Water and wastewater treatment, textiles, pharmaceuticals and many other industrial processes fundamentally rely on precisely controlled delivery of chemicals. However, while the requirement for dosing equipment is clear, making the right choices to specify and install the appropriate system that will deliver accurate dosing, reliability and long-term service is challenging.
Part of that challenge is the scale of equipment choices available. The wide range of industries and applications with their differing output volumes, system pressures and site conditions, is matched by the large number of original equipment manufacturers and the hundreds of product variants available.
System owners must select the right equipment for their application, site and process conditions, to achieve the required functionality at optimum CapEx and OpEx.
Considering chemical dosing
Alongside dosing pumps, components and accessories associated with the balance of system equipment such as storage tanks, valves, pipework, manifolds and control and instrumentation systems must also be considered.
A number of key criteria must be assessed when specifying chemical dosing system components. Each application will use specific chemicals at particular concentrations, so it is important that the system is specified to include materials that are capable of handling such chemicals in the concentrations to be used.
Some materials used in storage tanks, pumps and pipes can perform adequately at low concentrations but suffer greatly when exposed to higher concentration chemicals. It is also important to consider the complete system.
Even small elements with the system equipment, such as the seals and diaphragms within valves, pressure gauges and pumps, can rapidly degrade under particular conditions and thus impact reliability.
Alongside chemical concentrations, other physical properties also have an impact on materials selection. In some instances, there may be very viscous liquids or suspended solids and the system may operate at either high or low temperatures. Some chemical reactions are exothermic or perhaps produce gas which must be accommodated.
In addition, specific requirements for flow volumes and system pressures will form part of the determination of the pipework diameter, for example, and specifications of components such as tank sizes.
Any pump selected must be able to generate a sufficient head of pressure for the required lift given the physical limitations of the actual site. Dosing pumps may have to be installed high above the storage tanks.
It is also necessary to evaluate environmental conditions and constraints. For instance, storage tanks may be located externally and exposed to UV, and there are often mandatory regulatory considerations associated with chemicals, such as the need for additional spill containment.
Many processes, for instance in the domestic water supply sector, require continuous operations and redundancy in chemical metering systems. Even the availability of skilled operations and maintenance personnel should be considered and accommodated at the system design stage.
Chemical metering pumps for chemical dosing systems
At the heart of the chemical dosing system are the metering pumps that are designed to deliver precisely measured quantities of chemical. The pump extracts the chemical from its storage location and imparts sufficient pressure to transport the required volume through the distribution pipework to the required points of use.
There are three broad types of metering pump, diaphragm, peristaltic and progressive cavity. Each has particular characteristics, advantages and drawbacks.
For example, basic mechanical diaphragm pumps typically offer the lowest capital purchase cost and the simplest control and installation. Such pumps use a reciprocating action to move a flexible diaphragm, pushing a known volume of chemical into the distribution pipework in a series of pulses.
The diaphragm may be driven by an electrical solenoid, a stepper motor, or a mechanical linkage from a standard electric motor.
However, while solenoid pumps produce high discharge pressures, they typically have relatively low peak flow rates and can be noisy in operation due to their rapid oscillation. Solenoid pumps in particular also tend to have a shorter operational lifespan than higher cost alternatives.
Peristaltic pumps feature a rotor moving over a flexible tube which squeezes the chemical out. As there is no reciprocating motion the chemical flow is continuous while the absence of valves make peristaltic pumps more tolerant of liquids containing suspended solids or those which liberate gases such as the sodium hypochlorite commonly used in water treatment.
Advanced peristaltic pumps use stepper motors to produce a wide range of output capacities. Robust and easily designed for tolerance to the pumped chemical, peristaltic pumps do however produce limited discharge pressures and require periodic replacement of the pump tube.
Progressive cavity pumps
Using a helical rotor in a specially-shaped stator, progressive cavity pumps work well with slurries, viscous materials and chemical products that are sensitive to shear. But, progressive cavity pumps have a limited range of capacity adjustment and the complex geometry and requirement for extremely precise manufacturing tolerances means high capital and maintenance costs.
Hydraulic diaphragm pumps
Hydraulic diaphragm pumps are very accurate and have a long operating life as the diaphragm is supported by hydraulic fluid rather than a mechanical linkage. Diaphragm pumps are good with hard-to-handle materials such as slurries, but hydraulic diaphragm pumps are also expensive and have limited lift capacity. Such systems also have additional maintenance requirements associated with the hydraulic components and fluid.
Alongside the selection of the basic pump design, factors such as the turn-down ratio must be considered too. This can vary considerably across different models and between manufacturers. In addition, system designers and owners must consider instrumentation and controls. Increasingly, chemical dosing systems are automated and controlled via a networked SCADA system via WiFi or bluetooth interface.
Given the broad range of design choices and the multitude of suppliers and models available, developing a complex chemical dosing system is challenging. Developing a chemical dosing system which is optimised to the required process in terms of size, capabilities and costs places huge emphasis on making the right design choices and the need to gather independent advice of the kind provided by WES Ltd and others.
With the right support, the choice of the technologies, approaches and chemical dosing solutions that best meet the unique process needs can be relatively straightforward in even the most complex and demanding applications.