2 replies to this topic

[KxP]

Dear All, 

 

Could someone please explain what is the difference between a Joule Thomson expansion Valve (JT valve), which is used as a by-pass to the turboexpander, and a normal throttling pressure reducing valve. Both are used for reducing the pressure of the fluid stream. When both the valves does the same job, why is a JT valve much more costly than a PRV? 

 

Thanks,

Kiran

[PingPong]

The cost of any control valve is determined by:

- body and flange sizes

- special design of its internals

- material of constuction

- flange rating (design pressure)

- design temperature

- noise limitations

- ..........

 

The name of the control valve is not part of that list.

[Art Montemayor]

Kiran:

 

PingPong rightfully points out the basic reasons why you should have a difference in price.  You are making reference to what is a cryogenic valve design applied to an adiabatic expansion service.  Please refer to the attached illustrations of a cryogenic valve design and note the difference in physical design.

• A normal throttling pressure reducing valve does not have to withstand the stresses and temperature differences imposed on a cryogenic valve;
• A J-T valve design is used as a by-pass around any cryogenic expansion engine or turboexpander because usually a complete liquefaction of the entire gas stream cannot be allowed to occur within the isentropic expansion device due to the formation of excess harmful/destructive liquid within the machine.  Therefore, a J-T valve is applied to a part of the total stream – even though the adiabatic isenthalpic expansion is much less efficient than the isentropic expansion device doing useful work.
• In carrying out a J-T effect, the expansion valve is subjected to cryogenic temperatures and these require a special design that allows it to function normally when required.
• An extended stem is a physical requirement for any cryogenic valve in order to insulate the valve body from the required, ambient manual or automatic controls.
• Due to the cryogenic temperatures, the J-T valve requires exotic materials of construction (stainless steels) that will maintain their tensile strength throughout the cryogenic range.

 I have used many J-T valves during my time and most have been encased in a cold box.  The extended stem allows for this feature.  I have specified and used a type of J-T design that I have found to be more functional than the usual.  I prefer a 90^o valve body design, with the inlet stream at the side of the valve body and the exiting, low pressure, 2-phase stream at the bottom of the valve body.  This configuration allows me to orient the valve stem in a horizontal manner, allowing easy stem control and the resulting 2-phase stream to enter the vessel (distillation column, 2-phase separator, etc.) downstream with greater ease and less possibility of erosion, flow changes, and plugging due to formed solids (water ice, CO₂, etc.).  this configuration, of course, has its trade off: the extended stem design is subjected to the inlet pressure rather than the outlet pressure and consequently requires more robust design.  But I have found this configuration design has justifiable merits in cryogenic and other refrigeration applications.

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