Vacuum Sewer Systems (cont)
Operation and Maintenance
Vacuum stations function as transfer facilities between a central collection point for all vacuum sewer lines and a pressurized line leading directly or indirectly to a treatment facility. Vacuum pumps are needed to produce the vacuum necessary for liquid/air transport. They may be either the liquid-ring or sliding-vane type, although most recent systems use the sliding vane type. Efficiency in the normal operating range is often cited as the reason for this. The optimum operating range is 16-20 in. of mercury (Hg). The vacuum pumps, however, should have the capability of providing up to 25 in. of mercury (Hg) as this level is sometimes needed during emergency conditions and in the troubleshooting process. Redundancy is required, as design capacity must be met with one pump out of service. Discharge pumps are required to transfer the liquid that is pulled into the collection tank by the vacuum pumps to its ultimate point of disposal. Dry pit pumps have been used extensively, although submersible sewage pumps located on guide rails within the collection tank may be used as an alternative. The most frequently used pump has been the non-clog type. Redundancy is required, with each pump capable of providing 100 percent of the design capacity. The level controls are set for a minimum of 2 minutes pump running time to prevent excessive pump starting and related, increased wear. The pumps should have shut-off valves on both the suction and discharge piping to allow for removal during maintenance without affecting the vacuum level. Check valves are used on each pump discharge line or on a common manifold after the discharge lines are joined to it. Equalizing lines are to be installed on each pump. Their purpose is to equalize the liquid level on both sides of the impeller so that air is removed. This ensures that the impeller is filled with liquid, which allows the discharge pump to start without having to pump against the vacuum in the collection tank. Since this setup will result in a small part of the discharge flow being re-circulated to the collection tank, a decreased net pump capacity results.
Discharge pumps are typically located at an elevation below the collection tank to minimize the net positive suction head (NPSH) requirement. In conjunction with net positive suction head requirements, pump heads (TDH) must be increased by 23 ft to account for collection tank vacuum. Both vertical and horizontal pumps can be used. Materials of construction for discharge pumps are commonly cast iron with stainless steel shafts. Cast aluminum, bronze, and brass should be avoided. Double mechanical seals, which are adaptable to vacuum service, should be used. An emergency (or standby) generator is a must. It ensures that on-lot flooding or backup will be prevented through the continuing operation of the system in the event of a power outage. Standard generators are available from a variety of manufacturers. The wastewater is stored in the collection tank until a sufficient volume accumulates, at which point the tank is evacuated. It is a sealed, vacuum-tight vessel made of carbon steel, fiberglass, or stainless steel. Fiberglass or stainless steel tanks are generally more expensive, but do not require the periodic maintenance of a carbon steel tank, which may require painting every 5 to 6 years. Vacuum, produced by the vacuum pumps, is transferred to the collection system through the top part of this tank. The part of the tank below the invert of the incoming vacuum collection lines acts as the wet well. A bolted hatch provides access to the tank should it be necessary. Most collection tanks are located at a low elevation relative to most of the components of the vacuum station. This minimizes the lift required for the sewage to enter the collection tank, since sewage must enter at or near the top of the tank to ensure that vacuum can be restored upstream. This may result in a deep basement required in the vacuum station. Vacuum switches located on the collection tank control the vacuum pumps. The usual operating level is 16-20 in. of Hg with a low level alarm of 14-in. of Hg. Seven (7) probes, one for each of the six (6) set points of the pumping cycle and one (1) as a ground, are located inside of the collection tank and control the discharge pumps.
The vacuum system control panel houses all of the motor starters, overloads, control circuitry, and the hours run meter for each vacuum and sewage pump. The vacuum chart recorder, collection tank level control relays, and fault monitoring equipment are also normally located within the vacuum system control panel. Fault monitoring systems include telephone dialers or other telemetry equipment including radio based SCADA systems, digital or fiber optic based SCADA systems and telephone based SCADA communications systems.
Vacuum gauges, required to allow the operator to monitor the system, are used on all incoming lines as well as on the collection tank. These gauges are very important in the troubleshooting procedures. Chart recorders for both the vacuum and sewer pumps are needed so that system characteristics can be established and monitored.
It is standard practice in the U.S. for the vacuum station equipment to be supplied by the vacuum manufacturer who pre-assembles and tests the equipment and then ships it to the job-site on a skid(s). These skids can then be lifted into the building and connected to the incoming vacuum mains and the outgoing force main.
The vacuum station equipment must be installed in a protective structure. Materials of construction are the choice of the consulting engineer and typically are selected to match the architecture of the surrounding community.
All of the major vacuum system components are sized according to peak flow, expressed in gallons per minute (gpm). Peak flow rates are calculated by applying a peaking factor to an average daily flow rate.