What is gas ballast, and when is it used?

Gas ballast helps a vacuum pump handle condensable vapours. It admits a controlled amount of gas into the compression stage, helping the pump discharge vapour before it condenses inside.

On an oil-sealed rotary vane pump, this can reduce liquid contamination of the oil. The trade-off is that the pump will generally reach a higher ultimate pressure with gas ballast open than with it closed.

Why vapour can become a problem

A pump takes gas from a low-pressure inlet and compresses it before discharge. If that gas contains water vapour, compression can bring it to the point where it condenses. Water that entered as vapour then remains inside as liquid.

In an oil-sealed pump, water can mix with the oil and impair its performance. Other condensable vapours can cause similar difficulties, depending on their properties and the operating conditions.

Gas ballast introduces additional gas at a controlled point in the cycle. This helps the mixture reach discharge pressure before the vapour’s partial pressure becomes high enough for condensation. It is part of the pump’s compression process, not a vent opened on the vacuum chamber.

When might you need it?

Drying processes and equipment exposed to moisture are common examples. Gas ballast may also form part of the specified procedure for handling other vapours.

Check the pump instructions for whether ballast is required, the setting to use, operating temperature, permitted vapour load and the gas to supply. Air is not suitable for every process. An inert ballast gas may be specified, and the pump and gas supply must suit that duty.

Why pump temperature matters

Vapour is more likely to condense in a cold pump than one at its intended operating temperature. Instructions may therefore specify a warm-up period before admitting a vapour load.

The procedure varies by model. A warm-up time quoted for one pump should not be assumed to apply to another. Temperature alone does not solve the problem either: the load must remain within the pump’s capability.

Does gas ballast reduce the vacuum?

An open gas ballast usually raises the pump’s ultimate pressure. Manufacturers may give separate figures for operation with and without it. Comparing an open-ballast reading against a closed-ballast specification gives an unfair comparison.

During a process releasing vapour, accepting a higher pressure may be necessary to limit condensation. Once the vapour load has subsided, the operating procedure may allow the ballast to be closed. Follow the process conditions and equipment instructions when choosing the setting.

Can it clear contaminated oil?

For some pumps and types of vapour contamination, the manufacturer specifies running with gas ballast to help remove vapour from the oil. This cannot restore every contaminated oil charge.

Oil affected by particles, chemical degradation or heavy contamination may need replacement, and the pump may require cleaning or servicing. Cloudy oil can indicate liquid contamination, but appearance alone does not identify the cause. Use the pump and oil manufacturer’s guidance.

What gas ballast cannot do

Gas ballast has a limited vapour-handling capacity. It does not make a pump suitable for drawing in liquid, handling any solvent or accepting unlimited vapour flow. A suitable condenser, separator or trap may also be required.

Send Girovac the pump model and process details if you need help assessing the arrangement. Include the substance being handled, current ballast setting and any change in oil condition or performance.