5 replies to this topic

[Simonpen2001]

Dear all,

I am new here. I have just started to work for a company that design and build Organc Rankine Cycle. I am involved in PSV design for heat exchanger protection. I am trying to figure out how to calculate the needed mass flow to be discharge from a PSV in case of abnormal process heat input.

 

In shell side I have a hydrocarbon mixture;

in tube side hot steam+NCG (170°C)

 

In case of steam side stop valve fault which will be the flow rate that I have to discharge? It will simply be heat input from steam divided by latent heat of hydrocarbon at shell side design pressure?

 

Is there others method of calculation? Because in this way we calculate a big PSV....

 

Thanks in advance for your valuable help and sorry for my bad english...

 

Simone

[fallah]

Simone,

 

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

 

However, in the way you suggested for PSV relief load might a much conservative value for latent heat of the hydrocarbon mixture be used leading to a big size PSV.

[Simonpen2001]

Fallah, 

thanks a lot for your very quick response.

 

Hereunder a simplified sketch of the power cycle. As you can see, if during a plant trip, the on/off valve at tubeside kettle inlet doesn't work properly, the fluid in the shell side will see its temperature rising. 

 

 

Attached Files

•  foto.JPG   125.46KB   51 downloads

[fallah]

Simone,

 

Thanks for uploading the sketch...

 

Did you check about the used HC latent heat value?

[Simonpen2001]

Yes I checked...

 

My question is if there any other methods to calculate the mass flow rate to be discharged, or if the one that I used is correct and I have to go with the valve that I have calculated.

 

I mean....with a blocked valve on the cold side of an heat exchanger and an open valve on the hot side, is it correct to calculate the discharge flow rate as heat input divided by latent heat value at Prelieving?

 

An other question is: if the cold fluid has a complex molecule and a very low compressibility factor (z=0.4 for example) and an high cp/cv, I cannot use the simplified formula in API 520 that is for ideal gas and I have to use analytical one that can be find in Appendix B? Or can I use the simplified one with no big problem?

 

Thanks a lot once more.

[fallah]

I mean....with a blocked valve on the cold side of an heat exchanger and an open valve on the hot side, is it correct to calculate the discharge flow rate as heat input divided by latent heat value at Prelieving?
 
An other question is: if the cold fluid has a complex molecule and a very low compressibility factor (z=0.4 for example) and an high cp/cv, I cannot use the simplified formula in API 520 that is for ideal gas and I have to use analytical one that can be find in Appendix B? Or can I use the simplified one with no big problem?

 
Simone,
 
Appears what you described as above is different with what you mentioned in your first post in side (hot/cold) blockage standpoint...
 
However, supposing blockage at the cold side to be occured. Then, we suppose the cold side is to be in liquid state (you didn't clarify). A general answer to your query:
 
-When hot side temperature is less than the saturation temperature of the cold side, there would be a liquid release due to thermal expansion...
 
-When hot side temperature is more than the saturation temperature of the cold side, the relief load is based on generated vapor rate once the cold fluid would has been heated up to relieving conditions. It would be obtained by exchanger's heat duty divided by latent heat of cold fluid at relieving conditions...

Edited by fallah, 29 May 2013 - 01:43 AM.