Best practices: Turbo-compressor control monitoring
Coverage of turbo-compressor surge and turbo-compressor condition monitoring tends to be popular, but topics related to turbo-compressor control can be just as vital. Here are some of the best products and practices used in the field.
Literature on turbo-compressor surge and turbo-compressor condition monitoring tends to be popular, but topics related to turbo-compressor control have not seemed to draw that much attention. The best method of turbo-compressor control is one that provides the greatest efficiency for the desired level of controllability. An important concept is that regardless of arrangement of turbo-compressors, each compressor should have a control system that performs the anti-surge and required-control actions in a decoupled fashion without interactions. The general strategy is to first prevent any turbo-compressor from coming too close to the surge curve, and second, to operate any compressor as close as possible to the best efficiency point (BEP).
Commonly used products and their functions:
Inlet throttle valve (ITV)
The “Inlet Throttle Valve (ITV)” capacity control method is still in use for small and non-critical compressors. This is often used for constant-speed turbo-compressors below 1.9 MW. These are usually fixed-speed electric-motor driven compressors. The simplest form of this control system is accomplished by controlling the discharge pressure of the compressor at a constant value.
Inlet guide vane (IGV)
The variable geometry of compressor components can be used for the capacity control. The most common form is the inlet guide vanes (IGV). The IGV system gives pre-swirl to the flow entering turbo-compressors that can rotate and narrow the flow passage, allows capacity control of fixed-speed turbo-compressors.
Achieving reasonable compressor efficiency at part-load operations requires a reduction in the compressor work input. A variable-speed operation is a mechanism that can allow the reduction in the compressor work input required to achieve a good efficiency at part-load. Operating a turbo-compressor with a variable-speed drive is a good option for most applications. The variable-speed operation is more effective in head reduction than in flow reduction creating a turbo-compressor map that sometimes does not allow efficient operation at low flow and high head conditions, needed for some applications.
Variable-speed versus inlet guide vane (IGV)
Compressor applications with part-load conditions requiring a proportional relationship between flow and head show the largest benefit from the variable-speed operation. Compressor applications that require an almost constant head (with variable capacity) or an almost constant flow (with variable head) may not benefit much from the variable-speed operation. This is particularly correct for compressor applications that need relatively constant head and variable capacity. These machines often require full-speed compressor operation.
Independent control loops
On a low-flow demand, a compressor can be damaged by surge. On high-flow demand, there are usually driver limits. For example, electric-motor driven compressors can be damaged by excessive electric motor current. There is also choke limit (or “stonewall” limit). When a compressor is operating in the “choke” condition, the discharge pressure is no longer controllable. The discharge pressure can drop almost to the suction pressure for a slight increase in flow at a given speed or IGV position.
To prevent surge, in many small-size and medium-size compressors, a separate (independent) anti-surge loop is provided. When the surge controller becomes active (independently), this often interacts with the capacity control (such as an inlet throttle valve capacity control system) to cause unstable operation. The anti-surge controller can disturb the discharge pressure controller because the inlet throttle valve can affect both flow and pressure. Two loops acting simultaneously produce a sustained oscillation that is characteristic of physically-coupled, independently-controlled loops.
Variable-speed compressors have the potential of substantially improving compressor part-load efficiency and have become the selected option for many applications. The application of variable-speed to some turbo-compressors may require additional knowledge of compressor behavior as well as testing and verifications. Variable-speed turbo-compressors can offer an excellent capacity control performance.
Amin Almasi is a senior rotating equipment consultant in Australia. He is chartered professional engineer of Engineers Australia (MIEAust CPEng – Mechanical) and IMechE (CEng MIMechE) in addition to a M.Sc. and B.Sc. in mechanical engineering and RPEQ (Registered Professional Engineer in Queensland).
Edited by Jessica DuBois-Maahs, associate content manager, CFE Media, firstname.lastname@example.org.