Gas Compressors Control System

The electrical control system, operating on 24 Vdc power, provides for automatic starting, acceleration to operating speed, engine and driven compressor monitoring during operation. Control system protects the turbine engine and the compressor from possible damage resulting from hazards such as engine overspeed, high engine temperature, low lubricating oil pressure, and excessive oil temperature. See figure 7.
Gas Compressors Control System
 A) Drive Engine
                                 The drive engine is basically a gas turbine. Thermal energy is generated and then converted into mechanical energy through the application of thermodynamic process, arranged to occur in a cycle of events (see figure 1) as following:
 Compression; atmospheric air is compressed.
Combustion; fuel is added to the compressed air and ignited.
Expansion; air and combusted gasses expand through nozzles.
Exhaust; air and combusted gasses discharged into atmosphere.
 Turbine Engine Major Components
   The major components of the Centaur turbine-driven compressor are;

  • Centaur Type H two-shaft industrial turbine engine, and
  • Turbine control console.

A standard engine support system includes; a complete start system, a fuel system, an air system with associated pneumatic controls, a lubricating oil system with related subsystems, and an electrical control system.
1.1- Start System
The start system includes a starter and associated control devices. It provides the torque to initiate rotation and assists the engine to self-sustaining speed of 60 percent. At this time the starter shuts down, the starter clutch overruns, and the engine accelerates under its own power to loading speed.
Electro-hydraulic start system includes cranking, purging and ignition. The major components of the system are hydraulic starter motor, hydraulic pump with an accessory charge pump, and electric drive motor with space heater.
Description; as illustrated in figure 3, level switch S.326-2 monitors the oil level in the reservoir start permissive will suspend if the level sensed low during a start sequence, the solenoid valve L-327-2 kept energised to open. The hydraulic cylinder CYL-923 will keep the oil pump stroke in neutral position. Once the main motor B-330 starts, the charge pump will put hydraulic oil into associated discharge lines and pressurise up to 300 psig. Then the solenoid valve L-327-2 closes to pressurise the cylinder, so that the main hydraulic pump stroke is changed from neutral to C. The pump discharges a hydraulic pressure of 5000 psig, into the transmission lines. This power transmission moves the hydraulic motor that is coupled to the turbine engine.
The speed will increase as the transmission continuos. It shouts reach 3000 RPM in 15 seconds time. Then the other stages of start sequence will continue.
Once the engine speed reaches 60% i.e. 9000 RPM, then the hydraulic system discharges from the engine L-327-2 energies to make the pump stroke to neutral.
At 90% of the engine speed, the electric motor stops.
1.2- Fuel System
The fuel system controls the fuel pressure and automatically regulates the fuel flow according to operating requirements.
Fuel gas control system, in conjunction with the electrical control system and the air system, automatically schedules the fuel during acceleration and modulates fuel flow during operation, also provides over temperature and overspeed topping control of fuel flow, and includes automatic shutdown in the event of fuel component malfunction.
Description; the important components of this system as illustrated in figure 4, are the strainer, priming gas control valve (shut-off valve) V2P-932, The fuel gas pressure regulator VGF-931 and the gas throttle valve AO-931.
A small stream with a pressure control valve PCV-930 and shut-off valve L-340 also part of the system which used during the start-up of ignition.
As a part of start-up sequence logic performs a check on the fuel valves to ensure a tight shut-off. Once the healthiness is confirmed, the sequence proceeds for cranking and purging stages. A 4 minute purge removes the traces of fuel from the engine, then the ignition of torch will take place by opening L-340 solenoid valve and sparking voltage applied for 10 seconds period, during this time the fuel shut-off valves also open the throttle valve ramps up slowly.
The combustion zone temperature rises as soon as the ignition took place and at 350 degree F, the ignition spark supply voltage stops and the throttle valve settles to a new ramping speed.
The turbine speed is monitored and combustion zone temperature also has a great importance in controlling the fuel.
The engine compressor inlet guide-vanes opens to allow more quantity of air at 80% of the speed, as a result of this the compressor discharge pressure PCD will increase. PCD will control the fuel control valve VGF-931 to allow fuel in proportion to the air pressure. The turbine speed control system throttles the fuel valve as required to match the load. Fuel gas pressure monitored in the supply for high/very high and to trip the machine.
1.3- Air system
The air system pressurises oil seals in the turbine engine, cools the turbine rotor, and supplies reference pressure to the fuel control system and other pneumatic devices in related systems, also is required to make the fuel a combustible mixture.
Description;  as illustrated in figure 5 the air pressure in the combustion chamber is reduced by leaving bypass valve (Bleed valve) open during the start-up and at 80% of the engine speed it will close.
The inlet guide valve (Vanes) at the minimum open during the start-up and at 82% of the speed they start opening wider proportional to the engine speed up to 90% speed maximum open.
The compressor discharge pressure PCD controls the fuel gas pressure to the combustion chambers.
1.4- Lubricating Oil System
                                                          The lubricating oil system circulates oil under pressure to the various working parts of the turbine engine and driven equipment. In addition, the system supplies oil at engine inlet pressure to related hydraulic subsystems. Proper oil temperatures are maintained by an oil tank heater and by an oil cooler and thermostatic oil control valves.
 Description; as illustrated in figure 6, the major components of the system are the main lube-oil pump P-901, the pre-post lube oil pump P-902 and the post-backup P-903, Temperature control valve TCV-901, Pressure control valve PCV-901 and filters FS-901.
When the engine starts initiated, the pre-lube oil pump P-902 will start and builds up pressure of 40 psig and supplied to all the components. As the sequence proceeds further, the engine driven main lube oil pump P-901 will build up pressure and then pre-lube oil pump stops automatically.
The oil flow will be regulated as per the temperature through the cooler HX-901 and partly it is bypassed. Also the header pressure is automatically controlled by a PCV at 55 psig, Oil is filtered in a 5 micron duplex filter before injecting to the bearings. Lube oil pressure transmitter TP-380 provides control on pre-lube permissive start-6 psig. Oil pressure low alarm 42 psig and a shut down at 25 psig. Other alarm and shut down switches on the lube oil system are:
