Suction pressure throttle valves

A suction pressure throttling valve can also be installed to protect the compressor from too high a suction pressure. This is typically a butterfly valve that is placed in the suction piping. As flow rate to the compressor increases, the valve will close slightly and maintain a constant suction pressure. This will automatically limit the flow rate to exactly that rate where the actual volume of gas equals that required by the cylinder at tlie chosen suction pressure setting. It will not allow the suction pressure to increase and the compressor cylinder to thus handle more flow rate. 

In the third e.vample, the line terminates at 53%. This means DO NOT run this pump at less than 53% of the BEP. 53% of 4500 gpm is 2385 gpm. Because this is a firewater pump and because firemen need to throttle the nozzles on their fire hoses, then we need to install a pressure relief valve on this system with a discharge bypass line so that the pump dumps the restricted water (less than 2400 gpm) back into the suction tank or lake. If not, this firewater pump is likely to suffer bearing failure during an emergency. 

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. 

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. 

If the molar mass increases by 20 % and we control the suction drum pressure by recycling exit gas to the inlet (spill back control) the motor amps increase by 20% if control is by throttle of the suction line, the motor amps increase by 10%. For every 10% decrease in the total number of moles compressed, the amps load on the motor drive decreases by 5 %. For control valves on the suction or discharge side, allow a Ap of 5 % of the absolute suction pressure or 50 % of the dynamic loss, whichever is greater. 

When the compressor first operates, the incoming pressure is well above the suction set point, and the valve upstream of the suction knockout pot (PV-1/200) must throttle the flow. As the line pressure falls, the valve continues to open. When it is wide open and can no longer hold the desired suction pressure, the compressor recycle valve (PV-2/200) must open. Finally, at some level of vacuum on the source, the system reaches its limits and can no longer pull chlorine from the container. Then comes shutdown. 

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. 

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. 

A simple method to influence the operational behaviour of a jet compressor is to throttle the motive pressure by a control valve. With a control as shown in Figure 4.9, the motive steam pressure can be reduced and thereby the motive mass flow rate is similarly reduced. This means that for lower motive flows the critical discharge pressure is also reduced. But as previously explained, this has no direct influence on either the suction capacity or the suction pressure, as long as the actual discharge pressure is lower than the critical one. 

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. 

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

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. 

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 ...

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). 

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