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JT Valve Working Principle

JT valve works on the principle that when the pressure of a gas/Liquid changes, its temperature also changes.

The Joule–Thomson effect describes the temperature change of a gas or liquid when it is forced through a valve, while kept insulated. ( insulation is required to avoid influence of surrounding environment effects).

Joule–Thomson coefficients for various gases

Fig: Joule–Thomson coefficients for various gases at atmospheric pressure.

Joule–Thomson coefficient

The rate of change of temperature (T) with respect to pressure P is a Joule Thomson process (at constant enthalpy).

μJT = (∂T/∂P) H

μJT = Joule Thomson Coefficient expressed in   °C/bar or K/Pa

∂T = Change in temperature

∂P = Change in Pressure.

H= Constant Enthalpy.

All real gases have an inversion point at which the μJT changes sign. The temperature of this point the Joule-Thomson inversion temperature, depends on the pressure of the gas before expansion.

First case:

In a gas expansion the pressure decreases below the inversion temperature – the sign ∂P is negative by definition.

With that in mind, if the gas temperature is below the inversion temperature then μJT is positive since ∂P always negative thus ∂T must be negative. So the gas cools.

μJT > 0, then temperature decreases, gas cools.

Second case:

In a gas expansion the pressure decrease and the gas temperature is above the inversion temperature then μJT is negative, ∂P always negative thus ∂T must be positive. So the gas warms.

μJT < 0, then temperature increase, gas warms.


JT valve is used to avoid formation of hydrates down stream of control valve in winter.

R. Jagan Mohan Rao

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Madhu January 2, 2019 at 3:42 pm

Helpful Points on JT Valve :

1. A normal throttling pressure reducing valve does not have to withstand the stresses and temperature differences imposed on a cryogenic valve;

2. 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.

3. 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.

4. 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.

5. 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.

R.Jagan Mohan Rao January 5, 2019 at 2:42 pm

In Oil or Gas pipe lines, if a Control valve is not operated for prolonged time, in down stream of the valve there are more chances of freezing during winter.

There is a need for more discussion or comments on JT valve with reference to Oi & Gas industry.


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