Many industrial liquid-handling applications require the precise dosing and injection of chemicals and other raw materials on the way to creating a usable end product. This demand for precise flow rates at predetermined schedules and pressures relies on a very specific pumping technology, one that can promote adherence to the injection schedule and liquid volumes.
With this in mind, mechanically, hydraulically and electronically actuated metering pumps have become a first-choice technology for dosing and injection applications, with today’s metering-pump technology able to deliver exact volumes according to a set schedule and at varying pressures.
However, because of the many different styles and modes of operation available, special care must be taken to specify the proper metering pump for the dosing application. When determining which pump is the best option, Neptune™, which has produced a Sizing and Selecting Metering Pumps Guide, recommends that the following variables be taken into consideration:
- Flow Rate: One tenet regarding metering-pump operation is that they should not be oversized, and they should be sized so that the maximum expected flow rate is 85% to 90% of the pump’s capacity, which will leave room to add additional capacity in the future, if needed.
- Materials of Construction: Metering pumps are available in a variety of materials, most commonly 316 stainless steel, C-20 stainless steel, PVC and Kynar® PVDF. The corrosion, erosion and solvent action of the liquid being handled, along with any suspended-particle levels that may be encountered, will help determine the best material of construction for the application.
- Liquid Makeup: Liquids can range from extremely thin to highly viscous, while some will be classified as a slurry or will off-gas when transferred. Standard metering pumps are designed to handle clear liquids with viscosities ranging from water-like to 1,500 cPs. Liquids with viscosities approaching 5,000 will require special liquid ends. “Those with viscosities more than 5,000 cPs, and contains solids, will require a different solution like a Peristaltic pump. And for liquids such sodium hypochlorite that off-gas will require automatic venting options in the wetted end of the pump. This can be achieved by using an acrylic degassing liquid end that can automatically remove the accumulated gas formed inside the liquid end.” said Gurmeet Kharbanda, Director Product Management - Neptune.
- Motors: Metering-pump motors can be powered by electricity, water, gas, air or solar power, and must be selected according to the utilities that are available. Environmental hazards in the operating area must be considered, with pumps used in harsh operating environments potentially needing to be inspected more often than those used in controlled locations. “Certain pump applications that need to operate in an hazardous area will require explosion proof motors.” said Kharbanda.
- Environment: If used outdoors, the pump must be sheltered from direct sunlight. Any pumps used in freezing temperatures can only pump liquids that will not freeze at that temperature. In indoor applications, humidity and the operational effects of any surrounding equipment must be considered.
- Method of Control: The user must know if the pump is compatible with the application’s operating cycle, which can run a range from manual continuous operation to automatic on/off operation or operation that is governed by a process signal.
While metering pumps have proven over the years to deliver the required method of operation in highly sensitive dosing and injection applications, there is a long list of important operational variables must be considered and satisfied before the proper pump can be confidently specified.