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WAS Pumping/Thickening Controls

Control Strategies PIDs
WAS and Scum Pumping PID
  Thickening and Dewatering PID
Polymer System PID
 
WAS Pumps
                         WAS Pumping Control Modes
Mode Description
Flow Ratio Control Mode Control of the WAS withdrawal rate from the respective secondary clarifiers is automatically adjusted to maintain an operator-entered ratio between the two clarifiers.
Total WAS withdrawal rate is entered as part of the gravity belt startup sequence.
Remote Auto Control Mode The WAS is automatically adjusted to maintain an operator-entered WAS flow set point (gpm).
Remote Manual Mode The WAS Pump speed is manually adjusted through the PCS.
Local Manual Mode The WAS Pump speed is manually adjusted on the pump drives.
 
The WAS pump is normally controlled automatically as part of the operation of the GBT. To operate the pump automatically with the GBT, a two-position switch on panel LP‑8080 adjacent to the GBT must be placed in the GBT position. The HOA switch at LP‑8080 must be in the AUTO position, the OR switch at the pump must be in the REMOTE position, and the VFD LOCAL/REMOTE switch must be in the RE­MOTE position. The VFD is located in the Blower Building.
 
Polymer System 
Operation

Each two-tank polymer batching system is designed to feed an uninterrupted supply of polymer by automatically switching the polymer feed pump suction to the alternate tank when one tank becomes empty of polymer solution. The system also auto­mati­cally alerts the operator of the need to mix a new batch in the empty tank.

The actual batching operation is only partially automatic, and requires that the opera­tor manually initiate the batching sequence for an empty tank, and manually meter dry polymer into the tank. Tank filling and tank mixer starting and stopping are automatic functions, once the batching cycle has been initiated by the operator.

When a tank is not in use for batching polymer, the solenoid valve on the water line to the tank will be automatically closed. The manual valve on the water line to the eductor should be left closed by the operator, and the manual valve on the rapid fill line should be left open. The manual valve on the rapid fill line is intended to be used to adjust the rate of tank filling and to balance flow between the eductor and the rapid fill line while dry polymer is fed.

The PLC opens and closes the dilution water valves with the starting and stopping of the polymer pumps. The OCA switches for these valves are located at panel LP‑8080 (GBT) and panel LP‑8090 (BFP). Polymer pump speed is set manually from panels LP‑8080 and LP‑8090. The PLC sends the speed control signals to the pump variable speed drives via the SCP.

Polymer Pump No. 2 is a spare pump for either the GBT polymer system or the BFP polymer system. A THICKENER/PRESS selector switch is located at the SCP.

Secondary dilution water flow rate is measured by a rotameter and can be manually adjusted with a valve. A solenoid valve automatically turns the secondary dilution water flow on or off with the polymer pumps.

Manual switches permit all polymer related automatic functions to be performed manually from the SCP. When polymer is batched manually, the BATCH READY light illuminates on the basis of liquid level in the polymer tank.

Batch Make-up
1. The operator pushes the BATCH START button on the Sludge Control Panel (SCP) for the mixing tank to be used first. The solenoid valve on the tank automatically opens and water begins to flow into the tank through the rapid fill line. A BATCH RUNNING status indicating light illuminates at the SCP.

2. When the water level reaches an adjustable mid-level point (about 30 inches), the tank mixer automatically starts. The tank mixer runs for a set period of time adjustable by the operator through the PCMS. The mixer's HOA switch on the SCP must be in the AUTO position, and the mixer's OR switch at the mixer must be in the REMOTE position for these functions to occur.

3. After the mixer starts and a strong vortex is formed, the operator will begin manually feeding dry polymer to the tanks. The manual valve on the water line to the eductor is opened by the operator. Dry polymer is conveyed through a flexible tube with one end attached to the suction port on the educ­tor and the other end free to be held into a polymer container by the operator. The strength of the polymer solution is controlled manually by the operator, who observes the reduction in the weight of the polymer bag as he feeds the dry polymer into the tank.

4. When the desired quantity of dry polymer has been fed to the tank, the line is purged by removing the tube from the polymer bag while water continues to flow through the eductor. The operator then closes the manual valve on the water line to the eductor. The partial automation of the system permits the operator to leave after adding dry polymer, until the MIX NEW BATCH indi­cator illuminates at the SCP.

5. When the desired amount of dry polymer has been fed and the tube purged, the manual valve on the water line to the eductor is closed by the operator. Water continues to flow into the tank through the rapid fill line.

6. When the liquid level in the tank reaches an adjustable high level point, the solenoid valve will automatically close.

7. After the mixer timer has run out, the mixer automatically shuts off. A timer for batch aging, that is adjustable through the PCMS, starts after the mixer shuts off.

8. While the polymer is aging, a BATCH READY status indicating light will blink. After the aging timer has run out, the BATCH READY status indi­cat­ing light will be continuously illuminated, and the BATCH RUNNING indi­cating light will turn off.

