Slide 1

Sequence Control

Sequence control uses an automated system and operating instructions. The instructions are listed in the order required to perform a certain task to reach a desired result.

The sequence may begin with the operator pushing a button in a remotely located control room.

( Contd.)

Control System: Components

Control system components and instruments include the following devices:

sensing monitoring protective sequencing regulating optimizing

Information about each component follows.

Sensoring & Monitoring Devices

Sensing devices are the most basic control system elements. The figure shows a partial list of common sensing devices.

These devices only provide information to the control systems. They do not make adjustments.

Sensing devices provide information to the monitoring devices.

Monitoring devices include gauges, lights, and distributed control systems. Monitoring devices provide detailed information about operating conditions

Sensoring & Monitoring Devices

Protective devices include shutdown and alarm devices. If operating conditions are sensed to be at, or in excess of, an emergency shutdown setpoint, the equipment is automatically shut down.

Start-up and shutdowns must be done in a particular order to ensure safety of personnel and prevent equipment damage. Sequencing devices perform the necessary steps to accomplish start-up and shutdown.

Protective & Sequencing Devices

Regulating devices receive information from sensing devices and adjust the process to achieve a certain setpoint.

Regulating devices such as the anti-surge controller make adjustments based only on the current measurement and do not predict trends, as does an optimizing system, which is discussed next. (Contd.)Optimizing devices are the most complex devices in a control system. Optimizing systems can look at one variable or can be connected to a computer-based system that controls the entire process. The programmable logic computer (PLC) is an optimizing device.Compressor capacity control is discussed next.

Vibration Monitoring

Several protective systems are used to alert operators to abnormal operating conditions that could result in damage to the turbine or other equipment.

Vibration is one of the critical operating parameters that is monitored by a protection system.

A vibration monitoring system is usually a part of the gas turbine's programmable logic control and operator terminal.

The figure shows typical vibration detector locations in relation to the rotor.Vibration Monitoring

Vibration Monitoring System: Purpose (Contd.)

The last topic discussed in this lesson is the vibration monitoring system. The purpose of the vibration monitoring system is to help in preventing abnormal operating conditions.

The rotating shafts of any machine or gearbox have a tendency to move axially or radially as a result of speed, loads, worn internal parts, unbalance, or other reasons.

Vibration Monitoring System: Purpose

Axial and radial shaft movement is called vibration. Vibration is a continuing periodic change in a displacement from a fixed reference.

Excessive vibration is an abnormal operating condition that can result in equipment damage. Excessive vibration is a symptom of other abnormal conditions.

A bent shaft or improper shaft alignment could be the source of vibration.

Shaft Movement

Vibration monitoring systems are installed on gas turbines and driven equipment to monitor and sometimes record axial and radial shaft movement.

Shaft movement is monitored in either displacement (mils), velocity (length/unit-time), or acceleration (g's).

(Contd.)One mil equals 0.001 of an inch. A shaft movement of 5 mils could generate an electrical impulse of one volt. Either of these measurements may be used as setpoints to initiate an alarm or shutdown.

Vibration Monitoring Probes (Contd.)

In the gas turbine, vibration probes are installed in the bearing housings near the shaft.

The probe tips operate on 24-volt DC power to establish a magnetic field between the probe tip and a burnished area on the shaft.

As the distance between the probe tip and the shaft changes, the strength of the magnetic field changes.The probe senses fluctuations in the magnetic field, and the monitoring systems uses this information.

The figure illustrates a typical single and double radial probe installation in a bearing.

Vibration Monitoring Probes: Function

In the figure, four probes monitor the radial movement of a gas turbine shaft and two probes monitor the shaft axial location.

Axial position probes 1 and 2 monitor shaft axial movement in two places at the thrust collar.

Probes 3Y and 4X measure radial movement at the low pressure end of the compressor. (Contd.)Probes 5Y and 6X measure radial movement at the high pressure end of the compressor.

The probes are placed 90 degrees apart to monitor relatively both horizontal and vertical radial movement.

One probe monitors the X axis, and the other monitors the Y axis.

Lube Oil Instrumentation IntroductionThe preceding lessons in this series discussed the lube oil system, its operation, and individual components of the system.

This lesson provides information about lube oil system pressure and temperature controls, instruments, and alarms.

Proper lubrication of gas turbine engines is critical to engine performance and operating life.

Proper lubrication depends on oil pressure and oil temperature.

High oil pressure can damage lube oil system components. Low oil pressure can prevent oil from reaching internal engine parts.