 S-388-1, 2,3; reservoir level alarm and shut down 
S-397-1, 2; high differential pressure across filters, 50 psid / 80 psid alarm and shutdown.
S-324-1, 2; reservoir vent pressure high, 8.5 inch H2O, 10 inch H2O alarm and shutdown.
S-322; Pre-lube oil pressure 6psi.
S-381-2; Lube oil temperature 82 degree Celsius shutdown.
 1.5 Electrical Control System
                                                               The electrical control system, operating on 24 Vdc power, provides for automatic starting, acceleration to operating speed, engine and driven compressor monitoring during operation. Control system protects the turbine engine and the compressor from possible damage resulting from hazards such as engine overspeed, high engine temperature, low lubricating oil pressure, and excessive oil temperature. See figure 7.
The electrical control system is operated from a remotely installed control console. A turbine engine and compressor control panel, with all necessary switches, meters, and indicators for engine and compressor operational status, are installed on the front of the control console. The engine and accessories control system is operated and controlled by the turbotronic control system located in the turbine control console in a safe place.
 Turbotronic control system monitors and react to package operating conditions. The control system is controlled by the programmable logic controller module (PLC). The PLC interfaces with package functions with communication interface module by scanning input data from discrete and analog modules. Storage of data received, and computing the desired output is performed by PLC module and transmitted to appropriate components using discrete and analog output modules. Operator control of the package is by the trouble control panel and display terminal.
With the help of these devices the PLC will control and maintain the engine speed and temperature at safe levels.
 Governor is a speed-sensing device driven by the engine itself or by some mechanical part, such as the gearbox, coupled directly to it, or else it drives from the engine a signal, which represents the engine speed. It actuates directly, or through an amplifier, the fuel control to the engine.
The governor system is incorporated into the software of the PLC computer. It compares input from temperature sensors, Gas producer and power turbine speed, to determine the signal output to the fuel actuator.
 1.6 Engine Temperature Control System
                                                                                      Engine temperature control is monitored by the temperature control subroutine program block within the PLC software. The millivolt input module responds to temperature signals from six thermocouples located at the turbine engine, PLC temperature control generates appropriate signals for discrete and analog output modules to display appropriate status indication, on the display monitor and actuate proper controlling components
.Turbine Engine Start Sequence Description
                                                                                            Fuel gas supply pressure should be within operating range upon start PB on the control panel.
Pre lube cycle will start, the pre-post lube oil pump will start and a timer will keep track on the lube oil pressure, the pressure should be more than 6 psi in 25 seconds timeout.
 Cranking; As soon as the pressure reached to 6 psig, the main starter motor will start, with the pump stroke on neutral position, to charge the transmission lines. Then the pump stroke changes to C position to develop 5000 Psig  hydraulic pressure, this power transmitted to the hydraulic motor along with the start up of the main hydraulic starter, the valve check sequence also starts. The primary fuel gas shut-off valve will open for 5 seconds and then closes. The fuel gas pressure switch sends a positive pressure signal.
 The hydraulic motor rotates the rotor of the compressor and the engine turbine, the speed of the turbine should be more than 15% within 15 seconds time, to proceed further.
The cranking will put the hydraulic system pressure to 3000 to 3500 Psig. Engine driven lube oil pump will develop oil pressure to circulate on to the servo operated control devices. The pre/post lube oil pump stops at 30 seconds run time.
 Purging;  once the speed is greater than 15% , and the lube oil pump stopped, the secondary fuel shut-off valve opens to  release the trapped gas pressure, within 15 seconds time, the gas pressure should fall to less than 30 Psi and the purging cycle continues for about 4 minutes.
 Ignition; at the end of purge time-out, the ignition sequence begins, the primary fuel shut-off valve opens, the torch solenoid and ignition exciter are energised. The fuel throttle valve starts from minimum opening at a ramp-up speed rate of 7 mA/Sec. T5 temperature will be monitored for 10 seconds to check the tight-off, once the temperature reaches 350 degree F, it is considered that the ignition is complete. The torch solenoid valve and the ignition exciter are denergised. The fuel throttle valve starts at 20 mA signal at a new ramp rate of 0.25 mA/Sec.
 Acceleration; the T5 temperature will be monitored continuously while the acceleration is in progress. If the temperature reaches 800 degree F, the fuel will be throttled so that it is maintained at this point for 2 minutes, then the ramp up continues to increase the speed up to 60%. At 60% speed, the hydraulic starter pump stroke changes over to neutral position, then the engine will continue to ramp up in speed increase mode at a rate of 1% per second.
Once the speed reaches to 80%, the bleed valve will close and the air inlet guide-vanes opens proportional to the speed.
 At the engine speed 92%, the guide-vanes are full open, the hydraulic pump electric motor stops, machine can be put on load.
At every time, the T5 temperature is constant watch if it exceeds for the short time (less than 5 sec.) the control returns to normal otherwise slow stop will be initiated.
 Turbine Engine Shut-downs
 Normal Shutdown sequence involves a cool-down period in which the engine is allowed to run at idle speed for a period of 10 minutes before speed is reduced further. In normal shutdown, the control system activates a normal engine shutdown sequence by pressing the stop switch on the turbine control panel.
There are two types of cool-down stop; Cool down Stop Non-lockout (CSN), and
Cool down Stop Lockout (CSL).
Fast shutdown sequence is carried out without the cool-down period, and is used only when plant conditions require an immediate shutdown. The control system activates a fast engine shutdown sequence by pressing the fast stop switch on the turbine control panel. There are two types of fast shutdown; Fast Stop Non lockout (FSN), and Fast Stop Lockout (FSL).
 3.1 Engine Shutdown items;
 High engine temperature (FSL),

