Metering pumps are used to accurately displace a pre-determined volume of liquid within a specific time frame. You might use a metering pump for adding a chemical to a process, for example. It’s logical that most positive displacement pumps, rotary and reciprocating types, are used for metering because these pumps are designed to deliver a fixed volume of liquid per revolution or stroke of the pumping element(s).
When selecting a pump for a metering application, what do you take into consideration? Read on to find out what considerations Viking Pump recommends when sizing and selecting a pump for your metering process.
- Determine if metering accuracy requirements are realistic. When you exaggerate metering accuracy specifications, you could rule out several viable solutions to the metering problem.
- Define all possible conditions under which the pump is required to perform. It's very difficult and expensive to fix a problem resulting from application conditions that are overlooked.
- Minimize differential pressure to reduce “slip”. In this regard, avoid cavitation under all operating conditions by ensuring there is sufficient NPSHA. And, as noted in previous blog posts, reduce pump speed for viscous applications.
- With respect to the pump's normal operating speed, carefully take into consideration an application requiring very slow operating speeds. "Slip" becomes more apparent at extremely slow speeds and can be detrimental to metering accuracy.
- In discussing possible applications, be sure to point out that the pump is being considered for a metering job. More often than not, it's desirable to forego extra clearances normally applied to pump parts for high liquid viscosity at the expense of slightly higher drive horsepower requirements in the interest of greater metering accuracy.
- Consider the long-term effects of abrasive or corrosive wear on a pump’s internal parts.
- Some high temperature metering applications may require extra clearances within the pump. Remember that high temperature applications have lower viscosities. If substantial differential pressure is present, “slip” factor should be considered.
- Consider the accuracy of speed variation and repeatability of the type of drive used to power the pump. Obviously, the pump cannot be more accurate than its drive.
- Follow recommended mounting and piping practices. In spite of rugged construction, mounting and piping strains upon a pump casing can have serious effects on the pump’s ability to perform.
Metering is quite precise, so it's important to consider the right factors when sizing and selecting a pump for a metering application. Never go it alone, however. Contact your local pump distributor to receive an appropriate metering pump recommendation as some pumps are specifically designed for high-accuracy metering applications.
