panching - vol 3 of 4 - sludge dewatering plant - 130827

7/27/2019 PANCHING - Vol 3 of 4 - Sludge Dewatering Plant - 130827

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PANCHING WATER TREATMENT PLANT

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PROCESS PLANT O&M MANUAL VOL. 1 OF 3 1 | P a g e

OPERATION AND MAINTENANCE MANUAL

This document is a set of Operation & Maintenance Instruction Manuals for

Panching Water Treatment Works and comprised the following

SLUDGE DEWATERING PLANT & EQUIPMENT

1.0 INTRODUCTION

The sludge dewatering plant system is designed to handlethe sludge generated by the plant. The complete systemincludes a sludge cake handling system to transfer thedewatered sludge from the mechanical dewateringequipment into bins where it will be transported out fromthe building storage yard.

The plant consists of the following equipments:-

- Wash water recovery pumpset- Decanters- Submersible pumpset- Submersible mixers- Sludge scraper- Decanter feed pumps and holding tank feed pump

1.1 OPERATING PRINCIPLESThe plant is designed for 24 hours continuous operation inautomatic mode as programmed by the PLC at therespective plant. When required, the plant can also beoperated under full manual mode by operator intervention.


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1.2 TREATMENT PRINCIPLESThe plant is designed with the following treatmentprocesses to dewater the sludge prior to transfer out to thelandfill:-

1.2.1 -Enhanced flocculation process. Polyelectrolyte isdosed as a flocculent aid. The dosing points are atthe pipeline entering into thickener tanks andanother at the pipeline entering decanters.

The dosing of polymer solution is to aid in sludgethickening and to enhance the dewatering process.

2.0 SLUDGE DEWATERING PLANT DESCRIPTIONAND CONTROL PHILOSOPHY

2.1 DESCRIPTION

Settled sludge from the 10 sedimentation tanks is piped to2 Nos. sludge balancing tanks (1 duty & 1 standby) whilefilter backwash waste water is discharged into 2 Nos. wash-water recovery tanks (WWRT) (1 duty & 1 standby). OneWWRT is designed to hold the waste water from 2 filterbackwashing. After the WWRT has collected 2 lots ofwastewater, the inlet penstock is closed and the content issettled. The supernatant is pumped back to the inlet of

aerator through a floating suction. The sediment from theWWRT is pumped to the sludge balancing tank. Settlingoperation at WWRT is carried out batch-wise.

The sludge balancing tank (SBT) receives sludge transferredfrom the WWRT as well as sludge and chemical wastedischarged directly from the 10 sedimentation tanks andchemical building respectively. Sludge particles are keptsuspended in SBT by 2 submersible mixers. Sludge fromSBT is fed to the sludge thickener tank (STT) by 2 Nos.

submersible thickener feed pumps (TFP). In operation,either 1 or 2 feed pumps will run, depending on level at the


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SBT. When level is too low, both TFP will stop, but thesubmersible mixers are kept running.

Sludge thickener tank (STT) concentrates the watery sludgeto at least 4% dry solids. The sludge rake runs continuouslyas long as there is water or sludge in STT (liquid level abovea pre-set level). Supernatant from the STT overflows and isdiverted back to the WWRT. Solids carried over at STToverflow outlet is manually monitored and checked. Thethickened sludge is transferred to the sludge holding tank(SHT) periodically by means of PC pumps under levelcontrol at the SHT. The PC pump and sludge transfer pipeshall be flushed whenever the PC pump stops and theflushed water is discharged directly into sludge holdingtanks.

The sludge holding tank (SHT) supplies a steady stream ofsludge to the centrifuge decanters. Two PC pumps deliversludge from a SHT to 3 units of decanters. Normally, onlyone decanter runs during the day and afternoon shifts; butif sludge load is heavy, 2 decanters may be running.

The dewatered sludge cake from the decanter is collected ina dedicated arm roll bin by operating a screw conveyor. The

collected dewatered solids are disposed out from thetreatment works by trucks. The centrate from the decanteris discharged to the surface drainage.

There is a polymer solution preparation system and dosingpumps at this sludge treatment plant. The dosing pumpssupply polymer solution to the STT feed to assist in sludgethickening and to the centrifuge decanter to enhance thedewatering process.

2.2 WASH WATER RECOVERY TANK (WWRT)

2.2.1 FUNCTION

The WWRT serves as a holding & sedimentation tank forfilter wash-water. It enables the supernatant to be returnedto inlet of the aerator and settled sludge to be transferredto SBT.


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2.2.2 OPERATION

For each WWRT, there is an Auto/Manual selection switchon the SCADA screen to control the respective WWRP andSTP. The position of the selection switch (on the SCADAscreen) will determine how the associated equipmentresponse. Normally, the Auto position shall be selectedunless that WWRT is not available or under manual testing.

When a WWRT is selected to Manual at SCADA, start/stopof the associated WWRP and STP shall be done on a fullymanual basis via the SCADA or from the respectivestart/stop push-buttons at the MCC. The PLC controlprogramme is not available at the Manual mode.

