Sewage pumping stations

Chapter 7: Sewage Pumping Stations

This chapter covers the design of sewage pumping stations and forcemains. General requirements such as location, types, flows, reliability, emergency operations, controls and alarms and other special design details for pumping stations are presented. Design flow criteria are provided for pumping stations serving sanitary sewer systems. The design flow criteria for pumping stations serving combined sewer systems are identified where applicable. Note that the design flows are defined in Section 8.3 - Definition of Terms.

7.1 General

7.1.1 Station Capacity

Sewage pumping stations serving sanitary sewer systems should be able to pump the design peak instantaneous sewage flow. Sewage pumping stations should be designed so that they can be upgraded to handle future peak flows from the ultimate tributary area with minor modifications (e.g. pumps, motors or impeller changes). An economic evaluation may show that there are no savings by initially providing the design peak instantaneous capacity, then increasing the capacity at a later date. It is preferred that the ultimate anticipated peak flows from the tributary area could be handled with the addition of another pump and/or forcemain and other modifications. Oversizing the pumping station may impact station operations and should be evaluated during the design.

The capacity of a sewage pumping station serving a combined sewer system should be designed to pump all of the dry weather flow (DWF) plus 90% of the volume resulting from the design peak wet weather flow (WWF) that is above the design DWF (for an average year flow) to satisfy the requirements of ministry Procedure F-5-5, Determination of Treatment for Municipal and Private Combined and Partially Separated Sewer Systems. Higher pumping capacity or other control measures may be needed for swimming and bathing beaches affected by the overflow associated with the pumping station serving combined sewers in accordance with Procedure F-5-5.

7.1.2 Flooding

Sewage pumping station structures and electrical and mechanical equipment should be protected from physical damage by the 100-year design flood event. Sewage pumping stations should remain fully operational and accessible during the 25-year flood event. Regulations/requirements of municipalities, provincial and federal agencies regarding flood plain obstructions should be considered.

7.1.3 Accessibility and Security

The pumping station should be readily accessible by maintenance vehicles during all weather conditions. The facility should be located off the traffic way of streets and alleys. It is recommended that security fencing and access hatches with locks be provided.

7.1.4 Grit

Where it is necessary to pump sewage prior to grit removal, the designer should give special consideration to the wet well and pump station piping design to avoid operational problems from the accumulation of grit. This can include divided wells (i.e., for isolation and cleaning), aerated wells (i.e., to keep grit in suspension), steep wet well sides (i.e., to reduce area for accumulation) and desludging valves on pumps (i.e., to re-suspend and pump grit).

At some pump stations it may be beneficial to use bar screens, grinders, or comminutor devices. Design of bar screen facilities should include odour control and a method for handling the screenings.

7.1.5 Safety

Sewage pumping stations should be designed in such a manner as to ensure the safety of the operators and maintenance staff in accordance with the Confined Spaces Regulation (Ontario Regulation, or O. Reg. 632/05) made under the Occupational Health and Safety Act (OSHA). Typically, the following points should be considered:

• Any moving equipment should be covered with suitable guards to prevent accidental contact;
• Equipment that starts automatically should be suitably signed to ensure that operators are aware of this situation;
• Local lockouts on all equipment should be supplied so that maintenance personnel can ensure that they are completely out of service;
• Provision of fire/smoke detectors, fire extinguishers and sprinkler systems (where appropriate);
• All stairways and walkways should be properly designed with guardrails; and
• Confined spaces should be minimized, where applicable.

It is prudent to discuss risk management issues with the utility insurer to design safety and equipment components with their input and thus reduce long term risk. Also see Section 8.9 - Safety.

7.2 Design

7.2.1 Types of Pumping Stations

There are four major types of sewage pumping stations that the designer may consider for site specific conditions: wet well/dry well, submersible, suction lift and screw pump. In a wet well/dry well pumping station, the pumps are located below grade in a dry well immediately adjacent to the wet well. In submersible pumping stations, submersible pumps are used in order to locate the pumps in the wet well, with the motor control centre mounted above-grade. Some submersible pumping stations come as a factory assembled unit with two pumps and motors pre-configured in a single well. Suction-lift pumping stations incorporate self-priming pumps in order to locate the pumps above the water level and either eliminate or decrease the depth of the dry well. Screw lift pumping stations use an Archimedean screw with the motor mounted above grade.

7.2.2 Structures

Dry wells, including their superstructure, should be completely separated from the wet well and common walls need to be gas tight.

Provision should be made to facilitate the removal of pumps, motors and other mechanical and electrical equipment. Individual pump and motor removal should not interfere with the continued operation of remaining pumps.

Suitable and safe means of access for persons wearing self-contained breathing apparatus should be provided to both dry wells and wet wells. Access to wet wells containing either bar screens or mechanical equipment requiring inspection or maintenance should conform with all safety requirements. Screens and other equipment located in pits more than 1.2 m (4 ft) deep need to be provided with stairway access.

Fresh air needs to be forced into the enclosed area (i.e., wet well). Where continuous ventilation is required at least 12 complete air changes per hour needs to be provided. Where continuous ventilation would cause excessive heat loss, intermittent ventilation of at least 30 complete air changes per hour needs to be provided when personnel enter the area. Switches for operation of ventilation equipment should be marked and located conveniently. Explosion proof gas detectors need to be provided. Also refer to Section 7.2.10 (Safety Ventilation).

For built-in-place pumping stations, a stairway to the dry well should be provided with rest landings at vertical intervals not to exceed 3.7 m (12 ft). For factory-built pump stations over 4.6 m (15 ft) deep, a rigidly fixed landing should be provided at vertical intervals not to exceed 3 m (10 ft). Where a landing is used, a suitable and rigidly fixed barrier should be provided to prevent an individual from falling past the intermediate landing to a lower level. A manlift or elevator may be used in lieu of landings in a factory-built station, provided emergency access is included in the design.

Where high groundwater conditions are anticipated, buoyancy of the sewage pumping station structures should be considered and, if necessary, adequate provisions should be made for protection.

Materials should be selected that are appropriate under conditions of exposure to hydrogen sulfide and other corrosive gases, greases, oils and other constituents frequently present in sewage. This is particularly important in the selection of metals and paints. Contact between dissimilar metals should be avoided or other provisions made to minimize galvanic action.

If more than one sewer enters a site, a junction manhole is preferred so that only one inlet to the wet well is required.

7.2.3 Pumps

Multiple pumps should be provided. Where only two units are provided, they should be of the same size, to provide a firm capacity with one unit out-ofservice and at least capable of handling the 10-year design peak hourly flow. The designer should ensure that all pumps will be subjected to hydrostatic and operating tests performed by the manufacturer.

Pumps handling flow from combined sewers should be preceded by readily accessible bar racks to protect the pumps from clogging or damage. Where a bar rack is provided, a mechanical hoist is needed. Where the size of the installation warrants, mechanically cleaned and/or duplicate bar racks should be provided. The designer is referred to Section 10.1 - Screening Devices for more information.

Pumps handling sanitary sewage from 750 mm (30 in) or larger diameter sewers should also be protected by bar racks. Appropriate protection from clogging should also be considered for small pumping stations served by smaller sanitary sewers.

Except where grinder pumps are used, pumps handling raw sewage should be capable of passing spheres of at least 80 mm (3 in) in diameter. Pump suction and discharge openings should be at least 100 mm (4 in) in diameter.

The pump should be so placed that under normal operating conditions it will operate under a positive suction head, except where suction-lift pumps are used.

Each pump should be equipped with a time totalizer and provision for automatic or manual alteration of the lead pump.

Each pump should have an individual intake. Wet well and intake design should be such as to avoid turbulence near the intake and to prevent vortex formation.

A sump pump equipped with dual check valves should be provided in the dry well to remove leakage or drainage with discharge above the maximum high water level of the wet well. All floor and walkway surfaces should have an adequate slope to a point of drainage. Pump seal leakage should be piped or channeled directly to the sump. The sump pump should be sized to remove the maximum pump seal water discharge that would occur in the event of a pump seal failure. See also Section 7.6 - Alarm Systems.

Pumping station designs should be based on system-head calculations and curves for three conditions using appropriate Hazen-Williams factor “C” as follows:

1. Low sewage level in the wet well, C = 120;
2. Median sewage level over the normal operating range in the wet well, C = 130; and
3. Overflow sewage level in the wet well, C = 140.

System-head curve (b) should be used to select the pump and motor since this will reflect the normal operating condition. The extreme operating ranges will be given by the intersections of curves (a) and (c) with the selected pump curve. The pump motor should be able to operate satisfactorily over this full range (i.e., between conditions (a) and (c)).

Although it is normal to size pumps and motors for design peak instantaneous flows, consideration should be given to how the future and ultimate sewage flow requirements can be handled. Ultimate sewage flows would account for the build-out of the catchments area. These operating points should also be shown on the system-head curves.

Where pumping stations are discharging directly to a sewage treatment plant or into a pumping station (i.e., forcemain directly into wet well of a downstream pumping station), some means of flow pacing is needed. This is provided most commonly by variable speed drives, depending upon the degree of flow pacing necessary. If even minor pump surges will have serious effects, variable speed pumps should be used. If small surges can be tolerated, two-speed or multiple speed pumps can be used.

The pumps and controls of main pumping stations and especially pumping stations discharging to or operated as part of a sewage treatment plant, should be selected to operate at varying delivery rates. In addition, where practical, such stations should be designed to deliver as uniform a flow as feasible in order to minimize hydraulic surges. The firm design capacity (with the largest unit out of service) of the pumping station serving sanitary sewers should be based on design peak instantaneous flow and should be adequate to maintain a minimum velocity of 0.6 m/s (2 ft/s) in the forcemain.

7.2.4 Electrical Equipment

Electrical systems and components (e.g. motors, lights, cables, conduits, switch boxes, control circuits) in raw sewage wet wells, or in enclosed or partially enclosed spaces where hazardous concentrations of flammable gases or vapours may be present, should comply with the Electrical Safety Code (O. Reg. 164/99) under the Electricity Act, 1998 for Class I, Zone 1(old Division 1), Group D locations. In addition, equipment located in the wet well should be suitable for use under corrosive conditions. Each flexible cable should be provided with a watertight seal and separate strain relief. A fused disconnect switch located above ground should be provided for the main power feed for all pumping stations. When such equipment is exposed to weather, it should meet the requirements of the National Electrical Manufacturers' Association (NEMA) for weatherproof equipment NEMA 3R or 4. Lightning and surge protection systems should be considered. A 110-volt power receptacle to facilitate maintenance should be provided inside the control panel for lift stations that have control panels located outdoors. Ground Fault Circuit Interruption (GFCI) protection should be provided for all outdoor outlets.

Consideration should be given to the efficiency of the pumps, motors and drives to reduce energy requirements and cost. It is recommended that evaluation of such energy efficient units include both capital and operation and maintenance costs or life-cycle costs to provide an accurate evaluation of the benefits for such equipment.

7.2.5 Controls

Sewage level control sensing devices should be so located as not to be affected by turbulent flows entering the well or by the turbulent suction of the pumps. Bubbler type level monitoring systems should include dual air compressors. Provision should be made to automatically alternate the pumps in use. Suction-lift pump stations should be designed to alternate pumps daily instead of each pumping cycle to extend the life of the priming equipment. Float controls should be at least 300 mm (12 in) vertically and 450 mm (18 in) horizontally apart and positioned against a wall away from turbulent areas.

To minimize pumping costs and wet well depth, normal high sewage level (lag pump start elevation) may be designed to be above the invert of the inlet sewer(s), provided basement flooding and/or solids deposition would not occur. Where these problems cannot be avoided, the high sewage level (lag pump start elevation) should be approximately 300 mm (12 in) below the invert of the inlet sewer.

Low sewage level (pump shut down) should be at least 300 mm (12 in) or twice the pump suction diameter (D) above the centre line of the pump volute. The bottom of the wet well should be no more than D/2, nor less than D/3 below the mouth of the flared intake elbow.

7.2.6 Valves

Shutoff valves should be placed on the suction line of dry pit pumps.

Shutoff and check valves should be placed on the discharge line of each pump (except on screw pumps). The check valve should be located between the shutoff valve and the pump. Check valves should be suitable for the material being handled and should be placed on the horizontal portion of discharge piping except for ball checks, which may be placed in the vertical run. Valves should be capable of withstanding normal pressure and high-pressure transients.

All shutoff and check valves should be operable from the floor level and accessible for maintenance. Outside levers are recommended on swing check valves.

7.2.7 Wet Wells

Where continuity of pumping station operation is critical, consideration should be given to dividing the wet well into two sections, properly interconnected, to facilitate repairs and cleaning (including automatic cleaning devices). Divided wet wells should be considered for all pumping stations with firm capacities in excess of 100 L/s (1600 USgpm).

The design fill time and minimum pump-cycle time should be considered in sizing the wet well. The effective volume of the wet well should be based on design average daily flow and a filling time not to exceed 30 minutes unless the facility is designed to provide flow equalization. Other factors that should be considered include volumes required for pump-cycling, dimensional requirements to avoid turbulence problems, vertical separation between pump and control points, sewer inlet elevation(s), capacity required between alarm levels and basement flooding and/or overflow elevations, number of and horizontal spacing between pumps. The minimum surface plan area of a wet well should be 4.9 m² (53 ft²) [i.e., 2.5 m (8.2 ft) diameter or 2.25 m (7 ft) square]. Wet wells should not provide excessive retention times, due to potential odour problems. For details of odour control the designer is referred to Section 4.4 - Odour Control and Abatement Measures.

The designer should ensure that easy and efficient removal of pumps, motors and other mechanical and electrical equipment is provided. A suitable and safe means of access for persons wearing self-contained breathing apparatus needs be provided to wet and dry wells and valve chambers. Equipment such as access hatches, ladders, service platforms, guards, grates and handrails, should be constructed of a suitable material when exposed to wet and/or corrosive conditions.

For pumping stations equipped with 50 kW (67 hp) or smaller pumps, the wet well should be of sufficient size to allow for a minimum cycle time of 10minutes for each pump. To achieve this minimum detention time in a 2-pump station using constant speed pumps, the volume in cubic meters (m³), between pump start and pump stop should be 0.15 times the pumping rate of one pump, expressed in L/s. For two-speed or variable speed pumps, pumps over 50 kW (67 hp), or for other numbers of pumps, the required volume depends on the operating mode of the pumping units. The pump manufacturer’s duty cycle recommendations should be utilized in selecting the minimum cycle time. When the anticipated initial flow tributary to the pumping station is less than the design average daily flow, provisions should be made so that the fill time indicated is not exceeded for initial flows. When the wet well is designed for flow equalization, as part of a sewage treatment plant, provisions should be made to prevent septicity.

The wet well floor should have a minimum slope of 1 to 1 to the hopper bottom. The horizontal area of the hopper bottom should be no greater than necessary for proper installation and function of the inlet. The cross-sectional area of the wet well above the benching should be constant for the full depth of the wet well.

Access to the wet well should always be from the outside. An access ladder should be provided from the top of the slab to the service platform and a separated ladder from the platform to the bottom of the well.

The opening to the wet well should be no smaller than 750 by 900 mm (30 by 36 in), or 900 mm (36 in) in diameter. The cover should be equipped with a lock and pry lip and include a safety rail around the access. The opening edge should be flush with the vertical wall of the wet well. The opening to the wet well should be on the wall giving access to float controls, bubbler lines and similar equipment, without the necessity of entering the wet well.

The need for and type of screening facilities required for pumping stations varies with the characteristics of the sewage. For submersible pumping stations, screening may not be required, but for wet well/dry well stations, it is generally accepted practice to provide screening in the form of a basket screen or a removable bar screen. Although some basket screens may be cumbersome to remove and empty, their installation provides the advantage of not requiring entry of operating staff into the wet well for cleaning operations. With basket screens, guide rails should be tubular and similar to submersible pump rails. Manually cleaned bar screens should be sloped at 60º and have 38 mm (1.5 in) clear openings. The vertical sides should be solid. The minimum width should be 600 mm (24 in). A drain platform should be provided for screenings.

All wet wells need to be provided with ventilation. Natural ventilation will usually suffice for small pumping stations where access is limited. This can be achieved through two 100 mm (4 in) diameter vent pipes. Vents should be equipped with a gooseneck at the top, extending 900 mm (36 in) above the top of the slab of the wet well. The vents should be equipped with an insect screen. One vent pipe should extend within 0.3 m (1 ft) of the crown of the inlet sewer and the other should terminate on the underside of the roof slab. Natural ventilation can be supplemented with portable ventilation units. Adequate provisions for fresh air entry of all wet wells should be followed. In some cases mechanical ventilation may be preferred (see Section 7.2.10 Safety Ventilation).

In wet well/ dry well installations, the air bubbler line (if used) and sump pump discharge should be raised above the overflow elevation and should cross between the wells below the frost line.

A service platform is normally required to allow for servicing of equipment and bar screen cleaning (if used).

7.2.8 Suction and Discharge Piping

Pump suction lines should be designed with the following features:

• Inlets consisting of 90° short radius down turned flared elbows;
• Suction velocities for 20-year or greater pumping requirements; preferably in low end of 0.8 to 2.0 m/s (2.6 to 6.6 ft/s) range;
• Flanged wall pipe with water stop collar;
• Gate valve (flanged);
• Flanged eccentric reducer; and
• Minimum pipe size of 100 mm (4 in).

Pump discharge piping should be designed with the following features:

• Velocities for the 20-year or greater sewage flow pumping needs, preferably in the low end of 0.8 to 4.0 m/s (2.6 to 13.1 ft/s) range;
• Flanged, concentric increaser;
• Spacer 150 to 300 mm (6 to 12 in) long with one flanged end and one grooved end for Victaulic coupling;
• Elbows (as necessary);
• Check valve (flanged), preferably horizontally placed;
• Gate valve (flanged);
• Flanged double branch elbow (for 2-pump station);
• Riser pipe; and
• Magnetic or other type of suitable flow meter and recorder (or pump timers for small, constant speed stations where accuracy of flow measurement is not critical - 3 timers minimum, one for each pump and one for pumps operating in parallel).

7.2.9 Dry Wells

The designer should also refer to information on dry wells in Section 7.2.2 Structures and Section 7.2.3 - Pumps. Some additional design features are listed below:

• Ventilation, heating and dehumidification equipment should be provided to protect electrical control equipment from excess moisture;
• A lifting beam complete with permanently attached trolley or hook should be provided directly above the pump/motor assembly at a minimum height of 1.2 m (4 ft) above the motors to facilitate removal of the pump motors.

7.2.10 Safety Ventilation

Adequate ventilation should be provided for all pump stations. Where the dry well is below the ground surface, mechanical ventilation is needed. If screens or mechanical equipment requiring maintenance or inspection are located in the wet well, permanently installed ventilation is needed. There should be no interconnection between the wet well and dry well ventilation systems. Also, under no circumstances should wet well vents open into a building or connect with a building ventilation system.

In dry wells over 4.6 m (15 ft) deep, multiple inlets and outlets are desirable. Dampers should not be used on exhaust or fresh air ducts. Fine screens or other obstructions in air ducts should be avoided to prevent clogging.

Switches for operation of ventilation equipment should be marked and located conveniently. All intermittently operated ventilation equipment should be interconnected with the respective pit lighting system. Consideration should be given to automatic controls where intermittent operation is used. The manual lighting/ventilation switch should override the automatic controls. For a two-speed ventilation system with automatic switch over where gas detection equipment is installed, consideration should be given to increasing the ventilation rate automatically in response to the detection of hazardous concentrations of gases or vapours.

The fan wheel should be fabricated from non-sparking material. Automatic heating and dehumidification equipment should be provided in all dry wells. The electrical equipment and components should meet the criteria described in Section 7.2.4 - Electrical Equipment.

Wet well ventilation may be either continuous or intermittent. Ventilation, if continuous, should provide at least 12 complete air changes per hour; if intermittent, at least 30 complete air changes per hour. Air should be forced into the wet well by mechanical means rather than solely exhausted from the wet well. The ventilating fan should be oriented to blow fresh air into the wet well at a point 900 mm (36 in) above the alarm level. The air change requirements should be based on 100 percent fresh air. Portable ventilation equipment should be provided for use at submersible pump stations and wet wells with no permanently installed ventilation equipment.

Dry well ventilation may be either continuous or intermittent. Ventilation, if continuous, should provide at least 6 complete air changes per hour; if intermittent, at least 30 complete air changes per hour. A system of two-speed ventilation with an initial ventilation rate of 30 changes per hour for 10 minutes and automatic switch over to 6 changes per hour may be used to conserve heat. The air change requirements should be based on 100 percent fresh air.

Additional safety consideration can be given to installing cameras in dry well areas to allow for remote monitoring.

7.2.11 Flow Measurement

Suitable devices for measuring sewage flow should be provided at all pumping stations. Indicating, totalizing and recording flow measurement should be provided at pumping stations with a 75 L/s (1200 USgpm) or greater design peak hourly flow (DPHF). Elapsed time meters used in conjunction with annual pumping rate tests may be acceptable for pump stations with a DPHF of up to 75 L/s (1,200 USgpm) provided sufficient metering is configured to measure the duration of individual and simultaneous pump operation. Overflow volumes should be measured with instrumentation that can accurately monitor and continuously integrate the flow rate for an overflow event.

7.2.12 Water Supply

There should be no physical connection between any potable water supply and a sewage pumping station which under any condition might cause contamination of the potable water supply. The supply line should be equipped with a reduced pressure principle backflow preventer^{footnote 1[1]}. Where a potable water supply is to be used for human purposes (i.e., washrooms, sinks, showers, drinking fountains, eye wash stations and safety showers), a break tank, pressure pump and pressure tank needs to be provided. Water needs to be discharged to the tank through an air gap at least 150 mm (6 in) above the maximum flood line or the spill line of the tank, whichever is higher.

7.3 Suction-Lift Pumping Stations Special Considerations

The designer should consider the applicable design recommendations provided in Section 7.2 - Design except as modified in this section.

1 Consists of two spring loaded check valves operating in series and a diaphragm-activated, pressure differential relief valve, located between the check valves. Two shutoff valves with test cocks complete the device. Recommended for high health hazard risk where it would be impractical to have an air gap separation. Malfunctioning of this device is indicated by discharge of water from the relief port. The backflow preventers require periodic inspection, maintenance and induce high pressure loss. Cannot be installed below ground level and should be protected from freezing. Space for maintenance and testing should be provided.

7.3.1 Pump Priming and Lift Requirements

Suction-lift pumps should be of the self-priming or vacuum-priming type.

Self-priming pumps should be capable of rapid priming and repriming at the “lead pump on” elevation. Such self-priming and repriming should be accomplished automatically under design operating conditions. Suction piping should not exceed the size of the pump suction and should not exceed 7.6 m (25 ft) in total length. Priming lift at the “lead pump on” elevation should include a safety factor of at least 1.2 m (4 ft) from the maximum allowable priming lift for the specific equipment at design operating conditions. The combined total of dynamic suction-lift at the “pump off” elevation and required net positive suction head (NPSH) at design operating conditions should not exceed 6.7 m (22 ft).

