Section 95 - Effluent Pumping

95.01 General Overview

The purpose of effluent pumping is to ensure proper discharge of the plant effluent through the outfall. During normal plant operation, the disinfected effluent will exit the Parshall flume into the effluent pipeline and bypass the pump station to the outfall through a flap gate located in the Effluent Junction Box. The pump station will be utilized during periods of high plant flows, high Columbia River water levels, or certain combinations of the two. The pump station will operate whenever the water elevation downstream of the Parshall flume is greater than 24.5 and the differential head between the Parshall flume and Effluent Junction Box is less than 2 ft. The pumps have a common intake header and a common discharge header that discharges to the Effluent Junction Box. When the effluent pumps are operating, the hydraulic pressure of the pump discharge will cause the bypass flap gate to close, ensuring that all effluent passes through the pumps.

The design criteria for the Effluent Pumps are presented below.

95.03 Computer Assisted Control Strategies

95.03.a. General Overview

At the MPWRF, the Effluent Pumps are automatically controlled through the PLC based on the difference in level between the effluent Parshall flume and Effluent Junction Box. The operator selects which mode the PLC uses to run the Effluent Pumps from the SCADA computer located in the Marine Park Administration Building or in the Operator Lab. The operator uses the SCADA computer to monitor alarms and equipment status.

95.03.b. Control Strategies

Operation:

1. A timer on the flap gate sensor monitors the time after effluent begins flowing through the effluent pumps. This sensor activates the flap gate open alarm if the gate is still open an adjustable period after the timing sequence is initiated.

95.03.b.2. Effluent Parshall Flume and Effluent Junction Box

Operation:

1. The water levels downstream of the Parshall Flume and within the Effluent Junction Box are monitored to control the operation of the effluent pumps in REMOTE mode. A signal will trigger pump start if the differential head between the two locations is less than 2 ft.
   
   
2. When the LEAD pump starts, it will operate at a minimum speed and will not pump any effluent. When the water level downstream of the Parshall flume exceeds 24.5 ft, the pump speed will be increased and the pump will begin pumping. A warning is initiated if the LEAD pump isn't running and the differential head between the Parshall flume and the Effluent Junction Box is less than 2 ft.

95.03.b.3. Effluent Pumps

Operation:

95.04 Operating Strategies

95.04.a. General Operating Strategy

In REMOTE mode, each pump can be started and stopped and pump speed can be adjusted by the PLC/SCS system. The LEAD pump may be changed by operations using a SCS CRT hand switch. If the LEAD pump fails, the backup pump is started if it is in REMOTE mode.

There are two modes of REMOTE influent pump sequencing and control, MANUAL and AUTO. The mode selected is used for control of all four effluent pumps. There is a level controller faceplate for AUTO control and sequencing and a manual loading station for each pump at the SCS CRT. When in AUTO mode the level controller will indicate the setpoint and current pump speed. The setpoint cannot be adjusted through the SCS CRT. One of the three setpoints is automatically set based on the number of pumps running. The manual loading stations can be used to adjust individual pump speed in MANUAL mode. The PLC/SCS is programmed for bumpless transfer between modes. Current pump speed is maintained when modes are changed. The number of pumps running may be changed to satisfy AUTO control requirements when selected to AUTO.

When selected to MANUAL mode, the operator may set the speed of pumps and individually start and stop the pumps through the SCS CRT. From the SCS CRT the operator may start a maximum of three pump(s) together.

The pumps are sequenced automatically when selected to AUTO mode. The operator may select the sequencing mode through the SCS CRT. The pumps are started and stopped by the PLC based on the level differential level between the Effluent Junction Box and the level downstream of the Parshall flume as indicated in Table 1 for the respective sequencing modes. The basic strategy is to start pumping when the level downstream of the Parshall flume exceeds 24.5 and the differential between the Effluent Junction Box and the Parshall flume is less than 2 ft. The pumps will be sequenced to maintain the level downstream of the Parshall flume. The pumps will be stopped when the differential level increases above 2 ft and the level downstream of the Parshall flume decreases below 24.5.

95.04.c. Alternate Operating Conditions

95.05. Troubleshooting

95.06.a. Confined Spaces

95.06.a.1. Effluent Junction Box

The operator is responsible for reviewing the SOP on Entering the Effluent Junction Box. A confined space entry form must be filled out and properly signed before entry in the junction box.

95.06.b. Lockout/Tagout

95.06.b.1. Effluent Pumps

This section is not used.