Spill-back pressure control

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... 

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. 

In a process, energy losses consist of both thermal and mechanical losses. Thermal losses are typically originated from column overhead condensers, product mn down coolers, furnace stack, steam leaks, poor insulation of heat exchangers/piping and vassals and so on. Mechanical losses could also be significant, which usually occurs in rotating equipment, pressure letdown valves, control valves, pump spill back, heat exchangers, pipelines, and so on. Some of the thermal and mechanical losses are recoverable with a decent payback of investment but many others do not. 

Let s look at Figure 25-2. The discharge pressure of the compressor is fixed by the absorber fuel gas pressure control (PC). If we start to raise the absorber pressure by closing this PC, the compressor will have to develop more differential pressure. That means it will have to produce more feet of polytropic head (Hp). As shown in Figure 25-1 the compressor will be backed up on its curve. The volume of gas compressed will be reduced. The pressure in the wet gas drum will then rise. The other PC on the spill-back line will then start to close in order to reduce the volume of gas flowing back to the compressor s suction. 

Observe the spill-back valve start to close-off to keep the pressure at the spill-back control point from rising. 

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. 

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