When a WWRT is selected to AUTO at SCADA, theoperation of associated WWRP and STP will be automaticallycontrolled base on current PLC control programme. Theselected WWRT will undergo an automatic Filling,Settling and Disposal sequence. The auto sequence willbe described later.

All WWRP and STP shall be provided with field mounted

emergency stop push-button stations.

The operation at WWRT is carried out batch-wise. There are3 stages in this treatment process: (1) Filling with filterwash-water, (2) Settling the waste water and (3) Disposalof the settled waste-water.

Under Auto mode, only 1 WWRT is selected to Fillingstate. At the Filling state, the WWRT inlet penstock isopen to receive filter wash-water. After receiving 2

continuous filter backwashes and the level electrode detectshigh water level, the inlet penstock of the second WWRT isopened and the inlet penstock of the first WWRT is closedsimultaneously. It will also trigger the "Settling" timer.[Note: Ultrasonic level sensor shall be used to monitor thewater level of WWRT.)

If there is any fault at the actuators of inlet penstock orpenstock status not in line with sequence, correspondingalarms will be raised. Auto control sequence will still

continue even when there is alarm.


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When the first WWRT enters the Settling stage and thesecond WWRT will go to the Filling state. Settling is purelytime base and the pre-set time can be adjusted fromSCADA. Suggested settling time is 120 to 240 minutes. (tobe refined & checked during testing & commissioning). Mostof the sludge solids in the filter wash water will settle undergravity during the prescribed duration.

After the first WWRT has completed its Settling, it moveson to the Disposal state. In Disposal state, the dedicatedwash-water recovery pump (WWRP) will start and thedischarge valve of the WWRP open. Status of WWRP & itsdischarge valve are monitored, and alarm is raised whenunusual condition arises. The supernatant from the WWRTis recycled back to the inlet of the aerator. The recycle flowrate is monitored by a magnetic flow-meter & display on thesystem. The recycle flow is manually pre-set by throttling avalve.

In case the recycle wash-water flow exceeds a pre-set highflow value (SCADA adjustable) for more than 5 sec, analarm Recycle Flow High will be raised. Pumping shall stillcontinue even in Hi Flow condition.

Auto-stopping of WWRP is by level electrode. When the lowlimit contact is activated, the WWRP stops and thedischarge valve is close.

If WWRP fails before the water reaching low level, the NoWWRP Available alarm will be raised and the controlsequence will be suspended. Operator interference isrequired to rectify and normalise the situation. [One unit ofWWRP is dedicated to each WWRT, there is no standby

unit.]

Under normal condition when the WWRP has stopped, theduty submersible sludge transfer pump (STP) willautomatically start run until the low-low limit contact (oflevel electrode) is activated. Status of the STP is monitored.The stand-by STP will run if the duty pump were to fail.

After the STP has auto stopped (when WWRT has beenemptied), Disposal stage is completed and the WWRT is

put to Stand-by (inlet penstock continues to remainclose). It will remain in this state until the other WWRT is


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fully filled (water reaches high limit contact of levelelectrode).

2.2.3 Process Interlocks & Alarms / FailureConditions:

A.The WWRP is dedicated to one unit of WWRT. The 2STP from the same WWRT may run togetherautomatically (by manually switching both STP to'Available to start').

B.Control sequence will normally continue even whenthere is an active alarm (except as in (C) below).

C.Process control sequence is suspended with alarmraised when the WWRP or both the STP fail together.In this case, manual intervention is required.

D.WWRT is not available when :(a) Inlet penstock or valve is not remote selected

at actuator; or(b) The associated WWRP or both STP are not

available, fail/fault.

E. Alarm to be raised when :(a) Any pump, valve or penstock fails during

operation.(b) Alarm is raised when both duty & standby pump

fail.(c) Alarm is raised when both the WWRT are NOT

available.

2.3 SLUDGE BALANCING TANK (SBT)

2.3.1 FUNCTION

Sludge Balancing Tank (SBT) acts as a buffer tank forstorage of sludge from clarifier, chemical waste fromchemical building and sludge transferred from WWRT beforeit is fed to the thickener tank. This serves to even out thesludge feed rate to the 2 sludge thickening tanks.Dimension of a SBT = 9.00mW x 10.685mL x 2.80m Depth


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2.3.2 OPERATION

When a SBT is put to Service from the SCADA screen, theassociated inlet penstock will open. When the SBT is put toStandby or Out of Service, its inlet penstock will beclosed. It is possible that while a SBT is Out of Serviceand yet its associated submersible thickener feed pump(TFP) and submersible mixers (SMX) are still running onManual

For each SBT, there is an Auto/Manual selection switch onthe SCADA screen to control the respective TFP and SMX.The position of the selection switch (on the SCADA screen)will determine how the associated equipment response.Normally, the Auto position is selected unless that SBT isnot available or under manual testing.

When a SBT is selected to Manual at SCADA, start/stop ofthe associated TFP and SMX shall be done on a fully manualbasis via the SCADA or from the respective start/stop push-buttons at the MCC. The PLC control programme is notavailable at the Manual mode.

