8 replies to this topic

[chemks2012]

Dear all

 

I need your help/input.

 

The vessel protected has design pressure of 6barg and design temperature of 150deg C.

 

The safety valve set pressure of 4.5barg which is designed for external fire for vessel content being water.

The relieving pressure is 4.95barg [10% overpressure] and at this pressure, the relieving temperature is 158.62deg C. As you can see, the relieving temperature is higher than the design temperature of vessel.

 

Do you think this is allowed? OR do I need to set the relief valve at low set pressure?

 

Thanks

[aqassam]

Generally you can re-rate the vessel for a higher design temperature. Or you can look at the stress curve for the vessel material and rationalize the acceptance of a relieving temperature greater than the design temperature. 

[chemks2012]

Hi aqassam

 

Thanks for your input but I am not comfortable with the relieving temperature higher than vessel design pressure.

And I am looking into changeing the set pressure to about 3barg.

 

However, I am wondering what's the point that the vessel is desinged for 6barg while we can't set the relief valve at 4.5barg even?

 

Input from other memebers would be appreciated.

Edited by chemks2012, 19 February 2014 - 11:38 AM.

[fallah]

 

Thanks for your input but I am not comfortable with the relieving temperature higher than vessel design pressure.

And I am looking into changeing the set pressure to about 3barg.

 

However, I am wondering what's the point that the vessel is desinged for 6barg while we can't set the relief valve at 4.5barg even?

 

 

Hi,

 

Relieving temperature of the fire case shouldn't normally be considered in mechanical design of the relevant vessel, because in most cases it is assumed the vessel would be failed due to a fire case even with considering a PSV for protection against relevant overpressure...

Then, not only you don't need to lower the PSV set pressure but also you can increase the set pressure up to 6 barg indeed will lead to smaller size PSV...

[RoyenG]

Hi,

 

you have design pressure of 6 barg, but you want to lower the PSV set pressure?

 

How about the PAH/H? and normal operating P? of course nobody want to interrupt process so often and operator do not want to hear alarm.

 

for fire case, the design temperature can be considered as the maximum temperature.

 

Regards,

Royen   

[aroon]

Hi,

 

ChemKS?

 

I would like to highlight on some basics, which should be first checks as follows:

1. Basis of setting PSV at lower than design pressure is not clear. You need to double check the limiting scenario because generally there are many basis of lowering the set pressure such as to protect your vessel in the event of relieving temperature (other than fire case), to protect system located at the downstream this vessel, etc.

2. You can go up to 21% overpressure in the event of fire, which is general practice as per API/ASME. What is the basis of 10% which you are using?

 

As per Fallah, fire case can not be basis to set design temperature. You will have to check your design temperature philosophy.

 

In addition, if the fire is only applicable scenario on this vessel then 1st point mention above is not valid then you can set PSV at design pressure and 21% overpressure also can be used for relief valve sizing.

[chemks2012]

Thanks Fallah, RoyenG and aroon

 

Can anybody please quote me a section in the standard codes [e.g. API] which says that it is OK to have relieving temperature higher than design temperature? It would be highly appreciated.

 

Also, up to what margin this is allowed? For an example, in above case, the design temperature is 150deg C and I am sure, it's not safe to have relieving temperature of about 500deg C?

 

PS: I liked that smiley aroon....

 

Regards,

KS

[aroon]

Refer second paragraph of API 521 Clause 3.15.1.1: Effect of fire on the wetted surface of a vessel. First statement itself will clarify your concern.

 

There is as such no definite margin. You have to provide protective measures listed in Section 3.15.4 (mainly depressuring/blowdown valve) if it is mandatory as per your project specifications.

 

By the way, how are you getting relieving temperature of 500°C with water being a liquid inside vessel? You shall have maximum relief temperature of around 172°C at 7.26 barg (6 + 21% overpressure).

 

For your additional information on high temperature, there is high possibility to have super-critical fluid at relieving condition. You have to check fluid properties at relief first to check relieving condition. If relieving conditions are in super-critical zone then you will have to perform super-critical fire calculations, which are time dependent successive expansion.

[chemks2012]

Thanks aroon,

 

Yes, I am reading API521 now and agree with you.

 

That 500deg C was just an example just to get an idea of what margin is allowed but as you said API521 does not say anything specifically as usual and creates more confusion!!

 

Anyways, thanks all for your help.

 

Regards,

KS