Saturday, March 13, 2010

Wastewater Lift Station Design Guidelines

Wastewater lift station structures, equipment, piping, controls, and accessories must be engineered according to City requirements, standard guidelines, and in conjunction with the Public Works Design Manual.

General Requirements:
*Lift stations will not be allowed where an acceptable alternative gravity route exists.
*All wastewater lift stations shall have multiple pumps and shall be capable of delivering the design flow rate with the largest pump out of service.
*Pumps and related equipment must be designed so that it can be removed from the wet well with a vehicle mounted crane or other lifting device.
*Lift stations may be submersible pumping stations, package wet well / dry well stations or site designed vertical, dry pit, non-clogging, centrifugal pumping stations, depending on station size, head requirements and motor horse power. All pumps must be capable of passing a minimum three (3”) inch diameter sphere and shall be single speed. They must be designed specifically for handling raw, unscreened domestic sanitary wastewater.
*Above grade stations are preferred. They shall have a finished concrete floor with floor drains and be housed in an easily removable, pre fabricated fiberglass enclosure unless otherwise specified by the City. Below grade pump stations shall be reinforced concrete and shall extend at least 6 inches above finish grade.
**Adequate access, lighting, ventilation (minimum 10 air changes per hour), heating, Net Positive Suction Head (NPSHA), and potable water supply shall be provided to all wastewater lift stations.

Wet Well Design
*Wet wells shall be considered a hazardous environment. Whenever practical wastewater lift station wet wells shall be constructed of pre-cast reinforced concrete and shall be circular to a minimum of seventy two (72”) in diameter with 4-hour capacity or as necessary to accommodate the influent sewer, provide for adequate pump suction pipe or pump submergence as recommended by the pump manufacturer and to provide adequate volume to prevent the excessive cycling of pumps.
*Every effort will be made to prevent wastewater in the wet well from becoming septic. The wet well shall contain adequate vertical room for level sensing adjustments above and below the design levels.
*Primary high water alarm shall be set to wet well influent invert.
*Wet well interior walls and ceiling shall be lined with a material that is resistant to hydrogen sulfide and sulfuric acid. (i.e. fiberglass) and shall have a water proof system if anticipated to be below the water table. Regardless of the elevation of the water table, all joints in the concrete and all penetrations through the concrete shall be grouted with non-shrink grout on both sides of the joint or penetration.
*Access to well shall be through a top slab opening with aluminum hatch cover and frame. It shall allow for the removal of all equipment from the wet well, in no case smaller than 36 by 36 inches.
*Each wet well shall contain a sump (2 feet wide and 12 inches deep minimum) immediately underneath the inlet pipe to help assist in trapping large items to prevent them from entering the pumps.
*A sixty (60”) inch diameter approach manhole shall be constructed upstream of all wet wells, serving as a common point of connection for all sanitary sewer pipes tributary to the pump station. A single pipe shall extend from the approach manhole to the wet well. The approach manhole shall be located within the site fencing of the lift station.
*Provide restrained flexible couplings on all outlet piping within two (2’) feet of the station wall and a resilient-seat gate valve on the line into the wet well.
*Provide a bypass and a magnetic flow meter on the discharge of the pump station within a vault.

Pump Selection and Design Criteria (Consult Local Requirements)
Station Type / Influent Flow Range (gpm) / Maximum TDH / Maximum Motor HP
**Packaged wet well/ dry well; Up to 3,000 gpm (influent); Up to 45 feet (TDH); 100HP@1450 rpm
**Vertical centrifugal; no restrictions for influent flow; no restrictions for TDH; No restrictions for Max. HP
**Submersible Pump; Up to 2,000gmp (influent); Up to 160 feet (TDH); 100 HP @ 1800 rpm

TDH = Total Dynamic Head (estatical lift+ minor losses (i.e. valves) + mayor losses (i.e. pipe friction).
HP = Horse Power = {(Flow (Q gpm) x TDH)/ (3956 x Efficiency of the Pump -╬Ěp)}

1. Submersible Pumps
The lift station will consist of a minimum of two submersible centrifugal sewage pumps, guide rails, wet well access, discharge seal and elbow, motor control center (MCC), starters, liquid level control system and all hardware necessary to make a complete working system.
Pump volute, impeller and motor housing shall be of cast iron construction. Submersible wastewater pumps shall be fitted with leakage sensors for detecting the presence of water in the oil and/or stator housing. (Consult pump manufacturers such as ITT Flygt, Gorman Rupp, Goulds pumps, Peerless Pumps).
Each pump will be furnished with a discharge connection system, which will permit removal and installation of pump without the need for the operator to enter the wet well.

2. Self Priming Centrifugal Pump
The lift station will employ vertical, dry pit, single stage non-clogging centrifugal sewage pumps with motors totally enclosed, fan cooled, and premium efficiency.
The pumps shall be of standard cast iron construction with ductile iron impeller, oil lubricated mechanical seal, and shall include casing wear rings to maintain sealing efficiency between the wear ring and impeller faces.
Design of lift station enclosure for vertical centrifugal stations will be coordinated with the City and Fire Departments with respect to occupancy class and electrical and HVAC system design.

Emergency Station Operation
To ensure that utility power or equipment failures do not cause sewer system overflows, provisions to maintain wastewater pump station including standby power and emergency storage shall be made.
*A diesel engine emergency electric generator shall be provided for all wastewater lift stations. An automatic transfer switch shall be provided to switch to emergency power on a power failure or a drop in any phase voltage to 70 percent of line voltage.
*Emergency storage capacity shall be provided to hold a minimum of 1 hour of peak hour design flow. The wet well, collection system and emergency storage containment can all serve as the emergency storage provided that the 1 hour requirement is met without a spill occurring. The emergency storage must be available above the high water alarm elevation in the wet well and must be continuously available without the need for an operator to switch valves or diversions.

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