In addition to selecting wetted end materials to handle the liquid to be pumped, several other factors need to be considered when a specifying a metering pump. We have prepared the following list to help you make the right metering pump choice that will satisfy the needs of your application.
In our first blog article on specifying a metering pump we stressed the importance of matching the liquid to be pumped with wetted end materials of construction that can safely and economically handle the liquid. Of course you will also want to prevent leaks and spills, and specifying appropriate materials will accomplish that goal. As a reminder, the wetted end is any part of the pump that comes into contact with the liquid being pumped.
Here are some additional factors to take into consideration as you are specifying a metering pump:
- Flow rate requirements: how much liquid needs to be pumped? What is the maximum and minimum volume required? Obviously the pump model selected will need to have a capacity to handle the specified range. The normal turndown ratio for a Madden metering pump is 10:1. That means that a pump with a maximum capacity of 60 gallons per hour will be adjustable down to as low as 6 gallons per hour. If the required flow range is outside the flow rate capacity of any single pump, then multiple pumps may need to be used, one for high flows and one rated for lower flows, to enable the plant to cover all required flow rates.
- Discharge pressure requirements: a positive displacement pump, like the Madden diaphragm type metering pump, will be able to push the liquid to overcome system pressure up to the design limits of the pumps. For example, a Madden model no. MF136C pump is rated for up to 36 gallons per hour, and a maximum discharge pressure of 300 psi with a metal wetted end installed. We generally limit the plastic wetted end construction to 150 psi.
- Liquid viscosity: is the liquid runny like water, or highly viscous like some resins? Madden pumps are rated for volume pumping water. Liquids with viscosity up to 400 cps can usually be pumped successfully with little deterioration in capacity. If the viscosity is above 400 cps it would be best to contact the factory, and maybe even arrange for some tests, to determine how the higher viscosity will impact the rate of flow that the pump can produce. Higher viscosity liquids create drag in the piping, and inside the pump, which can reduce the volume of liquid that is pumped. We should also note that some liquids, such as caustic soda, can change in viscosity with the temperature, and that can definitely impact the pumping volume.
- Liquid temperature: Madden diaphragm metering pumps work well when the temperature of the liquid is from 30-200 degrees F. Some plastic wetted end materials of construction, such as PVC, are limited to 140 degrees F. If your liquid is outside of the normal temperature range noted above contact the factory for assistance.
These are just some of the factors to consider when specifying a metering pump. We will discuss additional elements in future blog articles. For additional help in specifying a metering pump to your your application, consult the Madden Pump Selection Guide on our website, call us, or send us an email inquiry. We are always quick to respond.