This is the first of eight articles in the series Sizing & Selecting the Right Chemical Metering Pump.
Before we even start with sizing the pump, it is important to plan the application. We take into consideration various things such as the chemical, its physical and chemical properties, storage tank and its location, day use tank and its location relative to the pump and more especially its suction port, the injection quill and its distance from the pump, electrical power requirement etc.
The first thing we take into consideration is the properties of the liquid we are pumping.
Viscosity – If the liquid is viscous it would need to be pump with a slow velocity to allow for the flow of that liquid. If you pump too fast the liquid could cavitate or stall the flow.
Temperature – If the liquid is too hot or too cold it will affect the rest of the pump components. A very hot liquid could need a cooling jacket or materials that can withstand those temperatures. A liquid such as chocolate will solidify and clog the pump and may require a heating jacket to keep it flowing.
Specific Gravity – Heavier liquids will require flow capacity adjustments.
Off Gassing – Chemicals such as Bleach (sodium hypochlorite) could generate gas bubbles and cause air lock in the pump. These may require a de-gassing valve to be incorporated in the diaphragm head.
Particles – Particles in the liquid could clog the pump internals as well as the pipe. So design will have to keep in mind that the smallest areas in the pump will pass the particles with ease.
Liquid Fumes – If there is a possibility of liquid fumes to be present especially in a closed area, you will have to use an Explosion proof motor to prevent the ignition of these flammable fumes due to arcing inside the motor.
Chemical Composition – This is the most important piece of information to ensure the pump parts do not corrode. In addition to the exact composition of the chemical, we also need to know the exact concentration. We will then have to choose all wetted parts that are corrosion resistant to that chemical.
Location of the Pump
In relation to the supply tank and the injection point or the discharge line.
Most positive displacement metering pumps prefer a flooded or positive suction head. If you locate the supply tank below the pump suction line, the pump may not perform accurately. One of the most common causes of a metering pump not functioning properly is the inadequate suction line pressure. So if the supply tank is located too far, the pump will starve due to the suction line losses which will not only affect the performance and accuracy, but also destroy internals such as the diaphragms. Additionally, if the pump is located too far from the delivery point (injection quill), we will have to take into account the loss of pressure through that long pipe or hose.
Ambient conditions such as temperature and exposure to sun and rain will have to be accounted for to choose the right protection for the pump.
The voltage and hertz of the power supply is important consideration in choosing the right motor. If you have a 50 Hz. power supply you will have to adjust the capacity of the pump as the pump will be running at a slower speed.
When you have all the above information you are ready to consult with a pump expert to help size and select the right pump.