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Set Pressure Of Cascade Thermal Relief Valve

thermal relief valve solar set pressure

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#1 potorozz

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Posted 21 December 2016 - 03:59 AM

Dear all,

 

I am designing the thermal relief valve in long pipeline (10 km) as picture in attachment. There are some block valve that to be closed during normal operating. Therefore, cascade of thermal relief valve shall be installed in this system. 

 

I have some question for this design.

1. The set pressure of each safety valve? Can i set the pressure of all relief valves at 18 barg? 

2. Superimpose back pressure of relief valve-01? Should it be the set pressure of relief valve of next valve or normal operating pressure?

3. Relief load of PSV-02?, Relief load should be combined relief load from PSV-01 or not?

4. Can i refer to any standard or design practice?

 

Thank you in advance.

Attached Files



#2 fallah

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Posted 21 December 2016 - 06:25 AM

 

I have some question for this design.

1. The set pressure of each safety valve? Can i set the pressure of all relief valves at 18 barg? 

2. Superimpose back pressure of relief valve-01? Should it be the set pressure of relief valve of next valve or normal operating pressure?

3. Relief load of PSV-02?, Relief load should be combined relief load from PSV-01 or not?

4. Can i refer to any standard or design practice?

 

 

potorozz,

 

1-Yes

 

2-It can be the static pressure at downstream of the first block valve when it's closed, provided that second block valve is open at the same time.

 

3-Relief load isn't so much to be considerable.

 

4-You can refer to API 520 or practices of reputable vendors.



#3 potorozz

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Posted 21 December 2016 - 10:32 PM

Thank you Fallah

 

I have checked in API520,521 but not found the guideline for this case. 

I would greatly appreciate it if you could suggest me about guideline in API or company standard.



#4 AlertO

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Posted 21 December 2016 - 11:44 PM

Hi Potorozz

API 521 Section 4.4.12.2 (version 2014) just mentions to aware about the backpressure, but not mention into details. As per my understanding, Shell DEP has mentioned about this for multi- thermal relief valve system.

#5 potorozz

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Posted 22 December 2016 - 04:53 AM

Thank you very much for your suggestion.

 

I have API 521 with old version (2007) that it is not mention in 4.4.12.2

If anyone have the API new version or Shell DEP. Kindly share to me.



#6 mirandomka

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Posted 22 December 2016 - 09:35 PM

The fundamental is both PSVs need to set high enough to avoid premature opening.

Then, you have to keep in mind conventional type PSV spring can be adjusted. Even if your back pressure is higher than 10% SP, depends on how you set your spring(lower the set pressure), you can still use conventional PSV.

I would first try to set the 1st one at 18barg, second one at 1.8 barg

Next, check the max normal operating pressure at the 2nd PSV. It has to be high enough to avoid premature opening.

If you have problem, you have to raise it to acceptable set pressure, then go back and lower the 1st PSV set to avoid go above 18barg

If you can provide, operating pressure at the PSV location and tank level info. We can have a more solid answer.

#7 mirandomka

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Posted 22 December 2016 - 09:45 PM

I disagree with the idea of setting both PSV at the same pressure unless you have pilot type PSV.

You have to make sure your pressure is not exceeding 18barg at any condition. If you set both PSV at 18barg, when you 2nd PSV is relieving the block valve in between is closed, the portion between the pump discharge and the block valve can go above 18barg, and indeed up to 36 barg, if it is a conventional PSV.

#8 mirandomka

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Posted 22 December 2016 - 09:56 PM

For question number 2, it really depends on PSV type, how you want to adjust the spring pressure for.
For question number 3, you need to be sure the 2nd PSV has enough capacity to handle the first PSV required load. Since it's for thermal relief, you have good reason to believe it is.

For question 4, API520. There is an example at the Annex explaining how spring adjustment will work. Also, details of how different types of PSV work and code requirements are explained.




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