  • Over speed(FSN), engine speed more than 108%
  • Back-up over speed(FSL), engine speed more than 110%
  • Under speed (FSN),engine speed drops less than 90% during normal run
  • Fail to crank (FSN), during start up, speed less than 15% in 20 sec.
  • Fail to start (FSN), engine speed does not reach 66% after starter motor run.
  • Ignition failure (FSN), T5 does not reach 350 F, within 10 sec of ignition.
  • High vibration engine (FSN), 125 micrometers (4.9 mils)
  • Critical shutdowns (FSN) stop due to activation of emergency P.B.
  • Low lubricating oil pressure (FSN), oil pressure less than 25Psi.
  • High oil temperature (CSN), more than 74 C.
  • Low oil level (CSL), oil QTY drops below 131 gallon.
  • Low pre-lube oil pressure (FSN), oil pressure less than 6 Psig during 30 sec.
  • High starting fuel flows (FSN), fuel pressure less than 8 Psig prior to ignition.
  • High gas fuel pressure (FSL), gas supply more than 230 Psig.
  • High gear box vibration (FSN), vibration is more than 30G’s on gearbox.
  • T1 RTD fail (FSL), RTD signal is lost.
  • T5 T/C fail (FSL), two or more T/C signals lost.
  • Back up over speed system fail (FSL), loss of signal to over speed system.
  • Gas monitor fail (FSL), loss of power supplies to the gas monitor.
  • Fire detected by CLV sensor (FSL), fire detected by UV scanners.
  • Fire detected, thermal sensor (FSL), heat detected in enclosure.
  • Enclosure gas level high (FSL)
  • Air inlet filter high D. pressure (CSN), filter DP is greater than 6 inch H2O.