When a SBT is selected to AUTO at SCADA, the operationof associated TFP and SMX will be automatically controlledbase on the current PLC control programme. The PLCcontrol programme is basically a level based program tostart and stop the TFP and SMX. Further, whenever a TFPruns, the duty selected Thickener Polymer Dosing Pump(TPDP) shall be auto started and the polymer solutiondosing valve shall open.

The polymer solution will be dosed to the common TFP

outlet. When two TFP on a same line are running, thecontrol system shall automatically double the polymerdosage (double the TPDP running speed).

All TFP and SMX shall be provided with field mountedemergency stop push-button stations.

Operation at a SBT is essentially based on level statuswithin the tank. The primary control element in each SBT isthe level electrodes. Each SBT feeds into a dedicated sludge

thickener tank (STT); SBT#1 feeds STT#1 and SBT#2feeding to STT#2; cross over operation is not possible.


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Sludge solids are prevented from settling on SBT tank floorby two submersible mixers which are running perpetually aslong as that SBT is put to Auto and the water level isabove preset. (The said SBT can be put Out Of Service)

Level electrode function:L1 : Longest electrode, serves as reference (100mm ftf)L2 : both SMX stop below this level (350mm ftf)L3 : both SMX start to run & both TFP stop (400mm ftf)L4 : First TFP auto starts on reaching this (1200mm ftf)L5 : Second TFP auto runs on reaching this (2000mm ftf)L6 : SBT Overflow alarm (2900mm ftf)

ftf = from the tank floor

Capacity of each TFP = 35 m3/hr @ 10mPlan area of each SBT = 9,000 x 10,685 mmSBT tank floor level = IL 31.00mNormal TWL at SBT = TWL 33.80mEstimated TFP run time = 30 minutes

When a TFP runs, the associated motorised butterfly valveshall open. When the TFP stops, the discharge valve will

close.

The TFP can run as long as the dedicated sludge thickenertank (STT) is put to Service. Operation of TFP is notaffected by level status or sludge density / concentration atthe STT; it is only dependent on level at the SBT. Either oneor two TFP may run, depending on level at the SBT.

The operation of SMX is only dependent on the level at SBT.Both SMX will run or stop together. The spare unit is an off

line item supplied loose.

Both the TFP and the SMX may be started/stoppedmanually from the SCADA control screen.

If the duty TFP fails before the pre-set level is reached, thestand-by TFP auto starts & continues to run until the pre-setlevel is reached. If the stand-by TFP is not available or failsafter running for a while, No TFP Available alarm will beraised and control system is suspended. Operatorinterference is required to normalise the fault condition.


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A.One SBT will serve only one dedicated STT. No crossover is permissible.

B.One or both TFP may run simultaneously, dependingon level control at the SBT.

C.When a TFP runs, the duty selected thickener polymerdosing pump shall be running and the correspondingthickener polymer dosing valve shall open.

D.Control action is still active even when there is anactive alarm.

E. SBT is not available when :(a) Inlet penstock is not remote selected at

actuator; or(b) Both the TFP or both SMX are not available,

fail/fault.

F. Alarm to be raised when :

(a) Any pump, valve or penstock fails duringoperation.

(b) Alarm is raised when one SMX or both duty &standby pump fail.

(c) Alarm is raised when both the SBT are NOTavailable.

(d) When a TFP has started and none of TPDP isrunning, alarm Thickener Polymer Dosing Failshall raised. System is allowed to continue.

2.4 SLUDGE THICKENER TANK (STT)

2.4.1 FUNCTION

The Sludge Thickener Tank (STT) increases the solidsconcentration of sludge before it is pumped to the holdingtank. The clear supernatant overflows the weir and isreturned to WWRT by gravity. The STT can also act assludge storage tank in case the sludge holding tank is full ortemporary out of service.


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The STT is dedicated to receive sludge from only one SBTand the thickened sludge will only be pumped to one of the2 sludge holding tank (SHT).

Diameter of one Sludge Thickening Tank = 25.00mdiameter x 3.50m depth

Volume of sludge, 1.0m depth in STT = 750 cu meter.

2.4.2 OPERATION

For each STT, there is an Auto/Manual selection switch on

the SCADA screen to interlock with the TFP. The position ofselector switch (on SCADA screen) will determine how theTFP response. Normally the Auto position is to be selectedunless that STT is not available or under manual testing.

When the STT is put to Auto from SCADA, the sludgescrapper or rake will run continuously; and the 2corresponding TFP will receive a start permissible signal;i.e. it is free to start & stop depending on level at thecorresponding SBT.

When a STT is put to Manual, the sludge scrapper willretain its last status before the switch-over (i.e. scrapperwill be running if it was running before or the scrapper willbe stop if it was not running earlier) and the correspondingTFP is NOT start permissive (i.e. the TFP will not autostart/stop with level changes at the corresponding SBT.).However, the TFP can still be manually started if so wishes.

There is no In service /Stand-by /Out of Serviceselection for the STT; the only available selection is Autoor Manual.

The sludge feed to STT and discharge from a STT is notcontrolled by status at the STT itself. When the TFP runs,supernatant from the STT will overflow and return to theWWRT by gravity. When the SHT feed pump (SHTFP) runs,the concentrated sludge will be transferred to the sludgeholding tank.

