Properly using a gas ballast valve as found on a mechanical oil sealed vacuum pump has been considered by some to be an arcane art and practiced blindly much of the time. Actually, gas ballasting is a very simple operation and when properly applied can keep vacuum pumps in optimal conditions even when used in wet processes that can potentially contaminate the oil with condensation.
When vacuum pumps have a faulty gas ballast valve that has no stopping point, a variety of issues can arise. As the valve is released, it could eventually pop right off allowing the full measure of air to enter the pump. Operating with a fully-opened gas ballast increases the flow rate, while limits the ability to achieve the maximum ultimate vacuum.
You can usually find indications for the maximum water vapor a pump can handle in the vacuum pump specification sheets. This is usually measured in grams (or ounces) of water per hour. If there is a chance that the pump will be experiencing higher levels of water than the specified limitations, a condenser might be a good solution.
Oil in Vacuum Pumps
Oil is required by both piston and rotary vane vacuum pumps for a variety of operations:
- To seal the inside of pumping mechanisms and stop pressure from leaking to the low side, the input, from the high-pressure side, the exhaust.
- To lubricate the moving parts where metal works against metal, although some pumps use polymer materials.
- To carry heat from certain areas back to the oil reservoir, where it can be cooled.
- To feature a low vapor pressure that allows the creation of lower pressures as needed.
- To reduce the noise generated by the pump.
If the oil supply vital to all these processes becomes contaminated with water vapors, its efficacy is compromised. The oils will not be able to properly lubricate moving parts, cool the pump, seal pressures or reach low enough pressure points to complete the desired process. If contaminated oil is in the pump and pump is in operations, the results can be devastating. Water can cause certain vane types to swell causing premature failure, can cause internals to rust, and can limit the ability of the pump to achieve its ultimate base pressure amongst other issues.
Where Does The Water Vapor Come From?
Water vapor is in the air all around us. If the large vacuum chamber is exposed to the atmosphere this moisture will collect in monolayers within the walls inside the chamber. As the chamber is subjected to the vacuum the collected water will desorb and enter the vacuum pump.
Then there are some materials applied in the construction of vacuum furnaces that are highly porous and collect water, like the insulation or fire brick. The water vapor molecules that collect inside these porous materials become lodged in well and can it take a long time for them to be fully released. But, there is the heat from the vacuum furnace that will help to expedite the desorption process.
Where Is The Gas Ballast Valve Installed?
The visible interface of the gas ballast valve is typically a tunable knob or tap located at the ends or top of the vacuum pump. This is typically a hand operated device that can be turned into two or three positions and determines the flow of air as needed. Some gas ballast valves from specific manufacturers have three positions which are “closed”, “medium-flow” and “maximum-flow”.
There is a small passageway on the inside of the pump mechanism that leads from the exterior gas ballast valve to the compression side of the pump. This passageway can allow air to enter the compression side close to the location of the exhaust valve. The entry point to the compression chamber must be located so that the volume of gas is isolated from the volume of the pump inlet. The intake passage can be a simple hole drilled into the pump casing or fitted with a small metal pipe, depending on the situation.
A proper non-return check valve is another important component of all gas ballast valves and is located between the exterior and the intake into the stator volume. It is important to the process that air enters the pump, but we don’t want vapor mixtures and exhaust gasses to leak out through the gas ballast valve. This is prevented by the non-return check valve closing the ballast line once the point in the process where pressure exceeds atmospheric pressure has been reached and exhaust valves opened.
To avoid the added work of hand-operating the gas ballast valve, this task can be handled by a solenoid operated valve that can be connected to the main process control system and controlled automatically. This is convenient because there are stages during the process where this valve should be opened and other times when it should remain closed. There are also some processes that involve hazardous gases. Because both oxygen (O2) and water (H2O) are constituents of the air around us, they can react with some chemicals in a process, for this reason, nitrogen can be used because of its inert qualities.
What Happens When The Gas Ballast Valve Is Open?
The effects are the same when used with a rotary piston or rotary vane pump. The gas ballast can be utilized when there are medium or larger amounts of condensable vapors, like water, in the system.
When the gas ballast valve is opened it will prevent the vacuum pump from fully achieving the vacuum. Once the cycle has neared its end and a lower pressure is needed, the gas ballast can then be closed once all of the water has been pumped from the system.
At regular temperatures, water vapors will begin to desorb from the surface of the vacuum chamber once the pressure has reached about 30 Torr. Water vapor pressure at this temperature is at 18 Torr so pressures below this marking will allow the greatest amounts of water to be evolved. The combinations of water vapor and gas will enter the vacuum pump near the inlet side and will then be sent into the high-pressure side through the pumping action.
Soon the volume of dry gases and vapors will be reduced by the mechanisms which are increasing pressure on the exhaust. This mixture is combined with the partial pressures of both gas and vapor constituents that make up the sum pressure of the entire chamber. Once the gas ballast has opened air will be introduced to the chamber and restore normal atmospheric pressures.
The air entering from the outside will be made primarily of oxygen and nitrogen and the ratio of water vapor to the total pressure will drop considerably. Because the valve to the gas ballast will remain open, this water will not have a chance to condense into liquid form but will remain a vapor and be exhausted from the system.
The gas will also be compressed to the point where exhaust valves are activated and may attempt to flow back up the gas ballast line and escape. Here it will be prevented from leaving the system through the activation of the non-return check valve. These are typically simple mechanisms with a spring-loaded valve that opens to allow passage but then closes to prevent gas from escaping.
Cleaning Oil Contaminated With Water
If the oil in the systems has been contaminated with traces of condensed water vapor, gas ballasting the pump will remove these contaminants. The entire process can take up to a couple of hours, less or more depending on the size of the pump and the extent of the contamination. As the gas ballast valve is kept open greater quantities of air will enter the vacuum pump creating an oil mist at the exhaust. This mist can be separated with an oil mist filter and if it is still clean it can be reused in the machine. This process can reduce the oil levels of your pump and cause potential damage, this is something to consider if you plan on leaving the vacuum pump ballasting overnight.
If there are water droplets in the oil they will soon spread out to the oil in the entire system, Oil runs ubiquitously throughout the entire system and all mechanisms are in contact with this oil. If the oil is contaminated it will soon reach the interior of the vacuum pump at the low-pressure inlet side. Here the water will be evaporated and driven into the exhaust side of the pump where the pressure is higher, here it can be exhausted as vapor if the gas ballast is kept open. As this process continues soon all the water vapors will be purged from the system and your system will be good to go. If the oil has become very contaminated, it may change colors and become green or whitish. If this happens the only option is to drain the contaminated oil, clean the system and begin pumping in clean oil.
In the end, a gas ballast valve is a very simple device that can be utilized to drive off condensables when operated properly.