A guide to effectively executing compressor control retrofits
Improving existing compressor control systems improves operations and energy efficiency
By Rick McLin
Compressors are major pieces of capital equipment with long, effective lifespans. Unfortunately, control system obsolescence, plant reconfigurations, and changes in process requirements can all drive the need for a control system retrofit over the lifetime of the compressor, and the multivariable, nonlinear system architectures required for compressor control can cause fear and uncertainty for the engineers responsible for executing the project.
However, by avoiding fundamental mistakes during the design phase, engineers can instead consider any retrofit as an opportunity to improve compressor operation and efficiency. This article will explore major areas to make compressor control improvements, including:
- Compressor control algorithms
- Control system interactions, including capacity control
Improving compressor control algorithms
The first area to investigate is the compressor control algorithm itself. If the compressor installation is more than ten years old, the control system involved is likely based on outdated anti-surge techniques that may not be as efficient as those in common use today. With current technology, it is no longer necessary to sacrifice process stability to protect the compressor. Today’s control hardware capabilities allow the use of rigorous models to optimize compressor performance, as well as tighter integration of the compressor controls into the overall process itself.
Performance maps supplied by the compressor vendor provide the base for compressor control algorithms. These maps typically represent flow along the X-axis, while discharge pressure, pressure ratio, or head are located along the Y-axis. Engineering units used on the X- and Y-axis can be (and often are) just about anything. Indeed, the only flow measurement not previously seen seems to be cubic furlongs per fortnight—anything else is apparently fair game. A typical compressor performance map depicts a variable-speed machine with performance shown as polytropic head on the Y-axis versus flow on the X-axis, measured in thousands of cubic feet per minute. Compressor flow and pressure follow a speed line until they reach a surge point. The flows and pressures change as the speed of the compressor changes. Another set of curves defines compressor efficiency at various speeds and flows. Compressor impellers are normally designed to achieve maximum aerodynamic efficiency near the center of each speed line. If the compressor does not normally operate near its area of maximum efficiency, consider a compressor rerating.
The phenomenon of surge
Surge occurs when the kinetic energy imparted into the gas by the impeller is less than the potential energy in the discharge. When this occurs the flow of gas will reverse direction.
When a compressor approaches the surge point along a speed-line flow, the compressor will reverse direction. This flow reversal happens at the speed of sound, far too fast for instrumentation to detect, and, once started, cannot be stopped. A surge cycle will repeat unless the surge control system intervenes. Repeated surge cycles can seriously damage or even destroy a compressor, so predicting the onset of surge is essential in modern surge-control algorithms.
What happens during a surge event
Figure 2 is a representation of a surge event. Only a single-speed line is shown for clarity. Assume that initially the compressor is operating at point (1) in the diagram.
The compressor is operating at its maximum flow capabilities at point (1). As the discharge pressure increases, the work the compressor must accomplish also increases. This pushes the compressor operating point along the speed line to point (2). If the discharge pressure continues to increase, the compressor operating point will move to point (3). If the control system cannot reduce the discharge pressure, the operating point will cross the surge line and flow will reverse through the compressor. When the flow reverses through the compressor, the compressor operating point will rapidly move to point (4). The surge event reduces the discharge pressure and increases the suction pressure of the compressor. The compressor will then re-establish forward flow and the operating point will move from point (4) back to point (2). Total time for a surge cycle is one to three seconds, but the flow reverses through the compressor in under a millisecond. This cycle will repeat until the compressor controls can intervene to change the operating conditions and stabilize compressor operation.
Common compressor control algorithms
Minimum flow recycle
There are several control approaches to prevent surge in compressors. The oldest and least efficient method is minimum flow recycling. This approach simply picks a flow rate that guarantees the compressor will not surge. If the flow drops below the set flow rate, the recycle or blow-off valve opens and maintains a redefined minimum flow through the compressor.
While this approach can be effective, it is not efficient. When the compressor operates at lower speeds, it requires a large flow to protect the equipment. This approach also does not take into account changes in gas properties, which may alter the compressor surge line.
However, while inefficient, this approach can be useful as a fallback algorithm in an advanced surge-control application. Fallback algorithms are used when field instrumentation faults prevent an accurate calculation of the compressor operating point. In fact, with degraded field instrumentation, minimum-flow fallback may be the only practicable control algorithm.
