Frequently Asked Questions

What is forced oil flush and how do you do it?

Forced oil flush is a procedure for oil sealed rotary vane vacuum pumps. The flush replaces used oil (and chemical contaminants) with new oil. This maximizes service intervals and ensures optimum pump performance.

The video here explains and demonstrates the forced oil flush procedure.

How do I know what is in a kit?

Welch Vacuum continually updates information about our products. One example of a kit is here. The picture shows the content of a kit. The product manual for the corresponding pump will typically spell out the content within a kit.

Different pump technologies use slightly different names for kit content. Below is a general summary:


Typical Content

CRVpro Service Kit

Includes seals and high wears parts such as vanes that may need to be changed when performing maintenance.

CRVpro Seal Kit

Includes all internal pump seals which should be changed when performing maintenance. Does not include lip seal.

CRVpro Lip Seal Kit

Lip Seal between oil shaft and oil case.

CRVpro Fastening Kit

All fasteners used in a CRVpro pump.

CRVpro Oil Drain Kit

Ball Valve used in place of standard oil drain plug.

Diaphragm Pump 1, 2, 4 and 8 Headed Service Kit

Diaphragm, valves and o-rings for each diaphragm pump head.

DuoSeal and Chemstar Oil Sealed Pump Minor Repair Kit

Shaft seal, gaskets, springs, spacers for pump module.

DuoSeal and Chemstar Oil Sealed Pump Major Repair Kit

Vanes, Shaft seal, gaskets, springs, spacers for pump module.

WOB-L Complete Rebuild Kit

Connecting rod, valve plate assembly, seals, piston cup for each pump head.


What do I do if my oil sealed pump is leaking oil?

Clean up oil seen and monitor for additional oil. Check to see if oil is leaking from middle of pump (shaft seal leak) or from the exhaust filter or sides of the oil case (oil splash due to too full, or oil coalescing in an exhaust filter).

Chemical contaminants can degrade oil performance and degrade the elastomers in the lip seal. Consider the entire application, particularly the cold trap.

Welch offers several styles of exhaust filers, including a filter with drainback feature and directional exhaust filters. Filters with drainback feature coalesce the misted oil, and enable the oil to drain back into the oil case. Directional exhaust filters ensure oil mist are directed to a suitable location such as a fume hood.

Are Welch Diaphragm Pumps rated for positive pressure?

No, Welch diaphragm (membrane) dry vacuum pumps are not rated to serve as pressure pumps. These chemical duty dry vacuum pumps have very flexible membranes that are part of their ability to reach deeper vacuum. As such, they should not be compressed by attempts to use as pressure pumps, and also not be challenged by flooding inside the pump mechanism when extreme vapor loads lead to condensing fluids internally. Flooding and excess pressure can rupture membranes, cause vacuum to fail, and lead to other mechanical issues with the pump.

Welch does have 3 models that are rated for use also as pressure pumps, using WOB-L® piston technology: 2522, 2534, 2546 series. All models have pressure gauges, pressure control, and overpressure relief valves.


What is the difference between the three grades of Welch pump oil?

The performance of an oil sealed rotary vane pump is only as good as the pump oil. Welch offers three grades of oil to ensure performance of its pumps. All Welch pump oil lots are tested for performance.

Welch Duoseal® Oil is suitable for use in belt driven vacuum pumps. The DuoSeal oil is a double distilled hydrocarbon oil specifically designed for low rotational speed pumps.

Welch Premium Oil is suitable for direct drive pumps. It is a triple-distilled hydrocarbon oil using severely hydrotreated base stock and is designed to resist breakdown at higher RPMs and operating temperatures found in direct drive pumps. No additives or inhibitors are added. The hydrotreating virtually eliminates aromatics and sulfur to give good resistance to sludge and varnish formation over time in corrosive environments. The oil is light yellow in color.

Welch Gold Oil is suitable for belt-driven and direct drive pumps in applications with corrosive gasses or vapors. It is a double distilled synthetic hydrocarbon oil that has no aromatics or sulfur, and no additives or inhibitor added. The result is excellent stability in chemical environments.

Can I use oil sealed pumps to pump oxygen?

Typical atmospheric oxygen is no problem with the hydrocarbon oils used in our oil sealed pumps. However, hydrocarbon oils cannot be used for concentrated oxygen service. Welch can point you to a service center who can convert an oil sealed pump to a Fomblin® sealed pump. You must consult your authority having jurisdiction (often the facility’s safety manager, fire marshal, OSHA representative, or insurance underwriter) for compliance on flammable processes.

What is gas ballast?
Gas ballast prevents condensable vapors (e.g. water vapor) from getting trapped in the pump oil. It works by introducing a small amount of outside air into the compression stage of a rotary vane vacuum pump.
How do I get my pump serviced?

If there is a potential warranty problem, Welch wants to know about any potential problems. The service section describes our process from start to finish.

For pumps outside of the warranty period, Welch can point you to a service center who can do any service work for you.


How do I change the oil in a rotary sealed vacuum pump?

Any of our operation manuals give a process for changing the oil in an oil sealed rotary vane vacuum pump.

We have also created a video that walks you through the process of changing the oil.

How often should I change my vacuum pump oil in an oil sealed rotary vane vacuum pump?

Change your vacuum pump oil in your Welch pump when it has degraded slightly.  Change when oil is warmed up (30 minutes or more of operation with inlet "blanked off" (stoppered, forline valve closed, etc.).

Use only Welch DuoSeal or Premium or Gold Oil in DuoSeal Pumps, use only Premium or Gold oil in Direct Drive Pumps, use only Gold oil in Chemstar Belt Drive pumps.

If you can measure vacuum pressure at inlet, take a reading there as "base pressure" and record. When base presure exceeds 0.100 torr (100 microns), it's time to change the oil, since oil is part of the pump "mechanism" and compromised oil doesn't seal as well, causing elevated base pressures.  If you cannot measure inlet base pressure, start with an oil change after 1 month of service, note oil color.  Change oil when color has darkened even slightly, do not wait for dark oil or viscous, sticky blobs of polymerized oil.

How do I determine if I have adequate oil in the oil case of an oil sealed rotary vane vacuum pump?
After old oil, warmed up, is fully drained, close oil drain valve and fill in oil fill port on top of oil case, using a funnel, typically. At first the oil level will not show up in the sight glass, then as you near the full mark it will show up.  Fill until meniscus is just below the top (Oil Level) line.  If the sight glass is stained with polymerized oil, use a bright flashlight to view actual oil level.  Next, do NOT top off oil in a pump that has been unoperated for several weeks.  Oil has filled pump mechanism cavities and operating the pump will push this oil out of these void spaces and raise oil level seen in the sight glass. If after 1 hour or more of operation, oil level still appears low, then add oil to near the full line and monitor.