Gas Vessel or Air Chamber
This control device functions similarly to a surge tank but its potential energy is stored as compressed air. The air chamber is usually used in a high-head pumping system. It should be located close to the pumping station and inside an enclosed building. Auxiliary equipment such as compressors are also required.
A differential orifice can be installed to minimize the chamber size by creating greater head losses for inflows to the vessel than to outflows entering the system. For a system with a high friction head, one should consider optimizing the chamber by installing several clusters of probes, each throttling and/or starting (or stopping) a specific number of operating pumps. Figure 14-17: Output of Bentley HAMMER V8i Run for an Air Chamber shows the effectiveness of a gas vessel in controlling hydraulic transients.
Figure 14-17: Output of Bentley HAMMER CONNECTRun for an Air Chamber
- If the downsurge head drops to or below the pump station elevation, part of the pipeline may already be subjected to subatmospheric pressures or even a full-vacuum condition. This may defeat the purpose of an air chamber installed to protect against the downsurge.
- Normally, an air chamber requires a high static head to be practical. If the downsurge head drops to the pump station, a large upsurge head can also bounce back, considerably higher than the static head. This may also defeat the purpose of its upsurge protection.
- Air inside a gas vessel (air chamber) is always contained by a thick metal shell and separated from atmospheric pressure by piping and a reservoir. With an air-inlet valve mounted on the top, during the downsurge period a large quantity of air at atmospheric pressure can rush into the chamber. During the upsurge (or even possibly during normal operation) period, the huge pressure difference between the inside and outside of the chamber provides a high possibility that a large volume of air could escape through a leak in the inlet valve. Since an air chamber is a pressure vessel, pressure inside the chamber is many times greater than atmospheric pressure outside the chamber. The mechanical part of the air-inlet valve can leak or fail.
When a significant volume is required, two smaller gas vessels should be considered to provide redundancy whenever one unit has to be maintained, or in case one loses its gas volume and is ineffective during a transient. The following appurtenances require careful design:
- There should be two or more redundant air compressors, each equipped with a tank to store enough air at the required pressure to supply the gas vessel for short times after a power failure. Compressors should be capable of running from generators during an extended power failure if diesel fire pumps will be running.
- Level-control probes should be set for high and low level, high and low alarm, and drain or fill. Compressors should be started and stopped according to these levels. Avoid setting high- and low-level probes too close to the normal operating range to avoid spurious warnings-this can cause operators to ignore more serious low- or high-level alarms.