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Psv Requirement


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#1 yog123

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Posted 21 March 2019 - 04:59 AM

In one of our project we got HAZOP recommendation to install PSV. Can any one suggest the solution so that applicability of this PSV can be checked.

 

My system is as follows:

I am getting process fluid from one pump which is having shut off pressure of 34 barg. This fluid is pass through cold box, get vaporizes and then sent to one of vessel. now on the outlet of cold box, one control valve is present. If this control valve is failed closed, then vaporization of liquid will start building a pressure. However, cold box is designed for 37.8 barg and safety valve set pressure is also same. so during blocked outlet scenario, does heat from cold box will raise the pressure upto the PSV set pressure?



#2 fallah

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Posted 21 March 2019 - 06:04 AM


 

Hi,

 

Please upload a simple sketch of the system you described...



#3 Chemitofreak

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Posted 22 March 2019 - 02:22 AM

Hi,

 

Please upload a schematic

 

On one hand you say there is a recommendation from HAZOP to install PSV and on the other hand you say you have a PSV installed.

 

Thanks.



#4 yog123

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Posted 22 March 2019 - 09:25 PM

PSV is proposed in HAZOP during BEP stage. For your ready reference sketch is attached 

Attached Files


Edited by yog123, 22 March 2019 - 09:26 PM.


#5 fallah

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Posted 23 March 2019 - 12:54 PM


 

Hi,

 

Lack of info for the other streams passing through the cold box...but, assuming 37.8 barg is the DP of tube side in cold box, in general appears the PSV has to be installed there for over pressure protection due to the blocked outlet scenario...



#6 abhi_agrawa

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Posted 24 March 2019 - 04:58 AM

Yog123,

 

When the control valve closes, you have a confined volume of liquid. The other heating streams to the cold box would continue to flow. Therefore, the maximum temperature that this confined volume can theoretically see would be the supply temperature of the hottest stream to the cold box. In my opinion, if the design pressure of this pass is set higher than the dew point pressure of this stream at the temperature of the hottest stream to the cold box, you would not need a PSV for this case only. You should still analyze the system to verify if PSV is not required for any other case.

 

Hope this helps,

Abhishek



#7 yog123

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Posted 25 March 2019 - 12:33 AM

Hi,

 

Lack of info for the other streams passing through the cold box...but, assuming 37.8 barg is the DP of tube side in cold box, in general appears the PSV has to be installed there for over pressure protection due to the blocked outlet scenario...

hi.

 

Thanks for your reply.

 

Pump shut off pressure is 34 barg which is less than the PSV set pressure. Do you think still PSV will see the over pressure during blocked outlet scenario? 



#8 Chemitofreak

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Posted 25 March 2019 - 02:22 AM

If it is suggested to install PSV during HAZOP, you should check the HAZOP Report.

 

They should have specified the scenario for which the PSV is required. Check the node and upset scenarios. 



#9 fallah

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Posted 25 March 2019 - 03:44 AM

 

Pump shut off pressure is 34 barg which is less than the PSV set pressure. Do you think still PSV will see the over pressure during blocked outlet scenario? 

 

 

Hi,

 

It depends on the temperature of the hottest stream passing through the cold box in the blocked outlet case while the trapped fluid is being heated to create the over-pressure scenario...



#10 yog123

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Posted 25 March 2019 - 07:42 PM

If it is suggested to install PSV during HAZOP, you should check the HAZOP Report.

 

They should have specified the scenario for which the PSV is required. Check the node and upset scenarios. 

Hi

 

I already checked the HAZOP report. They mentioned that check the requirement of PSV for the blocked outlet scenario. 



#11 yog123

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Posted 25 March 2019 - 07:48 PM

 

 

Pump shut off pressure is 34 barg which is less than the PSV set pressure. Do you think still PSV will see the over pressure during blocked outlet scenario? 

 

 

Hi,

 

It depends on the temperature of the hottest stream passing through the cold box in the blocked outlet case while the trapped fluid is being heated to create the over-pressure scenario...

 

Hi

 

As i mentioned in sketch, inlet of cold box is liquid and outlet will be vapor. 

 

As you know cold contains many streams, so is it fair to consider hottest stream temperature for PSV requirement check?

 

I am little bit confuse about overpressure. As this stream is getting vaporize in normal operation only, so even if this stream gets blocked and hot streams are continuous, will it create sufficient pressure to generate ovepressure case? 

 

Sorry, May be i am asking more questions but i am not clear about the overpressure case. 



#12 yog123

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Posted 25 March 2019 - 07:53 PM

Yog123,

 

When the control valve closes, you have a confined volume of liquid. The other heating streams to the cold box would continue to flow. Therefore, the maximum temperature that this confined volume can theoretically see would be the supply temperature of the hottest stream to the cold box. In my opinion, if the design pressure of this pass is set higher than the dew point pressure of this stream at the temperature of the hottest stream to the cold box, you would not need a PSV for this case only. You should still analyze the system to verify if PSV is not required for any other case.

 

Hope this helps,

Abhishek

thanks for your reply.

 

i checked the dew point pressure. it is about 74 barg which is higher than the PSV set pressure. However, i have one doubt. Operating temperature is -45.8 °C. So is it fair to consider hottest stream temperature for dew point check? Because, hottest stream temperature is 41.6°C and as cold box contains many streams, this temperature will not provide heat only to the concern stream.



#13 abhi_agrawa

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Posted 09 April 2019 - 03:00 AM

Yog123,

 

The Cold Box is a difficult piece of equipment to analyze. Just some thoughts from my side:

- Control Valve closes and we have a confined volume.

- Hot stream(s) continue.

- Other cold streams that can possibly act as heat sink continue as normal (do not consider double jeopardy).

 

In this situation, only the Cold Box vendor can predict the outlet temperatures and the temperature to which your stream would reach.

 

Due to multiple streams there is a time element as well. Depending on the other streams and the layout of the Cold Box, the heat pick-up may be slow or fast. Again only the Cold Box vendor can possibly advise on this.

 

You need to closely interact with the Cold Box vendor for this case. Alternatively, you can take an extremely conservative approach and say other cold streams are not there and provide a PSV accordingly.

 

You should also consider start-up and other upset scenarios as well.

 

Abhishek






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