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Pressure Vessel Design Temperature And Design Pressure

pressure vessel design

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#1 Abdelilah

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Posted 06 August 2019 - 12:55 AM

Dear all,

 

Please, need some clarification from you guys.

 

Our contractor is design a Mercury bed inlet KO with two design pressure and two design temperature. This KO drum is installed at the downstream of compressor air coolers.

 

For information:

OP = 71 barg @ 62°C.

PSV set @ 78 barg.

 

He considered two scenarios for that :

 

Scenario 1 :

Operating pressure (71 barg) with failure of the air cooler (120°C). Design condition : 77 barg @ 120°C

 

Scenario 2 :

Operating pressure (78 barg) with blocked outlet @ (62°C). Design condition : 78 barg @ 100°C

 

However, as per my understanding, pressure vessel shall be designed with coincidental conditions. in our cas, it shall be (78 barg @ 120°). But the contractor insist that this is not correct since blocked outlet+air cooler failure is not envisaged as double jeopardy.

 

So, please can some explain which scenario shall be considered for the (datsheet, name plate,..) why and any standard that treat this subject.

 

Thanks in advance.


Edited by Abdelilah, 06 August 2019 - 01:21 AM.


#2 Bobby Strain

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Posted 06 August 2019 - 11:21 PM

You probably have already wasted money on an oversized separator. You would have been wiser to look carefully at the size, and hence the cost. It's your money. Apply the golden rule: The client has the gold, she can make the rules.

 

Bobby



#3 Abdelilah

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Posted 07 August 2019 - 04:31 AM

Bobby Strain

 

it would be nice if you share a technical response instead of philosophy.

 

Thanks



#4 fallah

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Posted 07 August 2019 - 12:19 PM

 

Our contractor is design a Mercury bed inlet KO with two design pressure and two design temperature. This KO drum is installed at the downstream of compressor air coolers.

 

For information:

OP = 71 barg @ 62°C.

PSV set @ 78 barg.

 

He considered two scenarios for that :

 

Scenario 1 :

Operating pressure (71 barg) with failure of the air cooler (120°C). Design condition : 77 barg @ 120°C

 

Scenario 2 :

Operating pressure (78 barg) with blocked outlet @ (62°C). Design condition : 78 barg @ 100°C

 

However, as per my understanding, pressure vessel shall be designed with coincidental conditions. in our cas, it shall be (78 barg @ 120°). But the contractor insist that this is not correct since blocked outlet+air cooler failure is not envisaged as double jeopardy.

 

So, please can some explain which scenario shall be considered for the (datsheet, name plate,..) why and any standard that treat this subject.

 

 

Hi,

 

The vessel you did describe has two design conditions, even though the temperature difference also pressure difference between two cases are so small to be able to affect the pressure vessel design.

 

Nonetheless, based on the general engineering practices, you can calculate the thickness of the shell/head of the vessel in two mentioned conditions and select the conditions of the case leading to a higher thickness as governing design pressure/temperature of the vessel.



#5 PhilippM

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Posted 07 August 2019 - 12:29 PM

I think here in Germany it is possible to assign two different combinations of PS/TS (maximum allowable pressure/temperature as defined in the Pressure Equipment Directive). However, this is very rarely done, as it causes additional effort for the documentation.

One example would be a reactor which needs to be regenerated during a special operation mode with lower pressure but much higher temperature than in normal operation.

In this case it can be ensured that after regeneration the temperature is lowered for a sufficiently long time for all the steel to cool down so that it does not exceed the TS of the normal operation design case.

The case you are describing would normally not be dealt with this way, as you can't really make sure that scenario 2 does not occur right after scenario 1.

 

With the process data you've stated however I wonder why anyone would even think about this. I can't imagine that a design temperature of 120°C vs. 100°C would have any significant impact on the cost (unless the KO drum will be made of plastic).

The size of the drum will have a vastly larger impact on the cost and should therefore be scrutinized, instead of haggling about a difference of 20°C.



#6 rikakose

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Posted 09 August 2019 - 08:32 AM

There is no difference of cost between 100ºC and 120ºC.


Edited by rikakose, 09 August 2019 - 08:39 AM.


#7 Abdelilah

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Posted 08 October 2019 - 02:54 AM

Dear All,

 

Many thanks for your valuable replies.

 

The problem is not about the cost.

 

The contractor has made an input mistake for the design temperature (100°C instead of 120C) in the mechanical datasheet.

 

Therefore, the manufacturer calculated the thickness according to 100°C @ 78 barg instead of 120°C @ 78 barg and brought the metal sheets for fabrication.

 

When the contractor noticed the mistake to the manufacturer, it recalculate the new thickness and found that the brought sheet cannot withstand 78 barg @ 120°C.

 

Therefore, the contractor asked us to solve the problem by specifying two design conditions in the datasheet for the vessel. Since this vessel is located downstream of air cooler, he argue that:

 

Condition 1: 120°C (design temperature) can occur only during air cooler failure during normal operating pressure (69 barg).

 

Condition 2 : 78 barg (design pressure) can only occur during blocked outlet at operating temperature (70°C)



#8 PhilippM

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Posted 13 October 2019 - 08:34 AM

What were the specifications that the client agreed to and that were used for the HAZOP?

The client has to decide if he wants to pay for a vessel that does not meet the specifications.

Usually you have a single MAWP and a single MAWT and not a MAWP that is only valid at a temperature below MAWT.

 

If I'm not mistaken having two sets of design data is possible (depending on the jurisdiction and applicable codes and standards) but causes additional costs and problems.



#9 Abdelilah

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Posted 17 October 2019 - 02:05 AM

What were the specifications that the client agreed to and that were used for the HAZOP?

The client has to decide if he wants to pay for a vessel that does not meet the specifications.

Usually you have a single MAWP and a single MAWT and not a MAWP that is only valid at a temperature below MAWT.

 

If I'm not mistaken having two sets of design data is possible (depending on the jurisdiction and applicable codes and standards) but causes additional costs and problems.

 

I fully agree with you sir. I wanted to have some advice from expert mechanical engineer to follow my actions against the contractor which seems have xxx relationship with the client project manager.

 

Thanks for alls.

 

Best Regards






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