Design temperature for nozzle load check

[PAN]

My contractor and consultant propose to use operating temperature for nozzle load check and spring hanger selection. They refer to ASME B31.3 (App.S Para S301.5).

However, I disagree with them due to the design temperature is not for short term consideration. In my limited experience, the design temperature should be used in this case. Please comment.

 

[BigInch]

I feel that operating temperatures can take precedence in pipe stress. It makes little sense to design to temperatures that have a low probably of ever being seen during the lifetime of the installation whereas operating temperatures and pressures are continuous. There may also be some questions as to how reasonable the actual design criteria represent the maximum expected excursions from operating conditions. Many piping specs require that piping be designed to certain pressure and temperature ratings corresponding to an ANSI Class break, even though for a given system that class break may be well above any possibility of the actual operating pressure and temperature ever reaching those values. Also keep in mind that para. 302.2.4 even permits occasional excursions above the design parameters, under certain conditions.

Para. 302.2.4 "Occasional variations of pressure and/or temperature may occur in a piping system. Such variations shall be considered in selecting design pressure (para. 301.2) and design temperature (para. 301.3). The most severe coincident pressure and temperature shall determine the design conditions unless all of the following criteria are met."

The best method may be to select hangers and springs and set loading positions based on operating conditions, then check maximum travel loads and stresses based on maximum stress limitations in accordance with the design condition excursions permitted by 302.2.4.

Going the Big Inch!

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[PAN]

In my understanding, design temperature is not for short term operation.

The occasional variations in ASME B31.3 mean "above" design condition.

 

[BigInch]

I don't know what you mean by "design temp is not for short term operation".

What are your design and operating temperatures and pressures?

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[PAN]

BigInch,

I do not have specific question for the design and operating temperature. In my understanding, the piping system should be designed to withstand the design temperature in all cases.

 

[BigInch]

My question was in regards to trying to determine if your operating pressure and temperature are reasonable for setting springs and hangers and determining nozzle loads in relation to your design temperature, or if they are far apart.

You do know that, if there is a problem between loads at operating temperature and design temperature, you can have springs that, once set, maintain a constant load on the pipe and equipment at all temperatures.

I guess I really don't see the problem. IMO and from what limited information you are willing to say, your consultant is trying to make the right decision for you.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[PAN]

BigInch,

I am willing to say. However, this is very difficult to explain in this forum. My English language may not be good enough.

I used to read the meaning of stable operation in an ASME's article (by Vincent Carucci - Carmargen Engineering). I agree with his concept to use design temperature rather than operating temperature for process flexibility.

 

[StressGuy]

I'm not familiar with the article. However, you should be clear in understanding that there is a difference between design temperature and pressure for pipe wall thickness design and the flexibility temperature that the system should be checked at for flexibility stresses and related items such as equipment nozzle loads.

As BigInch has indicated, in many cases, the pressure and temperature used for wall thickness calcuations are often a padded number above the maximum operating conditions.

The thermal flexibility temperature should be a realistic maximum operating case that the system can see, and not some arbitrarily rounded up number.

Now, that said, since you are speaking of your "contractor and consultant", I am taking it that you are a client/owner of the piping system to be designed. Ultimately, you can have it designed to whatever temperature you want - but you will pay for it.

A stress analyst can design flexibility for whatever condition you specify. But, if that temperature is artificially inflated, the complexity and associated costs of the system will increase accordingly. It's your money.

Now, that said - when designing a spring support, it should be designed for the normal operating temperature as you want it to carry it's hot design load and the temperature the system will normally operate at. You would normally check the spring movement under emergency/upset conditions to insure that it will not exceed it's travel range.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[PAN]

StressGuy,

In my understanding, ASME B31.3 -2002 Edition allows for short term operation above design conditions in Para. 302.2.4 (f). This means we have to consider the design temperature for continuous operation (not for short term). This should not the additional requirement from ASME B31.3.

 

[BigInch]

I don't agree. Where's the logic in that? If you size your springs and cans for design temperature (maximum allowed temperature) it may be much higher than the normal operating temperature. Thus you will have the wrong spring travels and unproportional forces imparted to the pipe by the springs for 99.9% of the time. The good thing is that your design will be exact for that 0.01% of the time that temperatures take the high spike.

What would you do in a situation where normal operations (operating for 8750 hours of any given year) are conducted at 250ºF, but for about 10 hours each year, the temperature might run up as high as 500ºF, so the code says 500ºF is the "design temp"? Will you design your springs and cans for 500? You can obviously see that the forces and movements you allow and impart using your method could be stressing the pipe much more than necessary for normal operations.

I maintain that you should design for normal operating temperatures, (determine your normal travels and spring forces, spec the springs and set the rod lengths to that pipe position), then check the travel and forces in the extreme temperature condition and be sure your cans allow that travel and the pipe will take the forces imparted by the springs at that extreme travel. There is nothing wrong with using that method; you are covered in both situations and you will have the proper amount of travel and force on the pipe for 8750 hours of the year and you know the pipe will survive the other 10 hours.


BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[PAN]

BigInch,

I have no objection to your concept for spring hanger selection with check maximum travel load from design condition.

How about nozzle load and anchor point? I still believe that design temperature should be used in this case.

 

[BigInch]

Given that design temperature represents a reasonable maximum, yes, the design temps and pressures must be used to check those max load points. Sometimes you find design pressures and temperatures that do not represent realistic maximums and that is what can make trouble. A common example is one that I harp about often, where a system might only operate at a maximum of 100ºF and 800 psig, but someone saw the ANSI#600 class covers that for flanges and had the idea that pipe should match flange pressures and consequently 1460 psig and 125ºF are the "design" conditions (agreed that the +25ºF isn't much in my poor example). Most equipment, especially pumps, can't take much load and if design conditions arn't realistic, considerable effort and unnecessary expense may be required to "make it work". To be technically correct under those circumstances, the design conditions should be re-evaluated and perhaps changed (via the project procedures) to realistic values. But... right.. otherwise, you have no choice except to use the design conditions.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com