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Design Temperature For Power Failure Condition


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#1 Travesh

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Posted 22 September 2008 - 08:51 AM

Hi,

Please assist.

Usually design temperature is set at the maximum operating temperature plus an allowance.
During a power failure scenario, for instance if the fans on an air cooler fail, then the operating temperature of the air cooler will be above the design temperature momentarily.

Should the design temperature be increased to accomodate for this? Are there any documents/standards that govern this?

Thank you.

Regards,
Travesh

#2 fallah

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Posted 22 September 2008 - 10:38 AM

Design temperature of an air cooler as outdoor equipment should be considered equal to sunshine radiation temperature in location the air cooler to be installed.

#3 ARAZA

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Posted 22 September 2008 - 03:44 PM

Yes, you are right, the design temperature is always a extra margin over the operating temperature. The margin could be 50 deg F OR even 100 deg F based on the engineering judgement and how extreme the operating parameters are.

Power failure OR any utility failure in the plant DO NOT govern the design conditions. But these upset conditions matter a lot when the piping designer does the stress calculations. The pipe need to be flexible enough for the upset case. You should let your piping designer know that this is the case.

In your case, power failure would lead to failure of the fan and loss of cooling. BUT there is still some cooling (25%) available by natural convection. You should compare the flange rating at the design (operating + margin) temperature and the upset (power failure scenario) temperature, if the flange rating do not change, than you are good to go. If the upset temperature forces you to go to the next flange rating, than you should consult ASME B 31.6 and see how many hours in a year you can exceed the design temperature OR design pressure.

I would recommend get advice from your piping engineer and try to follow your company's standards you might have on this subject.

Hope this helps.

ARAZA

QUOTE (Travesh @ Sep 22 2008, 09:51 AM) <{POST_SNAPBACK}>
Hi,

Please assist.

Usually design temperature is set at the maximum operating temperature plus an allowance.
During a power failure scenario, for instance if the fans on an air cooler fail, then the operating temperature of the air cooler will be above the design temperature momentarily.

Should the design temperature be increased to accomodate for this? Are there any documents/standards that govern this?

Thank you.

Regards,
Travesh


#4 shan

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Posted 23 September 2008 - 08:03 AM

Hi Travesh,

You should use the hot process inlet temperature as design temperature reference for you air cooler to begin with. Loss of cooling due to air cooler power failure should be the contingence considered by the downstream equipment designs.

Regards

Shan

#5 Padmakar Katre

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Posted 23 September 2008 - 09:09 AM

Dear,
Its not a specific question where some one could give you the right answer. So many things are not clear like where is your air cooler located no sketch to comment. Does it have any back up of the trim cooler/condenser? What scenario you are describing here is it unit power failure or the whole complex power failure. I just assume certain points and will reply your query. You can specify the max operating temperature plus some value (10,15 or 20 whatever is in your design basis) as the design temperature. If your air cooler comes after a cooler then you can specify the inlet temperature of the upstream cooler as the design temperature of the air cooler as it will experiance that temperature in the event of the upstream cooler failure scenario. Here make sure that the upstream cooler inlet temperature value is more than the inlet temperature plus certain value as per your design basis. In simple words that if the design basis says that all heat exchange equipment to be designed for 20 C plus temperature than the maximum operating then make sure to account the Delta T in your upstream exchanger. Whichever is more specify that.
If my post doesn't make any sense please ignore.

#6 JoeWong

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Posted 23 September 2008 - 06:16 PM

The air cooler design temperature should be at least the upstream maximum temperature. Margin subject to designer...

Donwstream of air cooler subject to natural cooling or provision of high temperature trip...

#7 djack77494

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Posted 24 September 2008 - 11:20 AM

I concur with the suggestion made by shan and JoeWong, but would add a bit more detail. I am accustomed to using the higher of either the "normal" operating temperature plus a predetermined margin OR the maximum temperature that might be expected under off-design (upset) conditions. Note that no margin need be used in the latter case. Also note that fire is NOT included as an off design case - you cannot design for fire temperatures.

You still have the problem of specifying these off-design conditions. As suggested by others, many companies will permit you to take advantage of reduced cooling by natural convection during a power failure. I agree that this is a reasonable approach.

#8 JoeWong

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Posted 24 September 2008 - 01:30 PM

QUOTE (djack77494 @ Sep 24 2008, 12:20 PM) <{POST_SNAPBACK}>
...using the higher of either the "normal" operating temperature plus a predetermined margin OR the maximum temperature that might be expected under off-design (upset) conditions. Note that no margin need be used in the latter case. Also note that fire is NOT included as an off design case - you cannot design for fire temperatures.


Agree.

QUOTE
You still have the problem of specifying these off-design conditions. As suggested by others, many companies will permit you to take advantage of reduced cooling by natural convection during a power failure. I agree that this is a reasonable approach.


API allow as well...




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