Metering or dosing pumps are positive displacement pumps designed for accurate delivery of a mix of media for a wide variety of applications. Understanding their delivery capability as well as the scope of configurable functions now available offers the process design engineer an additional tool worth considering.
In the pump family there are two main segments namely, dynamic and displacement. Under displacement there are reciprocating and rotary. Metering pumps fall into the category of reciprocating and can be piston, plunger or diaphragm operated. Reciprocating metering pumps may be considered as a precise means of feeding a predetermined volume of liquid into a process or system. They also differ from other classes of pumps by having controllable stroke length and stroke rate for volume control that allows predetermined accuracy and repeatable delivery.
Generally they are designed for the handling of a wide range of liquids including those that are toxic, corrosive, dangerous, volatile and abrasive. They are positive displacement and able to provide significant delivery pressure to enable injection of the liquid into the process stream or system. To meet all these attributes there are a number of options available and careful selection based on a clear understanding of the application is necessary for reliable performance, much like the specification of a flowmeter for example. All process parameters need to be known as do details of the liquid to be dosed and consideration must be given to the pump suction requirements and limitations. Delivery side line loses and overall back pressure must be known, resulting in NPSH considerations with which the equipment supplier will be able to assist. Many accessories are available including pulsation dampers, pressure relief and keeping valves, dosing/injection valves and supply side kits that include suction filters, foot-valves and level alarms. (It should be noted that all diaphragm metering pumps are safe to run dry.)
Metering pumps today are light in weight, performance enhanced and capable of accurately operating in flow ranges from 0,4 to 4000 l/hr with size dependent delivery pressures up 100 bar. Self-venting for liquids with low vapour pressure, diaphragm rupture monitoring and stroke length/frequency detection are incorporated. Modified drive technology such as low speed single stroke control provides delivery advantages, while electronic modules provide control capability. These modules offer a selection of functions like digital or analogue inputs, batch operation, calibration, run hour totalisers, menu assistance, programmable I/O, Profibus interface, digital outputs and external connections for level and flow monitoring. Pump delivery flow monitoring and basic flow measurement are part of the available options. Displays also provide diagnostic information on pump condition and provide operating data in plain language.
Many industrial applications are found for metering pumps, in the power industry these include the dosing of chemicals for boiler tube de-scaling and all forms of water treatment such as demineralisation and effluent control and many more. A wider appreciation of the capability of modern metering pumps will enhance overall process design through elimination of unnecessary items, placing the pump in its rightful place as a piece of precision process control equipment.
For more information contact Wally Feldon, sera DoseTech SA, +27 (0)11 397 5120, [email protected], www.sera-web.co.za
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