Pressure vs. Temp. rating

[jtk921]

I have a new heat exchanger with both shell and tubes 304L and rated at 150 psi @ 350 F, water on tube side, steam on shell side (ASME Section VIII, Div 1). Does the MAWP change if the operating temperature is 160 F? How can I calculate it? Is there a graph or equation? Steam will be regulated through a control valve, but max. upstream pressure is 160 psig. Would I need a PSV on the steam side if the exchanger were blocked in? If I size a PSV for thermal expansion on the tube side, would I use the saturation temperature at 150 psi?

 

[Ashereng]

Some quick answers:

Yes. The allowable pressure varies with temperature.

I don't know if thre is a graph or equation. I use a table.

Yes, you will most likely need a PSV, even if the exchanger is not blocked it - like for a tube rupture.

I have no idea what temperature you would use for thermal expansion on the tube side - I usually get that info given to me by my chemical engineer (which, by the way, I see you are).

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[jtk921]

What table do you use? Do you have a link?

 

[Ashereng]

ASME B16.5 I think.

There is no link - you need to pay for it. Copyright infringements and all that.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[djack77494]

Design pressure would change based on the change in allowable stress as your temperature changed. However, over the range of temperatures (about 160 - 366F) you're considering, and taking into account the material you're dealing with (304L SS), I would expect the change in allowable stress to be nil.

With saturated steam at 160 psig or less, its temperature will be <=366F. You must provide tube side relief because you could block in a water filled tube side and then heat it with shell side steam. Size the PSV by considering how much heat will be transfered and use the thermal expansion of liquid water to figure out the volumetric flowrate to relieve. As long as you do not have superheated steam, you should not generate vapor (steam) on the tube side. (But, there may be other relieving scenarios such as fire where steam could be generated.)

You need to examine all possible overpressure scenarios to decide if you need shell side relief. Right now, I don't see a relief situation, but I have only a few facts. If the shell side could contain liquid, then there would likely be a fire case.

HTH,
Doug

 

[jtk921]

Thanks Doug.

I've taken care of the tube side PSV sizing. On the shell side, there is a bypass valve around the sat. steam regulator. What if this were to be inadvertently opened while the shell side was blocked in? Is this not a relief case? Also, I take it from your first paragraph that the MAWP is going to be essentially the same at 160 F. Currently we pump the hot water at up to 185 psig discharge pressure on the pump. Would we now have to throttle the flow in order not to exceed the MAWP of the tubes?

Jeff

 

[pleckner]

You cannot change the stamped MAWP for your convenience no matter what the actual operating conditions are. You must not use a PSV set pressrue greater than this stamped value but you can set your PSV for a lower pressure if you want, but I wouldn't recommend this.

Yes, this could be one of your relieving scenarios as well as a control valve failure of the steam (although the sizing is the same).

If you can exceed the design pressure of the tubes,then you must protect them. For this application, I would have made sure I had enough losses between the pump and the exchanger so that the PSV set pressure is the greater of 25 psi or 10%, more than the maximum expected operating pressure. The delta pressure I state is to ensure that I don't operate too close to the PSV set pressure, which is not a good thing.

So if your head difference between the pump and the exchanger can't meed a reasonable pressure difference, then yes, you must throttle to reduce the pressure to an acceptable level. You need to consider pump shut-off as part of your relief scenario audit.

Oh, for thermal relief due to water block-in on the tube side, a 3/4" x 1" PSV is usually good enough and no calcualtions are required.