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Five FAQs about chemical dosing pumps
30/01/2020

To deliver liquid chemicals into a process or treatment system in a controlled manner, you need chemical dosing pumps. These are the essential bit of kit that determines how much of the chemical agent is delivered at a given pressure and over a certain period of time.

Chemical dosing pumps come in several different design formats and there's plenty to learn about choosing the correct type for the task in hand. That's why we've answered five of the most common FAQs about chemical dosing pumps. For the sake of clarity the advice we give here relates to the dosing of general chemicals for water treatment applications.

1. What types of dosing pumps are available?

The main differences between types of dosing pumps is the way the pump is driven, the construction materials of the liquid-end and how the pump output is controlled.

Diaphragm dosing pumps are the most common type available for general commercial and industrial use with millions units in operation around the world.

For general water treatment there are four main types of dosing pumps to choose from:

  • Motor driven diaphragm pumps: Powerful dosing pumps which are commonly used for dosing applications in the range of 40 – 250 litres/hour. They are ideal for almost all general/ disinfection chemicals. These pumps are often used with a simple on-off 230V input with the output controlled by manual adjustment of the stroke control, but versions are available which use a permanent 230V AC supply, controlled with 0/4-20mA, pulse frequency or volt free switch contacts.
  • Solenoid driven diaphragm pumps: An economical pump ideal for lower output/pressure uses, typically in the range of 1-20 litres/hour. Normally permanently connected to a 230V supply and controlled using volt-free switch contact. Solenoid pumps generally emit more noise than other types of dosing pumps, so the intended location of the pump must be considered
  • Stepper-motor diaphragm: A higher cost option, stepper motor pumps offer smooth output with precise digital control. They are ideal for applications which require highly variable and proportional dosing output, commonly in the range of 2 – 60 litres per hour. Stepper motor pumps are ideal for use in sensitive areas where noise and pulsation of the chemical feed line need to be minimised.
  • Peristaltic (stepper motor driven): Peristaltic pumps work very differently to diaphragm pumps, moving liquid by using a flexible dosing tube and pinch rollers, explained below. These pumps are ideal for continuous, precise dosing of very small quantities of non-abrasive chemicals at low pressure, typically at 1-1200ml per hour at 1.5 bar. Peristaltic stepper motor pumps offer a perfect solution for dosing flocculants or additives in low pressure applications such as in commercial swimming pools, drinking water and food process applications. They are also extremely easy and quick to service.

We're always happy to help you decide which is best suited to your needs - just ask if you need any help.

2. How do dosing pumps work?

Diaphragm dosing pumps

In simple terms, most dosing pumps utilise a reciprocating diaphragm (4) which forms a robust but flexible seal between the pump drive mechanism (1) and a liquid chamber or pump head (5) which is sealed against the pump body (2) using four or more set-screws (7). The diaphragm is constructed from highly durable and flexible materials, commonly PTFE and EPDM rubber. The purpose of the diaphragm is to draw-in and pressurize the dosing liquid within the pump head. The pump head is equipped with one-way suction and discharge valves which allow liquid to be drawn into the pump head by the retracting stroke of the diaphragm, then pressurised and discharged by the forward stroke of the diaphragm. The output and pressure capability of a dosing pump is essentially determined by the power of the drive mechanism, the volume of the pump head and robustness of the diaphragm and pump head.

Peristaltic dosing pumps

Peristaltic pumps work in a completely different way to diaphragm pumps. A rotating roller passes along the length of the tube, pressing it against the surface of the cylindrical tube chamber, forcing the liquid through the tube and creating a temporary seal behind it. Due to this design, the pressure generated by this type of pump is generally lower than that of diaphragm pump, so it's capability is better suited to low pressure applications below 1.5 bar.

3. What are the key considerations when selecting a dosing pump?

There are some very important points to consider when selecting a chemical dosing pump:

  • Pressure rating – the pump will need to be able to overcome the pressure at the point of injection, also allowing for head pressure losses due to the length and elevation of the chemical feed hose.
  • Output capacity – the sizing of the pump should allow for the maximum dose rate, plus 20-30% headroom. The diaphragm of a dosing pump will last much longer if the output setting of the pump is around 70% of capacity.
  • The chemical resistance of the pump – choose the pump head, diaphragm and valve options that suitable for your chemical?
  • Accelerated head pressure - The length and internal diameter of the chemical feed hose can have a considerable impact on the pump's lifespan, servicing frequency and the performance of the pump. Long lengths of feed hose can generate very high back-pressure pulses which can damage the pump diaphragm or internal components. Always follow the manufacturers/supplier's advice.
  • The type of control function - power on/off, switch contact, pulse frequency or 0/4-20mA control? Check the compatibility of the pump with the device being used to control it.
  • Make sure the pump suction and discharge valves are suitable for the media. For example, to prevent frequent valve blockages, avoid using calcium hypochlorite or other sediment containing liquids with pump valve sizes smaller than DN6 or 6x9mm hose.

We're here to help if you need any advice.

4. How should dosing pumps be located and installed?

Good quality chemical dosing pumps are designed to work in arduous conditions and in a wide range of temperatures, however, it makes sense to locate the pump in the best position to avoid extreme conditions, frequent chemical splashing or risk of physical damage. Commonly, chemical dosing pumps are installed directly onto the top of a chemical day tank, or ideally, on a pump shelf above or close to the day-tank. Here are the most important points to take on board:

  • Follow the manufacturers instructions for the orientation and mounting of the pump so that it performs correctly – make sure the pump is positioned so that the suction and discharge valves are vertically to allow any entrained air in the dosing media to be easily ejected from the pump head.
  • Make sure that the pump is positioned to allow enough space for tool access to the pump head/diaphragm during servicing.
  • Keep the pump suction hose lines as short as practicable; generally less than 2 metres is ideal.
  • Always use a foot-valve (non-return valve) and strainer screen in the suction line; it will help to stop debris from entering the pump valves and also prevent the backflow of liquid through the pump and back to the day-tank in the event of an injection valve failure.
  • Locate the local power supply isolator or socket to avoid contact with liquids or chemicals in the event of a leakage or chemical spillage. Use the correct type to suit the environmental conditions.

5. What type of pump accessories are available or recommended?

  • Pressure Relief Valves, sometimes called multi-function valves, are an important safety device and are recommended for all types of diaphragm dosing pump installations. Automatic versions often provide venting of the dosing liquid (back to the day tank) in the event of injection point blockage, and backflow prevention. Manual PRV versions are also available and may be used for chemicals such as acids, where there is no possibility of scale or sediment problems, but where release of the dosing hose pressure is necessary for safe servicing.
  • Electric and manual mixers can be used to aid dissolving of chemicals in the day tank. Tablet baskets are also available which allow tablet chemicals to be contained whilst they dissolve gradually, avoiding any potential damage to the electric mixer propeller.
  • Chemical feed lines and injection points should be labelled and colour coded for easy identification

A range of durable accessories are available to help you complete a perfect installation.

Gaffey supply the ChemiDOS range of diaphragm dosing pumps. These pumps are of the highest industrial quality and are suitable for virtually all water treatment uses. Larger sizes and additional options are too many to list here, so please don't hesitate to contact us with your requirements.

Our Product Catalogue provides a comprehensive list of pumps, spares, accessories and many other products for disinfection dosing, on-site chlorine and chlorine dioxide generation and disinfection measurement and control.

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