Air bubbling up in reservoir.

Air bubbling up in the reservoir can be a sign of a worn metering pin and/or metering pin insert inside the pump. In a healthy pump when the pump strokes it wants to push fluid out past the outlet check valve. If clearances around the metering pin are too great, the fluid will take the path of least resistance. Instead of pushing out past the outlet check valve the fluid will just move back up into the reservoir. This would be more common on the standard pump which relies on a tight fit between the metering pin and the metering pin insert to create good pumping pressure.

There have been instances where fluid has corroded the internal passages inside of a manifold and connected the pressurized inlet air supply to the fluid supply and caused pressurized air to be fed into the reservoir.

Initial Troubleshooting Information:

• General Operation: The Unist metering pump is a pneumatically operated positive displacement pump. It requires an adequate air pressure source to properly operate the pump. Pumps can be driven by a pulse generator (PG) which cycles the pump at an adjustable rate as long as air pressure is applied to the system, or an OR element that strokes the pump once every time air pressure is applied to the system. There are two style pumps: Standard and MV. The two style pumps look different, but operate on very similar principles.
• Air Pressure: The pumps need a minimum of 60 psi to operate properly. 80-100 psi is preferred. Less than 60 psi supply pressure to the pumps may affect cycle rate (pulse generator driven pumps), the pump’s ability to stroke, and/or how quickly the pump can be cycled. Reduced air pressure can also reduce air output at the nozzle tip.
• Fluid Pressure: Most pump based systems are gravity fed from a reservoir at atmospheric pressure. If a system is fed from a pressurized tank it should have an air trap on the top of the pump stack. The air trap requires occasional maintenance to ensure proper fluid level. Pressurized fluid fed to a pump stack should typically be set at 5-10 psi. Fluid pressure should never exceed 10 psi.

Air bubbling in the reservoir can be a sign of worn pump or an internal manifold passage issue. Follow the troubleshooting flow chart below to help determine the root cause: