Vacuum Priming System
Vacuum sewers offer many advantages over gravity systems. These includes fast installation and lower installation costs due to smaller diameter piping and shallower burial depths. The heart of vacuum priming 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).
The Rotary Vane Vacuum Pump vs the Oil-Sealed Liquid Ring Vacuum Pump
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). On the negative side, there are problems with the vulnerability of these pumps if water accumulates into these pumps. Considering this situation, taking the pumps out of service removes the liquid; the result is a shortened service life. By contrast, the liquid-ring pump usually withstands accidents of this type with very little damage.
However, during evaluations of the vacuum systems, it does not consider the life cycle costs including maintenance (labor, oil, filters, repairs). The rotary vane vacuum pumps, on clean service, should 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, oil changes occur 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. However, the Vmax oil-sealed liquid-ring vacuum pump use external greased bearings and the oil seals the pump, not lubricates (see How a Liquid Ring Pump Works).
Advantages of the Vmax
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