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Reciprocating Compressor OptimizationDefinition Of Important TermsReciprocating Compressor. Sometimes called a piston compressor, is a positive displacement compressor that uses pistons driven by a crank shaft to deliver gases at high pressure. The intake gas enters the suction manifold, then flows into the compression cylinder where it is compressed by a piston driven in a reciprocating motion via a crankshaft and is then discharged. Picture Of A Reciprocating Compressor

Picture Of A Reciprocating Compressor

Definition Of TermsOptimization: Finding an alternative with the most cost effective or highest achievable performance under the given constraints, by maximizing desired factors and minimizing undesired ones.Introduction And ObjectiveOptimizing a reciprocating compressor can have different meanings based on your operating goals and/or conditions. We are interested in maximizing the throughput, reducing the fuel usage, lowering emissions and reliability and maintenance. Being a positive displacement compressor, reciprocating compressors are flexible and are said to be one of the most efficient type of compressors. They can be single or multi staged and can operate under a wide range of discharge pressure varying from 100psig to 1000psig. In natural gas industry they can range from 50HP to over 3000Hp on some applications and are typically driven by combustion engines or electric motors.Maximizing Throughput.This means maximizing the capacity of the compressor. To achieve this, there are two concepts in play. First, the gas has to go through the first stage cylinder(S). The capacity can be impacted by clearance devices such as variable volume clearance pockets(vvcp) or fixed volume clearance pockets(FVCP) that add clearance to the head end of a cylinder. The running speed of the compressor and single acting the crank end or head end of a cylinder will also influence the capacity of the machine. The second concept is to increase the capacity of a compressor by increasing the suction pressure; which means that the suction pressure is dictated by the flow of the compressor or vice versa.Optimized Performance Of A Three Stage Unit Discharge Pressure Of 1000Psig The cylinder capacity portion of the curve implies a cylinder capacity utilization of 100%, which means no clearance is added to the first stage cylinder(S) and the driver is running at maximum speed.The horsepower section of the curve indicates a horsepower utilization of 100%. In this portion of the curve, clearance devices are generally used to unload the driver and this is translated as an increased capacity while maintaining the horsepower usage at maximum.The knee of the curve is the only point on an optimized loading curve where the cylinder capacity utilization and horsepower used is at 100%.

Case Study- Maximizing Throughput On A Field Gathering CompressorA gas producer has a 3 stage, 4 throw back package with a capacity of 4 mmscfd, a suction pressure of 50 psig and a discharge pressure of 1000 psig. Table 1 shows the current operating conditions of this unit.As shown in Table 1, the pockets are opened 0.5 inch on both first stage cylinders and the running speed is 900rpm out of 1200rpm. According to the producer the suction pressure can be lowered and the field is sensitive to suction pressure.

Maximizing Throughput On A Field Gathering Compressor.The curve beside shows the potential potential production gains of the current running conditions. Depending on the well response, by closing the pockets and speeding the driver up to 1200 rpm the compressor will either see a decrease in suction pressure, an increase in flow or most likely a combination of both as shown in figure 2.

Maximizing Throughput On A Field Gathering Compressor.As indicated on figure 2 and 3, this optimization resulted in a decrease of 7 psig on suction pressure and increased flow of 0.9 mmscfd.

Fuel EfficiencyOptimizing a reciprocating compressor for fuel efficiency can be translated to reducing the fuel consumption of the driver for same conditions of operation. This generally occurs when the unit is underutilized with the bypass valve opened. In a gas gathering application a unit is identified as underutilized when its potential capacity is above the amount of gas the field can deliver.In North America, there are good number of natural gas fields that are depleted; this situation is resulting in multiple compressors being utilized. This fact highlights the importance of optimizing the compressors for fuel efficiency and ensuring that the bypass valves are closed. There is almost always a better solution than bypassing.To optimize a compressor for fuel efficiency, the capacity of the compressor needs to be adjusted to the amount of gas the well can deliver. On a reciprocating compressor the capacity can be typically controlled by varying running speed, clearance and cylinder action.ConclusionRegardless of natural gas price it is imperative that compressors remain operating in their optimized state. This paper explored different meanings of optimization for reciprocating compressors from maximum throughput, to fuel gas savings, to maintenance optimization and condition monitoring.