Vacuum-priming pumping stations should be equipped with dual vacuum pumps capable of automatically and completely removing air from the suction-lift pump. The vacuum pumps should be adequately protected from damage due to sewage. The combined total of dynamic suction-lift at the “pump off” elevation and required NPSH at design operating conditions should not exceed 6.7 m (22 ft).

Suction-lift pump stations using dynamic suction lifts exceeding the limits outlined above may be considered with the factory certification of pump performance and detailed calculations indicating satisfactory performance under the proposed operating conditions. Such detailed calculations need to include static suction-lift as measured from “lead pump off” elevation to centre line of pump suction, friction and other hydraulic losses of the suction piping, vapor pressure of the liquid, altitude correction, required NPSH and a safety factor of at least 1.8 m (6 ft).

7.3.2 Equipment, Wet Well Access and Valves Location

The pump equipment compartment should be above grade or offset and should be effectively isolated from the wet well to prevent a hazardous and corrosive sewer atmosphere from entering the equipment compartment. Wet well access should not be through the equipment compartment and should be at least 900 by 900 mm (36 by 36 in) hatch or larger. Gasketed replacement plates should be provided to cover the opening to the wet well for pump units removed for servicing. Valving should not be located in the wet well.

7.4 Submersible Pumping Stations - Special Considerations

The designer should consider the applicable design recommendations provided in Section 7.2 - Design except as modified in this section.

7.4.1 Construction

Submersible pumps and motors should be designed specifically for raw sewage use, including totally submerged operation during a portion of each pumping cycle. An effective method to detect shaft seal failure or potential seal failure should be provided. Small pre-fabricated submersible pump stations are available with pumps and motors pre-designed into a single well.

7.4.2 Pump Removal

Submersible pumps should be readily removable and replaceable without personnel entering or dewatering the wet well, or disconnecting any piping in the wet well. Pump removal should include an engineered hoist.

7.4.3 Electrical Equipment

Electrical supply, control and alarm circuits should be designed to provide strain relief and to allow disconnection from outside the wet well. Terminals and connectors should be protected from corrosion by location outside the wet well or through use of watertight seals.

The motor control centre should be located outside the wet well, be readily accessible and be protected by a conduit seal or other appropriate measures meeting the requirements of the Ontario Electrical Safety Code, to prevent the atmosphere of the wet well from gaining access to the control centre. The seal should be so located that the motor may be removed and electrically disconnected without disturbing the seal. When such equipment is exposed to weather, it should meet the requirements of weatherproof equipment NEMA 3R or 4.

Pump motor power cords should be designed for flexibility and serviceability under conditions of extra hard usage and should meet the requirements of the Ontario Electrical Safety Code standards for flexible cords in sewage pump stations. Ground fault interruption protection should be used to de-energize the circuit in the event of any failure in the electrical integrity of the cable. Power cord terminal fittings should be corrosion-resistant and constructed in a manner to prevent the entry of moisture into the cable, should be provided with strain relief appurtenances and should be designed to facilitate field connecting.

7.4.4 Valves

Required valves should be located in a separate valve chamber. Provisions should be made to remove or drain accumulated water from the valve chamber. The valve chamber may be dewatered to the wet well through a drain line with a gas and water tight valve. Check valves that are integral to the pump need not be located in a separate valve chamber provided that the valve can be removed from the wet well. Access should be provided in accordance with Section 7.2 - Design.

7.5 Screw Pump Stations Special Considerations

The designer should consider the applicable design recommendations provided in Section 7.2 - Design. Screw pumps range in size, based on the diameter of the screw, from a minimum of 0.3 m (1 ft) to a maximum of 3.7 m (12 ft). The efficiency of the screw pump increases from its minimum capacity to its rated capacity based on the fluid level in the influent well. These units are well suited for variable speed capacity operation because the rate of discharge is controlled by the fluid level at the screw inlet. No variable-speed device is required for these pumps.

7.5.1 Covers

Covers or other means of excluding direct sunlight should be provided as necessary to eliminate adverse effects from temperature changes.

7.5.2 Pump Wells

A positive means of isolating individual screw pump wells should be provided.

7.5.3 Bearings

Submerged bearings should be lubricated by an automated system without pump well dewatering.

7.6 Alarm Systems

Alarm systems with a backup power source should be provided for all pumping stations. The alarm should be activated in cases of power failure, dry well sump and wet well high water levels, pump failure, unauthorized entry, or any other cause of pump station malfunction. Pumping station alarms including identification of the alarm condition should be transmitted to a municipal facility that is staffed 24-hours a day. If such a facility is not available and a 24-hour holding capacity is not provided, the alarm should be transmitted to municipal offices during normal working hours and to the home of the responsible person(s) in charge of the pumping station during off-duty hours. Audio-visual alarm systems may be acceptable in some cases in lieu of a transmitting system depending upon location, station holding capacity and inspection frequency.

