Capacity control of reciprocating compressors

Large reciprocating compressors have high power requirements, sometimes running in the range of megawatts. Integrating them into complex processes with side streams or multi-stream compression requires precise control and operational flexibility. Advanced technologies for compressor capacity control can provide flexibility and save massive amounts of energy, but it can be difficult to achieve precise control. Large reciprocating compressors are a poor match for variable-speed drives because the large motors make the drives costly and the high torque requirements require large flywheels. Traditional capacity control methods waste electric power by using recycle valves or recycle valves in combination with step control.

Today, with increasing capacities and power ratings of new reciprocating compressors, advanced methods such as stepless capacity control and split-range control systems can help to eliminate use of recycle valves, save energy, improve controllability and save costs. This article explains the basic theories of traditional and advanced capacity control methods for reciprocating compressors.

Traditional methods used to work

In recent years, many refineries were upgraded to produce clean fuels or process feedstock with higher sulphur content. The sole capacity of one compressor running at full load was sufficient to meet the hydrocracker or hydrotreater hydrogen demand. A second compressor was kept in standby mode only to ensure equipment redundancy. Traditional methods were well-suited for these conditions:

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On/off control: This method is used for intermittent demand, where the compressor wastes power if run continuously. The capacity can be controlled by starting and stopping the motor, which is done manually or by a pressure control loop at certain pressures as per process requirements. This type of control would suffice for processes where continuous use is less than 50% of compressor capacity. Nowadays, with increasing capacity requirements, this method has limited use.

Bypass or spill-back method: The most common way of controlling the flow of a reciprocating compressor is the recycle, or spillback control, method (Figure 1). Bypass control is preferred compared to other methods because of its smoothness, simplicity, low initial cost, high practicality and ease of application. It can cover a capacity range from 0 to 100%.

A bypass with a control valve is used alone or in combination with another, energy-saving type of capacity control, such as an unloader valve, clearance pocket and/or step-less method. For safe operation, the bypass valve is designed for the full-load capacity of the compressor. To reduce the flow to process, the bypass or spill-back control valve is opened to divert excess flow back to the compressor suction. This control scheme is extremely inefficient because backflow gas is recompressed, and compression work is totally wasted.

While it's an inefficient method for controlling compressor capacity, it's always good to have spillback valve as a backup to other capacity control methods for safe operation. If other control methods fail, the spillback or bypass valve is activated for safety. It's also used during compressor startup and shutdown.

Constant-speed step control: Capacity variation is achieved by unloading one or more cylinder ends. The most common schemes are three- and five-step unloading. More steps saves more horsepower because they more closely match compressor output to demand.

In three-step unloading, capacity increments are 100%, 50% or 0% of maximum flow. The driver operates continuously, at constant speed, and capacity is varied in discrete steps by holding cylinder suction valves open on the discharge stroke or by opening clearance pockets in the cylinder. This method of unloading is accomplished by using valve unloading in the double-acting piston. At 100%, both cylinder suction valves are closed during the discharge stroke. At 50%, one cylinder suction valve is open during the discharge stroke, wasting half the capacity of the machine. At 0%, both cylinder suction valves are held open during the discharge stroke, wasting total machine capacity.

In five-step unloading, a clearance pocket is used in addition to cylinder suction valve control (Figure 2). The capacity increments are 100%, 75%, 50%, 25% or 0%. At 100%, both cylinder suction valves and the clearance pocket are closed. At 75%, only the clearance pocket is open. At 50%, only one cylinder suction valve is open during the discharge stroke. At 25%, one-cylinder suction valve and the clearance pocket are open. At 0%, both cylinder suction valves are opened during the discharge stroke.

It's an inherent disadvantage of step control that this method is only efficient as long as the required process gas flow is equal to the adjusted load step on the compressor. Otherwise, excessive gas has to be recycled through a spillback control valve, automatically or manually, resulting in energy losses.

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Advanced methods improve control and efficiency

Stepless capacity control uses hydraulic power and electronic time control to define the motion of the compressor valve (Figure 3). It provides a control range of 0 – 100%, from idling to full load, by regulating the gas volume compressed per stroke. A hydraulically actuated unloader delays the cylinder suction valve closing, allowing part of the gas in the cylinder to flow back into the suction chamber. This is also called reverse-flow, as excess gas goes back to suction.

The hydraulic actuator consists of a high-pressure piston generating a force acting on the unloader. The oil flow and valve motion is controlled by an extremely fast switching solenoid valve. After closing the cylinder suction valve, the remaining gas in the cylinder is compressed. Therefore, the compressor capacity is defined by the closing time, which is set electronically.

Stepless capacity control is an efficient, dynamic and fully automated control system. Only the required amount of gas needed is being compressed, unlike other control systems like bypass valves that recirculate compressed gas, or stepped control that compresses more gas than needed. At full load, the gas is compressed during the entire compression stroke. At part load, it holds the suction valve open at the start of the compression stroke and automatically closes the valve when the desired volume of gas is in the cylinder. Only the gas required is compressed to discharge pressure.

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The stepless control method reduces capital costs of efficient multistage compressors because the cost of a compressor with stepless control is less than a compressor with step loader. Capacity variation is bumpless compared to step control, so minimum intervention is required for operation. It also reduces CO2 emissions by saving energy, and can reduce the need for gas flaring since only the volume of gas required by the process is compressed. Thus, it reduces environmental impact and aids compliance with environmental regulations, reducing CO2 emission costs.

Split-range control: The main objective of this control loop (Figure 4) is to maintain suction and discharge pressure of the compressor header. Priority is given to suction pressure of two stages, rather than the discharge pressure. This is achieved by providing low-selecting switches (LSS) between suction pressure (PIC A) and discharge pressure (PIC B) for the first stage. The split-range control loop and minimum selector are implemented in the DCS.

During startup, the recycle valve (A) is fully open. During normal operation, PIC A is set at 5 Kg/cm2, and its output is low selected by LSS to maintain the pressure by modulating recycle control valve A and compressor A. If the pressure is below the PIC A setpoint, as its output increases from 0-50%, recycle valve A will close. From 50-100%, compressor A will start loading. The controller action of PIC A for the recycle valve is reverse, and direct for compressor A. This will mean using recycle valves and wasting energy. The recycle valve will only be operative during safety or start-up and shutdown sequences. The compressor will ramp up and shut down within few seconds, which will minimize use of recycle valves.

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With today’s growing demand, the sole capacity of many single compressors is no longer sufficient, and therefore many standby compressors have now become main units to satisfy added demand. For new plants, a standby unit often isn't foreseen due to the desire to limit capital cost and take into account today’s higher reliability.

Typically, the output capacity of two compressors running in parallel is higher than the maximum anticipated process requirement at full production output. Traditional methods do not allow for effective process control: some enable only incremental adjustment of the capacity, while others drastically decrease the efficiency of the plant by recycling the compressed gas back to the suction side of the machine.

In future, similar scenarios with reciprocating compressors, stepless capacity control with a split-range control system can be used to match the prevailing process gas demand, and help eliminate the use of recycle valves to save energy and costs and improve controllability.

References:

1. Liptak, B. G., Section 8.15 “Compressor control and optimization,” 2006
2. Liptak, B. G., “Integrating Compressors into One System,” Chemical Engineering, January 19, 1987.
3. Hoerbiger Corporation of America, Inc.