Hot Sustained Stress (Lift-Off) Checking in Caesar II

Hot Sustained Stress Checking in Caesar II

In Layman’s term Sustained means always present. So sustained stresses are the stresses which are present in the system throughout its operating cycle. Weight of piping system and Pressure inside the pipe are examples of sustained loads which generate sustained stresses in the system. So what is hot sustained case?

While analyzing a piping system you many times will come across with few supports which will take load in sustained case but are not taking load in operating and design temperature cases (Refer Attached Fig. 1 and Fig. 2 for one such typical example). The support is lifting at that point in temperature case i.e. supports are not contributing in load and stress distribution while in in operating condition. Still in that situation the weight of pipe and pressure inside the system will induce sustained stresses. So in my opinion, hot sustained stress is the sustained stress in pipe operating situation. And we must ensure that the system stress will not fail because of those supports not sharing any load. That is why many organization make it mandatory to check sustained stresses.

 

hot sustained 1
Fig.1: Caesar II Restraint summary showing lifting supports

Methods of Sustained stress checking:

I have come across with two different methods of hot sustained stress check in various organization:

1. Conventional Method followed by many organizations:

  • In first method the analyst has to run the static analysis as per conventional method.
  • Now go to restraint summary and note down the support nodes which are lifting or not taking any vertical load (Sometimes small positive value may be there due to guide and line stop frictions, in that case check the vertical displacement if it shows positive value consider the same as lifting).
  • Make a separate Caesar file with name FILE NAME_HOT SUSTAINED.
  • Open the input screen and delete all lifting supports from the nodes you noted down. Delete only +Y support, Guide and line stops will be there.
  • Run the analysis and check sustained stress.
  • If sustained stress is within allowable limit accept the file as it is else change the support location or routing to make the system safe.
hot sustained 2
Fig.2: Caesar II Plot showing lifting support

2. New method followed by very few organizations:

  • In this method analyst will check the hot sustained stress in the same main file (No need to create separate file). Some additional load cases are required. Lets assume we will check hot sustained stress in design temperature, T1 condition (means we will check which supports are lifting in design temp case). So the below mentioned cases is required for hot sustained stress checking

L1:                                W+T1+P1                                     OPE
L2:                                        T1                                OPE/EXP
L3:                                  L1-L2                                          SUS

  • Check the stresses for load case L3, if the same is within allowable limit then accept the file else make the system safe.

Notes:

  1. Now you may be thinking whether to mark deleted supports in isometric or not. You must mark those supports. As we have not deleted the supports in actual practice. Supports will be there at site, We simply ensured that without those supports also system will be safe. However if you want to delete those supports that can be done if all other stress criteria can met.
  2. Whether we need to check expansion stresses in hot sustained file too? In my opinion if we are using liberal stress for expansion stress range checking then it is better to check expansion stress (along with sustained stress) in hot sustained file. Otherwise it is not required as system won’t fail in expansion case even after removing those supports.

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.

9 thoughts on “Hot Sustained Stress (Lift-Off) Checking in Caesar II

  1. Neither the “traditional” approach nor the “new method” are valid from a mathematical or engineering standpoint. The traditional approach overlooks a very obvious and important aspect of load distribution in a nonlinear analysis, and that is the fact that piping loads can redistribute significantly without completely lifting off. A pipe support which supports 4,000 lbs in ambient condition may support only 500 lbs under W+T1. It didn’t “lift off”, but the weight redistribution was significant, and traditional approach ignores this.

    The new method is similarly flawed. Linear algebraic subtraction between nonlinear load cases is invalid.

  2. I agree with Mikey. In my opinion, if the pipe is lifting off support by less than 3 mm, ignore displacement it, And if the lift is more than 3 mm, either adjust support location or use spring.

  3. I agree that the approach may not be (structurally) valid but that’s not the point. There are piping codes that have the engineer review stress due to sustained loads for every support configuration that may occur (e.g., B31.3). This article reviews two methods addressing this code requirement.
    I like Prasad’s 3mm check but that number could be argued.

    We have always considered a change in restraint configuration and stress redistribution to be associated with the thermal component in such evaluation; “L1-L2” is the way to go.
    A good engineer should be able to determine when these extra analyses are actually necessary.
    One note on method two – this method is incomplete where friction is considered.

  4. I tried to set load cases as Raghavendra explained
    L1 W+P1+T1 (OPE)
    L2 W+P1 (SUS)
    L3 L1-L2 (algebric) (EXP)
    L4 L1-L3 (algebric) (SUS)
    but I obtain the same result when I use the tradional load cases
    L1 W+P1+T1 (OPE)
    L2 W+P1 (SUS)
    L3 L1-L2 (EXP)
    In both cases +Y support are not removed.
    Could you tell me where I’m wrong?

  5. When there is a lift up at a support location due to expansion load and you need to relocate the support. Does one have to reduce the support span by moving the support closer or increase the support span by moving the support away. Please advise what is the best practice.

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