Hello,

I have a question about allowable stress of plastic piping.

In B31.3 - 2010, Appendix B, Table B-1 is the Hydrostatic Design Stress (HDS) of thermoplastic pipe.

I am not sure it is the allowable stress or not, please help.

Thank very much.

If you're talking about FRP you don't want to use B31.3, use ISO-14692. If you're talking about an isotropic plastic material, then you can apply B31.3 rules.

In either case, you need to obtain the material properties for flexibility analysis from your vendor.

Hi Richard,

Thank for your answer, because my partner cant provide the allowable stresses so I have to find the closest values ^^

Maybe many people have the same problem with me.

Regards,

As Richard said, the HDS is a design stress for fiber reinforced plastics. If you want to do a stress analysis for that material, you can use B31.3, but it is better to use UKOOA, BS7159 or ISO14692.
If you want to do stress analysis for thermoplastic piping you better do a search on this forum for older posts about this same topic as there are some issues:
- material behaviour is not elastic-plastic, but elastic only. So normal B31.3 stress checks (SUS, OCC and EXP) are not applicable.
- material expansion will be large/huge. The used calculation method in CAESAR II may not be sufficient to correctly calculate displacements (and thus stresses).

Dear Corne,

thank for your answer, i think i will use the data ^^.

but my project is to evaluate the dangerousness of an as-built pipe line.

is it possible to use the code stress ratio to judge whether the components in the system are critical or not. Because i've read some document and it says the code stress is for new design piping system only?

Best regards,

Giang, it is not possible to use B31.3 stress ratio to check thermoplastic piping.
Keep in mind:
- thermoplastics are pure elastic materials. No elastic-plastic behaviour. So a combination of all stresses should be checked (OPE cases).
- material strength as defined by suppliers is a very short term tensile strength. This is much higher than long term tensile strength.
- depending on the content of the pipe, ageing will happen faster/slower.
- use correct safety factors for a thermoplast material.
- keep in mind the calculation method used. If (very) large deformation happens, CAESAR II will give wrong displacements and stresses.

You can check for example EN-1778 code for stress allowables for thermoplastics.

Dear Mr.Corn,

Thank for your advice, I will check it again. Because we cant get the basic data so it is very difficult. The only workaround is to find material properties on the internet.

But any how, thank very much.

Oh, sozi,

About my question above. If the material is A53-B, so can I use the stress ratios (sustained and expansion) to judge the risk of piping components?

Please be noted that, this is a pipe line which is being work, it in not the future piping (I mean being designed), I am very confused about this matter, so please help.

Best regards,

Be careful with the strength values of thermoplastics you find on the internet. These are always very short term strength values. Allowable design stress may be only 1/10 or less of these values.

You can analyse an existing system using the code formulas. Allowable stresses are the same for new and in-use material. Only when you have a large amount of cycles, or you are in the creep range, then you have to take into account the already lived cycles to determine the remaining life.
Of course you have to inspect the material to determine wall thickness, material degragation etc.

If you mean that you want to use A53-B to evaluate if the thermoplastic line is OK. Don't do that.

Corne,

Thank you. I will notice about that. I am new to pipe stress analysis and have to be in charge of 1 thermoplastic (4 different material PVC, CPVC, PE & PP) and 1 boiler piping (with Wye pieces and cut-short) systems

Wish they will not collapse before I get my graduation certificate ^^

Best regards,

If you are new to pipe stress analysis, and are in charge of these systems, I hope that there will be a senior who is guiding you and who will check your work. Otherwise, tell your boss you need a senior to do so!

Thank Corne,

I have only 2 choices, continue or stop the project (maybe stop working after that...). There is no option for a senior ^^.

So without any experience you have to check other peoples work? I can understand your personal reason to continu (you have to keep working and earn money), but do you also think about the possible risks?!

I suggest you to read ASME III Code Case N-755 for allowable stress method of HDPE piping. Please note that this code case is for PE4710. If you use PE100 you need to find out the limitations of this material in ISO standards.

You need to be very carefull about the terminology used, undestand it correctly, It has partial calculation method of calculating HDPE piping. The computer applications for pipe stress analysis is done by ADLPipe (by the originator of the code) for nuclear buried water pipe application. But it still requires hand/spreadsheet calculation for the combination of stresses. The HDPE material is not available in Caesar II and you need to create it. Be carefull about the shortpterm and long-term allowable of stresses, elasticity modulus, and poissons ratio. So you may have to use the same model several times for the new defined HDPE material and associated load cases.

There are some difficulties in the modelling of soil springs in the different directions in case you want to use the spring effect of the pipe shell since it may be significant for the low thickness piping. Combination of stiffnesses of soil and pipe wall is required, and application of this in Caesar II is too difficult unless you assign the node locations and spring stiffnesses by hand. If you do not use the pipe route parallel to the coordinate directions, you may mess up everything easily. So forgetting the wall spring effect may be conservative, but most of the case you will miss the usefull and forgiven effect of wall stiffness.

It is possible but require lots of involvement by designer and combinations of computer and hand calculations. As you can imagine it is too time consuming application if you use HDPE as a pipe material.

GRE/GRP material is a lot easier to implement in Caesar II, but you can only use one FRP/GRE/GRP material in a model since the FRP material properties are given by Configuration file only and limited to one.

Hope it helps.

Ibrahim Demir

Ibrahim Demir,

Maybe my description was not clear, the piping is above ground. So everything left is the allowable stress (Hot and Cold), I just need them to input in to my model.

I think you still can use or derive the allowable stresses with the guidance above. For the above ground piping you additionally need to take the environmental factor into consideration. Sun light thermal effect should not be ignored. Additionally you need to shield the pipe this material against other hot uninsulated piping or any other thermal effect which will increase the wall temperature to unticipated level.

