Vacuum Priming System
Vacuum Sewers offer many advantages over gravity systems including fast installation and lower installation costs due to smaller diameter piping and shallower burial depths. The heart of vacuum sewer systems are the vacuum pumps, which are either liquid-ring vacuum pumps or lubricated sliding vane (Rotary Vane) vacuum pumps. Liquid ring vacuum pumps can be either water sealed (requiring a continual water purge), or oil-sealed (known as Vmax).
Rotary Vane vacuum pumps are the most common type being installed over the last 5 to 10 years, primarily due to an emphasis on electrical efficiency, since rotary vane pumps are approximately 10-15% more efficient (CFM per HP). Per the EPA:”On the negative side, there have been problems with the vulnerability of these pumps should water be carried into (or accumulated in) them. In this situation, the pumps must be taken out of service to remove the liquid; the result is a shortened service life. By contrast the liquid-ring pump can usually withstand an accident of this type with very little damage.”
What is usually not considered in the evaluations, however, is the life cycle costs including maintenance (labor, oil, filters, repairs) of the vacuum pumps. Rotary vane vacuum pumps, on clean service, are recommended to change oil every 500 to 750 hrs of operation (about every 3 weeks for continuous service). The Vmax only requires oil changes once every year (10,000 hrs on clean service); for vacuum sewers- every 6 months. Water accumulation in the oil (very common problem), can damage the rotary vane pump because the vanes and bearings rely on the lubricating oil for operation, whereas the Vmax oil-sealed liquid-ring vacuum pump uses external greased bearings and the oil is used for sealing, not lubrication (see How a Liquid Ring Pump Works).
The oil-sealed Vmax offers many advantages in vacuum sewer applications, when low maintenance and high reliability are important.
- Lower maintenance (oil change once per year)
- Quieter Operation (78 dBA vs. 85 dBA)
- Higher Reliability (able in ingest liquid without damage)
- Small Footprint