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Biosolids Dewatering Controls

Control Strategies PIDs
Sludge Transfer PID
  Thickening and Dewatering PID
Polymer System PID
 
Sludge Feed/Load Pump and Grinder 
Automatic Control
The stopping and starting of the sludge feed/load pump is normally controlled auto­matically by the control system associated with the BFP. A switch on panel LP‑8090 adjacent to the BFP transfers control of the sludge feed/load pump to the control system associated with the GBT. Manual valve adjustment is also required to route the sludge discharged from the pump to the GBT.

If the switch is in the BFP position, the PLC will control the sludge feed/load pump during the BFP automatic cycle, as described in the following section. If the switch is in the GBT position, the sludge feed/load pump will start and stop in place of the WAS pump, that normally responds instead during the GBT automatic cycle (as de­scribed in a preceding section). All interlocks associated with the WAS Pump during GBT operation will also apply to the sludge feed/load pump if the switch is in the GBT position.

Flow rate for the sludge feed/load pump is adjusted manually at panel LP‑8090 ad­jacent to the BFP, even when the switch is in the GBT position.

When the BFP or GBT control system calls for the sludge feed/load pump to start, the PLC will start the grinder and, after a 5‑second delay, will start the sludge feed/load pump. Fifteen seconds after the PLC stops the sludge feed/load pump , it will stop the grinder. The length of the delays is adjustable at the PCMS. The grinder will come on even if the pump is operated by HAND (as long as the grinder's HOA switch is in AUTO).

The PLC will start the sludge feed/load pump only if the HOA switch located at panel LP‑8090 is in the AUTO position, the OR switch located adjacent to the pump is in the REMOTE position and the variable speed drive local/remote switch located near the pump is in the REMOTE position.
 
Fault Conditions
If the sludge feed/load pump is called to run automatically and does not come on after a short time delay, the PLC will alarm pump failure on the annunciator at LP‑8090. 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, the sludge feed/load pump will be stopped and an alarm will annunciate. To clear the low flow or high pressure alarms, reset push buttons must be pressed at the SCP and the VSD. If the pump is called to run by the PLC and, after a short time delay, the PLC does not receive an ON status indication signal, the PLC will annunciate an alarm.

If the sludge feed/load pump is being used to feed the BFP or GBT, the PLC will stop the pump if the liquid level in the sludge blend tank is below 1.7 feet.
 
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.

 
Belt Filter Press (BFP)
Automatic Control

To start the BFP, a start push-button is pressed at the vendor supplied panel. The following startup sequence is then initiated by the vendor supplied panel interfacing with LP‑8090:

   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‑8090 to start washwater 
       booster pump No. 2. 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 WATER ON status indication to the vendor sup­plied panel.

   2. The vendor supplied panel starts the belt. A PRESS RUN status is received continuously by the PLC from the
       vendor supplied panel while the belt drive is running. When the machine is ready to receive sludge, the vendor 
       supplied panel sends a signal to the PLC indicating PRESS READY. The PLC starts the polymer pump (either
       polymer pump No. 3 or No. 2 as selected at the SCP) and opens the polymer secondary dilution water valve 
       (provided that the OCA switch is in AUTO). The PLC then starts the sludge feed/load pump. 

   3. When the press is turned off at the vendor supplied panel, the sludge feed/load pump and the polymer pump 
       are automatically shut off, and the sec­ondary dilution water valve is automatically closed by the PLC. After 
       a timed washdown period, the START WASHWATER signal from the vendor panel to the PLC ceases, the wash­
       water pump is shutoff and the washwater pump discharge valve closes.

Adjustment of sludge feed rate and polymer feed rate will be made manually at LP‑8090, adjacent to the BFP.

Fault Conditions

Various lower level alarm conditions at the BFP vendor supplied panel will cause the machine to go into washdown mode. In this case, the PRESS READY signal from the BFP ven­dor supplied panel to the PLC ceases, the PLC generates a PRESS TROUBLE alarm and the sludge feed/load pump and the polymer pumps are shutoff. 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 BFP is running, and any of the following conditions occur, the PLC will shut off the sludge feed/load pump and polymer and will send a shutdown signal to the BFP vendor sup­plied panel:

   _ Sludge feed/load pump not running
   _ Washwater pressure low
   _ Polymer pump not running
   _ Low level in sludge blend tank (1.7 feet)


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Last Updated: 9/16/2013 11:31:59 AM
Version 4.0.1