Pressure control suction throttling

If the molecular weight rises by 20 percent and we control the drum pressure by suction throttle control, the motor amps will increase by about 10 percent. 

Figure 3.15. Control modes for the upper sections of fractionators, (a) Pressure control by throttling of the overhead vapor flow. The drawbacks of this method are the cost of the large control valve and the fact that the reflux pump operates with a variable suction head. The flow of HTM is hand set. (b) Applicable when the overhead product is taken off as vapor and only the reflux portion need be condensed. Two two-way valves can replace the single three-way valve. The flow of HTM is hand set. (c) How rate of the HTM is regulated to keep the pressure constant. One precaution is to make sure that the HTM, for example water, does not overheat and cause scaling. The HTM flow control valve is small compared with the vapor valve of case (a), (d) Pressure control is maintained by throttling uncondensed vapois. Clearly only systems with uncondensables can be handled this way. The flow of the HTM is manually set. (e) Bypass of vapor to the drum on PC ...

Suction throttling is usually done with a butterfly control valve, which has a very low pressure drop when it is wide open. 

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. 

Controlling the pressure in the drum by suction throttling allows the compression ratio P2/P1 to increase in response to an increase in the molecular weight of the gas. As the compression ratio increases, according to Eq. (28.2), the amperage on the motor driver also increases. But not by a lot. 

Both the suction throttle pressure control (PC) valve and the spillback [flow-control (FC)] valve are in a nice operating position. All we wish to do is save electricity. 

Controlling suction pressure by throttling in the suction line. 

Regenerator pressure is controlled by throttling the flue gas (305). A manually operated valve is sometimes used for rough adjustment and supplemented by an automatically controlled valve in a by-pass line for precise regulation (330). The automatic control may be arranged to maintain constant differential pressure between the reactor and the regenerator (68). Reactor pressure is controlled by holding a constant pressure at the suction side of the compressors that remove uncondensed vapors from the fractionator (202,330). 

Problems still arise when the gas goes on to processing. It can be difficult, for example, to transfer hydrogen from a zone at low pressure to a compressor with suction throttling while maintaining good control of cell room header pressures. 

A simple feedback system can control the process pressure if a throttling valve is installed in the suction line between the process and the pump, as shown in Figure 23-21. The main problem with this system is that the reponse time is directly tied to the system leakage rate. If the leakage rate is high, response if fast but if the leakage rate is low, the response time will be very slow. Unfortunately, low leakage rate and fast response time is the most desirable combination. 

By combining suction throttling with load gas as shown in Figure 23-25, the advantages of both can be retained. If the throttle valve is closed and the response time is too slow, load gas can be admitted to the system to bring the pressure back into equilibrium more rapidly. This method of control reduces the amount of any expensive load gas required. 

Positive displacement pumps are not supposed to be throttled or regulated on the discharge side. After the line has been walked and every valve has been checked, the pump can be started. Suction and pressure gauges should be carefully monitored, and flow rates tracked. Flow control loops are typically not used with PD pumps unless a series of relief valves and pressure control devices is used. A simple calculation should be made on how fast the tank will fill and how fast the suction tank will empty. Careful monitoring of liquid levels is important. Samples are frequently caught on the product lines and sent to the lab for quality checks. Some PD pumps are designed to be run liquid full at all times, whereas others can be run empty for short periods of time. 

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. 

Small pumps are usually driven by constant-speed electrical motors. Flow may be controlled by throttling a valve in the discharge line. The suction should never be throttled, because a centrifugal pump requires a positive suction head to operate. Low suction pressure causes cavitation and loss of flow. 

---