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Section 2: Pump Station Components

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Overview of Components

A full discussion of the design and specification of a pump station is beyond the scope of this manual. However, this section attempts to bring to the design engineer's attention the various components and the considerations for those components. Appropriate design specialists for the control, electrical, mechanical, and structural components of a pump station must to be consulted early in the decision process. A common reference for design of pump stations is FHWA Hydraulic Engineering Circular number 24 ( HEC-24).

The following are necessary considerations in pump station design:

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  • Property-An entire pump station generally requires more footprint than merely the pumps and wet well or sump. Other necessary parts of the station include the electrical service, system controller, motor control center cabinets, which must be in a separate, dry room, and standby power generation. Other considerations may be on-site storage and parking. A required consideration is maintenance access to the pumps and the standby generator; not just personnel access, but the ability and room to bring in suitable vehicles and equipment such as a boom crane to lift out pumps, generator, and electrical cabinets for repair or replacement.
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  • Arrangement-The wells and pumps may not need to be in the same place as the control house. An example of this is a set of wells with submerged pumps and discharge conduits located in a wide median of a depressed section of Interstate highway. The control house with the electrical service, standby generator, motor control center, and control circuitry is located along the frontage road out of the depressed section and away from buried or overhead utilities.
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  • Wet Well- The wet well receives the inflow of storm water prior to pumping. It must also be designed with a trash collection rack, room for sedimentation collection without diminishing the design capacity, and a sump pump to remove the bottom storage below the main pump level.
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  • Electrical-The appropriate electrical service for a pump station is usually 277/480-volt, 3-phase AC. For a typical pump station, the electrical service equipment includes large metal cabinets for the electrical metering, main circuit breaker, a transfer switch to isolate the station from the utility when the standby generator is powering the station, and the electrical distribution panel. The details of the electrical service equipment are the province of the electrical engineer. However, the project manager must understand that clearances and air space around electrical equipment are not options; they are mandatory safety requirements which may increase the footprint of the pump station, but cannot be ignored.
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  • Standby Power-The normal source of standby power is either a diesel or natural gas engine/generator set. Fuel cells are not suitable for pump stations because of the hours long start-up time they require. Battery technology is improving to the point where solar or wind power may become viable. For TxDOT pump stations, natural gas powered engines should be considered over diesel. Diesel is the more efficient fuel which allows for a smaller engine than natural gas, but diesel has many other problems. Modern diesel is intended to be used within a few months of production. Diesel fuel that sits in a holding tank for a long period is at risk of gelling, particularly if the fuel is warmed during the monthly or weekly scheduled test run of the unit. Sitting diesel fuel is also highly subject to moisture contamination from the atmosphere. Natural gas is not subject to contamination or breakdown in the pipeline. Destructive storms which may cause road blockages and delivery problems usually do not interfere with natural gas service.
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  • Pumps-Pump selection depends on station layout, required pump rate, wet well depth, and pump maintenance considerations. Pump selection includes the size, type, and number of pumps. For the most part, department pump stations use vertical propeller and submersible pumps. Pump sizes are usually selected to use multiple pumps rather than a single pump of appropriate size. Smaller pumps are usually less expensive to buy and operate, and with multiple pumps the loss of one will not shut down the entire pump station. A single, large pump is more likely to have long term maintenance problems from the frequent start up required to handle flows from smaller events. The sump pump is a much smaller pump, usually designed to handle small amounts of trash or debris loading without failing.
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  • Motors-Pump motors for department pump stations are usually 480-volt, three-phase electric motors. However, the specific voltage selected depends on the power available from the utility and on what pump-motor combinations are commercially available. The size of each motor depends on the pump size, flow rate, pressure head, and duty cycle. The hydraulic engineer specifying the pumps must work together with the electrical engineer specifying the motors and the control system to insure compatibility of components.
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  • Control and Communication Systems-The control system for a pump station is more than the sensor and circuitry to activate the pumps when the water in the wet well reaches a predetermined height. The control system includes a large cabinet for the motor control center (MCC) to operate and protect all the motors in the station, separate cabinets for the variable frequency drives (VFD) for the pump motors or any motor that may be expected to operate at less than full speed, and a separate cabinet for the programmable logic controller (PLC). [NOTE: A traffic signal controller is a specialized PLC.] The PLC monitors all signals and controls the sequence of operation of the pumps, activation of the standby generator when necessary, deactivation when the flood event has passed, and operation of any night security lighting. The PLC may also include automatic communication with the District and/or Maintenance Office to report the station's status regarding water levels, pump readiness, utility electrical power status, standby generator battery status, fuel status, security, and other central office concerns. The PLC can be integrated with the ITS to warn motorists of water over the roadway in the event of extreme rain events that exceed the capacity of the pump station. The design of the controls and communications is also the province of the electrical designer. However, the design is dependent on the input information from the hydraulic designer such as wet well capacity, allowed pump discharge rate, desired pump discharge rate, and specific communications.
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  • Control Board-The pump station should have a central control board for starting or stopping some processes and verifying the various components' conditions, whether "running", "standby", or "off". In addition, although the station may be operated by a control system (PLC or other), a manual override for each component is highly recommended for maintenance and testing. This must be designed by the electrical engineer with input from TxDOT maintenance personnel.
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  • Structures-The structure must meet requirements for public safety, safety codes, local extreme weather conditions, site security, and maintenance operations. Maintenance requirements may be oversized doors to move equipment in and out or a movable roof to allow crane access. Aesthetics and the possibility of future expansion should also be considered.
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  • Discharge Conduits-The collected waters are usually discharged to a storm drain system, although sometimes the discharge point is a wetland, mud flat, or creek. The designer must consider whether the receiving location is suitable for the anticipated pump rate, whether it is available during flood events, and whether flood water discharges from the pump station are allowed.
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  • Acceptance test-A full run acceptance test should be performed successfully before the pump station is accepted. A full run test procedure consists of running the pumps at maximum capacity for at least 6 hours and testing the control systems. During this procedure, the standby generator should be used to power the full station for at least 6 hours which will test the pumps and generator at full load. The discharge conduits can be arranged with a diverter or bypass to pour the pumped water back into the wet well to maintain the full run test.
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  • Scheduled Maintenance-Pump stations, unlike other hydraulic structures, require scheduled cleaning and maintenance. The trash rack should be cleaned after each storm, while the wet well sump must be cleaned whenever the sediment reaches a set point. The standby generator must be exercised at least once a month for a minimum of 30-minutes run time. The entire system including pumps should be exercised under full load at the same schedule to assure reliability. The discharge diverter or bypass from the acceptance test should be maintained so that it can be used in the scheduled maintenance monthly test.
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