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Optimising turbomachinery energy savings with control technologies

Virtually every plant manager who works in a hydrocarbon processing, power-generation or other industrial facility can relate to the need to optimise energy consumption.

Pier Parisi, Serge Staroselsky and Emily Hoop
Compressor Controls Corporation
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Article Summary
Opportunities to reduce unnecessary costs exist in various areas of production and operation, driven by the availability of new technologies and a need to improve the economic bottom line of the company.

Improvements in plant data capture and storage, mostly driven by advances in process historian and plant optimisation technologies, allow us to shine a spotlight on areas of potential energy savings and to take action with results that were, until recently, unachievable. The past two decades have witnessed the adoption of process optimisation technologies that delivered handsomely in terms of cost reduction and improved throughput. The next frontier in plant optimisation will be obtaining greater operational effectiveness from critical turbomachinery so that a plant can achieve maximum energy savings from both process and asset performance.

One area where significant improvements are now being demonstrated centers around the operation of compressor, turbine and motor control systems. Control systems technology can offer reliable information and energy savings improvements that can be safely predicted, and even guaranteed, in several scenarios.

To translate this concept into reality, a five-step methodology spanning assessment through successful commissioning and operation of the plant has been implemented. This approach has highlighted the following areas as the most common optimisation opportunities in the
• Lessening or elimination of recirculation and throttling of compressed gas
• Shorter startups and shutdowns
• Increased overall precision of control functions

Reduction in recycle rates.
Surge protection margins may beset too conservatively due to inadequate design and functionality of the antisurge control system. As a result, the unit’s power consumption may exceed process demand due to the excessive recycle flow. In such cases, the best approach is to evaluate the existing system and provide recommendations as to the possible improvements. These can include:
• Modifying the recycle valve for improved performance by updating the actuation, changing the flow capacity and/or relocating the valve
• Modifying and/or adding instrumentation that provides measurements into the antisurge control system for improved antisurge control
• Changing antisurge control strategies to encourage more robust algorithms, which will increase the compressor’s operating envelope by having tighter control margins and integrated control.

Automating capacity control.
While most control systems were originally designed for automatic control, some are still operated in manual control by plant operators. One of the most common reasons for manual operation is the poor behavior of the speed governor. Older hydraulic and mechanical governors are often unable to match a varying setpoint without significant overshoot and/or undershoot. This frequently leads to a process upset or a trip.

Consequently, operating personnel may run the unit at higher constant speeds and adjust capacity via recycling. In this case, the savings will come both from the reduction in operating speed, as well as the elimination of recycle. An evaluation of the speed governor design will determine what type of governor retrofit is required to accommodate automatic capacity control.

Another reason for manual operation is to avoid exceeding process or mechanical constraints due to an inadequately designed control system. A review of the existing control system and the system constraints will lead to identifying where and how limiting control should be implemented to have reliable automatic control within the boundary constraints of the system. Often, the operating boundary of the compressor can be increased when proper integration of capacity, antisurge and limit control is implemented.

Figure 1. shows an example of the power savings achieved by reducing recycle and lowering speed. Initially, the compressor operates at point A, with flow equal to WA. The process requires the flow to be equal to WB at discharge pressure PD. However, due to various reasons, such as poor governor performance and unreliable antisurge control, the compressor operates with recycle flow equal to WR. Using the compressor performance data, the savings in the consumed power can be estimated from the elimination of recycle flow and subsequent reduction in compressor speed.

The reduction in the consumed power, ?J, may be translated into reduced steam consumption, using turbine performance data, and then into reduced operating costs. The calculations for multi-section machines may be more complex, as performance changes need to be calculated for each section due to the effect the sections have on each other’s inlet and outlet conditions.

Rotation frequency control improvement
Rotation frequency control improvements provide not only an increase in compressor efficiency, but also in compressor productivity. This is achieved by installing a rotation frequency controller and/or a valve management controller. Efficiency can be increased by improving the accuracy of speed control to very tight margins (in this case, .02% of rotating speed). Further improvements to the hydraulic equipment associated with the control system also provide benefits in this area.

Antisurge control system hardware and software integration

System integration with loop decoupling technology among controllers delivers reduced variable deviations that are caused by process disturbances, as compared to independent operating controllers. This provides the additional value of reducing the incidence of process disturbances that are negatively affecting compressor operations, leading to additional energy savings.

Load sharing among compressor trains
System load sharing relates to groups of compressors operating in parallel and/ or in series. Many different strategies have been employed to unload and load a compressor station. One common method base loads the most efficient compressors either at their maximum flow or at their point of maximum efficiency. The swings in load demand are then met by modulated control of the less-efficient compressors, or by on-off control of these compressors. This system is not energy efficient or sufficiently reliable.

Proper load sharing can increase the efficiency of a group of compressors by:
• Preventing the compressor’s antisurge valve or valves from opening while one or more compressor(s) in the group can have a reduction in throughput without recycling
• Loading compressors in a group by utilising procedures that concurrently minimise total energy consumption.
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