Turbo Tips: Key Considerations & Risks in the Operation and Control of Compressors

Any change in operating conditions that causes the operating point to move on the turbocompressor map is known as a disturbance. Disturbances can be categorized depending on their speed. An important factor is the speed of the disturbance relative to the overall speed of the compressor control loop—a disturbance can seem slower to the compressor control main loop if the loop is made faster using a faster controller, instruments/sensors, and actuators. A fast-problematic disturbance for an ordinarily slow-response turbocompressor can be easily managed in an advanced, fast-responsive turbocompressor.

Advanced, fast-responsive digital controllers facilitate great flexibility in compensating for fast changes in operating conditions, nonlinear performance curves, unexpected situations, and complicated control strategies.

THEORETICAL CURVE VS ACTUAL CURVE

A turbocompressor curve or map supplied by the manufacturer is usually an expected performance curve (or a theoretical curve) unless it is verified by a realistic (site) performance test, i.e., a test with final installations and job conditions and not a shop test with shop facilities and alternative gases. The same is true for the surge point/curve: A surge point (or surge curve) tested at the shop is only a rough approximation compared to the real surge point (curve) that is tested at the site with the actual job piping and facilities. In addition, the turbocompressor performance curve shifts with wear, degradation, fouling, etc.

DEGRADATION

During operation, a turbocompressor is usually affected by fouling due to compressed gas dirt and impurities, which causes its performance to degrade over time. Fouling is defined as degradation of the flow capacity and efficiency caused by contaminants on the gas flow path surfaces, which include both moving and fixed parts in contact with the com-pressed gas. Different kinds of impurities such as dirt, liquid, water, oil, salts, solid particles, etc., could be in the compressed gas.

Another type of fouling is caused by oil leaks. Lubrication oil leaks from the bearings of a turbocompressor or another compressor upstream are a common cause of fouling, especially when combined with dirt ingestion. The most important effect of compressor fouling is a decrease in the gas flowrate and compressor pressure ratio. This can result in some changes in the compressor curves and map and a re-matching of different compressor stages. This re-matching could affect the compressor efficiency and operation unexpectedly. However, in most cases, a decrease in efficiency reduces the performance and global efficiency of the facility. In extreme cases, fouling can also result in surge problems, as it tends to move the compressor surge curve to the right toward the operating points.

Solutions to reduce compressor fouling are offline compressor washing, online compressor washing, and advanced filtration systems. However, if any of these methods cannot be used or are not as effective as they should be, some changes in the operation of the turbocompressor should be expected. For example, the turbocompressor may consume more power when compressing the required volume of gas to the required discharge pressure. A turbocompressor control system should be configured to deal with reasonable fouling and wear on the compressor. The control system’s response to some fouling and degradation should be evaluated and confirmed in the design stage.

DOWNSTREAM CHECK VALVE

Most turbocompressors have a check valve installed on the turbocompressor discharge to prevent backflow during a surge, for example. It should be located as close as possible to the turbocompressor discharge nozzle to minimize the gas volume involved in the surge instability. The check valve is critical for a turbocompressor with a reactor downstream, for example, to prevent the backflow of catalyst (or harmful mixtures) to the turbocompressor. The check valve is also extremely important for turbocompressors in parallel operation.

A possible malfunction could be the failure of a downstream check valve or the downstream control valve. Proper risk assessment and failure scenarios should be defined, and consequences and mitigation methods should be evaluated. Some turbocompressors have two check valves in series.

OPERATING CONDITIONS VS SURGE CURVE

It is difficult to note a general narrative for how a surge curve may change due to a change in operating conditions. Such changes can be case-dependent and difficult to generalize. The following are based on some observations.

• A decrease in suction pressure can lower the surge curve on a turbocompressor map, which moves the operating point closer to the surge.
• An increase in the suction temperature can act in the same way and lower the surge curve.
• A decrease in molecular weight can lower the surge curve, resulting in a slight movement toward the surge.

ANTI-SURGE FLOW MEASUREMENT

A key question is where to locate anti-surge flow measurement—on the suction or discharge? The anti-surge flow measurement could be located at suction or discharge. The suction flow measure-ment is preferred because it measures the flow at the location where it’s needed. However, in some turbocompressors, due to practical or operational reasons, the flow should be measured at the discharge. For example, there may not be sufficient space or pressure at the suction to accommodate the suction flow measurement. It is difficult to make a general rule since both types are in use today. If the flow measurement is on the discharge, the discharge flow should be referenced back to the suction temperature and pressure. For such an anti-surge system, the temperature and pressure at the suction and discharge should be accurately measured. The discharge temperature measurement may not be fast enough to correct the flow signal for transient-operating situations due to heat transfer lags in temperature measurement devices.

Amin Almasi is a Chartered Professional Engineer in Australia and the U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.