Lesson 28Lesson 28

• CONTROL SYSTEMS I CONTROL SYSTEMS I

• Control systems on board ship are either Control systems on board ship are either automatic or remote or a automatic or remote or a combinationcombination of of these. these.

• The term ‘automation’, or automatic The term ‘automation’, or automatic control, generally is taken to mean the control, generally is taken to mean the operation of a machine by operation of a machine by a a self-self-regulatingregulating mechanism mechanism that that

• that senses the that senses the performanceperformance of the of the machine and, using a process called machine and, using a process called feedbackfeedback, compares , compares itit with with the intended the intended performanceperformance, ,

• and then adjusts the machine to reduce and then adjusts the machine to reduce the difference between actual and the difference between actual and intended performance. intended performance.

• In In a full automatic control systema full automatic control system, a , a computer is used to computer is used to supervisesupervise, , analyzeanalyze, , recordrecord and and displaydisplay the conditions of the the conditions of the machines, machines,

• and then to make necessary and then to make necessary decisionsdecisions and and take necessary take necessary actionsactions. .

• Remote controlRemote control means that the system is means that the system is being controlled by the being controlled by the operatoroperator who is who is situated remote from the system. situated remote from the system.

• Examples are steering gear, watertight Examples are steering gear, watertight door system, tanker cargo control, bridge door system, tanker cargo control, bridge control of engine, etc. control of engine, etc.

• The combination of The combination of automatic controlautomatic control an and d remote controlremote control results in the results in the centralizecentralized control(d control( 集中的控制集中的控制 )) of main and auxil of main and auxiliary machinery. iary machinery.

• Modern engine room has Modern engine room has machinery machinery control roomscontrol rooms from where every possible from where every possible operation will be performed remotely. operation will be performed remotely.

• The operation state and parameters of The operation state and parameters of working condition of machines can be working condition of machines can be recorded, displayed and controlled recorded, displayed and controlled automatically or manually in the rooms. automatically or manually in the rooms.

• An alarm system enables the monitoring An alarm system enables the monitoring of certain measured of certain measured variables(variables( 变量变量 )). .

• Where a fault condition is located, Where a fault condition is located, audible and visual alarmsaudible and visual alarms are given and a are given and a print-outprint-out of the fault and the time of of the fault and the time of occurrence is produced. occurrence is produced.

• The modern The modern improvementimprovement of control syst of control systems and the reliability of the equipment uems and the reliability of the equipment used have resulted in machinery space remsed have resulted in machinery space remaining aining unattended(unattended( 无人看管的无人看管的 )) for long for long periods. periods.

• In order to ensure the safety of the ship aIn order to ensure the safety of the ship and its equipment during nd its equipment during UMS (UnattendeUMS (Unattended Machinery Spacesd Machinery Spaces 无人机舱无人机舱 ) operation) operation,, certain essential functions must be provi certain essential functions must be provided. ded.

• Those are Those are bridge controlbridge control, , machinery machinery control room controlcontrol room control, , alarmalarm and and fire fire protectionprotection, and , and emergency poweremergency power. .

• UMS UMS are mainly unattended overnight are mainly unattended overnight and during this time the bridge officer and during this time the bridge officer on on watchwatch should have control of the main should have control of the main engine. engine.

• In the event of In the event of minor(minor( 较轻的较轻的 ) ) emergenciemergencies es when the machinery spaces unattendedwhen the machinery spaces unattended the engineer the engineer on cabin watchon cabin watch will be infor will be informed of malfunctions in the machinery by med of malfunctions in the machinery by his his cabin monitorcabin monitor. .

READING MATERIAL READING MATERIAL

• A. LUBRICATING OIL SYSTEM A. LUBRICATING OIL SYSTEM CONTROL CONTROL

• The lubricating oil is pumped from The lubricating oil is pumped from the the drain tankdrain tank, very often with , very often with a a magnetic magnetic filterfilter in the suction line, via main suction in the suction line, via main suction filters, fitted with a filters, fitted with a differential pressuredifferential pressure alarmalarm. .

• These filters may both be automatically These filters may both be automatically

cleaned cleaned oror one hand-cleaned and one one hand-cleaned and one automatically. automatically.

• In some cases, when the differential In some cases, when the differential pressure exceeds a pressure exceeds a pre-set valuepre-set value, , the the filter is automatically filter is automatically back flushedback flushed with with airair, ,

• the displaced oil going to an auxiliary the displaced oil going to an auxiliary drains tank where it is subsequently drains tank where it is subsequently filtered and pumped back into the filtered and pumped back into the system. system.

• Cooling is carried out by Cooling is carried out by a three way by-a three way by-pass valvepass valve in the oil line controlled from a in the oil line controlled from a sensor at the inlet to the engine. sensor at the inlet to the engine.

• This valve should be This valve should be fail-safe(fail-safe( 有故障保有故障保护的护的 )) with all the oil going through the c with all the oil going through the cooler. ooler.

• In some arrangements the filters are In some arrangements the filters are placed after the coolers. placed after the coolers.

