Reciprocating compressor's capacity control

1.0 Introduction

The Design Specification provides a description of the compressor process control scheme. It shall identify the controlled variable, the source of the control signal, and the control method, e.g., bypass control, unloaders etc. Fail safe modes for control elements should be shown.

2.0 Capacity Control Methods

The Design Specification identifies the method by which the reciprocating compressor flow capacity will be regulated to maintain control of the chosen process variable. Methods of controlling the compressor net capacity are as follows:

2.1 Bypass Control

An external bypass around the compressor is provided to recycle gas from the compressor discharge to the inlet or to the atmosphere in the case of an air compressor. Care must be taken to ensure that the bypassed gas is cooled before returning to the compressor inlet to avoid excessive stage discharge temperatures.

For operating flexibility, it is recommended that the recycle line, control valve, and cooling capacity be selected to handle the entire capacity of the compression service. It is also recommended that the recycle line return gas to the compressor upstream of the inlet knockout drum to remove any liquids and facilitate pipe cleaning during commissioning.

Bypass control is preferred because of its smoothness, simplicity, and low investment cost. It is, however, inefficient because excess compressor capacity is expanded across the control valve in the recycle line. For this reason, it is commonly combined with the use of inlet valve unloaders and/or clearance pockets which reduce compressor capacity in discrete predetermined steps. The plant operator may then choose to use these devices when the amount of recycle exceeds the capacity reduction achievable by activation of a valve unloader or clearance pocket.

2.2 Inlet Valve Unloaders

Valve unloaders are mechanisms which hold open or bypass one or more of a cylinders inlet valves at each end of double acting cylinders. This provides complete unloading of one or both ends of the cylinder. For a single cylinder compressor stage, valve unloaders can achieve three step unloading which provides nominal cylinder capacities of 100, 50, and 0 percent and two step unloading providing cylinder capacities of 100 and 0 percent.

Actuator sizing may limit the use of valve unloaders to services with inlet pressures at or below 1000 psig (70 barg). For higher inlet pressures, actuator sizing should be reviewed by a machinery specialist.

2.3 Clearance Pockets

Cylinder delivered capacity can be reduced by increasing the cylinder clearance volume. This is done by adding a clearance pocket. The fixed volume clearance pocket is a volume chamber separated from the normal cylinder end clearance volume by a valve or plug. Opening the pocket reduces the cylinder inlet volumetric flow by trapping additional gas in the now larger clearance volume at the end of the piston stroke. This trapped gas is not delivered through the discharge valves and the cylinder volumetric efficiency is reduced. Capacity reduction of 50-60% can easily be achieved; more reduction is possible.

Clearance pockets are more easily fitted to the outboard or head end of the cylinder than to the crank end or packing box end. Consequently, clearance pocket flow adjustment is frequently applied to the head end only, effecting a 50-60% reduction in the head end capacity and a 25-30% overall capacity reduction in a double acting cylinder.

Valve unloaders combined with clearance pockets can achieve five step unloading which provides nominal cylinder capacities of 100, 75, 50, 25 and 0 percent. These are the nominal capacities that will be provided when five step unloading is specified. 100 percent capacity is achieved without activating the valve unloaders or clearance pocket, 75 percent capacity is achieved by opening the head end clearance pocket, 50 percent load uses the crank end valve unloader, 25 percent load uses the crank end valve unloader and head end clearance pocket simultaneously, and 0 percent load is achieved by activating the valve unloaders on both ends of the cylinder. If for some reason, another set of nominal capacities is required, e.g., 100, 70, 50, 20, and 0 percent, it should be specified. Intermediate steps can be provided by unloading the cylinders sequentially when a process compression stage consists of two or more cylinders in parallel.

Actuator sizing may limit the use of clearance pockets to services with cylinder inlet pressure at or below 1000 psig (70 barg). For higher inlet pressures, actuator sizing should be reviewed by a machinery specialist.

Pockets can represent a significant volume relative to cylinder size in low compression ratio services. At compression ratios below 2, the feasibility of providing a pocket to achieve a given capacity reduction should be checked by a machinery specialist. Also bear in mind that pockets are sized for one set of operating conditions, and variations in operating pressures and gas composition will affect the amount of capacity reduction achieved. Clearance pockets can be arranged for local manual operation, manual/pneumatic operation, or automatic/pneumatic operation.