The only equipment that is controlled from the STT is the

sludge scrapper drive.


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Sequential time base control or level control is notapplicable to STT system.


The Auto/Manual selection state of a STT is interlockedto start permissive-ness of the corresponding TFP asdescribed earlier.

Whenever a Sludge scrapper fails or trips, an alarm must beraised. Sludge scrapper shall be provided with a remotemounted emergency stop button station.

If the torque switch on the sludge scraper gearbox isactivated, Alarm Sludge Scraper Tripped on Over-Torqueis raised.

2.5 HOLDING TANK FEED PUMPS AND SLUDGEHOLDING TANK

2.5.1 FUNCTION

A Sludge Holding Tank Feed Pump (SHTFP) transfers theconcentrated sludge from the bottom of a STT to the sludgeholding tank (SHT). There are 2 sets of dedicated STT,SHTFP and SHT; cross-over operation between these 2 setsof equipment is not possible.

The SHT stores and provides a steady stream of thickensludge of fairly uniform solids concentration for feeding to

the centrifuge decanters. The 2 submersible mixers (SSMX)at each SHT prevent the sludge particles from settling.

Dimension of a SHT: 7.50mL x 7.00mW x 2.35mD


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2.5.2 OPERATION

For each SHT, there is an Auto/Manual selection switch onthe SCADA screen to interlock with the SHTFP and SSMX.When Auto position is selected, the SHTFP can autostart/stop depending on the thickened sludge level in STTand sludge level at SHT, while the auto start/stop of SSMXdepends on the sludge level at SHT. [Note: Thickenedsludge level in STT to be monitored via sludge level probe.]

The operation of SHTFP is independent of the status at therespective STT; and there is hardwired safety interlock ateach SHTFP motor starting circuit to protect the pump. Incase of dry running, a temperature switch embedded in thepump body will send a signal to trip the affected pump.When pump discharge pressure is too high, the commonpressure switch will stop the pump.

When selected to Manual, SHTFP and SSMX can only bestarted or stopped manually. The hard-wired interlock onSHTFP will still function, but control from level electrodeswill be disabled.

When a SHTFP stops, the sludge delivery line and pump

shall be auto flushed before the pump is turned off. Whilethe pump is still running, the flushing water inlet valve isopen and the flushed water is discharged into SHT. After atime delay of several secs (adjustable) the commonmotorised pump suction valve is closed. Then the SHTFPruns for a further pre-set time (120 to 600s, adjustable)before the SHTFP is stopped. The flushing water inlet valveis closed after the pump has stopped.

In case there is a valve actuator failure during the pump

flushing sub-routine, the SHTFP will stop and the respectivevalve failure alarm is raised. Operator attention is requiredto check & reset.

There are 2 SHTFP in a system, when a SHTFP fails duringan auto running operation, the stand-by unit shall auto startand a SHTFP Failure alarm raised. Operator intervention isrequired on the failed pump to be repaired & manuallyflushed before returning to standby.

The suggested level control set points#1 at sludge holdingtank (SHT) for auto control are given below. These level


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set-points for electrode (measured from tank floor) are stillsubject to re-confirmation after site testing /commissioning.

L1 0.050 m Reference level electrodeL2 0.300 m Stop SSMX & start both SHTFPL3 0.400 m Start SSMXL4 0.700 m Start one SHTFPL5 2.300 m Stop all SHTFPL6 2.350 m SHT overflow alarm is raised


The Auto/Manual selected state of a SHT will eitherenable or disable SHT level control of the correspondingSHTFP and SSMX.

An equipment (pump, mixer & valve) failure alarm shall beraised whenever such an event arises. Except for SHTFP,the failure of the duty unit shall not cause the standby unitto run.

Before a SHTFP stops on auto, the pump and the sludgedelivery piping shall be auto-flushed as described.

2.6 DECANTER FEED PUMPS AND SCREWCONYEVOR

2.6.1 FUNCTION

The decanter feed pump (DFP) transfers the sludge fromthe sludge holding tank to the centrifuge decanter (CD) fordewatering. Dewatered sludge as discharged from decanteris transported by a screw conveyor (SC) to a collecting bin.

There are 4 decanter feed pumps and 3 centrifugedecanters in this system. One DFP shall run to supply thefeed to one decanter. Each decanter will have its dedicatedscrew conveyor to transport the sludge cake to an arm-rollbin.


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When any CD or DFP runs, the duty selected decanterpolymer dosing pump (DPDP) will auto start and thecorresponding polymer inlet valve to CD will open.

Normally only one centrifuge decanter shall be running, theother 2 units act as standby. However when the qualities ofraw water deteriorates and sludge volume increases, twodecanters may be operated in parallel (duty cycle).

The sludge cake collected in the arm roll bin shall betransferred and stored at sludge disposal yard periodicallyby the arm roll truck.

2.6.2 OPERATION

2.6.2.1 Selecting available equipment to duty

The suction pipeline is arranged in such that sludge holdingtank No.1 (SHT #1) supplies feed to DFP#1 and DFP#2;while SHT#2 serves DFP#3 and DFP#4. Individualdischarge pipeline of all DFP will join to a common 'ringmain' and then equally distributes the feed to each CD. Theprovision of 'ring main' allows duty-cycle operation of the

decanter feed pumps (DFP) and centrifuge decanters (CD).