9. Polymer solution is then ready to feed to the GBT or BFP from this tank. When the liquid level reaches 8 inches in the alternate tank, the discharge valve on the new full tank will open, provided that it is in BATCH READY sta­tus. (If it is not in BATCH READY status a BATCH NOT READY alarm will annunciate.) The valve OCA switch on the SCP must be in the AUTO posi­tion for this to occur.

10. A MIX NEW BATCH indicator light will illuminate at the SCP alerting the oper­ator to the need to mix a new batch in the empty tank.
 
Fault Conditions

If the PLC detects that any of the steps in the batching sequence are not executed, a BATCH FAILURE alarm will annunciate.

Polymer pump start/stop, and secondary dilution line valve open/close will normally be automatically controlled from the respective GBT or BFP panel. The pump will not be permitted to start if the following two conditions are not met:
    (1) at least one tank has a liquid level above the low level probe and is in BATCH READY status, and
    (2) the automatic valve on that tank is open.
If these conditions are not met, then a BATCH NOT READY alarm will annunciate.

The polymer pump will also be stopped and an alarm generated by the PLC if the PLC receives a low flow switch signal from the suction side of the pump or a high pressure switch signal from the discharge side of the pump. If the PLC calls for the polymer pump to run and after a short time delay does not receive the indication that it is running, the PLC will generate a pump failure alarm at the SCP.

 
Gravity Belt Thickener (GBT)
Automatic Control

To start the GBT, a start pushbutton is pressed at the vendor supplied panel. The following startup sequence is then initiated by the vendor supplied panel interfacing with LP‑8080 as described below:

   1. The vendor panel starts a hydraulic system pump for belt tensioning and tracking. After adequate hydraulic
       pressure is developed, the vendor supplied panel sends a signal to the PLC in LP‑8080 to start Washwater 
       Booster Pump No. 1. The PLC starts the washwater pump and opens the washwater pump discharge valve. 
       When the PLC receives a signal indicating that there is adequate water pressure (from the washwater pump
       discharge pressure switch), the PLC will send a WATER ON status indication to the vendor supplied panel.

   2. The vendor supplied panel starts the belt. A THICKENER RUN status is re­ceived continuously by the PLC from
       the vendor panel while the belt drive is run­ning. When the machine is readily to receive sludge, the vendor
       supplied panel sends a THICKENER READY output to the PLC. The PLC starts the selected polymer pump (either
       Polymer Pump No. 1 or No. 2 and opens the secondary dilution water valve (provided that the O-C-A switch is
       in AUTO). The PLC then starts the WAS Pump.

   3. When the thickener is turned off at the vendor supplied panel, the WAS pump and the polymer pump are 
       automatically shut off, and the secondary dilution water valve is automatically closed. After a timed washdown 
       period, the START WASHWATER signal from the vendor panel to the PLC ceases, the washwater pump is shut
       off and the wash­water pump discharge valve closes.

Various lower level alarm conditions at the GBT vendor supplied panel will cause the machine to go into washdown mode. In this case, the THICKENER READY signal from the GBT vendor supplied panel to the PLC ceases, the PLC generates a THICKENER TROUBLE alarm, the WAS and the polymer pumps are shut off, and the secondary dilution water valve closes. After washdown, the START WASHWATER signal from the vendor panel to the PLC ceases, the washwater pump is shut off, and the washwater pump discharge valve closes. When a higher level alarm condition occurs, the START WASHWATER signal ceases immediately, rather than after a washdown period.

When the GBT is running, and any of the following conditions occur, the PLC will shut off the WAS and polymer, and will send a shutdown signal to the GBT vendor panel:

   _ Washwater pressure low
   _ Polymer pump not running
   _ WAS flow below 40 gpm

Adjustment of sludge feed rate and polymer feed rate will be made manually at panel LP‑8080, adjacent to the GBT.

The thickened sludge pump removes sludge from the bottom of a thickened sludge hopper below the discharge of the GBT. A level transmitter mounted on the hopper sends a level signal to the PLC at LP‑8080 that will maintain a desired level in the hopper by automatically modulating pump speed.

The thickened sludge pump starts when the level in the hopper reaches a depth of 10 inches above the level of the level sensor and stops when it reaches 6 inches.

Fault Conditions

The PLC will also stop the thickened sludge pump, the WAS pump, and initiate the GBT shutdown routine if the PLC receives a low-flow switch contact input or a high-pressure switch contact input from the discharge side of the thickened sludge pump. To clear the low-flow or high-pressure alarms, the PLC must receive a status input at LP‑8080 from a reset pushbutton. High-level and low-level in the hopper are also alarmed by the PLC to the annunciator. High-level will also shutdown the GBT in the same manner as a THICKENER TROUBLE alarm. If the pump is called to run by the PLC and after a short time delay the PLC does not receive a On status, the PLC will alarm pump failure on the annunciator.


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Last Updated: 9/16/2013 9:46:09 AM
Version 4.0.1