7.7 Standby Power and Emergency Operation

7.7.1 General

The designer should evaluate the need for standby power at a sewage pumping station for each specific location and should confirm this assessment with the ministry. The objective of emergency operation is to prevent (and in the case of combined sewer system to minimize) the discharge of raw or partially treated sewage to any waters and to protect public health by preventing backup of sewage and potential discharge to basements, streets and other public and private property.

7.7.2 Emergency Pumping Capability

Emergency pumping capability is required unless on-system overflow prevention is provided by adequate storage capacity. Emergency pumping capability should be accomplished by provision of portable or in-place internal combustion engine equipment, which will generate electrical or mechanical energy, or by the provision of portable pumping equipment. For engine driven generating equipment, an automatic transfer switch should be provided to allow for bypass of unit for service. Such emergency standby systems should have sufficient capacity to start up and maintain the design capacity of the pumping station. Regardless of the type of emergency standby system provided, a portable pump connection to the forcemain with rapid connection capabilities and appropriate valving should be provided outside the dry well and wet well.

7.7.3 Emergency High Level Overflows

A controlled, high-level wet well overflow to supplement alarm systems and emergency power generation should be provided for use during possible periods of extensive power outages, mandatory power reductions, or uncontrollable emergency conditions. Where a high level overflow is utilized, consideration should also be given to the installation of storage/detention tanks, or basins, which should be made to drain to the pumping station wet well. Where such overflows may affect public water supplies or other critical water uses, the ministry should be contacted for the necessary treatment or storage requirements and in the case of combined sewer overflow the application of the ministry Procedure F-5-5 to the site-specific conditions.

7.7.4 Equipment Requirements

The following general requirements should apply to all internal combustion engines used to drive auxiliary pumps, service pumps through special drives, or electrical generating equipment:

• The engine should be protected from operating conditions that would result in damage to equipment. Unless continuous manual supervision is planned, protective equipment should be capable of shutting down the engine and activating an alarm on site and as provided in Section 7.6 Alarm Systems. Protective equipment should monitor for conditions of low oil pressure and overheating, except that oil pressure monitoring will not be required for engines with splash lubrication;
• The engine should have adequate rated power to start and continuously operate under all connected loads;
• Reliability and ease of starting, especially during cold weather conditions, should be considered in the selection of the type of fuel;
• Underground fuel storage and piping facilities should be constructed in accordance with applicable provincial and federal regulations;
• The engine should be located above grade with adequate ventilation of fuel vapours and exhaust gases;
• All emergency equipment should be provided with instructions indicating the need for regular starting and running of such units at full loads; and
• Emergency equipment should be protected from damage at the restoration of regular electrical power.

7.7.5 Engine-Driven Pumping Equipment

Where permanently-installed or portable engine-driven pumps are used, the following requirements in addition to general requirements apply:

• Engine-driven pumps need to meet the design pumping requirements unless storage capacity is available for flows in excess of pump capacity. Pumps should be designed for anticipated operating conditions, including suction lift if applicable;
• The engine and pump need to be equipped for automatic start-up and operation of pumping equipment unless manual start-up and operation is justified. Provisions also need to be made for manual start-up. Where manual start-up and operation is justified, storage capacity and alarm system needs to be provided to allow time for the detection of the pumping station failure and time to setup portable equipment; and
• Where part or all of the engine-driven pumping equipment is portable, sufficient storage capacity with alarm system needs to be provided to allow time for detection of pumping station failure and setup of portable equipment.

7.7.6 Engine-Driven Generating Equipment

Where permanently-installed or portable engine-driven generating equipment is required, the designer in addition to general design recommendations in Section 7.7.4 - Equipment Requirements should consider the following:

• Generating unit size should be adequate to provide power for pump motor starting current and for lighting, ventilation and other auxiliary equipment necessary for safety and proper operation of the pumping station;
• The operation of only one pump during periods of auxiliary power supply should be evaluated and justified. Such justification may be made on the basis of the design peak hourly flows relative to single-pump capacity, anticipated length of power outage and storage capacity; and
• Special sequencing controls should be provided to start pump motors unless the generating equipment has capacity to start all pumps simultaneously with auxiliary equipment operating.

Provisions needs to be made for automatic (i.e., automatic transfer switch (ATS)) and manual start-up and load transfer unless only manual start-up and operation is justified. The generator should be protected from operating conditions that would result in damage to equipment. Provisions should be considered to allow the engine to start and stabilize at operating speed before assuming the load. Where manual start-up and transfer is justified, storage capacity and alarm system needs to be provided to allow time for the detection of the pumping station failure and time to setup portable equipment. It is standard practice, when using diesel engines, to permit them to run for not less than 60 minutes to avoid sludging and other problems. For this reason the designer should provide a standby power system with manual start up or with automatic start up utilizing an adjustable delay timer to start up during momentary power failures which would prevent the diesel engine from unnecessarily running. Timers should also be provided to bring equipment on-line in such a way that the generators will not be overloaded by the starting current requirements of motors. Similar protection will be necessary to avoid overload of the normal electrical supply on resumption of power following a power failure.