When only one unit centrifuge decanter need to be run, anyavailable DFP and CD may be selected for the duty.Operator has to ensure the suction and delivery manualvalves are open and then the duty DFP and CD are selectedto Auto at the LCP or SCADA. The motorised suction valveof the duty selected DFP and the motorised inlet valve tothe duty selected CD will auto open. The sludge cake screwconveyor (SC) of the duty selected CD shall be put to

Auto as well, to enable remote control by the PLC.

When two decanters are required in operationsimultaneously, any 2 available DFP may be selectedmanually for the duty while the 2 duty CD units areautomatically selected by PLC (based on available runninghours). The setting of running hour is adjustable atSCADA/PLC. The default running hour setting shall be 8hours per day for each CD. Total running hours for all threeCD is 8 x 3 = 24 hours.


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When 2 sets of DFP and CD are running simultaneously,both DFP supplies feed to a common discharge line (ringmain) and then equally distribute into both CD. Therespective SC of both duty CD will automatically start viaPLC remote control.

2.6.2.2 Operation of centrifuge decanter & relatedequipment.

After the selected DFP, CD and SC are switched to Autoon the LCP and the correct isolation valves are open, thecentrifuge decanter may be manually initiated to start fromthe decanter local control panel by pressing a push-buttonon the decanter LCP control touch-screen or remoteinitiation from SCADA. The decanter will start and undergoa normal starting sequence as described later. Themotorised valves, feed pump and screw conveyor will beopened and started automatically at the correct sequence.Similarly, the decanter will go through an auto stopsequence when the Stop switch is activated.

When the decanter is switched to Manual position, thestandard decanter starting and decanter stoppingsequences will be both deactivated. The associatedmotorised valves, feed pump and screw conveyor must bemanually started or stopped at the proper sequence.

A low level condition (L2) at SHT (sludge holding tank) shallto trigger the centrifuge decanter connected to this SHT toundergo a normal stop sequence on Auto.

2.6.2.3 Centrifuge Decanter standard starting sequence

When a decanter is initiated to start, the standardcentrifuge decanter starting sequence is activated. A typicaldecanter starting sequence is as follows:

(1)Decanter start initiated from the LCP. (after checkingmanual valve position)


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(2)Start the main decanter motor drive & decanter scrollmotor run. Decanter start timer is started.

(3)Start the associated screw conveyor.

(4) When decanter motor has reached full speed, a DFPStart signal is issued from the CD control panel.

(5) With this DFP Start signal and the PLC control systemwill(a) Open the pump suction valve of the selected DFP.(b) Open the decanter inlet motorised valve(c ) Open the decanter polymer solution inlet valve(d) Check valve status, to sound alarm if required

(6) Start the associated DFP.

(7) Send start signal to duty DPDP (if dosing pump notalready in operation), or increase the polymer feed rateaccording sludge solids loading (when the 2nddecanteris started and the DPDP is already running).

(8) Thickened sludge & polymer solution enter the runningdecanter; dewatering of sludge will take place.

(9) Centrate liquid is discharged from the liquid end & thedewatered sludge cake drops from solid discharge portto the screw conveyor.

(10) The centrifuge decanter control system will adjust thespeed of decanter and monitor the differential speed inresponse to any variation in torque via automatictorque mode. Torque is affected by feed rate and solidscontent in feed. The decanter controller will monitor

these operating conditions while the unit is running.

(11) The decanter would remain in operation when there isa continuous stream of sludge feed and the decanter ishealthy.

(12) The decanter will undergo a normal stop sequencewhen the operator initiates the Stop button. TheEmergency Stop button shall not be used to stop theunit under normal operating condition, unless it is an

emergency. On pressing Emergency Stop, the unit


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will stop immediately without undergoing the flushingcycle.

2.6.2.4 Centrifuge Decanter standard stopping sequence

When a normal Stop command is received by the decantercontroller, a standard stop sequence is activated. A typicaldecanter stopping sequence is as follows:

(1) Decanter Stop is initiated from decanter LCP.

(2) The DPDP will immediately stop and the suction valveof DFP is closed. After 5 seconds, the polymer inletvalve to the CD will be closed.

(3) The corresponding DFP flushing water inlet valve isopen; while the DFP and CD is still running. Flushingwater displaces the sludge in DFP & delivery pipe to thedecanter when the DFP is running. The DFP & CDflushing duration can be set from the LCP. [Note: Iftwo decanters are running simultaneously, theinitiation of "Stop" will cease the feeding of sludge forboth DFP and flushing will take place in series for both

DFP and CD.]

(4) After the DFP & CD flushing duration has elapsed, theDFP will stop.

(5) After 5 s, the flushing water inlet valve to the DFP isclosed.

(6) After a delay of 3 minutes (adjustable from LCP), theCD inlet valve is closed & the motor drive of the CD is

stopped.

(7) After another delay of 2 minutes (adjustable) when allthe sludge cake has been transferred to the arm rollbin, the SC attached of the decanter is stopped.

