Engine-Driven Generating Equipment

Where permanently-installed or portable engine-driven generating equipment is used, the following requirements in addition to general requirements shall apply.

3.3.5.1 Generating Capacity

Generating unit size shall 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 must be justified. Such justification may be made on the basis of maximum anticipated flows relative to single-pump capacity, anticipated length of power outage and storage capacity. Special sequencing controls shall be provided to start pump motors unless the generating equipment has capacity to start all pumps simultaneously with auxiliary equipment operating.

3.3.5.2 Operation

Provisions shall be made for automatic and manual start-up and load transfer. The generator must 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 systems must meet requirements of Section 3.3.4.3

3.3.5.3 Portable Generating Equipment

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

3.4 INSTRUCTIONS AND EQUIPMENT

The operating authority of sewage pumping stations shall be supplied with a complete set of operational instructions, including emergency procedures, maintenance schedules, tools and such spare parts as may be necessary.

3.5 FORCE MAINS

3.5.1 Velocity

At design average flow, a cleansing velocity of at least 0.6 metres per second shall be maintained.

3.5.2 Air Relief Valve and Blowoff

An automatic air relief valve shall be placed at high points in the force main to prevent air locking. Drain or blowoff valves should be provided at all low points in pressure sewers.

3.5.3 Termination

Force mains should enter the gravity sewer system at a point not more than 0.6 m above the flow line of the receiving manhole. A 45° bend may be considered to direct the flow downward.

3.5.4 Design Pressure

The force main and fittings, including reaction blocking, shall be designed to withstand normal pressure and pressure surges.

3.5.5 Size

Force mains shall be sized to provide sufficient flow velocity, required capacity at the available head and to withstand operating pressures as outlined in Sections 3.5.1 and 3.5.4. In general, force mains shall be a minimum of 100 mm in diameter.

3.5.6 Slope and Depth

Force main slope does not significantly affect the hydraulic design or capacity of the pipeline itself. Under no circumstance, however, shall any force main be installed at zero slope. Zero slope installation makes line filling and pressure testing difficult, and promotes accumulation of air and wastewater gases.

A forcemain should have a minimum cover of 1.8 m.

3.5.7 Special Construction

Force main construction near watercourses or used for aerial crossing shall meet applicable requirements of Sections 2.9 and 2.10.

3.5.8 Design Friction Losses

Friction losses through force mains shall be based on the Hazen Williams formula or another acceptable method. When the Hazen Williams formula is used, the following values for "C" shall be used for design.

Unlined iron or steel - 100

All other - 120

When initially installed, force mains will have a significantly higher "C" factor. The "C" factor of 120 should be considered in calculating maximum power requirements for smooth pipe.

3.5.9 Separation from Water Mains

Water mains and sewage force mains are to be installed in separate trenches. The soil between the trenches shall be undisturbed. Force mains crossing water mains shall be laid to provide a minimum vertical distance of 450 mm between the outside of the force main and the outside of the water main. The water main shall be above the force main. At crossings, one full length of water pipe shall be located so both joints will be as far from the force main as possible. Special structural support for the water main and the force main may be required.

3.5.10 Identification

Where force mains are constructed of material which might cause the force main to be confused with potable water mains, the force main should be appropriately identified.

3.6 TESTING

3.6.1 General

The entire length of a force main shall be tested for leakage. If the length of a force main exceeds 400 m, the allowable leakage must not exceed the allowable leakage for a similar force main 400 m in length. All valves in the force main must be opened immediately prior to testing.

3.6.2 Leakage Test

The force main shall be filled with water, and a test pressure of 1035 kPa or equal to 1.5 times the working pressure shall be applied, measured at the lowest point in the test section. The pressure shall be maintained by pumping water from a suitable container of known volume. The amount of water used for a period of two hours shall be recorded.

3.6.3 Allowable Leakage

Allowable leakage for a force main shall be determined by the following formula: L = (SD) x P0.5

727,500 where:

L = allowable leakage in litres/hour S = length of pipe in metres

D = nominal diameter of pipe in mm P = test pressure in kPa

Allowable leakage for closed metal seated valves is 1.2 mL per mm of nominal valve diameter per hour. The maximum test section should be 400m or as directed by the regulatory agency having jurisdiction.