JT Unit Skids
“JT” is an abbreviation for Joule-Thomson effect. In 1854, James Joule and William Thomson proved that cooling occurs when a non-ideal gas expands from high pressure to low pressure. This cooling effect can be amplified by using the cooled gas to pre-cool the inlet gas in a gas heat exchanger. The efficiency of a JT unit is directly related to the efficiency of the gas heat exchange involved in the process.
The JT unit in the picture above consists of a gas-to-gas exchanger (long item), a JT valve (control or motor valve) and a two-phase separator. The additional instrumentation is associated with a hot gas bypass, the pneumatic methanol pump, and the methanol distribution system.
The process of expanding gas to produce cooling is not considered to be an energy efficient cooling process but can be very cost-effective when “free” pressure drop or excess pressure is available. “Free” pressure drop is associated with high-pressure gas reservoirs or pressure letdown stations, when the pressure drop associated with the cooling effect must be taken (such as taking a pressure drop across a choke), regardless of whether a JT unit or another process is utilized.
JT units become expensive to operate when the pressure reduction is no longer “free” and must be provided by mechanical compression. A JT unit can require anywhere from 100 psi to 800 psi differential pressure to operate. A well-designed unit will minimize this pressure differential through increased use of heat exchangers and operate in the 100 to 300 psi range. The cost savings associated with the reduction of compression horsepower and compression fuel will dwarf the incremental costs to upgrade a JT unit with the extra heat exchange.
JT units have a limited application. The cooling generated from the expanded gas is limited and will only condition a gas stream that is fairly low in heavier hydrocarbon components.
The C3-JT unit in the picture above uses three DPC “C” heat exchangers mounted on two skids to reduce the pressure differential across the unit. In this case, at 50 MMSCFD, the savings of 200 psi in pressure differential can reduce the compression requirements by 1350 BHP or approximately $70,000/month in compression fuel ($6.50/MMBTU) and rental.
DPC has JT units operating as warm as 60°F and as cold as -15°F.
- Warm Operations:
A warm operation is when the cold separator of a JT unit operates at anything 40 °F or higher. Warm operations are usually stable as long as the unit has no greater than 7#/MMSCF of water entering it. Methanol injection is used to handle water spikes above 7#/MMSCF.
- Cold Operations:
A cold operation is when the cold separator is operating between 30 °F and 20 °F. To successfully operate a unit in this range, the operator must either operate his TEG system to produce 3#/MMSCF water or rely on a good methanol distribution system. 7#/MMSCF water has a water dew point of approximately 30 °F at typical transporting pipeline pressures. Therefore, when operating at these temperatures with 7#/MMSCF gas, free water droplets will form. Free water will either form hydrates or freeze into ice. Without a good distribution of a suppressant like methanol, the JT unit will foul its heat transfer surface, plug off with ice and hydrates, and ultimately stop working. The lost sales associated with only a couple of hours of downtime will greatly offset any savings from the rental of a JT unit from a low-quality provider. This is the major cause of downtime with JT operations.
- Very Cold Operations:
DPC defines this as any temperature below 20 °F. In these cases special care must be taken. DPC highly recommends that gas water contents be lower than 2#/MMSCF for reliable operations.
METHANOL INJECTION IS ONLY A SUPPLEMENT TO GOOD DEHYDRATION OPERATIONS.