IntroductionProper lubrication also depends on oil viscosity, which is affected by oil temperature.

Cold oil is more viscous than hot oil. Cold oil also produces higher oil pressure at the pump discharge.

Hot oil loses film strength because of reduced viscosity.

Information about the devices used to control lube oil pressure and temperature begins the lesson.

Pressure Control Devices: Purpose

The purpose of pressure control devices is to regulate lube oil pressure. Excessive pressure in the lube oil system is prevented by relief valves.

The following lube oil system pressure control devices are discussed:

pump relief valve

pressure relief valve

diaphragm operated control valve

The figure illustrates typical lube oil system protection devices.

Relief Valves

Lube oil pumps are protected by a relief valve. If the oil pressure at the pump discharge outlet is high, the valve is lifted off its seat and some of the excess oil is returned to the pump inlet. Some systems return the oil to the reservoir.

A lube oil system pressure relief valve relieves excessive oil pressure.

Like the pump relief valve, this valve opens when oil pressure overcomes spring pressure, and excess oil is returned to the reservoir.The pressure relief valve can be adjusted by increasing or decreasing spring tension.

Diaphragm Valve

A diaphragm valve regulates lube oil pressure within a very narrow range. A diaphragm valve senses lube oil pressure in the bearing header and opens or closes to maintain the pressure in the correct operating range.

As mentioned at the beginning of this lesson, oil pressure and oil temperature are closely related.

Information about temperature control devices is presented next.

Temperature Control Devices: Purpose

The purpose of temperature control devices is to regulate lube oil temperature.

Such devices include:

oil cooler

thermostat

lube oil regulator assembly (thermal bypass valve and pressure regulator)The figure illustrates a typical oil cooler arrangement. Lube oil always flows through an oil filter, but it does not always flow through an oil cooler.

Oil Coolers & Thermostat

The figure illustrates two water-cooled oil coolers. The oil cooler at the top of the figure shows the oil temperature control valve in the cool oil mode. The oil cooler at the bottom of the figure shows the oil temperature control valve in the hot oil mode.

In the cool oil mode the valve is partially open. Most of the oil flowing through the valve completely bypasses the cooler.Contd.The oil that flows through the oil cooler is mixed with the bypassed oil at the control valve outlet.

The lube oil temperature control valve is an oil temperature thermostat.

The thermostat uses a heat sensitive spring to position the valve.

Oil Coolers & Thermostat

In the hot oil mode, the thermostat spring expands to hold the valve against the valve seat and stop the flow of oil through the bypass part of the valve.

When the thermostat is cold, the spring retracts to lift the valve off its seat.

This position allows the oil to flow through the valve and bypass the oil cooler.

As the thermostat warms, it begins to close the temperature control valve to force more flow through the oil cooler.

Lube Oil Regulator Assembly

A typical lube oil regulator assembly contains:

two thermal bypass valves a pressure regulating unloading valve

The figure shows these components and the flow of the oil through the lube oil regulator assembly.

(Contd.)

The thermal bypass valves located inside the lube oil regulator assembly serve two purposes:

control the lube oil temperature

protect the oil cooler against high oil pressure during a cold weather start

If lube oil temperature is below 60F, the thermal bypass valves are fully open and lube oil bypasses the oil coolers.

Lube Oil Regulator Assembly

When oil temperature exceeds 60F, the valves begin to close and are fully closed at 140F. At that point, all oil flows through the oil cooler.

This assembly also protects the oil cooler against high oil pressure during cold weather starts.

When the differential pressure across the bypass valves exceeds 50 psig, the valves are opened wide enough to maintain 50 psig differential pressure. ( Contd.)

Another component of the lube oil regulator assembly is the pressure regulating unloading valve.

This valve is pilot operated and spring closed. The valve also has a center passage with an oil metering opening.

Lube Oil Regulator Assembly

During operation, pressurized pilot oil flows through the opening to the back side of the regulating valve and assists the spring in keeping the valve closed.

Oil pressure is adjusted by means of the externally adjusted pilot oil pressure relief valve. Pressurized pilot oil comes from the lube oil filter outlet.

As the engine-driven lube oil pump pressure increases with engine speed, the unloading valve opens to maintain a constant lube oil system pressure.

Lube oil system instruments and alarms are discussed next.

Instruments & Alarms

Instruments provide information on the operation of the lube oil system. They monitor conditions such as oil level in the reservoir and oil pressure and temperature throughout the system.

Alarms alert the operator to out-of-limit conditions that must be corrected to ensure safe operations.

