LMI Metering Pump Not Priming? How to Bleed a FASTPRIME Liquid End

LMI solenoid pumps are everywhere in water treatment, dosing hypochlorite, acids and caustic. Run one long enough and you'll eventually hear it clicking away with nothing going out. When an LMI metering pump stops delivering, in our experience it's mostly gas in the head, and the rest is usually an air leak on the suction side or a control setting. The FASTPRIME liquid end was designed to make bleeding easy, and once you know how to use it, many of these calls are over in minutes.

Know Which Liquid End You Have

The P+, PD and EXCEL AD series all come standard with a FASTPRIME liquid end. The literature describes it a little differently for each: on the P+ it's a liquid end with a bleed port, on the PD it primes quickly through an integrated bleed valve, and on the EXCEL AD it primes without depressurizing the discharge line.

There's also the AutoPrime liquid end, built for chemicals that gas off. In P+ model codes, S means a FASTPRIME head with a multifunction valve, N a FASTPRIME head only, H an AutoPrime head with a multifunction valve, and A an AutoPrime head only. In the PD valve code, S means a four-function valve is fitted. One look at the model number on the nameplate tells you which bleeding method applies to your LMI metering pump.

Why an LMI Metering Pump Won't Prime: Gas in the Head

A solenoid LMI metering pump moves only a small volume each stroke. A pocket of gas in the head means the diaphragm is just compressing it, the check valves never open, and no chemical comes up. It happens on first start-up, after a tank change, when the suction line leaks, and on gassing chemicals such as sodium hypochlorite.

Bleeding a FASTPRIME liquid end

It isn't complicated. With the LMI metering pump running, open the bleed valve on the head or the bleed knob on the multifunction valve so gas and chemical run back to the tank through the return tube. When the return tube shows a steady flow of liquid, close it. Always plumb the return tube back to the tank rather than letting it spill on the floor.

The point of FASTPRIME is that you don't remove the discharge tube or depressurize the discharge line. Many older pumps had to have the discharge tube pulled off the injection valve to bleed, which is a messy and splash-prone job on a pressurized line.

Gassing chemicals: AutoPrime and StayPrime

With hypochlorite or hydrogen peroxide that gasses constantly, a manual bleed holds for a few hours and then the head locks up again. That's when AutoPrime is worth a look. According to LMI, an AutoPrime liquid end has three check valves: suction on the bottom, discharge on the front, and a bleed check valve on top. By design, part of the chemical continuously bleeds back to the tank through the top valve, so gas is relieved automatically, and rated maximum output is somewhat lower than the same size FASTPRIME head. Mind the pressure rating too: on the P+X1, FASTPRIME is rated 17.2 bar and AutoPrime 7.6 bar.

The enhanced PD also offers StayPrime degassing: after the LMI metering pump has sat idle for a while, it starts a priming cycle on its own, which cuts the risk of losing prime.

Suction Side: Lift, Foot Valve and Weight

The B and D series LMI metering pump data sheets give a suction lift of 1.5 m of water, and limited suction ability is common to solenoid pumps. The higher the pump sits above the liquid and the longer the suction tube, the harder it is to prime, so a low position next to the tank is best.

A P+ LMI metering pump ships with an injection valve, a foot valve and a ceramic weight. The weight keeps the foot valve down in the tank so it doesn't float up and suck air, but don't let it bury itself in sludge at the bottom either, or the strainer clogs. Keep suction fittings properly tightened; any leak lets air in. The P+ is rated for a maximum fluid temperature of 50°C, and warmer chemical flashes to vapor more easily.

LMI Pump Not Priming? Check Settings and Signals

If the pump is silent or stroking very slowly, look at the settings first:

The P+ runs 1-240 spm with stroke length from 30-100%. With both turned down low, the output is too small to notice;
The P+ can be controlled remotely by 4-20 mA or pulse, and switched on and off through the 5-pin external control connector. A lost signal or a stop signal that's always present leaves it idle;
The enhanced PD (PD7) has pulse input, remote start/stop and tank level input, and a miswired remote or level signal can also keep it from stroking;
The P+7 and P+8 can take an optional low-level float switch that alarms when the tank runs low, and a stuck float gives false alarms.

A multimeter on the signal wires and a glance at the panel will find these faster than taking the head apart.

Check Valves: Single Ball or Double Ball

On the LMI metering pump range, the P+ offers single-ball or double-ball check valves, and the EXCEL AD has double-ball valves as standard. Double balls dose more consistently, but with two balls and two seats there's one more place for crystals to stick. When cleaning, note the orientation and refit everything exactly as it was, because a valve fitted upside down stops the pump delivering. Balls are ceramic and seats are typically PTFE or Polyprel; replace scored or deformed parts with new ones from the spares kit for your LMI metering pump.

Keeping Prime Day to Day

Don't wait for the tank to run empty before swapping it. At low level the pump draws air and you'll have to bleed it again afterwards. If a level switch is available, connect it so you get a warning first. On crystallizing chemicals, flush the head and check valves with clean water before a long shutdown. Keep a spares kit on site so a bad ball or seat can be changed on the spot.

Conclusion

When an LMI metering pump won't prime, work in this order. Read the model code to see whether it's FASTPRIME or AutoPrime, then bleed the head with the bleed valve or multifunction valve. Check suction lift, the foot valve, the weight and the suction fittings. Verify stroke rate, stroke length, external signals and level switches. Clean the check valves last. For gassing chemicals, go straight to AutoPrime or an enhanced PD with StayPrime.

FAQ

What's the most common reason an LMI metering pump won't prime?

Gas in the head. One bleed through the FASTPRIME bleed valve or multifunction valve brings most pumps back.

Do I have to remove the discharge tube to bleed an LMI metering pump?

Usually not with a FASTPRIME liquid end. It's designed to prime without depressurizing the discharge line; just route the return tube back to the tank.

Hypochlorite keeps air-locking my LMI metering pump. What can I do?

Consider an AutoPrime liquid end, which continuously returns some chemical and gas to the tank, or an enhanced PD with StayPrime.

Can I replace the check valves on an LMI metering pump myself?

Yes. Buy the spares kit that matches your model, stop the pump, release the pressure and wear protection before opening it, and don't refit the valves upside down.

Why did output drop after switching to AutoPrime?

That's by design. AutoPrime constantly bleeds some chemical back through the top valve, so maximum output is lower than the equivalent FASTPRIME head. Size the pump on the AutoPrime figures.

Still not dosing? Send us the model, chemical and an installation photo through our contact page and we'll take a look.