6 replies to this topic

[SHAMI234]

Hi Guys,

I have gas scrubber connected to the upstream of compressor.I have psv installed at the inlet of the scrubber.Gas is propane.I sized my valve based on blocked flow as there is gas (propane) but I have been asked to size it for fire which seems very much unlikely to me.Valve set pressure is 250 psi and downstream of the valve is venting to atmosphere.

Attached Files

•  Capture.JPG   84.41KB   4 downloads

Edited by SHAMI234, 27 August 2014 - 11:44 PM.

[Marc-Andre Leblanc]

Hello,

 

Without as sketch or plan it is hard to see the whole picture properly since some crucial information are missing.

 

It is normally considered in the fire case that the all of the equipement inlet and outlet manual valves are closed.

 

In your case this will most likely isolate your blocked discharge PSV on the inlet piping from the scrubber and this PSV wont be available to protect the equipement.

 

In general this will mean the sizing of a PSV for the fire case because its impractical to garantee that 100% of the time this manual valve will stay open.

 

A good exemple is during the maintenance of the scrubber ...

 

Regards

 

Marc-Andre

Edited by Marc-Andre Leblanc, 27 August 2014 - 03:34 PM.

[fallah]

SHAMI234,

 

You should. at first, specify all csedible scenarios and relevant relief loads; then specify the governing case and finally size the PSV based on specified governing case...

[SHAMI234]

Hi ,

 

 

Yes i attached the snapshot/

[SHAMI234]

Thanks Fallah,

 

Yes I'm doing calculaions for all  credible scenarios.

 

Doing sizing for fire for gas filled vessel im confused about surface area.Should i need to use total surface area? 

[Marc-Andre Leblanc]

Hello, 

 

It is unclear from your P&ID if the vessel V110 is located on the ground. 

 

From the look of the P&ID, the vessel seem to also serve a gas/liquid seperation purpose, If such is still the case, use the maximum level of liquid normally operated in the vessel, using the sight glass indication of such maximum level.

 

If that is the case, the impact of the fire on this liquid need to be considered, vaporisation normally generate more capacity requirement than thermal expansion of gas.

 

In both case, fire case normally consider the first 25 ft of elevetion to be exposed to fire. This will mean the total surface area of the pressure vessel if the total height is less than 25 ft.

 

Some Client standard will exempt the area behind the skirt from exposition to fire if only 1 opening of limited dimension is present, confirm if such standard is applicable to your case before considering using this exemption as it is not the conservative approach.

 

Regards.

 

Marc-Andre

Edited by Marc-Andre Leblanc, 28 August 2014 - 03:05 PM.

[fallah]

SHAMI234,

 

The A' in the relevant equation to calculate the effective discharge area is the area of the vessel can be exposed to fire, if fire case would be a credible scenario for that vessel. Then because there is no wetted wall area for gas filled vessel case to transfer the absorbed heat to the fluid inside the vessel effectively, total surface area is normally taken as the area of the vessel which is exposed to fire. Of course, in gas expansion case due to fire, the PSV cannot normally protect the tquipment from failure and this matter should be considered in evaluating the PSV selection and sizing for the relevant equipment...