Precise and effective control of mechanically driven gas compressor sets

Pressure variations resulting from the huge number of entry and exit points along a gas pipeline, pressure loss during gas transportation, temperature gradients as well as varying gas quality need to be monitored and compensated for in the compressor stations. In doing so, it is important to use as little energy as possible.

Different types of compressors are used to compress the gas to be transported; they are powered either by electric motors or by gas turbines or gas engines that are supplied with fuel directly from the pipeline.

Monitoring of a multitude of process parameters ensures that the system's performance always is adapted to the permanently and sometimes very rapidly varying operating conditions while at the same time maintaining high efficiency. This is a major challenge with mechanical propulsion systems in particular.

One of the most important process parameters is the propulsion power transmitted to the gas compressor. It is essential to control its generation in such a way that the minimum propulsion power required for the compressor's particular operating situation is permanently available, even with fast changing loads. At the same time, generating excessive propulsion power results in reduced efficiency and, moreover, in increased pollutant emissions (with mechanical propulsion systems) as well as in critical operating states (in particular with mechanical propulsion systems with gas engines). Permanent monitoring and control of the propulsion power of such systems therefore requires the compulsory measurement of rotational speed and, in addition, measurement of the torque transmitted to the compressor.

Contrary to rotational speed measurement that can be implemented directly and relatively easily, implementing torque measurement is rather difficult. For this purpose, measurement quantities such as cylinder pressure and temperature are often used as auxiliary quantities and as the basis for calculating torque and thus power. This method has been used for many years and has been permanently refined; however, it has the disadvantage that the measurement uncertainty related to torque significantly increases due to a number of parameters with higher tolerances used for this purpose and, moreover, usually cannot be convincingly proven. Greater tolerances of measurement quantities required for control purposes, however, inevitably result in larger deviations from the optimal operating parameters. This can have undesirable effects, in particular with the gas engine. This will be illustrated by the following chart:

Operating range of a gas engine
Source: Wärtsilä Corporation

It shows the relationship of brake means effective pressure (BMEP) and air/fuel ratio which enables knocking and misfiring areas to be visualized. In no case must the engine's operating point be in one of these areas; in particular, it is essential to make sure that, in all operating conditions, it does not shift to the knocking area, since this can cause damage to the engine.

The optimum operating window is the area between knocking and misfiring and gets narrower toward the top. This means that operating the gas engine at maximum power while maintaining low pollutant emissions requires a sensitive control mechanism with small tolerances. Higher tolerances necessarily mean reduced maximum power to increase the distance between the operating point and the knocking and misfiring areas. At the same time, it is essential that the control is very fast, since large pressure fluctuations can occur at the compressor within a small time frame, which become noticeable as load variations at the engine.

They need to be quickly and precisely compensated for by the engine to ensure that the operating point remains in the safe operating window. The following chart shows that these load variations can amount to about 50% of the system's capacity within a few seconds:

Load variations of a gas compressor set
Source: Wärtsilä Corporation

Besides the method previously described there are other methods for determining the torque transmitted to the compressor. This involves evaluation of the input shaft's elastic torsion resulting from the application of torque. There are various methods (for example, strain, displacement, angle, frequency measurement), all based on the measurement of an auxiliary quantity and the subsequent calculation of torque and therefore - provided that they are merely mounted onto the drive train - also must be considered indirect methods. In these cases too, the tolerances of the parameters to be taken into consideration (e.g. the material and the shaft geometry) result in relatively high measurement uncertainty of the measurand torque.

The indirect torque measurement methods based on the input shaft's elastic torsion can be translated into the direct torque measurement method by calibrating the measurement system for the measurand torque; as explained above, calibration for the respective auxiliary quantity is not sufficient. This requires that an input shaft section fitted with the measurement system is calibrated using a torque calibration machine to determine the exact relationship between the applied torque and the measurement system's output signal. This approach presents a number of difficulties.

• Adaptation of the shaft section during installation in the calibration machine
• Low elasticity of the shaft section resulting from the input shaft design resulting in low sensitivity of the measurement system
• The measurement system must not be dismounted from the shaft section after calibration since otherwise the calibration certificate is no longer valid.

1. Transient response behaviour of gas engines
Position paper by the CIMAC working group Gas Engines, April 2011

2. Wärtsilä 20 Dual Fuel (DF) Engine Presentation
Wärtsilä Corporation, 2010

3. LNG based concept development
 Tomas Aminoff, Wärtsilä Corporation

Klaus Weissbrodt

HBM Key Account and Project Manager
High-capacity Torque Applications