The lube oil system instruments and alarms used by G.E. and Solar perform the same basic functions. However, the schematics used by the two companies are different.

Schematics for both companies are included in this lesson.

G. E.: Level Indicator

The figure shows a typical G.E. lube oil system schematic. The devices explained are highlighted. The letter "L" in a circle represents the lube oil reservoir level indicator.

As drawn in the schematic, the level indicator is a local gauge. It also signals lube oil level information to the gas turbine control panel and to the distributed control system (DCS).

Various lube oil levels are listed to the right of the level indicator. When the oil level is 12 inches below the top of the tank, the lube oil reservoir is considered full.

G. E.: Level Indicator

When the level is 10 inches or less from the top of the reservoir, the high lube oil level alarm is initiated by the high level switch 71QH-1.

The level gauge indicates EMPTY when the oil level is 16 inches below the top of the tank.

If the oil level is 17 inches below the top of the tank, level switch 71QL-1 initiates the low lube oil level alarm and shutdown.

G. E.: Temperature Indicator & Relief Valve

The temperature indicator gauge is identified by the letter "T" in a circle.

This gauge indicates the temperature of the lube oil in the reservoir.

The relief valve VR1 regulates the discharge pressure of the main engine-driven lube oil pump, which is discussed next.

G. E.: Pressure Indicator

In the figure, the main lube oil pump is a shaft-driven gear pump.

This pump is attached to the accessory gearbox of the gas turbine.

Following the discharge line from the bottom of the main lube oil pump, the first instrument encountered is a pressure indicator identified by the letters "PI" in a circle.

Follow the pump discharge line almost to the oil cooler where another pressure indicator is located.

G. E.: Pressure Indicator & Switch

Low lube oil pressure switch 63QA-1 is located to the right of this indicator. During the shutdown sequence or whenever the main oil supply pressure decreases to approximately 75 psig, oil pressure switch 63QA-1 actuates. This action starts the auxiliary lube oil pump.

The auxiliary lube oil pump, identified as 88QA in the figure, is a motor-driven pump. A pressure indicator and another pressure switch are located between the pump and the check valve.

The pressure switch, identifed as 63QP, signals the control system when the auxiliary lube oil pump is operating.

G. E.: Pressure Indicator & Switch

The emergency lube oil pump is identified as 88QE.

An emergency lube oil pump is started when AC power is lost or the auxiliary lube oil pump fails.

The emergency lube oil pump is started by actuating a low lube oil pressure switch located on the gas turbine bearing header. This pump also has a pressure indicator and a "run" switch located between the pump and the check valve.

Pressure switch 63QE sends a "pump running" signal to the gas turbine control system when the emergency lube oil pump is operating.

G. E.: Pressure Indicator & Switch

In the G.E. lube oil system, a pressure indicator is installed in the discharge of each pump.

The indicator may be local, it may have a transmitter to signal the pressure information to the DCS, or both.

G.E. uses a pressure switch at the discharge of each motor-operated pump to initiate a "pump running" signal to the DCS.

G. E.: Differential Pressure IndicatorIn the figure, lube oil flows from the pumps to the oil coolers, oil filters, and the bearing header.

Lube oil for the generator is discharged from the main lube oil header.

The differential pressure indicator (PDI) is connected across the lube oil filter inlet and outlet lines.

The purpose of the differential pressure indicator is to monitor the pressure drop across the lube oil filter that is in operation.The device identified as 63QQ-1 is the main lube oil filter differential pressure alarm switch.

This switch initiates an alarm when the differential pressure reaches the setpoint.

G.E.: Bearing Header Pressure Regulator

The bearing header pressure regulator (VPR2) is located between the generator lube oil line and the gas turbine bearing header.

A diaphragm in the regulator senses the oil pressure in the bearing header and opens or closes the control valve to maintain set pressure.

G. E.: Emergency Pump Start Switch

To the left of the bearing header is the low lube oil pressure emergency pump start switch, 63QL.

On the right side of the gas turbine bearing header are three outgoing lines labeled hydraulic supply.

Lube oil from the bearing header provides oil to the hydraulic pumps which provides hydraulic pressure for starting, inlet guide vane operation, and fuel gas control valves.

G. E.: Temperature Switches

High lube oil temperature may indicate fouled lube oil cooler tubes.

A temperature indicator and several switches are located to the right of the hydraulic supply lines.

The temperature indicator is a local instrument as drawn. When used with a transmitter, this temperature can be signaled to the DCS.

