45 degree Bend?

  4. Andrew Corilla,
    I feel the sign convention matches with that of CAESAR II convention. the direction of PSV outlet piping determines the direction (sign) of the force acting on the first elbow. It will be in the direction from PSV outlet towards first elbow, with CII sign convention (positive or negative)
    I feel that entering force values in Vector 1 and Vector 2 at the same node is not correct. It has been done only for explanation, I feel by Mr. Dey. Vector 1 corresponds to F1 and should be entered for PSV-1 pop up case at elbow-1 node.
    Vector 2 corresponds to F2 and should be entered for PSV-2 pop up case at elbow-2 node.
    Hope this helps. Thanks.

  5. Dear Sir,

    This explanation is very useful for us could you please expalin how to give  the valve weight.i mean we considered two rigid to model psv so can we put the weght  equally or give total weight? 

  6. Dear Sir, Please give a detailed description about where to apply reaction forces in the open and closed system, whether in the discharge elbow or in the valve itself and also describe where to provide force in the piping input in which force vector FX, FY or FZ and the sign of the vector ( +ve or -ve). Kindly provide the answer for both open and closed system.

  7. Dear sir
    if we use proper guides to the inlet piping of psv why cant we provide the springs on the inlet piping. Sometimes in the temperature range of more than 330 deg cel resting support will not work in hot sustained case so we need to provide the springs. We cant use the loops as of limitations of the pressure drop.

  8. Dear Sir,
    In open system, also input reaction force at first bend to reached last point that open to atmospheric or not ?.
    In close system, also input reaction force at next bend to reached header or not ?

  9. Dear Sir,

    first of all thanks for this usefull text.
    would you please explain what is diffrence between “RV. POP CONDITION” AND “ESTABLISHED FLOW CONDITION” ?
    also what is diffrence between load cases for these two condition?

  10. Dear Sir,

    If possible, can you share sample closed loop PSV line analysis file?

    This would help me lot..Please.

  11. Closed Discharge System:

    On the figure above, it is shown that the PSV reaction force must be applied on the first elbow downstream of the PSV? Why only one elbow? Should we not apply the forces on other bends as well?

  12. Hi Anup,

    what could be the application of forces in open PSV system with 2 or 3 bend from exit point to PSV. Shall we apply at all elbows. I am very much interested on “which code is describes about this force application”.

  13. Dear sir
    Thank you for good text. I have wondering the open type SRV with vent pipe. Nonmandatory appendix of ASME B31.1 shows the calculation of the force in the vent pipe. The same rule could be applied to the vent pipe or any further consideration is required?

  14. One comment- The API 520 equations assume sonic conditions exist inside valve body and also at exit plane, which need not be the case. Overestimation of the reaction force should be avoided.

  15. Can I use this method to calculate the reaction force of an intermediate blocking valve on a duct that has a bypass with two vents activated by valves? thanks in advance

  16. Can I use this method to calculate the reaction force of an intermediate blocking valve on a duct that has a bypass with two vents activated by valves? thanks in advance

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