Selection criteria for chemical dosing systems are not as straightforward as might, at first, be assumed. Kevin Wheeler, Managing Director of chemical dosing systems specialists WES discusses the advantages and drawbacks of different pumping approaches and describes factors influencing pump choice.

Kevin Wheeler: Precise chemical dosing in industrial and water processes shares many of the characteristics of taking the right dose of medication.

Failing to take the correct medication or administering inaccurate dosages means the drug will not be at the right level in your bloodstream. Taking the medicine too soon could lead to drug levels that are too high, and missing a dose, or waiting too long between doses could lower the amount of drug in your body and keep it from working properly.

In the same way, the controlled delivery of chemicals is fundamental to many industrial processes and almost all municipal water or wastewater treatment regimes.

Challenge for system owners is to select right pump for the application

Chemical dosing is, in most cases, carried out using automated equipment, but there are different types, dozens of equipment manufacturers and hundreds of product variants available to choose from. The challenge for system owners is to select the correct one for their particular application.

A typical dosing system comprises several components, including a chemical storage tank, chemical metering pumps, and system componentry including control valves, switches, gauges, indicators and transmitters to indicate the presence or absence of flow, flow rates and operating pressures.

Chemical storage tanks are usually made from chemically-resistant plastics or stainless steel. Storage tanks selection must be based on the chemicals you intend to store and the chemical concentration.

Other tank selection criteria include:

• Internal or external location (effects of weather and ultraviolet light)
• Tank sizing dependent on fill cycle frequency
• Requirements for fill connection and level instruments
• Requirements for access to the tank top or manways for tank inspection
• Chemical compatibility and life expectancy
• Requirement for bunding as secondary spill containment
• Outlet valve and pipework configuration

Diaphragm pumps use a reciprocating action to move a flexible diaphragm, adjusting the volume of an internal tube or cavity. A known volume of chemical is drawn into the cavity through an inlet valve and then pushed out into the chemical delivery pipework through an outlet valve. Chemical flow is in a series of pulses.

Metering or dosing pumps are designed to deliver precisely measured quantities of chemical, as well as providing suction if required to extract the chemical from the storage tank and pressure to transport it through the distribution pipework to point(s) of use (see the box).

Chemical dosing pumps can be controlled manually, or via digital or analogue signals from one or more flowmeters and sensors in the process system, or via a signal from a networked SCADA system.

Components for dosing systems include pressure relief valves to protect pumps and other components from overpressure due to blockages or closed valves; pressure loading valves to maintain a small positive pressure immediately downstream of the metering pump and flow instruments to verify or measure the volume of chemical being discharged by the dosing pump.

They also include pulsation dampers which contain pressurised gas behind a diaphragm and act as shock absorbers to suppress flow and pressure spikes inherent with reciprocating chemical pumps, and calibration columns to allow manual calibration or verification of system output.

Other components take in flushing and drain connections to allow water to be flushed into the system to remove residual chemicals before maintenance; dosing lines to convey the chemical from the dosing system to the point of application; filters or strainers to prevent debris entering pumps or distribution pipework and static mixers to ensure rapid, thorough mixing of the chemical into the process fluid flow.

Injection lances / quills, meanwhile, deliver the chemical to the correct point of application into the process flow stream, and enclosures, shields and dual contained pipework ensure the safety of personnel and operators in the event of a line breakage or pump failure.

Types of pumps for dosing applications

Dosing typically uses one of three main pump types:

Piston-diaphragm pumps are applied to more arduous, high accuracy or higher pressure applications. In these pumps, the diaphragm is displaced by a secondary hydraulic fluid driven by a reciprocating piston.

Top-end options include hydraulic diaphragm pumps, which are very accurate, have a long operating life and can cope with hard-to-handle materials such as slurries.

Between these two extremes are motor-driven mechanically actuated diaphragm pumps offering reasonable capital cost, simple maintenance and good life expectancy at the expense of lower accuracy and pressure capability.

Peristaltic pumps use a rotary motor drive to move rollers over a flexible tube inside a rigid housing. The rollers squeeze the tube, forcing chemical out into the distribution pipework. The chemical flow is continuous and the absence of internal valves makes the pump more tolerant to liquids containing solids.

Their disadvantages include limited discharge pressure capability and the need for periodic replacement of the pump tube.

Progressive cavity pumps use a helical rotor turning inside a specially shaped stator. The motion of the rotor against the stator results in the creation of a series of fixed shape cavities that transport chemical through the pump to the distribution pipework.

Progressive cavity pumps work well with slurries, viscous materials and chemical products that are sensitive to shear, but have a limited range of capacity adjustment.

With over 25 years of experience, WES specialises in the design, engineering and supply of chemical dosing systems and ancillary products and services.

The range of services WES offers varies from a simple component through to the complete design, build and installation of bulk chemical dosing systems, including a wide range of readily available hire systems.

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