Hi shaff,
You're not the first one who ask this question:
The logic which you are concluding is not accurate, soft seat Plug valve is not (necessarily) requiring pressure balance.
Suggest to understand the terminology and design first (google it you will end up with so many good reference)
Lubricated (metal seated) Pressure balance Plug valves vs Soft Seated Plug valves
[ul]
[li]Plug valves: A quarter-turn (on-off valve) which have tapered Plug with variety of port types and not torque seated.[/li]
[li]Soft seated: Normally liner/insert on a sleeve which installed in the body cavity. Like any other soft material, it has (not so high) temperature constraint and restricted chemical compatibility.[/li]
[li]Metal seated : To compensate what soft seat cannot do (chemically and or temperature wise). However since it is metal to metal it will have higher friction. Relatively (comparing with the different type and same size), it has larger contact area (which contribute further to higher friction) between the Plug and body[/li]
[li]Lubricated : (pressurized) lubrication which establish a seal between the Plug and body (sealing area), protects it against corrosion and act as lubricant.[/li]
[li]Pressure balance : A design (vary between manufacturer) which allows top cavity pressure (more or less) self-equalizing with bottom cavity or distribute the friction between the Plug and body (more or less) uniform in whole contact area, therefore reduce the torque / friction whilst operating the valve.[/li]
[/ul]
Probably others have a better English word to explain or other reference.
Hope I inform you sufficiently.
Regards,
MR
Greenfield and Brownfield have one thing in common; Valve(s) is deemed to "run to fail" earlier shall compared to other equipments
