Compressors a general guideline
This short article discusses some general guidelines for industrial compressors. The purpose of this article is to provide a quick summary of findings for general understanding.
It is assumed that you have a general understanding of industrial compressors.
- Increasing compressor suction pressure and reducing discharge pressure will increase compressor capacity.
- Reducing system pressure drop to increase suction pressure or decrease discharge pressure allows more gas to be compressed through the compressor without modifications
- Lower discharge pressure reduces compressor head and driver power, which increases compressor capacity.
- Centrifugal compressors have performance curves similar to pumps. The major difference is that a compressor moves gas which is compressible, while the pump moves liquid that is not compressible.
- For a fixed speed, the curve shows that for a known inlet flow rate a fixed head is developed. Centrifugal compressor inlet flow rate increases as the head decreases.
- A compressor curve starts at the surge point and ends at stonewall, or choke flow.
- The surge point is the head at which inlet flow is at its minimum. At this point, the compressor suffers from flow reversal, which is a very unstable operation that is accompanied by vibration and possible damage.
- On the other end of the curve is the choke (or stonewall) point. At the choke point, the inlet flow through the compressor cannot increase no matter what operating changes are made.
- As compressor operation moves toward stonewall, decreasing head has less influence on inlet flow rate because the curve slope increases. As the stonewall point is approached, changes in head will have negligible effect on inlet flow rate.
- Centrifugal compressors do not develop a constant differential pressure; they develop a constant differential polytropic head at a given inlet flow rate.
- Reducing polytropic head will increase compressor capacity by moving the operating point to the right except at stonewall. The slope of the curve will determine the magnitude of the inlet flow rate increase resulting from a given polytropic head reduction.
- Reducing polytropic head lowers the compressor shaft horsepower.
Most gas lift, flash gas, and vapor recovery compressors require a recycle valve because of the unsteady and unpredictable nature of the flow rate. Indeed there may be periods of time when there is no flow at all to the compressor. At a constant speed, a constant volume of gas (at suction conditions of pressure and temperature) will be drawn into the cylinder. As the flow rate to the compressor decreases, the suction pressure decreases until the gas available expands to satisfy the actual volume required by the cylinder. When the suction pressure decreases, the ratio per stage increases and therefore the discharge temperature increases. In order to keep from having too high a discharge temperature, the recycle valve opens to help fill the compressor cylinder volume and maintain a minimum suction pressure,
As flow rate to the compressor increases, the suction pressure rises until the volume of gas at actual conditions of temperature and pressure compressed by the cylinder equals the volume required by the cylinder. A flare valve is needed to keep the suction pressure from rising too high and overpressuring the suction cylinder, creating too high a rod load or increasing the horsepower requirements beyond the capability of the driver. The flare valve also allows production to continue momentarily if a compressor shuts down automatically. Even in booster service it may be beneficial to allow an operator to assess the cause of the compressor shutdown before shutting in the wells. In flash gas or gas-lift service, it is almost always beneficial to continue to produce the liquids while the cause of the compressor shutdown is investigated. The flare valve must always be installed upstream of the suction shutdown valve.
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 the chosen suction pressure setting. It will not allow the suction pressure to increase and the compressor cylinder to thus handle more flow rate.
The pressure upstream of the suction valve will increase until sufficient back-pressure is established on the wells or equipment feeding the compressor to reduce the flow to a new rate in equilibrium with that being handled by the cylinder or until a flare valve or relief valve is actuated. Suction throttle valves are common in gas-lift service to minimize 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.
A speed controller can help extend the operating range and efficiency of the compressor. As the flow rate increases, the compressor speed can be increased to handle the additional gas. Compressor speed will stabilize when the actual flow rate to be compressed equals the required flow rate for the cylinder at the preset suction pressure. As the flow rate decreases, the compressor slows until the preset suction pressure is maintained. A speed controller does not eleminate the need for a recycle valve, flare valve, or suction throttling valve, but it will minimize their use. The recycle valve and suction throttling valve add arbitrary loads to the compressor and thus increase fuel usage. The flare valve leads to a direct waste of reservoir fluids and thus loss of income.