Maximum discharge pressure
This approach to surge control relies on the relationship between the maximum achievable discharge pressure a compressor produces at various temperatures. Discharge pressure control is commonly used on constant-speed, packaged-air compressors (typically integrally geared machines) where suction pressure does not vary. The advantage of this approach is that it is extremely inexpensive (read: cheap) to implement because minimal instrumentation is required: just a discharge pressure transmitter and an ambient temperature. There is not even a need to measure flow through the compressor.
There are several compressor maps supplied by the manufacturer to relate the maximum pressure the compressor can produce in both summer and winter conditions. During the winter, when air is colder and therefore air density is higher, the compressor can produce a higher discharge pressure before a surge occurs. In the summer, when the air density is lower, the compressor cannot produce the same high discharge pressure. As a result, control is very simple; with a discharge pressure proportional integral derivative (PID) featuring an adjustable variable pressure setpoint for ambient temperature.
Because variations in manufacturing require a conservative approach, many companies use discharge pressure control for packaged air compressors where the surge data is generic, rather than specific to a particular machine.
Unfortunately, this approach often wastes energy and does not provide adequate equipment protection. In addition, as the compressor impellers wear out or intercoolers become fouled, the maximum discharge pressure the compressor can achieve decreases. This requires a lower pressure setpoint to protect the compressor, which if ignored will cause compressor damage.
Similarly, for older packaged compressors using pneumatic controls, maintenance personnel often ignore outdated pneumatic temperature measurement because it is difficult to calibrate. This results in a pressure setpoint that does not change with temperature, which also can cause damage to the compressor.
More modern controls, even if ignored by maintenance personnel, allow for automatic adjustment of the surge line. Once a surge occurs, the margin can be adjusted to limit the number of surge cycles a compressor would experience. Multiple surge cycles also can be set to trip the compressor to help prevent damage.
Delta P vs. h
The Delta P vs. h algorithm, also known as Pressure Rise, was originally developed in the 1970s. It was developed based on observations that the pressure ratio across the compressor closely followed the measured differential pressure across a flow measurement device. Delta P vs. h is still widely in use today, due to its relative operational simplicity and low cost. In fact, this control method requires only a flow and a differential pressure measurement across the compressor to function successfully.
Unfortunately, while Delta P vs. h is a major improvement on minimum flow recycle, it still has significant problems. The method does not account for changes in gas properties and requires a suction pressure that does not significantly change during operation.
As control systems have become more advanced, it has become possible to implement more elaborate thermodynamic models for compressor control. As a result, some form of a compressor head model has largely replaced Delta P vs. h.
Compressor head vs. flow
The compressor head vs. flow algorithm calculates the head generated by the compressor and plots it versus the temperature- and pressure-compensated flow produced. Regardless of whether the algorithm is based on polytropic or adiabatic head, this approach can accurately predict the compressor operating point at various temperatures and pressures. In addition, it is not affected by changes in the molecular weight of the gas. The basic equations for this algorithm are shown below.
The basic equation for polytropic head is defined as:
Hp = Polytropic head
Pd = Discharge pressure
Ps = Suction pressure
Td = Discharge temperature
Ts = Suction temperature
n = Number of moles of gas in a given pressure/volume
Z = Gas compressibility
R = Universal gas law constant
MW = Molecular weight of the gas
The difficulty with using this equation for surge control is that not all the variables can be measured directly. Gas compressibility and molecular weight cannot be determined except by offline analysis.
To eliminate these variables from the equation, it is necessary to utilize the flow relationships of differential pressure-flow measurement devices. Orifice, venturi, annubar, and other head-type measurement devices have flow equations that include terms for molecular weight and compressibility.
For example, the classic orifice equation:
Q = Flow, in appropriate units
H = Differential pressure across flow measurement device (head)
K = Orifice coefficient, dependent on flow units and geometry
The term is present in both the head and flow equations. These equations are used to generate the plotting coordinates used on the polytropic head versus flow compressor control map. Changes in molecular weight, suction temperature, and compressibility affect the X and Y coordinates by the same amount, allowing the generation of compressor maps that are valid for variable composition gas streams.
This approach generates a compressor control map often referred to as a Universal Surge Curve. This approach is valid for all temperatures and pressures as defined by the manufacturer’s compressor map and accounts for changes in molecular weight. Accommodate changes in gas properties, such as the heat capacity ratio, by incorporating the thermodynamic relationships derived from Charles and Boyle’s law.
Control system interactions (capacity control)
An operating compressor is an integral part of the process in which it is installed. Control of the volume of gas delivered by the compressor is necessary to match process requirements. As a result, capacity control, while often handled by other controllers or a plantwide distributed control system (DCS), is best handled by the compressor controller.
