JT valve downstream temp control

[Renoyd]

Merry christmas,

I need to control the downstream temperature of a JT valve by opening or closing the valve itself. Is this applicable? How to realize it? Thanks.

 

[MortenA]

It dosnt make (much) sense.

The downstream temperature is controlled by two things: Upstream temperature and pressure drop. The temperature drop is proportional with the dP for a wide range of pressure when condensation is NOT experienced.

So unless you can control your upstream temperature, pressure or downstream pressure then you cant control your temperature. One of these has to be an independant variable. That not so common since pressure and flow is connected and normally you dont have infinite turn down on your flow.

Best regards

Morten

 

[JLSeagull]

An understanding of the application could be helpful. JT valves exist in refrigeration for HVAC, natural gas processing and other applications. Also, it would help to clarify whether the JT valve is manual like a fixed choke, or variable such as a control valve operated by an electro/pneumatic signal.

I worked on a fuel gas system where the initial startup conditions could exceed the carbon steel limits. This was due to changes in the installed case from the design case. One could control the downstream temperature as an override to maintain the outlet temperature above the minimum temperature limits if the downstream system is high carbon steel.

Install a temperature transmitter in the outlet stream as the input to a PID controller. Assume that the JT control valve is controlled by another variable, perhaps pressure or flow. For a fail-closed control valve, a signal selector could select the lower output signal of the primary process controller or the outlet temperature controller.

 

[Renoyd]

Thank you for your input. The JT control valve is used for natural gas dewpoint control. In its downstream is a low temperature separator, whose temperature has to be lower than - 13 C to achieve the sales gas dewpoint requirements. Currently the valve is tripped on downstream pressure, which can't always guarenttee the downstream temperature. So I think we could probably directly use the downstream temp as input to control the opening/closing of the valve. Consider the flow to be constant. Based on JLseagull, I need a temp transmitter downstream to send temp signal to a PID controller that will control the movement of the valve, right?

 

[MortenA]

There are four variables:

1) valve opening
2) Upstream Pressure
3) Downstream pressure
4) Flow

The pressure difference 2)-3) is proportional to temperature drop dT

Now you need to consider which variables are independent (controllable) and which are dependent:

1): Independent
2): Most likely dependant - if your valve closes pressure goes up?
3): Most likely fixed by valve downstream the separator or floating on export pipeline pressure (will not change fast)
4): Dependent on valve opening

In this case controlling the valve opening by using the valve upstream the cold separator will cause the upstream pressure to increase and the flow (most likely) to decrease in increased upstream pressure affects well operating conditions. This may be OK - and then you scheme will work

There are many things that I dont know about your system - but the main thing to consider is that controlling dT is actually controlling dP-

Best regards

Morten