The devices identified as 26QT are related temperature switches. If lube oil temperature in the bearing header increases to approximately 165F, a high lube oil temperature alarm is initiated. If corrective action is not taken and lube oil temperature increases to 175F, temperature switches 26QT-1A and 26QT-1B will trip the gas turbine.

G. E.: Thermocouples

The device labeled LT-TH-1A,B represents thermocouples located in the bearing header.

These thermocouples provide high temperature alarms and trip signals to the control system.

To trip the unit, the trip temperature must be sensed by at least two thermocouples.

This concludes the information about oil instruments and alarms in the G.E lube oil system.

The Solar lube oil system will be discussed next.

Thermocouples

The point where the two dissimilar metals are joined that will be most exposed to the heat of a fire is called the hot junction.

The cold junction, sometimes called the reference junction, is enclosed in dead air space between insulation blocks. ( Contd. )

Thermocouples

A typical thermocouple is installed in a protective well or cage. Other thermocouple cages have several passages that allow air (or gas) to enter the protective cage and surround the elements.

A thermocouple fire detection system has a different response to the thermal switch system. (Contd.)

Thermistor

A thermistor (thermal resistor) is a resistive circuit component. When cool, a thermistor has high resistance to current flow. As the temperature of a thermistor increases, its resistance decreases.

A thermistor fire/rate-of-rise detection system is a continuous loop system that usually surrounds the surveillance area.

(Contd.)

Thermistor

In both systems, the power lead is insulated from ground. The single-wire system uses an insulator of ceramic beads that are coated with a substance called eutectic salt. The two-wire system uses a thermistor material to insulate the wires.

Each of these materials loses electrical resistance when heated.

In the fire/rate-of-rise circuit diagram shown in the figure, 24V DC is supplied to the hot lead through an alarm relay coil.

When cool, the insulation does not allow current flow between the hot lead and ground.

Thermistor

When a fire condition heats the insulator material, it loses electrical resistance and a path is complete from the hot lead to ground.

The thermistor system, like the thermal switch systems, automatically resets when cooled.

A pneumatic fire detection system is discussed next.

Maintenance Management Overview

MM46SUPERVISION STAGESDIAGNOSTICSCONTROLFAILURE DETECTIONRECONFIGURATIONPROCESSInteligencia artificial para la deteccin y el diagnstico de fallosJoaqun Melmdez / Joan Colomer (Universitat de Girona)47CONDITION MONITORING SYSTEM (CMS)CONDITION BASED MAINTENANCE (CBM)

Impressions(human senses)Vibration monitorsChemical analysisI&C channelsThermographMaintenanceoperators

DECISIONSMAINTENANCE MANAGEMENTElectricalconsumptionOthersCumulatedExperienceCONDITION MONITORING SYSTEMCONDITION BASED MAINTENANCE481. CONDITION MONITORING SYSTEM1. TRADITIONALInformation source: MAINTENANCE OPERATORS OTHER PLANT SECTIONS I&C INDICATIONS / ALERTS / ALARMS / RECORDERS2. NEW TECHNOLOGIESInformation source: SIGNALS (I&C, ELECTRICAL / MAGNETIC MEASUREMENTS) NOISE ANALYSIS SUPERVISOR (VIBROMETERS, PROXIMITOR,) THERMOGRAPH TRIBOLOGY (LUBE OIL ANALYSIS OTHERS: all available equipment information2. CONDITION BASED MAINTENANCEMAINTENANCE MANAGEMENTMAINTENANCE ENGINEER EXPERIENCEANALYSISDECISIONS

COMPUTER MAINTENANCE MANAGEMENT SYSTEM (CMMS)

CONDITION MONITORING

DECISIONSMAINTENANCE MANAGEMENT Trends Analysis Alerts Store data Data management Experience databaseCumulatedExperienceCMMS49Refer to BS3811:2000

Maintenance is the work undertaken in order to keep or restore a facility to an acceptable standard level. MaintenanceWork undertakenAll activities (information, analysis, repair, etc.)To keepPlanned maintenance (Preventive, Predictive and proactive) policy for critical equipmentTo restoreUnplanned maintenance (Corrective or run to failure policy for non-critical equipmentFacilitySystem level (equipment, unit, plant)Acceptable standard levelAcceptable level at certain working condition (HSE, working hours, etc.)50CostPM CostTotal Maintenance CostCM CostBest level Down Time CostPM level

Reliability

Total Maintenance cost = Direct cost + Overhead cost + Downtime costor= PM cost + CM cost + Downtime cost Company LogoCriticality AnalysisHSE EffectStand By AvailabilityProcess EffectMajor(B)

