NLGI 3 grease follows this path: a sealed 1000 L flexible bag supplies an air-operated double-diaphragm (AOD) pump, the AOD pump feeds a progressive-cavity (PC) pump, and the PC pump supplies a dosage pump through an adjustable pressure bypass. Diagnose the path in that order. A bleed valve between the pumps removes trapped air, but it cannot correct an upstream leak or a persistent air channel in the grease.
Where does the grease path first lose continuity?
Run the AOD pump at its lowest practical cycling rate and record three observations: the condition of the grease at the bag outlet, the interstage pressure between the pumps, and the material discharged from the bleed valve. Stop at the first point where grease continuity disappears.
| Observation | Likely mechanism | Next check |
|---|---|---|
| The bleed initially passes grease, then changes to air | The AOD pump has reached an air pocket, an inlet channel, or a suction-side leak | Inspect the flexible bag and every inlet connection |
| The AOD outlet alternately expels and draws air | One or more check balls are not seating, often because the liquid chamber contains gas rather than grease | Restore a flooded inlet and prime before opening the pump |
| The AOD feeds grease alone, but interstage pressure collapses when the PC pump starts | The PC pump is withdrawing grease faster than the AOD pump supplies it during part of the AOD cycle | Compare pump rates and record interstage pressure through several strokes |
| Air continues with a sealed, flooded inlet | A suction fitting may leak under vacuum, or compressed air may cross an internally damaged diaphragm | Leak-test the inlet and inspect the AOD diaphragms |
| Priming restores operation only temporarily | Grease temporarily seals and wets the check balls, but the air-entry path remains | Find the recurring air source before changing check components |
An AOD pump depends on its check balls changing state on each stroke. During suction, the inlet check opens and the discharge check closes; during discharge, those states reverse. Gas in the liquid chamber is compressible, so chamber motion can compress and expand the gas without moving grease. Sticky grease and weak differential pressure can then leave a ball away from its seat. Filling the pump with grease restores hydraulic continuity, which explains why manual priming can make the system run without proving that the balls caused the original failure.
Is the flexible bag admitting air?
Inspect the source before dismantling either pump. In the resolved installation, the top cap on the 1000 L flexible grease bag leaked. AOD suction formed an air channel from that leak to the bag outlet. Because NLGI 3 grease did not flow back into the void, the channel remained open even after the bag stood idle. The pump then drew air through the established path instead of lifting grease.
- Stop both pumps and isolate stored pressure using the equipment procedure.
- Inspect the top cap, its seal, and any other closure that should remain airtight as the flexible bag collapses.
- Look for a continuous void or channel extending from the air-entry point toward the suction outlet.
- Remove free air from the bag using a method compatible with the bag construction.
- Close and seal the top cap before restarting the AOD pump.
- Bleed the AOD liquid side and interstage line until grease, rather than alternating grease and air, reaches the bleed point.
If air returns, observe whether it returns after a repeatable amount of bag drawdown. A recurrence tied to source level or bag shape points back to the container, pickup geometry, or cap seal. Immediate recurrence with a full, sealed bag moves the check to the inlet fittings and AOD pump.
Does the AOD inlet remain flooded and airtight?
Layer one is the physical suction path. High-viscosity grease creates a large inlet pressure loss, particularly through restrictions, long runs, small passages, sharp changes in direction, or partially obstructed fittings. The resulting vacuum makes a connection capable of holding positive pressure leak inward. Such a leak may admit air without showing grease outside the pipe.
Take a suction-pressure reading at the AOD inlet while cycling slowly. Compare the running reading with the pump manufacturer's allowable inlet condition rather than applying a generic limit. Inspect seals, clamps, hoses, pickup fittings, and the bag outlet while the pump is under suction. A pressure reading that becomes more negative as the problem develops identifies starvation or restriction. A stable flooded condition with continuing air shifts the decision to an inlet leak or internal diaphragm failure.
Heating the material to 30 or 40 degrees was proposed as a way to reduce viscosity, but those values are not a process specification. Change temperature only after checking the grease supplier's permitted range, the bag rating, seal compatibility, and the filling process requirements. Lower viscosity can reduce inlet loss, but it will not seal a leaking cap or repair a damaged diaphragm.
Are the AOD checks or diaphragms actually failing?
Inspect the nitrile balls and diaphragms only after restoring a sealed, flooded source. Otherwise, source-side air can imitate a check-valve defect. With the pump isolated and depressurized, examine each ball and seat for grease deposits, deformation, cuts, swelling, and incomplete travel. Verify that nitrile is compatible with the grease formulation and operating temperature using the component supplier's compatibility data.
A check problem becomes the leading cause when the inlet stays flooded, no suction fitting leaks, and reverse motion at the outlet continues after correct priming. If the pump manufacturer offers a spring-loaded check arrangement for this service, it can help establish positive seating when downstream backpressure falls between strokes. Do not add springs or substitute balls without the pump manufacturer's approved configuration; added closing force also increases the pressure needed to open the check in viscous service.