(8) The DC and its feed pump (DFP) are now in the stopposition.


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(1) When two decanters are required in operation, theselection of duty decanter feed pumps (DFP) is donemanually. The operator is required to check & confirmthe manual valves of selected duty DFP are openbefore the decanter standard start sequence isoperative.

(2) An equipment (decanter, feed pump, screw conveyor &motorised valve) failure alarm shall be raised wheneversuch an event arises. The failure or tripping of anequipment shall not cause the standby unit to run onauto. The operator has to go to the field to investigate,clear the fault and normalise the equipment beforestarting again. This applies to all motorised equipmentin this section, i.e. all equipment related to DCP andCD, except for the decanter polymer dosing pump(DPDP).

(3) When sludge holding tank level is at L2 or below (see

section 7.5.2 above), the operating centrifuge decanterconnected to it will undergo a normal shutdown, withDFP & CD flushing prior to shut down; if the CD isselected to Auto.

(4) When the screw conveyor (SC) associated with arunning decanter trips, the operating centrifugedecanter connected to it will undergo a normalshutdown, with DFP & CD flushing prior to shut down;if on Auto.

(5) When the DFP associated with a running decanter tripsor fails, the operating decanter connected to it will stopimmediately, without flushing (since flushing cannot becarried out when the DFP fails). At the same time theDPDP will stop or has its output halved if anotherdecanter unit is still running.

(6) Motorised valve failure(s) taking place after a decanterhas completed its standard starting sequence will not

cause the decanter to stop or trigger a CD stoppingsequence. If valve failure takes place during a CD


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starting or stopping sequence, the start/stop sequencewill be aborted and the affected unit will stopimmediately.

2.7 POLYMER DOSING PLANT

2.7.1 DESCRIPTION

Two units of Packaged Raw Water Polymer Batchingplant shall be installed for the works.

Polyelectrolyte solution is dosed at a controlled rate,proportional to the plant inlet flow (raw water), into thecommon inlet channel of the clarifiers before enteringthe respective clarifier via the respective inlet penstock.

Prior to dosing into the raw water clarifier inlet chamber,the liquid polyelectroyte is diluted with carrier water viaa common dilution water valve to assist dispersion. Theamount of dilution water is regulated by manual valveand indicated locally by rotameter. The dilution water

shall remain unaltered when changing the polyelectrolytedose.

From laboratory jar tests and pump calibration curves,the desired polyelectrolyte dose shall be set by thestroke adjustment hand wheel on the duty dosing pumpmanually. The dosing pump output can be periodicallychecked by using the portable Calibration Cylinderlocated at the suction pipework.

The control of the polyelectrolyte dosing plant will bemanual and automatic. Electrically operated valves shallonly be provided for the respective dosing pump on thesuction and storage tank outlet.


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2.7.2 CONTROL PHILOSOPHY

2.7.2.1 RAW WATER POLYMER PREPARATIONSYSTEM

Preparation of the polyelectrolyte solution is designed forautomatic batching, with control by the main PLC.

The polyelectrolyte batching plant comprise of a storagehopper of about 100 kg c/w a feeder mounted at thebottomed. A vacuum powder loader is provided at thehopper for loading bagged powder into the hopper. Thehopper shall be equipped with loadcells weigher for displayof content weight.Two nos. of SS316 mixing tanks of approximately 5 m3 areconstructed. A dedicated top mounted stirrer is provided forthe mixing tank.

The plant offered is designed for batch preparation of stocksolution. A fixed speed dry feeder is provided to deliver apredetermined quantity of Polyelectrolyte powder into themixing and mix with the incoming service water.

The batching plant comprise of a hopper to store therequired Polyelectrolyte powder. The powder storagecapacity shall be 100 kg. The hopper is equipped with avacuum powder for loading bagged powder into the hopper.A weighing system comprising of loadcells shall display thehopper content weight at the LCP.

The operator will be able to select the required polymerbatch strength (0.1 to 0.4% w/v). Upon selection of the

strength to batch, the PLC shall compute the quantity ofpowder required and the feeder shall deliver accordingly toachieve the selected strength.

The powder Polymer hopper shall fill by operating thevacuum powder loader with a local controlled switch at thehopper. The conveyor nozzle is manually inserted into a25kg powder bag to effect the transfer.


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The preparation of polymer solution, at a pre-selectedstrength is automatically controlled by the PLC.

The preparation sequence is initiated on low level only inthe Mixing Tank as detected by a level switch. Upon manualinitiation and a low level signal is detected, the sequenceshall proceed as follows:-

Service water inlet valve opens to fill the Tank via a wettingdevice. The stirrer shall start when the feed commencefeeding powder into the mixing tank. When a predeterminedquantity is delivered into the mixing tank, the feeder shallstop. Service water continues to fill the mixing tank untilthe required level is achieved.

FAILURE CONDITION

On power failure, the preparation sequence is interrupted.On restoration of power the batching sequence shall berestarted manually to continue.

2.7.2.2 POLYMER DOSING PUMPS

Two nos. of Polymer Dosing Pumps are provided with oneduty and one standby for the works.

