Metering Pump Basics


(Some Provide Soup to Nuts – Here You Just Get Nuts!)

This is not intended to be a comprehensive guide to the world of metering pumps. Instead, this will be a short form guide to the equipment you’re really going to have to deal with. Some of these basics may be of value to you, but most of what we’ll go over will be of the greatest value when you are teaching your new people.


I. Piston Pumps

These pumps work best with low viscosity chemicals, preferably those with no solids or abrasives and a minimum suction lift. One of their advantages is the ability to produce a high output pressure (1,000 PSI+). Expect the packing to leak, calling for regular maintenance.

II. Diaphragm Pumps

This type of positive displacement pump comes in a number of styles. As you would expect, each has its own advantages and disadvantages.

A. Air Operated Diaphragm Pumps (AOD)

This pump typically requires up to 100 PSI and 20 SCFM of available air. The double diaphragms shuttle back and forth in their suction and discharge cycle, and the large volume of their pumping chambers literally allow them to pump a rope! They are not, however, able to pump accurately at their bottom output end – they are not really a metering pump, but rather a transfer pump.

B. Gear Pumps

Gear pumps are often platform-mounted, so they can be driven by air motors, close-coupled electric motors, or even a motor with a flexible shaft. Their intermeshing gears propel the pumped fluid between them, and are versatile in that they can pump a full range of high specific gravity and high viscosity chemicals. They can produce medium levels of output pressure (100 – 200 PSI). Generally, though, these pumps are somewhat limited in their corrosion resistance and handling of shear-sensitive products like polymers, and are not really precise in their output.

H. Hydraulically Backed Diaphragm Pumps

This pump is characterized as a lost motion device, because its output is controlled by a variable oil bypass. While the design itself is somewhat aged, there is really no significant difference between the operation of this type of pump and that of a mechanically actuated diaphragm style of pump. One advantage of this pump is its ability to produce high pressures (1,000 PSI+) although that capability is needed less and less these days. Cost and low end performance are drawbacks.

M. Mechanically Actuated Diaphragm Pumps (MAD)

A direct coupling between the motor and pump diaphragm defines a MAD pump. Like gear and hydraulically backed pumps, DC motors are often employed to work with proportional 4-20 mA DC signals. In the mid-range pumps, however, there have been some recent breakthroughs in modulated AC drives, which significantly improve their overall economics. These pumps tend to prime readily in suction lift applications, even though they perform best with a flooded suction. They work well in municipal-sized jobs.

P. Peristaltic Pumps

These pumps squeeze the products they pump – sort of like compressing and pushing to do the pumping. As a result, they are good self-primers, even though they fall down at high pressure or high output. Their maintenance frequency can also be a drawback.

S. Solenoid Pumps

This last pump has been increasingly used to pump neat, or concentrated, chemicals. Being able to control both the stroke frequency and stroke length allows these pumps to operate over a very broad output range. In addition, within their range, they are very accurate and easily controllable. All of this and good economics has produced an increase in their use during recent years.

Future Technology

Computers are of course beginning to surface in pumping technology. PLCs (Programmed Logic Controllers) are often used now to accept or provide various signals, such as voltage conversions, on/off, 4-20 mA DC, or pulse outputs. PLCs can tie different information-gathering pieces of equipment (flowmeters, pH sensors, chlorine monitors, streaming current detectors and more) together with action pieces of equipment like pumps. All of this can result in a water treatment system whose treatment parameters vary according to the constantly changing needs of the water.

One of the next things we’ll begin to see is PID control (Proportional, Integral, Derivative). This type of controller takes different input signals (for example flow and pH), weights them for importance, and summarizes them into a single pump control output signal. Now picture this type of controller talking to all of the other controllers, pumps, mixers and valves in your system. Our treatment systems will balance themselves to provide a proportional treatment, set according to the parameters we establish.

Some controllers are already able to use modems and respond to remote control via telephone. Networked systems with built-in computers are already here, and they will grow on us. It won’t be too long until this technology expands to include pumps, even though pumps have never been a leading edge kind of item. Up until now, it has been more important for a pump to keep working on and on reliably. But the computers will catch up to us – and then they’ll go wireless!