PAM Dosing Pump: Diaphragm Metering Pump or Progressive Cavity Pump?

"As soon as the polymer is made up, the metering pump struggles and output halves." We hear that a lot from wastewater plants and sludge dewatering sites. Picking a PAM dosing pump isn't really about brand. It's about how thick your polyacrylamide solution is, how much you need, and where it's going. Diaphragm metering pumps and progressive cavity (screw) pumps can both do the job, but each has limits. Use the wrong one and you either lose accuracy or damage the polymer.

Why PAM Is Hard on a Dosing Pump

Polyacrylamide is a long-chain polymer, and its solution gets noticeably viscous as concentration rises. When thick liquid passes a ball check valve, the ball seats slowly and doesn't seal fully, so every stroke leaks back a little and output falls.

The second problem is shear. Hard mixing or high-speed squeezing can break the polymer chains and weaken flocculation. That's why makers of polymer make-up equipment use slow-speed agitators.

The third is undissolved "fish eyes" and gel lumps. They blind the foot valve screen and lodge on valve seats, so the PAM dosing pump keeps moving while nothing comes out.

Diaphragm PAM Dosing Pump: Dilute Solution, Accurate Dosing

A diaphragm metering pump doses accurately, doesn't leak and is easy to adjust. The NEWDOSE mechanical pump manual states steady-state accuracy within ±1%. As long as the polymer is dilute and the flow is modest, a diaphragm PAM dosing pump is a comfortable choice.

The configuration matters, though. Three points make the difference:

Choose a slow-speed model. With the same diaphragm and stroke, a slower pump gives more time for suction and for the balls to seat. The NDWS-143/0.6 and NDWS-213/0.5 both use a 105 mm diaphragm and 6 mm stroke, but the first runs at 118 strokes/min and the second at 177. For viscous liquid, pick the slower one. The NDWL-275/0.7 versus NDWL-410/0.6, and the ND1000-660/0.5 versus ND1000-1000/0.4, follow the same pattern.
Fit the multi-function check valve. NEWDOSE mechanical pumps can take check valves with a PTFE spring element. The valve closes by spring force instead of gravity, and the documentation says this keeps output and linearity steady with high-viscosity liquids.
Upsize the suction line and use flooded suction. The manual says to enlarge inlet piping for viscous liquids. Mount the pump below the tank level so solution flows into the head on its own.

Be careful with solenoid pumps. The NEWDOSE solenoid manual lists excessive viscosity as a cause of low output and simply says to contact the factory. A solenoid PAM dosing pump only makes sense for very dilute solution at small flow.

Progressive Cavity PAM Dosing Pump: Thick, High Flow, Shear-Sensitive

A progressive cavity pump moves liquid forward cavity by cavity as a rotor turns inside a rubber stator. Discharge is continuous with almost no pulsation, and shear on the liquid is low. Where polymer is stronger and flow is higher, for example feeding a dewatering unit straight from the make-up tank, many sites use a screw-type PAM dosing pump.

Know its weak points too:

It isn't a metering pump. Flow is roughly proportional to speed but shifts with discharge pressure and stator wear, so for accurate control you normally add a variable frequency drive and a flow meter for closed loop;
It must not run dry. The stator can burn quickly without liquid, so the tank needs low-level protection or the pump needs dry-run protection;
The stator elastomer must suit the chemical; check with the supplier before switching products.

When specifying a screw-type PAM dosing pump, give the supplier the approximate concentration, flow and discharge pressure, and let them set speed and size for the duty rather than matching it to pipe size.

How to Choose a PAM Dosing Pump

Work through it in this order:

Dilute solution, small to moderate flow, some pressure at the injection point, accuracy needed: a diaphragm metering pump, slow-speed model with spring-loaded check valves;
Thick solution, high flow, sensitive to pulsation and shear: a progressive cavity pump with variable speed and a flow meter;
Not sure of the viscosity: don't decide yet. Take a tank of made-up solution, run a trial on site into a measuring cylinder, and see how much the diaphragm pump's real output drops.

Some sites use both. A screw pump delivers stronger stock solution, and a dilution water line on the discharge brings it to the target strength before injection.

Piping details

Polymer sticks to pipe walls and builds up. For this kind of liquid the mechanical pump manual recommends fitting valves at elbows, or replacing elbows with plugged crosses, so lines can be flushed without dismantling, plus tees and shut-off valves near the pump head so the head can be flushed. Avoid omega-shaped suction runs, since a high bend traps air. If there's a back pressure valve on the outlet, add a pulsation damper and a diaphragm pump will deliver more smoothly.

Conclusion

The right PAM dosing pump depends mainly on viscosity and flow. For dilute solution and accurate dosing, use a diaphragm metering pump: slow-speed model, PTFE spring-loaded check valves, larger suction line and flooded suction. For thick solution, high flow and shear sensitivity, use a progressive cavity pump with variable speed and a flow meter. Solenoid pumps only suit very dilute, small-flow duties. When in doubt, a quick trial on site beats calculations on paper.

FAQ

Can a solenoid metering pump handle PAM?

For very dilute solution at small flow, yes, with the pump mounted below the liquid level. Once the solution thickens, output drops noticeably and you should move to a mechanical diaphragm or progressive cavity pump.

Can a progressive cavity pump be used as a metering pump?

It can dose, but it isn't a true metering pump, since flow depends on pressure and wear. For accurate dosing, pair it with a variable frequency drive and a flow meter in closed loop.

Why does my PAM dosing pump deliver less and less?

Usually gel lumps are holding check valves open, fish eyes are blinding the foot valve screen, or the solution is being made up thicker than before. Clean the valves and screen, then check the make-up concentration.

Will turning the stroke down help a diaphragm PAM dosing pump?

A very short stroke moves less per stroke, so valve leakage takes a bigger share and accuracy gets worse. Better to choose a slower, correctly sized model and run the stroke in mid range.

Should I flush a PAM dosing pump before shutting down?

Yes. Dried polymer leaves a film on valve seats and pipe walls that can jam the valves at the next start. For pumps standing idle for a long time, circulate clean water for a while before stopping.

Tell us your polymer concentration, flow and injection point, and contact us to decide between a diaphragm pump and a screw pump.