Where portable generating equipment or manual transfer is provided, sufficient storage capacity with alarm system needs to be provided to allow time for detection of pump station failure and transportation and connection of generating equipment. The use of special electrical connections and double throw switches are recommended for connecting portable generating equipment.

7.8 Operations Manual

Sewage pumping stations and portable equipment should be supplied with a complete set of operational instructions, including emergency procedures, maintenance schedules, tools and such spare parts as may be necessary. Documentation to be kept at the pumping station should confirm the level at which flooding, in particular basement flooding, will occur. This level should be provided as an elevation and also co-related to levels in the pumping station wet well. The designer is referred to Section 3.14 - Manuals & Training for more details on operations and equipment manuals.

7.9 Forcemains

7.9.1 Velocity and Diameter

At design pumping rates, a cleansing velocity of at least 0.6 m/s (2 ft/s) should be maintained or a range of between 0.6 and 1.1 m/s (2 to 3.6 ft/s); the maximum velocity should be limited to 3 m/s (10 ft/s). The minimum forcemain diameter for raw sewage should not be less than 100 mm (4 in), unless hydraulic computations are made. If the velocity in the forcemain is lower than 0.8 m/s (2.6 ft/s), hydraulic computations should be made to determine the pipe diameter if it is to be less than 100 mm (4 in), although the pipe diameter should not be less than 50 mm (2 in).

7.9.2 Air and Vacuum Relief Valves

An air relief valve should be placed at high points in the forcemain to prevent air locking. Vacuum relief valves may be necessary to relieve negative pressures on forcemains. The forcemain configuration and head conditions should be evaluated as to the need for and placement of vacuum relief valves.

7.9.3 Termination

The forcemain should enter the receiving manhole with a smooth flow transition to the gravity sewer system at a point not more than 0.3 m (1 ft) above the flow line. Corrosion protection should be provided where corrosive conditions are anticipated due to septicity or other causes. The forcemain length should be short to reduce dynamic headlosses and the production of odours and corrosive gases at initial and design flows, respectively.

7.9.4 Design Pressure

Pipe and joints should be equal to watermain strength materials suitable for design conditions. The forcemain, reaction blocking and station piping should be designed to withstand transient pressures and associated cyclic reversal of stresses that are expected with the cycling of sewage lift stations. The use of surge valves, surge tanks or other suitable means (e.g. slow closing check valves) to protect the forcemain against severe pressure changes should be evaluated. The designer should be aware of the reduced reliability of air and vacuum release valves and surge control valves when applied to sewage containing grease, grit and rags. The location of the pumping station or the forcemain should be such as to minimize intermediate high points that might result in column separation.

7.9.5 Special Considerations

Forcemain construction near streams or water works structures and at watermain crossings need to meet applicable provisions of Section 5.14 Protection of Drinking Water Systems.

The designer should provide an external bypass and portable pump connection for small pumping stations.

7.9.6 Design Friction Losses

Friction losses through forcemains should be based on the Hazen-Williams formula or other acceptable methods. When the Hazen-Williams formula is used, the value for “C” should be 100 for unlined iron or steel pipe for design. For other smoother pipe materials (i.e., such as PVC, polyethylene, lined ductile iron) a higher “C” value not to exceed 120 may be considered.

When initially installed, forcemains will have a significantly higher “C” factor. The effect of the higher “C” factor should be considered in calculating maximum power requirements and duty cycle time to prevent damage to the motor. The effects of higher discharge rates on selected pumps and downstream facilities should also be considered. In evaluating existing systems for expansion, the C-factors should be determined by actual tests wherever possible.

7.9.7 Identification

Where forcemains are constructed of material which might cause the forcemain to be confused with potable watermains, the forcemain should be appropriately identified. The designer should consider designing the sewage forcemain with materials not used for watermains at the same location (e.g. PVC) to avoid cross-connections.

7.9.8 Leakage Testing

Leakage tests should be specified, including testing methods and leakage limits.

7.9.9 Maintenance Considerations

Isolation valves should be considered where forcemains connect into a common forcemain. Cleanouts at low points and chambers for pig launching and catching should be considered for any forcemain to facilitate maintenance.

7.9.10 Cover

Forcemains should be covered with sufficient earth or other insulation to prevent freezing. The required burial depth (i.e., frost penetration depth) varies across the province from approximately 1.2 m (4 ft) to greater than 3.0 m (10 ft). The designer should refer to Chapter 6 - Challenging Conditions Affecting Servicing for information on climatic factors impacting the design of sewage forcemains.