• Temperature monitoring would be Temperature monitoring would be carried out on the carried out on the common inlet common inlet temperaturetemperature, , thrust inlet temperaturethrust inlet temperature, , and and blower outlet temperaturesblower outlet temperatures, all , all having alarms for high temperature. having alarms for high temperature.

• Low pressure is monitored at the Low pressure is monitored at the common inlet to the engine with common inlet to the engine with alarmalarm and and slowdownslowdown or or shut downshut down of the main of the main engine. engine.

• In some cases there may be an alarm only In some cases there may be an alarm only for low pressure on all supplies other for low pressure on all supplies other than to the main bearings, with alarm than to the main bearings, with alarm and shut down only on these. and shut down only on these.

• Low level is monitored on the Low level is monitored on the drain tankdrain tank and and header tankheader tank by by magnetic switchesmagnetic switches wi with a 20 second delay to prevent false alarth a 20 second delay to prevent false alarms due to ms due to surging(surging( 起伏、波动起伏、波动 )). .

• Failure of the circulating pumps is Failure of the circulating pumps is alarmed, alarmed, asas is high differential pressure is high differential pressure across the filters. across the filters.

• Crankcase Crankcase oil moist detection(oil moist detection( 油雾探测油雾探测器器 )) alarms on high density with engine s alarms on high density with engine shut down. hut down.

• The lubricating oil The lubricating oil purifierpurifier may take oil may take oil from the drain tank and discharge the from the drain tank and discharge the clean oil into the supply line. clean oil into the supply line. clarifier

• The purifier is provided with a level The purifier is provided with a level alarm. If the water seal falls below a pre-alarm. If the water seal falls below a pre-set level an audible alarm is given and the set level an audible alarm is given and the purifier stopped. purifier stopped.

• In some cases, if the sea water pressure In some cases, if the sea water pressure falls, this also operates an alarm. falls, this also operates an alarm.

• B. COOLING WATER B. COOLING WATER TEMPERATURE CONTROL TEMPERATURE CONTROL

• Accurate control of diesel engine cooling Accurate control of diesel engine cooling water temperature is a requirement for water temperature is a requirement for efficient operation. efficient operation.

• This can be achieved by a single This can be achieved by a single controller under steady load conditions, controller under steady load conditions,

• but because of the but because of the fluctuating(fluctuating( 波动、起波动、起伏、不定伏、不定 )) situation during manoeuvring situation during manoeuvring a more complex system is required.a more complex system is required.

• The control system shown in Fig.7 uses a The control system shown in Fig.7 uses a combination of combination of cascade(cascade( 级联控制、主从级联控制、主从控制控制 )) and and split(split( 分区控制分区控制 )) range control. range control.

Main enginecooler

Exhauststeam

Steamcontrolvalve

Seawatercontrolvalve

Seawaterinlet

Seawateroutlet

Pump

Slavecontroller

Mastercontroller

Headertank

Desired value

Temperaturetransmitter

Fig.7

• Cascade controlCascade control is where the output from is where the output from a master controllera master controller is used to is used to automatically adjust the desired value of automatically adjust the desired value of a slave controllera slave controller. .

• The master controller obtains an outlet The master controller obtains an outlet temperature reading from the engine temperature reading from the engine which is compared with a desired value. which is compared with a desired value.

• Any deviation acts to adjust the desired Any deviation acts to adjust the desired value of the slave controller. value of the slave controller.

• The slave controller also receives a signal The slave controller also receives a signal from the water inlet temperature sensor from the water inlet temperature sensor which it compares with its which it compares with its latest desired latest desired valuevalue. .

• Any deviation results in a signal to two Any deviation results in a signal to two control valves arranged for split range control valves arranged for split range control. control.

• If the cooling water temperature is high, If the cooling water temperature is high, the sea water valve is opened to admit the sea water valve is opened to admit more cooling water to the cooler. more cooling water to the cooler.

• If the cooling water temperature is low, If the cooling water temperature is low, then the sea water valve will be closed in. then the sea water valve will be closed in.

• If the sea water valve is fully closed, then If the sea water valve is fully closed, then the steam inlet valve to heater will be the steam inlet valve to heater will be opened to heat the water. opened to heat the water.

• Both master and slave controllers will be Both master and slave controllers will be identical instruments and will be identical instruments and will be two two term (P+I)term (P+I) in action. in action.

• Another method of temperature control Another method of temperature control involves the use of only a single involves the use of only a single measuring element. measuring element.

• A three way valve is provided in the A three way valve is provided in the cooling water line to enable bypassing of cooling water line to enable bypassing of the cooler. the cooler.

• The cooler is provided with a full flow of The cooler is provided with a full flow of sea water which is not controlled by the sea water which is not controlled by the system. system.

• A temperature sensing element on the A temperature sensing element on the water outlet provides a signal to water outlet provides a signal to a two a two term controller (P+I)term controller (P+I). .

• The controller is provided with a desired The controller is provided with a desired value and any deviation between it and value and any deviation between it and the signal will result in an output to the the signal will result in an output to the three way control valve. three way control valve.

