5 replies to this topic

[Asifdcet]

Hi All,

There's a desalter and its PSV inlet piping is not free draining. There's almost 3m of head. The case is that if there's fire while it is online then the vapors generated will displace the liquid. I want to explain this to the asset team that the risk is acceptable and the vessel will not see above design pressure. The design pressure is 20.5barg and relief valve js set at 18.6barg. relief valve is 20m high with 36m lenght if inlet piping.

[breizh]

Hi,

A sketch pls !

Breizh

[Pilesar]

The PSV should be located where it relieves vapor. Is the valve sized to pass the equivalent volume of liquid to the rate of vapor generated? If you find some relevant standard allowing the configuration you describe, please report that to this forum. I am currently aligned with your asset team.. 

[Asifdcet]

The desalter operates with liquid filled at 12 barg pressure. The governing scenario is blocked discharge with flow of 6000T/D and fite scenario flow is 107T/D. I don't know if the relief valve is sized for pass the equivalent volume of liquid to the rate of vapor generated or not and that's what I want to know how I can do that. My principal told me to find out the time at which the vapor will displace the liquid without reaching the relieving temperature. When it will reach the relieving temperature by that time line will be free of liquid and will not cause vessel rupture

Edited by Asifdcet, 07 April 2025 - 05:09 PM.

[Dacs]

The PSV should be located where it relieves vapor. 

 

Desalters are normally liquid full. Oil goes on top and water goes on bottom.

 

The design pressure is 20.5barg and relief valve js set at 18.6barg. relief valve is 20m high with 36m lenght if inlet piping.

 

We're talking about approx. 19 m of liquid height in water (less for crude oil). My take is if the static head (from PSV to top of vessel) + pressure drop when relieving is lower than this, then in principle you should be OK. The PSV inlet piping should be designed with liquid full anyway since it expects to handle blocked outlet.

[Asifdcet]

Description of the problem

 

The desalter relief valve inlet lines do not free fall back to the vessel and therefore in the event of fire, there is a reliance on vapour displacing liquid at the equivalent volumetric rate of fire vapour generation. The pressure drop of the relief valve inlet line is expected to increase the desalter vessel above 10% overpressure with potential for vessel failure during the fire. Note that as the desalter is normally open, this scenario only relates to desalter shutdown scenarios which occurs for only a short time duration when isolated for maintenance without having been drained.

 

As per DEP Shell section 2.15.2 Removing/isolating equipment from operation

1. Fire exposure of equipment removed/isolated from operation is not considered a credible overpressure scenario, subject to approval by the principal, if both of the following conditions are met:

a. Procedures are in place for promptly draining and depressuring equipment that is isolated for maintenance.

 

NOTE: This credit is typically taken only for spared equipment such as heat exchangers, reactors, filters, driers, etc. that are intended to be put into operation at a pre-determined frequency.

 

b. Fire relief load can be relieved elsewhere in the system during normal operation (when the equipment is lined up again).

 

The shutdown procedure for Desalter has all the necessary steps to ensure that desalter is drained and depressured immediately after it has been taken offline that eliminate the risk of liquid being trapped.

 

The desalter remains online most of the time and the scenario is related to desalter shutdown which occurs only for a short time duration when isolated for maintenance which is once every 5 to 7 years. Also, it is taken offline for short period of time for RVs changeover without having it been drained and that is also once in 5 years.

 

An additional layer of protection can be added by making inlet (already a CSO) and out valves CSOs to have precautions of draining and depressuring requirement when they are shut.

 

The RV set pressure has been reduced from 20.5 to 18.6 bar due to static head.

Edited by Asifdcet, 07 April 2025 - 05:10 PM.