A diaphragm breach creates a different path: compressed operating air can enter the grease chamber. Isolate the PC pump and test the AOD pump with a sealed, flooded inlet. Persistent gas generation that is independent of the source container calls for diaphragm inspection. Also examine the air exhaust for signs of grease, since product on the air side indicates loss of diaphragm separation.
Can the AOD pump keep the PC pump supplied?
Both pumps displace material, but their instantaneous flow patterns differ. The PC pump draws continuously while the AOD pump delivers in strokes. Between AOD discharge pulses, the PC inlet can lose pressure. If the PC pump removes even slightly more grease than the AOD pump supplies over time, interstage pressure trends downward until the PC pump pulls on a gas pocket, opens an inlet leak path, or starves.
| Reported setting or requirement | What must be identified | Engineering interpretation |
|---|---|---|
AOD pump running at 3 bar
|
Determine whether this is compressed-air supply pressure or measured grease discharge pressure | The value cannot be compared directly with the PC discharge requirement until its measurement point and medium are known |
PC pump discharge at 5 bar
|
Measure at the dosage-pump inlet under an actual filling cycle | The PC pump creates downstream pressure while its inlet remains at the interstage condition |
| Adjustable bypass | Record bypass position, return path, and pressure response as dosage demand changes | The bypass regulates downstream pressure but does not replace adequate PC inlet supply |
| AOD stroke rate | Record cycles and interstage pressure variation | A low cycling rate may improve check seating and reduce severe pulses, but average feed must still exceed PC withdrawal |
Fit a pressure instrument between the pumps and observe both the average value and the stroke-to-stroke minimum. A falling average indicates a capacity mismatch or inlet starvation. A stable average with deep periodic troughs indicates pulsation and insufficient interstage storage. A steady positive interstage condition with air still appearing redirects the check to the source, suction seals, or AOD diaphragm.
Will a pulsation damper correct the problem?
A diaphragm-type pulsation damper close to the AOD discharge can attenuate pressure fluctuations presented to the PC inlet. It does not remove entrained air and cannot correct the leaking cap that caused the loss of prime. Treat it as an interstage stability measure after the material path remains airtight.
NLGI 3 grease moves slowly through restrictive damper ports, so damper selection must account for grease rheology, AOD displacement per stroke, cycling rate, interstage operating pressure, pipe volume, and the allowable pressure variation at the PC inlet. The installation called for a relatively large damper, but no volume can be calculated from the supplied values. Obtain the damper size and precharge procedure from its manufacturer using measured pressure traces and pump displacement data.
Place the discharge damper as close as practical to the AOD outlet so the pulse reaches the compliant volume before traveling through the interstage line. A suction-side damper may help where inlet piping dynamics disturb the AOD inlet, but it cannot supply grease through a persistent bag channel. After installation, compare minimum, maximum, and average interstage pressure at the same pump rates used before installation.
How should the resolving branch be started and verified?
- Isolate both pumps and relieve material and air pressure according to the equipment procedure.
- Repair and close the leaking top cap on the 1000 L flexible bag. Remove trapped air from the bag without creating another open path to atmosphere.
- Check the bag outlet, suction hose, fittings, and seals for restrictions or inward leakage.
- Fill the AOD liquid chambers with grease if needed to restore check-ball wetting and prime.
- Open the interstage bleed and cycle the AOD pump slowly. Continue until the discharge contains uninterrupted grease.
- Close the bleed and run the AOD pump alone. Confirm that it continues drawing from the bag without forming a new air channel.
- Start the PC pump at a low rate. Record interstage pressure through multiple AOD strokes and adjust the pump-rate relationship so the average pressure does not decay.
- Operate the dosage pump under its real filling cycle and adjust the bypass to hold the required
5 barat the stated downstream measurement point. - If stroke-to-stroke pressure variation remains unacceptable while the grease stays air-free, size and install the interstage pulsation damper, then repeat the pressure trace.
Frequently Asked Questions
How do I stop an NLGI 3 grease pump from sucking air?
Seal the flexible bag cap and all suction connections, remove free air from the bag, prime the AOD pump with grease, and bleed the interstage line. In this installation, closing the leaking top cap after evacuating the bag removed the persistent air channel.
How do I tell whether the AOD check balls are stuck?
First prove that the inlet remains flooded and airtight. If the outlet still reverses flow after correct priming, isolate the pump and inspect the nitrile balls and seats for deposits, damage, swelling, or incomplete travel.
How do I match an AOD pump to a progressive-cavity pump?
Measure pressure between the pumps while both run and compare the average and stroke-to-stroke minimum. Reduce PC withdrawal or increase dependable AOD delivery when the average decays; address pulsation when the average is stable but the minimum repeatedly collapses.
How do I verify the grease pumping fix?
Run the bag, AOD pump, PC pump, bypass, and dosage pump through repeated production cycles. The final verification is uninterrupted air-free grease at the bleed point, no renewed channel from the sealed cap, stable interstage pressure, and 5 bar at the dosage-pump inlet during filling.