Demister freezing

[enguest]

Gents,

We are working on the plant air supply system and to avoid the free water entering the air dryer and nitrogen unit our client asked us to install the demisters in the air receiver vessels. After HAZOP it was identified that this demister can be freezed in the cold time of the year and so this can complicate the plant air supply to the air dryer and nitrogen units or even stop the air going to this equipment. Has anobody of you the experience of heat tracing the demister pad? Or maybe you can suggest anything else?

Thanks in advance.

 

[Zoobie]

Should the air dryer not be upstream of the receiver?

 

[enguest]

Zoobie,

No it should not be upstream of the receiver, firstly the free water is beeing separated from plant air and then this air is being sent to the air dryer for further drying of it up to negative dew point values.

 

[JLSeagull]

The pressure safety relief valve should be upstream of the demister.

 

[enguest]

JLSeagull,

Thank you this valuable comment. Yes we considered the safety relief valve upstream of the demister,this only protects the system from the overpressure but doesn't eliminate the blockage of the demister and stopping the supply of the plant air to the Air Dryer and Nitrogen generator.

 

[JLSeagull]

Your question pertains to freezing, one aspect of the demister that could cause loss of instrument air. Oil or other contaminants coulod plug the demister. Treating the demister issues is important.

A process hazard analysis could also consider the consequences of loss of air pressure. In some process applications, the loss of instrument air could justify SIL-3 solutions such as triple measurement devices etc. activating emergency shutdown of the units on pressure above where individual shutdown and control valves begin closing due to the supply pressure loss (at different pressures). Consider an orderly shutdown on low instrument air pressure instead of a more random fall-down.

 

[786392]

In Case the system is very precious to operate and

even the de-mister presence is felt as 'inevitable'then

some automatically controlled electrical heating system

along with sufficient "Control's hierarchy in-place"

Could prove to be a possible way out.

Best Regards
Qalander(Chem)

 

[Pons19]

We have done similar type of design before keeping the whole compressor, receiver and dryer package inside a temperature controlled room.

During summer, there will not be any problem but during the winter, we are advised to use a heater in the air intake filter during low ambient temperature conditions.

Kind Regards
Pons