Choosing the Right Chemical Metering Pump

Diaphragm Pump

Cooling tower water treatment systems “bleed,” or “blowdown” system water to reduce the level of total dissolved solids (TDS). Additionally, chemicals are added to prevent corrosion, scale, and microbial buildup on piping and cooling tower surfaces.  Corrosion is an obvious concern due to the high cost of system component replacement and downtime. Any buildup on surfaces will reduce system efficiency by restricting flow and preventing optimal heat transfer. Chemicals are injected into the system by chemical metering pumps, also referred to as chemical dosing pumps.  Each chemical has its own dedicated pump, which must be selected appropriately for the specific chemical used, required flow rate, and discharge pressure.

Peristaltic PumpDiaphragm Pump

Factors to Consider

There are several factors to consider when selecting the appropriate chemical metering pump for your application. To begin with, the pump needs to provide a flow rate that is adequate. In other words, it must be sized correctly.  Ideally, the maximum desired flow rate should be around 85% of the pump’s actual maximum capacity while the minimum desired flow rate should not be less than 10% of it.  Unlike other types of pumps, metering pumps should never be oversized. The desired pump capacity is determined by the site’s water treatment professional, based on local conditions, system size and usage. An equally important factor to consider is chemical compatibility.  There are several types of chemicals that may be used in a cooling tower water treatment system. Corrosion/scale inhibitors, algaecides, biocides and pH adjusters all have different chemical compositions and characteristics. It is crucial that the pump, fitting and line materials are selected appropriately. Manufacturers provide compatibility charts which show what materials perform best with any given chemical. Certain chemicals form gas when agitated which leads us to the two pump types used for water treatment chemical injection.    

Peristaltic Pumps

A peristaltic pump, also known as a hose pump, tube pump, or roller pump uses a flexible tube fitted inside a circular pump casing. Rollers compress the tube, trapping the chemical between them. The chemical is moved through the tube, toward the pump outlet. Peristaltic pumps are effective when pumping gas forming fluids since the gas is simply pushed through the tube.  Peristaltic pumps are simple and easy to use. They are self-priming and can handle more viscous chemicals since there are no check valves to become clogged.  They also operate better against higher back-pressure.

Peristaltic Pump Operation

Peristaltic pumps have one major drawback. The repeated squeezing eventually weakens the hose. The pump’s capacity will diminish over time, resulting in more energy use. There is also the possibility that after extended use the hose may rupture.  Manufacturers rate these pumps for a specific maximum runtime which should not be exceeded.

Diaphragm Pumps

A diaphragm pump, also known as a membrane pump, uses a reciprocating rubber or plastic diaphragm in combination with check valves. When the diaphragm expands, the pressure decreases and chemical is drawn into the chamber. When the diaphragm compresses, the pressure increases and the chemical previously drawn into the chamber is forced out the pump outlet. Although they are more complicated that peristaltic pumps, with proper maintenance they are cheaper to operate long term, are more efficient and less likely to leak.

Diaphragm Pump Operation

Diaphragm pumps have a few drawbacks. They use check valves, which can become clogged if not properly maintained. Also, they have a pumping cycle that consists of a suction and discharge phase, which can cause gas buildup in the pump head when used with certain chemicals. This buildup of gasses can result in vapor lock, or loss of prime.  This problem can be alleviated by using a degassing head option, which relieves gas buildup from the discharge outlet.

We offer Pulsafeeder and Seko chemical pumps.  Get in touch with us today and we’ll help you select the right pump for your application. 

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