Control of pumps and electrically operated valves are at theLCP. Two units of dedicated VSD are provided at the LCP tovary the pumping speed of the dosing pump via flow pacingcontrol through a central PLC. The pump stroke setting shallbe manually selected at the pump to suit.

One no. of Polymer Dosing Pumps is a duty pump and the2nd pump is served as a 100% standby dosing pump.

In manual mode i.e. Local Manual, the duty tank outlet

valve and the duty pump suction valve are operated at theLCP. The stroking speed of the dosing pump is directed


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varied at the VSD controller to achieve the required doserate. The stroke setting shall be manually selected at thepump to suit.

There are two auto modes available at the LCP i.e. LocalAuto and Remote Auto. By Local Auto control, dosingsequence is activated at the LCP only and by Remote Autocontrol; the dosing sequence and dosing parameter areactivated/changed at the Central SCADA only.

Switchover of duty pump to the standby pump is automaticif Local Auto or Remote Auto is selected and control bythe PLC timer.

Under Local Manual mode at the LCP, the operator is ableto select the duty dosing pump to start, as well as the dutyvariable speed drive units provided at the LCP. The dosingpump speed is manually adjusted at the VSD proportional tothe plant raw water flow and stroke is manually adjusted tosuit requirement.

Selection of duty tank and duty pump are at the discretionof the operator by opening the selected tank outlet valveand the selected dosing pump suction valve at the LCP

valves selector switches.

Under Local Auto mode at the LCP, the operator shall onlyneed to initiate an Auto start button. The PLC shall controlthe dosing sequence and dosing pump speed. Selection ofpump stroke is done manually to suit requirement. Dutychangeover of exhausted tank shall be automated by theduty tank level signal feeding to the PLC.

Under Remote Auto mode, the operation will be the same

as Local Auto except that the SCADA shall have fullcontrol of the plant i.e. duty tank and duty pump selectionas well as initiation of Auto start and change of dosingparameter.

Start and stop for the automatic mode are initiatedmanually by a Start/Stop button at the LCP or controlcentre MMI. Upon initiation of Start the auto sequenceshall commence upon detection of flow and stop when flowceased. Unless Stop is initiated, sequence shall resume

when flow is re-established.


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In the auto mode, the dosing pumps flushing sequence isautomatic each time the duty pump stops. The pumpsuction valve shall close and the flushing valve shall open toallow high pressure water through the pump and out forabout 15 to 20 seconds.

FAILURE CONDITION

On power failure the Polymer Dosing Pumps will stop and allvalves position will maintain. On restoration of powersupply, operation of the plant is reinitiated manually.

When a duty dosing pump fails during operation underLocal Auto or Remote Auto mode, the standby dosingpump will automatically take over the failed pump. Theappropriate valves will automatically open or closerespectively to correspond the operation requirement.

On dosing pump failure, an appropriate alarm is raised atthe LCP.

Electrically operated valve failure (fail to open and fail toclose), an amber failed light will be lighted on the respective

valve status lights and the operator shall investigate andtake appropriate action (to rectify or to switch over to nexttank or pump).

On VSD failure the appropriate alarm is raised at the LCP.The VSD shall be manually changeover.

2.7.2.3 POLYMER PLANT LCP

The facilities for Manual Control and Local Auto are locatedat the LCP and complete with indication lights. The LCPhave the following control functions and indications:

1)Power On status indicator lamp2)Alarm Accept and Lamp Test push-button.3)Hopper Weight Indicator 2 nos.

4)Running/Stopped/Tripped status indicating lights & Runand Stop buttons for :-


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a. Feeder No. 1b. Feeder No. 2c. Mixer No.1d. Mixer No.2

5)Open/Close/Fault status indicator lights & Open andClose push buttons for :-a. Pump Suction Valves 2 nos.b. Mixing Tank Outlet Valves 2 nos.c. Pump Flushing Valve 2 nos.d. Common Carrier Water Valve 1 no.e. Mixing Tank Inlet Valves 2 nos.

6)Running/Stopped/Tripped status indicating lights & Runand Stop buttons for :-a. Dosing Pump No. 1b. Dosing Pump No. 2c. Dosing Pump Starter 2 nos.

7)Dosing Pump Starters 2 nos.

8)Variable Speed Drive Units 4 nos.

9)Local /Remote Selector Switch.

10) Auto/Manual Selector Switch

11) Local Start/Stop initiation push button

2.7.2.4 ALARM SUMMARY

The following alarms shall be provided for Raw WaterPolymer Dosing Pump LCP:-

1)Hopper Weight Low2)Hopper Weight High3)Mixing Tank Level High4)Mixing Tank Level Low5) Common Failed for Pump/Valve/VSD


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2.7.3 POLYMER DOSING PLANT START UP

BATCHING PLANT START UP

The batching plant is only operated in automatic mode.The initiation of the batching done manually at the localpanel mounted on the skid. Selection of solutionstrength is also done on the local panel.

POLYMER DOSING PUMP START UP

AUTOMATIC MODE

Start-up of the plant in the automatic mode is initiatedmanually by a Start button at the LCP or at the controlcentre MMI. Upon initiation of Start button, the autosequence commences when raw water flow is detectedand stop when raw water flow ceases. Unless Stop isinitiated, sequence will resume when raw water flow is

re-established after the raw water flow ceased.

