PSVs vented to atmoshpere

[techv]

Hi Guys,

We have Ethylene Tank in offiste whose PSV in vented`tio atm. Whole ethylene plant PSVs are routed to Flare but not this tanks PSV as itb has no enough pressure. I understand this is a standard practice. Our Client questioned this and wants the tank to have dedicated flare. Our defence is that the area has no ignition sources to cause vapor cloud explostion when the PSVs are activated in emeregency.

Can you throw some light on this based on your facilities/designs?

Thanks
TechV

 

[dcasto]

Ethylene has a density close to air. Because it will be cold coming from a PSV, it will be heavier and sink to the ground. A vapor clound will form and if that cloud finds an ignition source, there will be a large vapour cloud explosion.

That is why people put ethylene vapors to a flare. In a plant where there is almost an unlimited supply of ethylene and lots of sources of ignitions, the flare is an absolute requirement.

A single tank at low pressure (you will have to explain this one, cryogenic, only C2= vapors or what), you could look at the size of a release and what ignition sources are available and decide what the risks are. I had a remotely operated ethylene pump station that had a flare on the pump blowdown. The blowdown had a RO so that the maximum blowdown rate was 5000 lb/hr, but if a valve on the station failed open, the whole 90 miles of 8" line could be vented. It was decided the risk was to high and a pizeoelectric ignitor was added to the blowdown vent.

 

[ash9144]

Ethylene is almost always stored under pressure, unless cryogenic as Dcasto mentions. So it would have plenty of pressure to vent.

Also in all the plants I have been in you must assume an ignition source, even remote locations could have a source through some sequence of events. With a heavy gas it will travel until it finds that ignition source.

 

[AndreChE]

The final ethylene storage, for example, at the harbour is at -104ºC. Intermediate storage is under pressure, depending on the consumers processes.

However, you cannot storage at exact atmospheric pressure, you need to control its pressure around 40-60 mbar above atm pressure, giving enough DP to send it to near and well designed flare.

Our operation at the harbour is:
control at 40 mbar
to flare at 60 mbar
only in emergency case: vent to atmosphere

Intermediate storage: 22 barg because polymer plants receives ethylene at 20 barg

AndreChE