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Relief Valve Adequacy Check


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#1 farid.k

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Posted 30 August 2014 - 04:36 AM

Hye All. I need your technical support regarding to adequacy check of exiting relief valve and also control valve. For information, the existing slug catcher undergoes retrofit (replacement of the internal part) for improving vapor liquid separation. The existing gas filter separator also has been upgraded with new filter (0.3 micron). My questions are:

  1. On what situation that we need to do adequacy check on the existing relief valve?? (Refer PSV 1, 2&3). Is it by having new flowrate (flowrate has been incresed)? Or the composition of the natural gas change? For me, let say the flowrate increased, the only relief valve that needs to be check is PSV 2 since the relief rate is based on actual flowrate. If we add the flowrate to some figure, the relief rate of the valve might be increased. For the fire case, even the flowrate has been increased, I think the relief valve size still maintain since the relief rate is based on expansion of the liquid inside the vessel of slug catcher and also Gas filter separator and the expansion of the liquid is based on wetted area exposed to the fire. So since the vessel is maintained, I conclude that the relief rates also maintain and the valve size is still maintained. Let say the composition of the gas only change (flowrate remain same), I think, no need to do adequacy check for all relief valve.
  2. It is expected the condensate flowrate increased after having retrofitting at slug catcher, and also due to evolution of feed gas composition (tends to being heavier). So for me, all the control valve need to be checked with new flowrate. If the flowrate increase not so significant, I think we can maintain the existing control valve isn’t?

 

Appreciate team to comment.

 

 

refer Doc1 for rough sketch. need further info pls ask,

 

 

Many Thanks

Attached Files

  • Attached File  Doc1.pdf   111.2KB   33 downloads


#2 fallah

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Posted 30 August 2014 - 05:08 AM

farid.k,

 

Due to limited info of the case you described, following general guidelines could be submitted:

 

If the wetted wall area in the slug catcher and filter not to be changed due to change in incoming flow rate and they could be isolated at starting the fire case, PSV 1, 3 might not need to be checked for new flow rate, but PSV 2 should be checked if would meet the new flow rate.

 

For composition change all PSVs should be checked for new composition, because such change on one hand affects the latent heat value of the fluid and on the other hand affects the required orifice area of the PSVs...



#3 farid.k

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Posted 30 August 2014 - 08:06 AM

Fallah,

 

Thanks so much for the reply. Could you please tell me what is the other info that you need? I will provide it soon.

 

Regarding to the slug catcher, yes, there is no modification in term of equipment size. The pipe size also remains the same. However, they plan to use same facility for other natural gas source. That’s why I said different composition.

 

Regarding to separator, the only modification that has been made is replacement of Filter Cartridges with 0.3 micron size cartridges. Condensate flowrate (at bottom) expected to be higher. It is because we plan to consider natural gas from other source using the same facility.

 

I don’t get what do you mean by “could be isolated at starting the fire case”. For psv sizing during fire case, normally during the fire, all feed and output streams to and from the system cease and all internal heat sources within the process are assumed to have stopped. Thus, the vapor generation and expansion are only a function of the heat absorbed from the fire and only the surface area wetted by equipment’s internal liquid contents is effective in generating vapor. Am I right?

 

Any comment regarding to control valve? I think should be re-check it since the flowrate is different isn’t?

 

farid.k,

 

Due to limited info of the case you described, following general guidelines could be submitted:

 

If the wetted wall area in the slug catcher and filter not to be changed due to change in incoming flow rate and they could be isolated at starting the fire case, PSV 1, 3 might not need to be checked for new flow rate, but PSV 2 should be checked if would meet the new flow rate.

 

For composition change all PSVs should be checked for new composition, because such change on one hand affects the latent heat value of the fluid and on the other hand affects the required orifice area of the PSVs...



#4 fallah

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Posted 30 August 2014 - 01:27 PM

farid.k,

 

To keep the conditions unchanged other than the incoming flow rate, i meant the slug catcher and separator should be isolated from the relevant process by the isolation valves at the moment the fire case getting started. Then it can be supposed flow increasing doesn't affect the relief load of fire case provided that the liquid level in those equipment would be kept at the previous value and the relevant wetted wall areas as well.

 

And yes, the control valve sizing should be rechecked for the change of flow rate and the fluid composition as well...



#5 farid.k

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Posted 31 August 2014 - 02:55 AM

hye fallah,

 

 

farid.k,

 

To keep the conditions unchanged other than the incoming flow rate, i meant the slug catcher and separator should be isolated from the relevant process by the isolation valves at the moment the fire case getting started. Then it can be supposed flow increasing doesn't affect the relief load of fire case provided that the liquid level in those equipment would be kept at the previous value and the relevant wetted wall areas as well.

 

And yes, the control valve sizing should be rechecked for the change of flow rate and the fluid composition as well...

 

Fallah,

Thanks again for your response and really appreciate it. To be honest, I never do sizing for gas service, fire case and multicomponent system. I do appreciate so much for your assistance. I don't know how to specify latent heat of multicomponent system. For single component quite straight forward. Can we specify it using hysys? I’ve run hysys to get the latent heat for multicomponent but I can’t find it. Regarding to the latent heat, does it we need it in order to calculate the rate of vapor or gas vaporized from the liquid?? If I not mistaken using formula:

 

 Mass flow in lbs/h= (Total heat absorption n of the wetted surface in BTU/h)/(Latent heat of vaporization in BTU/lb)

 

Am I right?

 

Appreciate also if you can share go-by for the fire case relief valve sizing if you have it.

 

Thanks so much for your time



#6 fallah

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Posted 31 August 2014 - 03:08 AM

farid.k,

 

1- You can use HYSYS to obtain the latent heat of multicomponent fluid...

 

2- Your formula to calculate the vapor relief load is correct...

 

3- Having the relief load, PSV sizing for fire case can be done same as other cases other than the allowable over pressure is 21% of MAWP instead 10% for single and 16% for multiple PSV...






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