 Turbine Engine Control System Maintenance
                                                                                                         Inspection and scheduled maintenance of the turbine engine control system is required at established intervals. A list of servicing needs will establish the most practical inspection and maintenance schedules. Through the scheduled maintenance at specific times, will minimize the need for corrective maintenance.
Scheduled maintenance frequency is based on hours of equipment operation per year and is divided into three categories; Operational, Intermediate, and Major.
4.1 Operational Maintenance                                                     
                                                      The equipment need not be shutdown, is a walkaround  inspection to ensure equipment is functioning properly and to detect early signs of deterioration. The operational inspection procedures need not be carried out daily. It is, however, recommended that they be done as often as practical.
4.2 Intermediate Maintenance
                                                          Intermediate maintenance requires that the equipment be shutdown for most of the inspection. It is recommended that this maintenance be performed after six months of operation. Maintenance intervals for subsequent operation should then be established on the basis of experience gained during the first year, with due regard to the possibility that changing operating conditions may dictate other and more practical intervals.
4.3 Major Maintenance
                                            Major maintenance involves disassembly of selected subsystem components for inspection, and visual inspection of engine gas path components with borescope instruments. Major maintenance should be performed at 8000 hour intervals. Elements that have malfunctioned or have been defective in the past, and all other discrepancies that have come to light during previous inspection, should receive renewed attention. It is important that detailed records be kept as a means of noting a trend of a component defect.  Attached is scheduled performance tasks listed in a table, showing the scheduled maintenance action and the assemblies required for each.

  1. B) Compressor Unit

                                         Turbo-compressors have a serious problem as pressure ratio is increased; at some limit of higher pressure ratio and reduced flow, flow becomes seriously unstable, and may even completely reverse. This condition, called SURGE.
 The following are the process condition that can lead to surge:

  • Rise in discharge pressure; if suction pressure (P1) and RPM are held constant, a rise in discharge pressure (P2) results in a fall in flow, ultimately reaching the surge limit.
  • Fall in suction pressure; if discharge pressure (P2) and RPM are held constant, a fall in suction pressure (P1) results in a fall in flow, ultimately reaching the surge limit.
  • Fall in RPM; if suction and discharge pressures are held constant, a fall in RPM can very quickly drop the flow to surge limit.
  • Drop in Mol Mass (Mol weight or specific gravity).
  • Rise in suction temperature.
  • Changes to equipment.

 Surge Maps; The most useful maps for consideration of surge control are the P2/P1 vs QSTD/P1 map, for operation at constant suction pressure, and P1/P2 vs QSTD/P2 map for operation at constant discharge pressure. Figure 8 shows typical map representing operation at constant suction pressure.
 Controlling surge is by measuring two variables; (a) differential pressure across the compressor and (b) suction flow to the compressor. (a) is used to generate a set point to control (b). by using this method some compensation for the effect of gas density is achieved.
 Anti-Surge Control System
                                                 Anti-surge control system can change either the compressor or the process to avoid surge or to move away from surge. A successful surge control system consists of :

  • An anti-surge controller, which has two principle parts; a calibration algorithm which determines where the compressor is operating relative to the surge limit, and a control algorithm which adjusts final control elements to prevent the operating point from crossing the surge limit.
  • Final control elements, which serve as levers on the process or compressor to move the compressor operation point away from surge, a common final control element is a recycle valve with associated control accessories.
  • Transmitters and flow element, which send information to the calibration algorithm.

  Calibration Algorithm
                                    Algorithm means mathematical model. A surge control system is unusual because it uses a model of physical parameter to be controlled(distance from the surge) rather than controlling a directly measured parameter. Figure 9 shows the typical simple anti-surge control scheme (ASC-M2 Algorithm); in which the calibration algorithm is as follows:

  • Compressor differential pressure provides a minimum Flow Setpoint to a controller.
  • Flow element differential pressure provides a Flow Measurement to the controller.

While compressor differential and flow element differential are actually being measured, compressor ratio and inlet volumetric flow are the actual resultant model parameters, which results from the flow element differential being pressure dependent.
Figure 10 shows the compressor surge line and the calculated surge control line for the same conditions at three values, the relative position of the two lines should result in satisfactory protection for all expected conditions or the algorithm or calibration needs to be changed.
 Control Algorithm
                               Control algorithm for a controller is to avoid overshooting past a limit, such as a surge controller, is different, where the purpose is merely to maintain a value at a selected setpoint, also must handle the transition from not being in control (high flow) to suddenly being in control, and also handle coordination with compressor running sequence. Surge controller has additional control elements such as:
Asymmetrical control, Anticipation, Adaptation, and backup high gain action.