MANUAL MODE

The Polymer plant is designed to run only in LocalManual and Remote Manual is therefore not used.

Start-up of the plant in the manual mode is done at thelocal control panel. The following steps shall be followedto commence the operation in manual mode:-

- At the LCP under Preparation System, please select theswitch to Local for duty tank

- Then select the selector switch next to it to Manual

- At the LCP under Dosing System section, please select

the switch to Local


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- Then select the selector switch next to it to Manual

- The operator shall at the field, check for manual valvesfor the storage tank and dosing pump that need to beopened

- The operator shall decide on the duty tank to be put inservice by opening the corresponding motorized tankoutlet valve at the LCP

- The operator shall decide on the duty pump to be put inservice by opening the corresponding motorized suctionvalve at the LCP

- Ensuring raw water flow is established, the operatorshall initiate Initial Start button at the LCP

- From the pump calibrated curve or chart provided, theoperator shall vary the pump stroke setting and VSDspeed manually to achieve the dosage that is required

- Once the pump is started, the operator shall at the LCP,opened the motorized carrier water valve and regulatethe flow if necessary

- A plant started in manual mode shall be monitoredclosely by the operator as there will be no pump failedchangeover or tank empty switchover that isautomatically done in auto mode.

2.7.4 POLYMER DOSING PLANT SHUTDOWN

AUTOMATIC MODE

Shutdown of the plant in auto mode is initiated manuallyby an Initial Stop button at the LCP or at the controlcentre. Upon initiation of Initial Stop button, the autostop sequence commences with the duty pump stoppingand follows by the respective closing of motorizedvalves. When raw water flow ceases or not detected, the

plant will also stop. Unless Initial Stop is initiated,


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sequence will resume when raw water flow is re-established after raw water flow ceased.

The dosing pumps shall be automatically flushed eachtime the dosing pump stops.

MANUAL MODE

Shutdown in manual mode is done at the LCP bypressing the Initial Stop button or at the controlcentre.

The following steps shall stop the plant operated inmanual mode:-

- After pressing the Initial Stop button, the duty pumpsuction valve shall be selected to Close to isolate thepump

- Select the duty tank outlet valve to Close to isolate thetank

- Then, select the common carrier water valve to Closeto stop the carrier water

The dosing pumps shall be manually flushed by selectingOpen on the respective flushing valve located at thepump suction for about 10 to 15 seconds and Closeafter that

2.7.5 POLYMER DOSING PLANT MAINTENANCE

The Polymer dosing plant basically consists of thebatching plant complete with mixing and storage tanks,level sensors, motorized valves, a transfer pump andmetering pumps. Manufacturers equipment manuals areincluded in this O&M manual for reference. Theoperators are advised to refer to the specific manual forproper maintenance procedure described in the relevant

manual.


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It is recommended that mixing tanks should be emptiedand cleaned on a half yearly basis to check and removethe hardened polymer stuck to the side of the tanks.

- On a monthly basis, the storage hopper should bechecked for hardened polymer powder and removedto prevent obstruction to free flow of powder in thefeeder screw.

- On a monthly basis, check the total weight of powderfeed by the screw feeder during a batchingsequence. For a 0.2% w/v solution strength, 1.2kgshould be fed by the feeder for a single batched.Readjust the feeder frequency if required.

- On a monthly basis, the metering pump should berecalibrated to ensure the pumping rate is maintained. Ifflow rate is noticeably different from previouscalibration, refer to manufacturer manual for pumpfault.

- On a monthly basis, the motorized valves should betested to ensure they are able to close and open fullyand the respective signal feedback status is displayed at

the SCADA.

- On a monthly basis, the mixing tank level probesshould be tested to trigger the high-high, high and lowlevel indication at the SCADA.


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2.7.6 POLYMER DOSING PLANT TROUBLE SHOOTING

PUMP NOT STARTING (OR STOPS ITSELF)

Possible Causes Remedial Action

1. Raw water flow ceased.

2. Raw water flow signalloss.

3. Service tank level

sensor detected levellow-low.

4. MCC power to pumpsisolated.

5. Pump emergency switchoperated.

6. Pump tripped/faulted.

1. Restart raw water

pump.

2. Check flow meter and

rectify.

3. Initiate batching of

polymer solution.

4. Switch on the power

to the pumps at the

MCC.

5. Reset pump

emergency switch.

6. Check for pump

overload or possibleshort circuit.


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PUMP FLOW LOW


1. Pump diaphragmdamaged.

2. Pump suctionchoked.

3. Check pump stroke

and VSD setting.

1. Inspect pump diaphragm

and replace if necessary.

2. Check suction valve and

pipe for blockage and

clean.

3. Readjust setting or

recalibrate pump if

necessary.

PUMPING SOUND ABNORMAL


1. Dampenerdiaphragm rupturedor deflated.

2. Pump suctionchoked.

1. Inspect dampener air

pressure, inflate or

replace where necessary.

2. Check suction valve and

pipe for blockage and

clean.

panching - vol 3 of 4 - sludge dewatering plant - 130827

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