Process Description (see Figure 1) The purpose of the process configuration of a Joule Thomson plant is to refrigerate the gas by expanding the gas across a restriction (commonly referred to as a choke valve ). Liquids that form in the gas stream due to the cooling are then collected and removed using a separator. The cool gas is then used to pre-cool the warm inlet gas ahead of the choke valve. Standard Control Instrumentation The purpose of the instrumentation for a choke plant is: Control the process temperature downstream of the choke valve. Maintain sufficient pressure drop across the control valve to ensure that the desired process temperature can be achieved by the expansion of the gas across the choke valve. Prevent no-flow or compressor overpressure conditions from occurring. Figure 1 Figure 1 shows a commonly used control scheme. The differential pressure controller is typically set for 300 psi pressure drop across the choke valve. The temperature controller operating the temperature control valve bypasses some of the residue gas around the shellside of the gas/gas exchanger. If the outlet pressure (sales pipeline) varies significantly, it is advantageous to use a differential pressure controller as shown in Figure 2.
Figure 2 Advantages of Figure 1 and Figure 2 control scheme: Separate control of T and P are easy to visualize and explain to operations staff. Disadvantages of Figure 1 and Figure 2 control scheme: Wasteful of compressor horsepower since compressor discharge runs always at highest operating pressure that ensures target process temperature, as set by the operator.
Figure 3 Figure 3 shows the energy saving control scheme which is the configuration recommended for Gequip Joule Thomson designs operating on compressor discharge. In this control scheme, the temperature controller, as before, controls the temperature of the gas downstream of the choke valve. However, it accomplishes this by direct control of the choke valve. The choke valve is only opened by the amount that allows the desired process temperature to be achieved. This allows the compressor to draw minimum horsepower and consume minimum fuel. The pressure controller is used to override the temperature controller signal under unusual process conditions such as warm start up when the temperature controller would be attempting to close the choke valve too much. Advantages of Figure 3 control scheme: Saves compressor horsepower and fuel. Save the cost of an exchanger bypass valve. Disadvantages of Figure 3 control scheme: Operators likely require some retraining if familiar with older choke plant control system to become comfortable with the variable discharge pressure concept. Requires a high quality temperature controller with adjustable PID parameters to control the slow rate of temperature change exhibited by most choke plants.
Frequently Asked Questions Q: My PC and TC seem to fight each other at startup. A: It is normal for the choke valve to alternate control between the PC and TC for two or three cycles during the cool-down phase when starting the equipment from a warm condition. The PC normally needs to be set so that there is adequate pressure drop across the choke valve to cool down both the gas and the steel in the plant. This is more pressure drop than is required for continuous operation. After cool-down, the plant will be controlled by the TC unless there is not sufficient pressure drop across the choke for the TC to achieve it s setpoint. Q: My PC and TC seem to fight each other all the time. A: In order to function properly, the choke valve must have equal percentage trim and be selected for design choke plant flowrate at approximately 25% to 35% open. If this is not the problem, then it is likely that the controllers are incorrectly adjusted. The order you should test them is as follows: 1. Check if choke pressure is high enough. See if PC can be set a bit higher. (Normally 50 psi above required pressure drop as given in the computer process printout or as determined by manual operation.) 2. Set PC proportional band narrower. (0 100% should only be about 20 psi.) 3. Increase proportional band of TC. (0 100% should be about 40 degrees F span, which is quite a bit wider than most instrument technicians tend to expect.) 4. Decrease integral action of TC to see if it helps. Very little integral action is required of a choke plant TC. 5. Mark original setpoint and then Increase or Decrease derivative action of TC to see if it helps. If not, return to original setpoint. Normally, very little or no derivative action is required of a choke plant TC. Fortunately, the problem of temperature control loops that are too fast is a common one faced by instrument technicians and calling one in to tune your loop is an appropriate course of action if you have tried the above steps 3, 4 and 5 a half a dozen times without success.
Q: In the afternoon, my compressor discharge pressure increases, but the sales pipeline pressure has not increased. A: This is the normal reaction of the energy saving control scheme. The compressor discharge temperature and, hence, the inlet temperature to the choke plant has increased due to increased outdoor mid-day temperature. The temperature control sensed the increase in temperature of the separator and pinched the choke valve so that the gas would cool a bit more. If the TC pinches the valve too far, the PC will take over control of the choke valve to prevent an excessively high compressor discharge pressure.