• If the measured temperature is low, more If the measured temperature is low, more water will be bypassed and its water will be bypassed and its temperature will therefore increase. temperature will therefore increase.

• If the measured temperature is high, then If the measured temperature is high, then less water will be by-passed, more will be less water will be by-passed, more will be cooled and the temperature will fall. cooled and the temperature will fall.

• A simple system such as this can be used A simple system such as this can be used only after careful analysis of only after careful analysis of the plant the plant conditions and the correct sizing of conditions and the correct sizing of equipment fittedequipment fitted. .

• C. BOILER WATER LEVEL C. BOILER WATER LEVEL CONTROL CONTROL

• A modern high pressure, high temperatuA modern high pressure, high temperature watertube boiler holds a small quantitre watertube boiler holds a small quantity of water and produces large quantities y of water and produces large quantities of steam. of steam.

• Very careful control of the drum water Very careful control of the drum water level is therefore necessary. level is therefore necessary.

• The reactions of steam and water in the The reactions of steam and water in the drum are complicated and require a drum are complicated and require a control system control system based on a number of based on a number of measured elementsmeasured elements. .

• When a boiler is operating, the water When a boiler is operating, the water level in the gauge glass reads higher than level in the gauge glass reads higher than when the boiler is shut down. when the boiler is shut down.

• This is because of the presence of steam This is because of the presence of steam bubbles in the water, a situation which is bubbles in the water, a situation which is accepted in normal practice,accepted in normal practice, 。。

• If, however, there occurs a sudden If, however, there occurs a sudden increase in steam demand from the increase in steam demand from the boiler, the water in the drum at the boiler, the water in the drum at the higher pressure will now ‘higher pressure will now ‘flash offflash off’ and ’ and become steam. become steam.

• These bubbles of steam will cause the These bubbles of steam will cause the drum level to rise. drum level to rise.

• The reduced mass of water in the drum The reduced mass of water in the drum will also result in more steam being will also result in more steam being produced, which will further raise the produced, which will further raise the water level. water level.

• This effect is known as ‘This effect is known as ‘swellswell 膨胀、隆膨胀、隆起起’’ . .

• A level control system A level control system which used only as which used only as a level measuring elementa level measuring element would close in would close in the feed control valve when it should be the feed control valve when it should be opening it. opening it.

• When the boiler load returns to normal When the boiler load returns to normal the drum pressure will rise and steam the drum pressure will rise and steam bubble formation will reduce, causing a bubble formation will reduce, causing a fall in water level. fall in water level.

• Incoming colder feed water will further rIncoming colder feed water will further reduce steam bubble formation and what ieduce steam bubble formation and what is known as ‘s known as ‘shrinkage(shrinkage( 收缩收缩 ))’ of the dru’ of the drum level will occur.m level will occur.

• The problems associated with swell and The problems associated with swell and shrinkage are removed by the use of a shrinkage are removed by the use of a second measuring element, ‘second measuring element, ‘steam flowsteam flow’. ’.

• A third element, ‘A third element, ‘feed water flowfeed water flow’ is ’ is added to avoid problems that would added to avoid problems that would occur if the feed water pressure were to occur if the feed water pressure were to vary. vary.

• A three element control systemA three element control system is shown is shown in Fig. 8. in Fig. 8.

P+Icontroller

Water level

Feed watercontrol valve

Feed water

Auto/manualselector

Steamflowtransmitter

Steam from boiler

Leveltransmitter

Desirevalue

Differentialrelay

Feed water flowtransmitter

• The measured variables or elements are The measured variables or elements are ‘‘steam flowsteam flow’, ‘’, ‘drum leveldrum level’ and ‘’ and ‘feed feed water flowwater flow’. ’.

• Since in a balanced situation steam flow Since in a balanced situation steam flow must equal feed flow, these two signals armust equal feed flow, these two signals are compared in e compared in a differential relay(a differential relay( 差动差动继电器继电器 )). .

• The relay output is fed to a two term The relay output is fed to a two term controller and comparator into which the controller and comparator into which the measured drum level signal is also fed. measured drum level signal is also fed.

• Any deviation between the desired and Any deviation between the desired and actual drum level and any deviation actual drum level and any deviation between feed and steam flow will result between feed and steam flow will result in controller action to adjust the feed in controller action to adjust the feed water control valve. water control valve.

• The drum level will then be returned to The drum level will then be returned to its correct position. its correct position.

• A sudden increase in steam demand A sudden increase in steam demand would result in a deviation signal from would result in a deviation signal from the differential relay and an output signal the differential relay and an output signal to open the feed water control valve. to open the feed water control valve.

• The swell effect would therefore not The swell effect would therefore not influence the correct operation of the influence the correct operation of the control system. control system.

• For a reduction in steam demand, an For a reduction in steam demand, an output signal to close the feed water output signal to close the feed water control valve would result, thus avoiding control valve would result, thus avoiding shrinkage effects. shrinkage effects.

• Any change in feed water pressure would Any change in feed water pressure would result in feed water control valve result in feed water control valve movement to correct the change before movement to correct the change before the drum level was affected. the drum level was affected.