A metering pump is a special class of reciprocating pump. It is used for injecting very small, precisely controlled volumes of liquids into products or processes. Some of the liquids that are handled by metering pumps are:
• Product additives.
• Corrosion inhibitors.
• Antifoam agents.
• pH control agents; acids or alkalis.
The rates pumped by metering pumps are usually below 1 gpm. However, pumps are available with capacities up to about 10 gpm. A metering pump is shown schematically in Figure 15. The cylinder has a single acting piston. There are ball type check valves at the suction and the discharge ends of the cylinder. In most metering pumps, a diaphragm isolates the piston from the pumped liquid. A hydraulic fluid is contained between the diaphragm and the piston. The pumped liquid is on the other side of the diaphragm. This configuration isolates the piston and other parts of the mechanism from the pumped fluid. Such protection is required when the pumped liquid is corrosive, abrasive, toxic, or of low viscosity.
In the case of extremely toxic or flammable applications, a double diaphragm, with a leak
detector between diaphragms, is used.
Metering systems are usually supplied as a package on a skid. Process engineers frequently have to troubleshoot these systems. Figure 16 shows a typical installation. The liquid is stored in an additive tank. The capacity of the tank is normally about one week’s supply. The additive is delivered by a tank truck or in drums.
The liquid flows from the additive tank through stainless steel tubing to the metering pump and then to the process. A strainer is installed just upstream of the pump to keep solids out of the pump.
Block valves allow the pump to be removed from service without disturbing the process. Normally, the spare for a metering pump is not installed in the line but rather is stored in the warehouse. A malfunctioning pump can be removed from the line and the spare pump can be installed quickly.
One optional feature is a diluent added to the line downstream of the pump. Diluent increases the flow rate and reduces the residence time between the pump and the process. It provides a volume flow rate that is large enough to make a spray nozzle operate properly.
A relief valve or safety valve is required. It may be external as shown in Figure 16, or it may be internal and supplied by the pump manufacturer. If the pump contains an internal relief, an external valve is not required unless the liquid is very flammable or toxic.
The calibration cylinder is used to verify that the proper flow rate of liquid is being delivered by the pump. To use the cylinder, close the valve between the additive tank and the calibration cylinder. Now, the pump will take suction from the cylinder alone. Time the rate at which the level falls in the calibration cylinder. The cross-sectional area of the calibration cylinder is known. Therefore, the delivery rate can be calculated.