Suction throttle valves

The final element is a suction throttle valve that reduces the flow of air into the compressor. 

A process pressure increase over a set value would cause a signal to reach the suction throttle valve and would close the valve in order to reduce the inlet pressure. 

Suction throttle valves are common in gas-lift service to minimi/c the action of the flare valve. Flow from gas-lift wells decreases with increased back-pressure. If there were no suction valve, the flare valve may have to be set at a low pressure to protect the compressor. With a suction valve it may be possible to set the flare valve at a much higher pressure slightly below the working pressure of the low-pressure separator. The difference between the suction valve set pressure and the flare valve set pressure provides a surge volume for gas and helps even the flow to the compressor. 

A speed controller does not ehminate the need for a recycle valve, flare valve, or suction throttling valve, but it will minimize their use. The recycle valve and suction throttling valve add arbitrary loads to the compressor and thus increase fuel usage. The flare valve leads to a direct waste of reservoir fluids and tlius loss of income. For this reason, engine speed control is rec-... 

Let s say we have a centrifugal refrigeration compressor, driven by a motor. The motor is tripping off because of high amperage. Should we open or close the suction throttle valve shown in Fig. 22.2 Answer— close it. Of course, both the evaporator vapor outlet temperature and the process fluid outlet temperature will increase. But that is the price we pay for having too small a motor driver on the compressor. Does this mean that when our home air conditioner gets low on freon, our electric bill drops Correct. But the price we pay is a hot home. 

Figure 28.6 is a partial answer to Jane s predicament. It is a suction throttle valve. Use of the suction throttle valve will partly reduce the... 

If the molecular weight of the gas increases, the gas density will increase. The AP will increase. The pressure in the wet-gas drum will drop. The new suction throttle PC valve will start to close. This will restore the pressure in the wet-gas drum, without increasing the flow of gas through the spillback valve. But what happens to Px How does closing the suction throttle valve affect the actual compressor suction pressure ... 

The suction throttle valve, shown in Fig. 28.6, is analogous to the dam on the River Yeo. When we close this valve, the pressure in the wet-gas drum will increase. But the pressure at Pv the compressor suction pressure, is completely unaffected by the movement of the suction throttle valve. 

For example, let s say that the molecular weight of the gas increases by 20 percent. The AP, developed by the compressor, would then increase from 90 to 108 psi. If the absorber pressure is fixed, then the wet-gas drum pressure would drop by 18 psi. To prevent this excessive decrease in the drum pressure, we throttle on the suction PC control valve. The pressure drop through this valve would increase by 18 psi. But the pressure at P1 will drop by 18 psi, just as if we never moved the suction throttle valve. 

Continue to pinch on the suction throttle valve until the compressor surge limit is approached, or until the spill-back is shut. 

There are a variety of computer software programs available to optimize the speed, suction throttle valve position, and spill-back valve position to minimize driver horsepower and keep a compressor safely av ay from its surge point while still controlling suction pressure. 

When 1 describe how to save compressor horsepower by suction throttling later in this chapter, you will understand the importance of the shape of the performance curve. The concept of the suction throttle valve, in fixed-speed centrifugal compressors, is the least understood aspect of all process equipment. 

Figure 41.6 is a partial answer to the problem of excessive power demand on the compressor s motor driver. We use the suction throttle valve (i.e., the "PC," control valve) shown in this figure. The use of the compressor suction throttle valve will partly offset the increase in the amp load on the compressor s motor driver, which results from... 

Notice how the problem of surge and the amp load on the motor driver are two ideas that interact together. It is kind of like solving two equations with two unknowns. The two objectives we are trying to optimize are the motor amps and avoidance of surge. The two handles we have on the problem are the suction throttle valve and the spillback valve. And just like solving the two equations, we have to optimize the position of the two valves simultaneously. 

Inlet guide vane position or suction throttle valve position will also affect compressor performance. Figure 7c shovii the effect of inlet guide vane position on compressor performance. The efiect of pinch suction throttle valve is similar to reduce compressor s speed. It will reduce compressor s head generation. 

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