HDPE can be safely used -20 deg C to +40 deg C in accordance with ISO and Australian standards. The Code Case N-755 allows to use it up to 60 deg C. If you use it you need to use different service factors other than water service. The Code Case uses service factor of 0.5 (derating factor) for water. It may go to 0.25 for extreme cases. Therefore this factor should be derived by the user of the pipeline if you cannot decide yourself easily. So you have to be careful in selecting or deriving the allowables. If you check the elasticiy modulus at this temperature it may discourage you to use HDPE at this temperature. You may have to support the pipe continuously. The poisson's ratio and elasticy modulus are very important factors in the design of the pipe, selection of span length. If you do not have concrete information about the material that you are using you need to get those from the manufacturer of HDPE including supporting distances etc...

I suggest you to go through the handbook/catalog of major suppliers to understand the material and the concept how to use it. Check Performance Pipe Handbook on the internet.

Regards,

Ibrahim Demir

Dear,

Thank for your kind help, but everything Ineed is the following information, I now have allowable stress for PVC only, but it seems to be HDS value in Appendix B2, B31.3 multiplied with derating factors.


Material P: PVC

Density = 0.053 (lb/in3)


Temp E Alpha Allowable
(F) (psi) (in/in/F) (psi)
73 4.30E+05 2.80E-05 2000
80 4.30E+05 2.80E-05 1750
90 3.70E+05 2.80E-05 1500
100 3.60E+05 2.80E-05 1260
110 3.40E+05 2.80E-05 1000
120 3.10E+05 2.80E-05 800
130 2.90E+05 2.80E-05 600
140 2.70E+05 2.80E-05 440

And the stresses will be checked will be OPE, SUS and EXP. If anyone has the above information for PE, PP, CPVC, please give me for confirmation because I will use HDSxDerating factor to get the allowable stresses.

Thanks,

There are several derating factors involved depending on the code that you are obligated to use. Service derating, temperature derating, location derating etc... B31.3 Appx B does not give you any of them. They are HDS values only and I guess B31.3 is still in development stage for plastic piping.

The most developed one is Code Case N-755-1 for nuclear applications and we were told it is going to be used for the other industrial applications. I guess you can use B31.3 with the information taken from the Code Case N-755-1. But you still need to know almost everything about the materials you are using. Without knowing the material and their behaviour in operation you should not directly do any calculation, the responsibility is great; you may not be able to handle alone. Ask the provider of the materials to give you hand on the material types, and their allowable. None of them will give you any service factor. Your client or you are to decide.

If there is a recognised ISO or European standard for the material you are trying to investigate, and this standard investigation is an option in your analysis program, I would suggest you to use it.
Additionally please note that all the pipes with the materials you mention above, if they are containing pressure, they expand under internal pressure similar to thermal expansion, and they may be as significant as or even large than thermal expansion. Assume 50 metres of PE pipe with 50 deg C of differential wall temperature; it expands around 500 mm under thermal load only. So you need to prepare your piping for the thermal+pressure expansion before start doing anything else.

Good luck.

Ibrahim Demir

Dear all,

Thank for your help. Finally, I have understood something about stress analysis, even though not much ^^, I will have to submit my work to my professor this Monday so I hope you can help me confirm my work is acceptable or not:

+ Pipe materials: PVC, C-PVC, PE, PP, assumed isotropic
+ Allowable stress: hot/cold=HDS*Temperature derating factor
+ Code used: B31.3
+ Stresses checked: Sustained and Expansion

Best regards,

And,

If anyone are willing to help, can provide the following information, it will be very great help, not only for me but other people who want to analysis thermoplastic pipe:

- Applicable code
- Allowable stress (base for this value) which are used to input into Cii, how the code calculates stress limit values (for SUS, OPE, etc.)
- Factors need to be checked (which code stress, strain,...) according to the chosen codes.

Best regards,

If anyone has the hot and cold allowable stress, which are used to input into CII, please send me, I will be very appreciated.

my email is progiangbk@gmail.com.

thanks

Sorry, I meant PE and PP

I found this:

"The method proposed by Coade, the software developer, in their April 1991 article in “Mechanical Engineering News” is adopted in this analysis. The method suggested by Coade for determining the allowable stresses for plastic pipe is based on the allowable span given by manufacturer. A straight pipe, filled with water, sitting on rest supports spaced at the maximum span stated by the manufacturers catalogue is analysed using Caesar II and the largest code stress and largest bending stress at the support location are taken as the limits to the bending and code stresses that should be allowed in the operating system. However, the allowable stresses for piping so obtained appear to be very low simply because the pipe allowable span table is too conservative."

I hope someone can clarify this more.

As said before: "You can check for example EN-1778 code for stress allowables for thermoplastics."

Mostly the strength values and spans given by the manufacturers are at 20 degr C (ambient temperature). If the temperature is higher allowable stresses will even drop further.

There are a lot of factors involved in allowable stress for thermoplastics. That is why you should read the code I mentioned above.

Corne,

We have chosen B31.3 from the beginning, and I see that people are still using it for plastic pipe so I want to continue using this code, moreover we are using academic version so only B31.3 available.

I found that your previous comments are right, for plastic piping systems, we have to check the following (according to Chapter 52, Structural Engineering, National Engineering handbook):

1, bending stress < (HDB/FS)*T = HDS*T with T is temperature derating factor

2, deflection < allow. 0.5% span for PVC and 1 inch for another plastic pipes. But the above allowable deflection seems no reasonable because it should be dependent on the pipe diameter and thickness?

3, Hoop stress < HDS

The long-term and short-term strength and modulus are applied differently for each cases depending on the load is long-term or short-term.

Please correct me if I am wrong.

Thanks.