Hydrocarbon Compression


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When a compressor is connected to a large system, which needs a large amount of gas for the process, the gas intake will vary with the process requirement. During startup, the gas demand will be very high. Hence the compressor will run at its full capacity.

As the process proceeds, the demand for gas will start reducing. If the system does not use the gas as quickly as the compressor delivers, the system pressure will increase.

With the increase in the system pressure, the resistance to discharge of the compressor increases.

Due to this, the compressor capacity reduces.

When the head required maintaining the flow increases above the maximum head of the compressor, the gas flow stops.

Under this condition, the pressure within the compressor becomes less than the system pressure. This results in the gas flow from the system to the compressor, called “flow reversal.”

When some quantity of gas has gone to the compressor section, the system pressure will drop.

When the system pressure becomes less than the maximum head of the compressor, the compressor starts delivering the gas to the system.

The compressor operates at a lower capacity and higher head/pressure.

If the system pressure system builds up again to a high pressure, the flow reversal will repeat. The cycle continues.

The rapid flow of gas back and forth the compressor is called surging.

Surging occurs when the compressor operates below the minimum capacity.

The rapid reversals of surging set up severe vibrations in the compressor and piping, which may cause damage to the compressor.

A compressor goes into surging because the flow of gas drops down below the minimum stable limit.

Most compressors are protected against surging by anti-surge control methods as explained below.

Refer to the figure shown below.


Y axis shows the percentage of rated head and x axis shows the percentage of rated capacity of the compressor. A point higher on the graph shows higher head. The compressor capacity varies between 40 to 110 percent. The curve shows, as the flow increases, head decreases.

The compressor is discharging into a system that requires 100 percent
of its rated head. According to the graph, the compressor is operating at
100 percent of its capacity.

Suppose the discharge system does not use as much gas as the compressor delivers the pressure at the discharge end of the compressor increases as may be seen from the graph.

When the gas flow reduces to 90 percent, the head developed increases to 102 percent of the rated head. As the gas discharged from the compressor reduces, the pressure developed by the compressor increases. When the flow reduces to 50 percent of the rated flow/capacity, the compressor no longer delivers the gas to the system and virtually the gas flow stops.

When the compressor pressure becomes lower than the system pressure, the gas will flow from the system to the compressor (flow reversal).

When this condition is reached, the system pressure will start dropping down, and the compressor will discharge gas to the system again.

This repeated process of forward and reverse flow is called “surging.”

Written by Jack

October 1st, 2021 at 2:59 am

Posted in Fundamental

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