Compressors Capacity limitation

COMPCP Compressor capacity limit PDS Product D to sales... 

There are at least three reasons for on-stream cleaning. The first is to restore the system s capability. If the unit is a driver, its maximum horsepower will probably drop as it becomes dirty. Cleaning will restore this limit. If the machine is a dynamic compressor, fouling may reduce its head, and therefore, the maximum gas flow rate. Cleaning will restore the capacity limit. 

Improving overhead cooling will increase the wet gas compressor capacity. Excessive pressure drop or limited cooling in the overhead system decreases the capacity. This can result from ... 

Gas recycle loops are normally set at maximum circulation rate, as limited by compressor capacity, to achieve maximum yields (Douglas doctrine). 

At Prudhoe Bay, the bulk of the natural gas is compressed by powerful compressors and reinjected into the gas caps. This reinjection is beneficial in that it maintains high reservoir pressures, thus keeping the oil production rates relatively high. However, as the oil is produced, the gas cap expands, the reservoir pressure declines and GOR increases and the gas cycling occurs. This "gas cycling" has already been observed in some wells at Prudhoe Bay. The production rate of oil then may be limited by the compressor capacity. This however does not imply that the ultimate oil recovery will be adversely affected. However, the cost of reinjection will increase. 

The condenser pressure controls the tower pressure and thus the feed tray pressure. There is a pressure valve in the overhead, which can be used to control tower pressure. The lower limit of the tower pressure is defined by the column overhead condensing duty, net gas compressor capacity, and column flood condition. During extended turndown periods, reducing pressure up against an equipment limit can avoid dumping. Many of the new APC systems have pressure control implemented. 

For compressors driven by an ordinary (nonvariable speed) motor, the speed of the compressor is fixed. However, there is a method one can employ to overcome capacity limitations imposed by fixed speed compressors (and compressors running at their maximum rated speed). This method is to change the composition of the circulating refrigerant. Details are discussed later in this chapter. 

Fig ure 5-2. Compressor boost capacity is limited by the surge line. 

Compressors driven by steam mrbines, gas turbines, hot gas expanders, or variable speed electric motors usually employ variable rotation speed for capacity control. Generally, the surge limit slope varies witli rotation... 

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