Major (A)Without(C)With(D)MinorMinorCentrifugal Pump(System level)CriticalityDrain systemCWater systemBOil systemASteam systemAFire-fighting systemAComparison of different maintenance policiesPolicyApproachGoalsReactiveRun to failure (fix-it when broke).Minimize maintenance costs for non-critical equipment.PreventiveUse-based maintenance program.Minimize equipment breakdown.PredictiveMaintenance decision based on equipment condition.Discover hidden failures and improve reliability for critical equipment.ProactiveDetection of sources of failures.Minimize the risk of failures for critical systems.GlobalIntegrated approach.Maximize the system productivity.53A comparison among proactive maintenance approachesPolicyApproachGoalsRCFAIdentification of root causes of failures.Eliminate failures. FMECAIdentification of criticality of failures.Improve equipment availability.HAZOPIdentification of hazards and problems associated with operations.Improve HSE effect.RCMDetermination of best maintenance requirements for critical systems.Preserve system function & improve reliability.RBIDetermination of an optimum inspection plan for critical systems.Improve system HSE and availability.54Comparison among global maintenance approaches

PolicyApproachGoalsOSMOptimization approach for the global maintenance system.Maximize reliability measures and minimize maintenance cost rates. TPMComprehensive productive-maintenance system.Maximize plant effectiveness and resource productivity.55Facility equipment are divided into four major categories:Run-to-Failure - Most cost effective to let equipment run unattended until it fails. Used on lowest priority equipment.

Preventive Maintenance - Perform maintenance tasks on a piece of equipment at regular intervals, whether the equipment needs it or not.

Predictive Maintenance - Perform maintenance based upon real-time data collected on a piece of equipment. This data shows the health of the equipment

Proactive Maintenance - Determine root causes of failure and implement fixes (e.g., redesigning the piece of equipment so that it does not break down as frequently)56Why PM should be done?To prevent equipment failuresTo detect early failuresTo discover hidden failuresTime-Directed Maintenance (TD)Condition-Directed Maintenance (CD)Failure Finding (FF)Preventive Maintenance:57Predictive Maintenance:Condition basedmanagementVisualInspectionVibrationanalysisUltrasonic Pressureanalysis

TemperatureanalysisOil analysisEfficiency analysisWearanalysis58Atypical machine condition-vibration trend

59PdM Planning:Best Method (vibration analysis, .. etc.)Best Frequency (inspection interval)Best LocationsBest ToolsInternational Standard (ISO10816, .. etc.)Standard LimitsSeverity ChartTrouble Shooting ChartReference CreationRegular Measurements (monthly, .. etc.)Analysis Decision MakingCorrective Actions - Good conditions, - Routine Maintenance, - Repair, or - Replace.60PdM Policy: Vibration analysis:1- Frequency: Every 300 Running Hours

2- Tool: Vibration Equipment: accelerometers, charge amplifier and analyser. Computer program for trend analysis and prediction.

3- International Standard: CDA/MS/NVSH107

4- Method:Record the vibration spectrum, specify the peaks corresponds to the bearing componentsRecord each component peak and frequency.By using the soft ware and the standard limits, determine the trend of each peak.Determine the bearing state(good need service need change)

5- Limits: According to CDA/MS/NVSH107 Pre-failure: vibration level5.6 m/s Failure: vibration level 5.610 m/s Near catastrophic failure: vibration level >10 m/s

6- Actions: Bearing is Good Call for bearing change Bearing must be changed immediately61Proactive Maintenance:Definition: Determine the root causes of repeated failures and address these rather than just treating the symptom. E.g., if seals keep failing on a certain pump, do not just keep rebuilding the pump, figure out why they keep failing. Is the type of pump wrong for the application? Is the seal material not compatible with the fluid being pumped? Is the pump grossly mis-sized for the duty? etc.

Some root causes of failure:Poor Design/Poor Manufacturing,Poor shipping, handling, and storage procedures, equipment becomes damaged or begins to degrade before it is installed,Poor Installation,Poor materials,Poor working conditions.62What is the Maintenance?How?1- How to keep or restore the facility at acceptable standard level in certain operating conditions? System description Main parameters Main items Functional block diagram Criticality Working conditions2- How to prevent the failures?Main failures:PM:3- How to discover the hidden failures?Main failures:Policy:4- How to detect the early failures?Main failures:Policy:5- How to minimize the risk of failures?Main failures:Risk:Policy: