Metering Pump Fundamentals

Metering pumps are the conduit between good chemistry and a controlled process. There are limitless applications for chemical metering equipment ranging from agriculture, car wash, and water conditioning to boilers, cooling towers, and waste water.

While the fundamentals of selecting the right metering pump are common throughout all applications, there are some application-specific considerations which can be overlooked. This article is intended as a reference to pump fundamentals as well as some application specific tips- a useful refresher for some and an educational reference for others. When working with metering equipment, ensure to take appropriate safety precautions. Wear personal protective equipment (PPE) when handling chemicals, while in loud equipment rooms, and when working on electrical equipment.

Core Metering Pump Technologies

A plethora of different technologies are available when it comes to choosing a chemical feed pump. Pump technologies include: diaphragm, peristaltic, piston, progressive cavity, gear, hose and more. Alternatives to metering pumps are also worth mentioning, such as a venturi, pot feeder, and solid feeders, such as a brominator. While my background with metering equipment gives me a bias, it is important to know all options and their respective strengths and weaknesses. With this knowledge, you can ensure to choose the right equipment the first time to balance cost, performance, and reliability. Nonetheless, the scope of this article will be limited to metering pumps, specifically diaphragm and peristaltic.

Diaphragm Pump

The rubber meets the road in the head of a diaphragm pump. The core components of a diaphragm pump are: the suction valve, discharge valve, and the reciprocating diaphragm. Irrelevant the drive mechanism, solenoid or motor driven, the principals of diaphragm pumps are the same. Fundamentally, all diaphragm pumps have a suction and a discharge stroke.

The suction stroke is when the diaphragm retracts backwards from the forward position, increasing the distance between the surface of the diaphragm and the inner wall of the head. This increases the volume inside the pump head and causes a decrease in pressure, which creates a vacuum, due to the Ideal Gas Law, P1V1 = P2V2. This vacuum, or negative pressure, creates suction, which applies pressure inward which pulls down on the discharge valve, creating a seal as the check ball seats firmly in the seat. Conversely, the vacuum also pulls up on the suction valve and lifts the check ball from its respective seat, allowing the negative pressure to be equalized by the higher pressure in the suction line.

The discharge stroke is when the diaphragm moves forward or decreases the distance between the surface of the diaphragm and the inner wall of the head. This increases the pressure in the head as volume decreases. As the pressure builds, the force pushes outward evenly. This pressure pushes down on the suction valve, firmly seating the check ball. The same pressure also pushes up on the discharge valve and allows the higher-pressure fluid in the head to diffuse into the discharge line. Diaphragm pumps only feed into the process during the discharge stroke. During the suction stroke there is no output. This causes a unique pulsating output. Adjusting the output of a diaphragm pump is done by either adjusting the stroke length, which changes the volume in the head displaced during every stroke, or by changing the speed of the pump stroke.

Peristaltic Pump

Another staple in the metering pump world is the peristaltic pump. The function of the pump is similar- both dose repeatable and accurate quantities of chemical. Contrarily, the different pump technologies operate quite differently. A peristaltic pump’s head consists of a suction and discharge side. The “wetted” liquid end of the pump is a flexible piece of tubing. Positive displacement is created as the rollers inside the head contact the tubing, creating a pinch point, and rotate around, causing suction on the suction side and positive output pressure on the discharge side. The pinch points in a peristaltic pump are the “seals” in the pump and the rotation of the pinch points that causes a volume change, creating the suction and discharge pressure allowing the pump to operate.

The volume displaced from a single head revolution in a peristaltic pump is fixed based on the size of the tube, but output can be varied by adjusting the speed or rotations per minute of the pump head. Considering the suction and discharge stroke occurs simultaneously you would assume the output of a peristaltic pump to be more consistent; however, the pump operation will create varying fluid acceleration and some pressure swings as each roller goes by the discharge valve.

Functional overview of a peristaltic pump operation.

Pump Selection

These differences in technology cause one pump to excel over another in a given application. From a chemical compatibility standpoint, you typically have better coverage on a diaphragm pump when working with more aggressive chemicals. Dialing in the head material, the elastomers and the check balls typically enable you to select a wet end compatible with the chemical being dispensed into the system. Peristaltic pumps on application considerations including pressure, flow, liquid end life, compatibility, life cycle cost and accuracy tend to be slightly less when compared with diaphragm technology. However, the “self-priming” nature of a peristaltic pump makes it a favorite for many in addition to the heightened performance with small suspended solids. Suspended solids will sit on the valves inside a diaphragm pump and interfere with the check balls seating, which prevents the pump from functioning. Lastly, when it comes to suction lift capacity or the ability of a pump to lift fluid up to the pump head, a peristaltic pump performs better.

You might have been working with metering equipment for 30 years or 30 days; however, no one is perfect. If you have not already, you will eventually make a mistake on choosing the proper equipment for a process or just installing the equipment incorrectly. Some mistakes could lead to not feeding any chemicals for a month, while others might cause a full tote of chemicals to be dumped preemptively into a process. It is difficult to predict every corner case in some applications that could adversely impact your metering equipment. Everyone develops a set of “go-to” equipment and even has a typical installation setup. This can help drive consistency in your various site visits. You have a pump you are comfortable with, know how to rebuild, and maybe already have spare parts lying around in case of an emergency. Sometimes the best pump is simply the one that is there and works; however, you are not always handed accounts that fit your typical equipment selection. Be aware of the dosing equipment needs in each process; factor in for the process variables like output, pressure and compatibility. Scope out your installation conditions, and minimize unnecessary chemical tubing runs. Make sure your equipment can interface with any other existing system when applicable and be aware of any unique environmental conditions which might lead to special equipment considerations.