by Sara Budinis
What is the purpose of this project within Energy SmartOps?
One of the targets of Energy SmartOps is to operate and control industrial compressors in order to guarantee their stable and safe operation while reducing their energy consumption. In order to achieve these objectives (good operation, safe control, reduction of energy consumption) it is important to understand the state of the art within the gas compression industry. This is the starting point of the project and our research partner ESD Simulation Training played a fundamental role during this step, providing tools and experience in this field.
What is a compressor and how do we control it nowadays?
A compressor is a machine that compresses a gas (the process fluid) in order to increase its pressure. Compressors are a vital piece of equipment in many industrial sectors and they are employed for gas treatment and transportation. Among the different types of compressors, centrifugal compressors are employed when a high flow rate of gas must be delivered. The compression ratio (i.e. the ratio between the outlet pressure and the inlet pressure) of a single stage centrifugal compressor can be low and for this reason, when higher compression ratio is required, more stages are placed in series and they constitute a multiple stage compressor (also called train of compressors).
A compressor must be controlled in order to guarantee its operation. Because the purpose of a compressor is to compress a gas either for treatment at high pressure or for its transportation, the outlet pressure of the gas must be at a desired value called pressure set point.
The first thing to know when we want to control a compressor is if it runs at constant speed or at variable speed. If it runs at constant speed a throttle valve can be manipulated in order to change the pressure of the gas. Instead if it runs at variable speed, the speed of the driver can be manipulated in order to achieve the pressure set point. The relation between flow rate of the gas, its inlet pressure, its outlet pressure and the rotational shaft speed is represented in the map of the compressor.
Therefore once the inlet pressure, the inlet flow rate and the set point of the outlet pressure are known, the rotational shaft speed can be changed in order to meet the process load.
The operating region of the compressor is limited by the minimum and maximum speed of the driver (represented by the minimum speed line and the maximum speed line) and by the surge line (the red line connecting D-C-B-A). Surge is a dynamic instability of the flow and it happens when the compressor cannot meet the process load. If the compressed flow rate is low and the pressure ratio is high, then the flow starts to reverse inside the compressor itself, causing vibrations and overheating that can seriously damage the machine. Therefore surge must be avoided and the operating point of the compressor must always be on the right side of the surge line on the compressor map.
In order to avoid surge, if the compressor gets closer to the surge region, part of the compressed gas is recycled back to the inlet of the machine. In this way the inlet flow rate increases.
Why does compressor operation require energy?
The operation of a compressor requires energy. This is due to the power consumption of the machine when it is running. This power consumption depends on many factors such as the design and selection of the compressor and its matching with the process application, on the operation of the compressor and on its maintenance. However the most energy consuming operation is the recycle of the gas for surge protection. In fact higher is the gas flow rate higher is the power consumption of the machine. Moreover when part of the outlet gas is recycled, it is not delivered as a product to the downstream section of the plant. For this reason gas recycling increases the power consumption of the compression section without contributing to its final goal.
How can we reduce the energy consumption?
The energy consumption of a compressor can be reduced by reducing the amount of gas that is recycled in the daily operation of the machine. In order to do that it is necessary to:
understand the effect of inlet and outlet disturbances on the operation of the compressor
understand the interaction between pressure controller and antisurge controller
implement better control systems that can provide overall control stability
focus on the specific application
analyse the dynamics of the compression system and the impact of the different key element.
Some work towards energy savings is the design and development of a control system able to guarantee safe and reliable operation even during fast process transients (Budinis and Thornhill, 2014). Figures 4 and 5 show how the outlet pressure of a compressor can be controlled in a better way thanks to the proposed solution. In fact while the gas pressure of the gas is kept closer to its desired value (Figure 4) the compressor is also protected from surge during the full transient (Figure 5). This means less gas can be recycled during off-design operation and therefore this contributes to save energy that would be otherwise spent during the compression process itself.
Figures modified from (Budinis et al., 2013) and (Budinis and Thornhill, 2014)
Budinis, S., Thornhill, N.F., 2014. An integrated control technique for compressor operation, 10th International Conference on Control, Loughborough University.
Budinis, S., Thornhill, N.F., Fabozzi, D., 2013. A Control Technique Based On Compressor Characteristics With Applications to Carbon Capture and Storage Systems, 2013 AIChE Annual Meeting, San Francisco CA.