Modern compressor controllers have the capability to incorporate capacity control, which allows compressor surge conditions to factor into the capacity control. By “decoupling,” the plantwide DCS then sends a setpoint for the capacity controller, which provides compressor protection while still meeting plant requirements. Decoupling (described in more detail later) prevents a process or capacity controller from pushing a compressor into surge by forcing it out of its operating envelope.
Methods of compressor capacity control
The flow rate and discharge pressure that a compressor can produce relates directly to the speed at which the compressor is being driven. The figure below illustrates the movement of the operating point when the speed is reduced. Reducing the compressor driver speed from point (1) to point (2) reduces the flow produced by the compressor. In this example, the pressure ratio across the compressor does not change, allowing the compressor to supply a lower flow rate at a reduced speed. Speed control also allows the compressor to stay in its most efficient operating range.
If the speed lowered rapidly, it is possible the compressor would pass the surge line and surge would occur. Instead, having the speed controlled by the compressor controller allows implementation of decoupling to protect the compressor from surge.
Inlet guide vanes
Inlet guide vanes are stationary blades with variable pitch that provide a mechanism to alter the swirl pattern on the inlet flow to a compressor. Commonly used on fixed-speed compressors, they increase the operating range of the equipment. Inlet guide vanes connect together with a mechanical linkage to allow all guide vanes to move together.
Inlet guide vanes can significantly increase the efficiency of the compressor and improve the turndown ratio of the machine. Figure 7 shows a compressor performance map for a constant speed compressor with inlet guide vanes. While this map looks similar to a variable speed map, there are significant differences in the control methodology used. For example, the slope of each guide vane angle line is usually much steeper than a speed line.
Guide vanes dramatically alter a compressor performance, and special control techniques can take guide vane position into account. Guide vane position feedback is critical, as maximum compressor discharge-pressure capabilities vary greatly with guide vane position. If the guide vane position is not accurately reported to the surge controller due to mechanical problems or incorrect calibration, severe damage to the compressor can result.
Suction or discharge throttling
The least efficient method of compressor capacity control is throttling. Throttling to lower compressor flow increases the pressure ratio that the compressor has to achieve. This causes the compressor to work harder than necessary.
However, because the gas has not been compressed first, suction throttling is more efficient than discharge throttling for lowering flow across a compressor. Suction throttling reduces suction pressure, which increases the pressure ratio and thereby reduces flow through the compressor. Discharge throttling increases discharge pressure, which also increases the pressure ratio.
If the compressor installation has a suction throttle valve to decrease suction pressure, rapid changes in the valve position can push the compressor into surge by increasing the pressure ratio above the surge line of the compressor. Similarly, if the compressor has a discharge throttle valve to increase discharge pressure, changes in valve position can increase the pressure ratio above the surge line of the compressor, causing surge.
In the instances above: reducing speed, changing inlet guide vane angle, or increasing pressure ratio by throttling, capacity controllers respond to a process demand. As mentioned previously, decoupling is a technique utilized to protect the compressor against being forced into surge by capacity controllers. It temporarily suspends the control action driving the compressor towards an unstable operating area while it establishes recycle flow to stabilize the compressor. Once stabilized, the capacity control action is once again allowed to satisfy the process demand.
Effective implementation of decoupling is only possible in the surge controller, as it is responsible for determining the compressor operating point, including when and how quickly a compressor is approaching the surge line. A DCS accomplishes capacity demand from a remote setpoint to the compressor controller, which can implement decoupling to protect the compressor from surge.
Managing a successful compressor retrofit
Compressor control system retrofits can be intimidating, but by following the basic design principles outlined here, they are not impossible to accomplish. Multivariable, nonlinear applications will never be easy, but systems can be designed that improve operations and energy efficiency. In fact, the current generation of programmable logic controllers offers amazing performance, including algorithm execution speeds that are better than made-for-purpose black box controllers, at a very effective price point.
ABOUT THE AUTHOR
Rick McLin, development manager, Turbomachinery Controls—Rockwell Automation, has more than 25 years of experience in turbomachinery controls (TMC) as an end user and leader of development teams in the U.S. and abroad. Rick spent 16 years at one of the largest oil companies in the world, developing surge control algorithms and implementing distributed control systems in oil and gas production facilities. Rick also led the development of the subsea compressor control and safety systems developed for Statoil in the North Sea.