LMI Pump Selection Guide: P+ vs PD Solenoid Metering Pumps
Here's a situation we see often. An old LMI P series pump has run for years and needs replacing, and the supplier offers two options: P+ and PD. They look similar and their flow ranges overlap, so which one? This LMI pump selection guide answers exactly that.
A rough rule for LMI pump selection first. If you need 4–20 mA analog control or flows in the teens of liters per hour, look at the P+. If you need higher pressure, a screen and calibration assist, look at the PD.
Where Each Pump Sits
Both are LMI solenoid metering pumps, with a solenoid driving the diaphragm, aimed at low-flow water treatment dosing. They're simple, compact, and mount next to a tank or on a wall. No difference there.
The P+ adjusts both stroke rate, 1–240 spm, and stroke length, 30%–100%, with two knobs controlling them separately.
The PD data sheet states plainly that it's an upgrade of the P series: an improved drive design, all-new electronics, and the FastPrime liquid end as standard. It uses a constant stroke length, adjusts flow electronically, and compensates dynamically for temperature and voltage changes.
LMI Pump Selection Step One: Flow and Pressure
- Maximum flow: P+ 15.1 L/h, PD 7.6 L/h;
- Maximum pressure: P+ 17.2 bar, PD 30.6 bar (316SS liquid end);
- Maximum stroke rate: P+ 240 spm, PD 160 spm.
The P+ has six output codes. The 0.9 L/h and 1.9 L/h steps reach 17.2 bar, and pressure falls as flow rises, down to only 1.0 bar on the 15.1 L/h step.
The PD has four steps: 0.95 L/h, 2.57 L/h, 4.20 L/h and 7.57 L/h, at 30.6, 17.3, 10.2 and 4.8 bar. Note that 30.6 bar is only reached with the 316 stainless liquid end. On the same step, the PVC liquid end is 10.2 bar and PVDF is 17.3 bar.
So above 7.6 L/h, it has to be the P+. Above 17.2 bar, it has to be a stainless PD. Where both qualify, the next LMI pump selection step is control method.
LMI Pump Selection Step Two: Control Is the Biggest Difference
In LMI pump selection this is the item people miss most, and the one point this guide most wants to flag.
The P+ has four control codes: 0 is single knob (fixed rate, adjustable stroke), 1 is dual knob, 7 is pulse input plus a low-level float switch, and 8 is 4–20 mA analog input plus a low-level float switch.
The PD comes as a manual version (PD0) and an enhanced version (PD7). The enhanced version has a 2.4 inch color LCD, external inputs for pulse, remote start/stop and level, plus calibration assist, a totalizer and StayPrime degassing. The PD data sheet doesn't list a 4–20 mA analog input.
If the site doses in proportion to a 4–20 mA signal from an online analyzer, the literature points to the P+ with control code 8. If a flowmeter sends pulses, either pump works.
Liquid Ends and Degassing in LMI Pump Selection
Both offer FastPrime and AutoPrime liquid ends. FastPrime has a bleed valve for fast priming. AutoPrime suits chemicals that gas off, like sodium hypochlorite and hydrogen peroxide.
On the liquid end side of LMI pump selection, the P+ adds one more layer of choice in construction: molded head with single-ball valves, molded head with double-ball valves, or machined head with double-ball valves. Head materials are PVC, PVDF, PP and acrylic, with ceramic or PTFE balls.
The PD enhanced version's StayPrime feature is also about gas. After the pump has been idle for a while, it automatically runs a priming cycle to reduce the risk of the head losing prime.
Housing and Environment
The P+ housing is a thick-walled reinforced polyaramid thermoplastic that resists vibration and impact, with IP68 watertight electrical connectors and a maximum fluid temperature of 50 ℃. It has an on/off button on the body, so you don't have to pull the plug to stop it during routine checks.
The PD has a NEMA 4X/IP65 enclosure that the data sheet says is suitable indoors and outdoors, a universal 115–230 V supply, and ±3% steady-state accuracy and repeatability. Listed certifications are ETL and CE, with NSF 61 and 50 marked as pending at the time, and the data sheet mentions a two-year warranty.
Differences in Commissioning and Daily Use
On the P+, flow comes from the rate and stroke knobs working together. During commissioning, set stroke at a sensible position first, then fine-tune with rate, and don't take stroke below 30%. External control and the low-level switch connect through the IP68 watertight connectors, 5-pin for external control and 3-pin for low level.
The PD has fixed stroke length and adjusts only rate, so there's one less variable to manage. Calibration assist on the enhanced version is intuitive: draw down chemical from the calibration column, change the estimated value on screen to the measured one, and calibration is done. Where operator turnover is high, the PD enhanced version is quicker to learn.
Common LMI Pump Selection Pairings
Putting it together, here are quick LMI pump selection calls:
- Cooling towers and small boilers dosing from water meter pulses, up to a few liters per hour: PD enhanced;
- Automatic dosing from a residual chlorine or pH analyzer's 4–20 mA signal: P+ with control code 8;
- Very small doses into a high-pressure line: PD with stainless liquid end;
- Manual fixed dosing around ten liters per hour: P+ dual knob.
Conclusion
The core of this LMI pump selection guide is simple. The P+ offers a bit more flow, more control options and 4–20 mA input. The PD offers higher pressure, a screen and calibration assist, and suits pulse control and outdoor installation. For any LMI pump selection, rule out by flow and pressure, check the control signal, then settle the liquid end material and whether you need AutoPrime, and you'll rarely choose wrong.
FAQ
My old P series pump failed. Will a PD fit straight in?
The PD is the upgrade of the P series, but check mounting dimensions and connections against the PD data sheet. Measure the existing mounting position first, then see whether the original piping connects directly.
Does a warm chemical make a difference in LMI pump selection?
The P+ literature gives a maximum fluid temperature of 50 ℃. Above that you'd need another series. The PD data sheet gives no separate fluid temperature limit, so confirm warm chemicals with the factory for the chosen liquid end.
Why is the PD accuracy listed as ±3%?
The data sheet gives ±3% for both steady-state accuracy and repeatability, which is the PD's rated value. Where tighter accuracy is needed, look at a motor-driven digital pump such as EXCEL XR, rated at ±1% steady-state accuracy.
Can the pump stop automatically on low tank level?
The P+ with control codes 7 or 8 can take a low-level float switch and alarm. The PD enhanced version has a level input.
What comes in the box?
The P+ literature lists an injection valve, foot valve, ceramic weight and 2 m of tubing. For the PD, confirm the included accessories when ordering.
Still stuck on LMI pump selection between the P+ and PD? Tell us your dosing signal, flow and line pressure through our contact page and we'll pick the model from the data sheets with you.