2.4 Stepless Capacity Control

A stepless capacity control system called Reverse Flow Control can provide a capacity range of approximately 40 to 100% of rated capacity. In this system, an unloading device is fitted to each suction valve. At partial load, the unloading device does not allow the inlet valve to close when the piston is in its bottom dead center position but delays the closing in a controlled way. Consequently, an amount of gas, which can be adjusted, is allowed to return to the compressor inlet manifold before the compression starts. As the piston velocity increases in the compression stroke, it pushes gas in reverse flow back out the inlet valve faster and faster. This creates a larger and larger gas drag force on the inlet valve plate which eventually overcomes the unloader force and closes the inlet valve. Application of this type of control requires economic justification unless, of course, it is an Owner preferred arrangement. The cost for valve unloading and control hardware may be justified by the energy savings achievable by operation at any discrete capacity in the 40-100 percent capacity range. This is as opposed to having to recycle flow when operating between unloading steps in the conventional three or five step unloading arrangements. Actuator sizing may limit the use of stepless capacity control to services with inlet pressures at or below 1000 psig (70 barg). For higher inlet pressures, actuator sizing should be reviewed by a machinery specialist.

If stepless control appears attractive, the Design Specification should, as a minimum, request it as an option to allow detailed economic analysis during compressor selection.

2.5 Variable Speed Drive

- Integral reciprocating gas engine drives provide a means of using variable speed for flow control. It may safely be assumed that integral gas engines can operate in a 75 to 100% speed range thus providing a capacity turndown capability to 75% of rated. Operation below 75% of normal speed should not be assumed without checking with the engine manufacturer.

Speed control is normally augmented by bypass control and inlet valve unloaders and/or clearance pockets to provide for unloading during startup and provide wide flow variation.

Steam turbine drivers and separate engine drivers should be assumed to operate at constant speed to avoid excitation of train torsional critical speeds. Before assuming any speed variation flexibility consult with a machinery specialist.

2.6 Inlet Throttling

This method of capacity control is not recommended as lower than normal inlet pressures cause higher than normal piston rod loads and discharge temperatures. As the result of rod load and discharge temperature limitations, the mass flow range which can be safely achieved is normally too low to suit process needs.

3.0 Startup Unloading

Some means of unloading for startup is usually required because drivers lack sufficient torque to accelerate the train rotating parts under positive displacement compression loads. Inlet valve unloaders and external bypasses are the two most commonly used methods for startup unloading. Where automatic reacceleration of motor driven compressors after a voltage dip is required for service continuity or where the compressor must auto start unattended, the unloading system must be arranged to operate automatically on a voltage dip or startup signal. The startup unloading method employed should be developed with the compressor vendor during detail engineering.

4.0 Specification of Control Method

The recommended control methods for reciprocating compressors may be summarized as follows:

4.1 Constant Speed Units

All reciprocating compressors, except integral gas engine driven units, are specified for constant speed operation to avoid excitation of torsional critical speeds. For steam turbines and separate engine drives, consult a machinery specialist before assuming speed variation flexibility.

For all constant speed applications, it is recommended that automatic bypass control be provided. See discussion of Bypass Control for recommendations on bypass sizing and arrangement. In addition, for applications with a stage inlet pressure less than 1000 psig (70 barg), compressors should also be specified to have three or five step unloading. Specification should state that step unloading will be accomplished manually. Above 1000 psig (70 barg) stage inlet pressure, the availability of pockets, unloaders, and stepless capacity control should be determined by consultation with a machinery specialist.

For a constant speed applications using Stepless Capacity Control, the stepless system should be supplemented with a bypass control arrangement. See discussion of Bypass Control for recommendations on bypass sizing and arrangement.

The stepless capacity control will control the capacity from 100-40% of capacity and the bypass, from approximately 40 to 0%. The bypass sized for the capacity of the entire compression service is recommended for maximum flexibility.

4.2 Variable Speed Units

Capacity control from 75-100% of rated is readily achievable with integral gas engine driven compressors. Additional flexibility may be specified, if needed, using the control systems covered for constant speed units to achieve capacities below 75% of rated.

Design Specification should state if capacity controls should be operable from the compressor local control panel and control house or locally only.

The Design Specification must also specify if the compressor is required to start automatically or, in the case of an electric motor driven unit, whether unit reacceleration is required after temporary electrical power failure. This will assist the compressor supplier in determining the need for automatic unloading of the compressor on startup and the required automatic operating features for auxiliary systems.

5.0 Conclusions

In can be concluded that above reciprocating compressor capacity control can be exercised by operators to optimize the compressor capacity control based on varying process demand. A trundown upto 40% can be achieved for a mid-capacity (e.g. 10 MMSCFD) reciprocating compressor with combination of above control mechanism.

Reference: Exxon Mobil Design Practices