How To Select A Pump - Part 2

Continuing from part 1...

Temperature. The fluid temperature inside the pump is very important because the vapour pressure can have big impact on the performance of the pump. Other fluid characteristics like the viscosity and the density can be affected as well by the changes in the temperature. For instance, in the case of the rotary lobe pumps, the cooling of the liquid in the discharge line will have a significant effect on the pumping process of the fluid. The temperature of the fluid could also have big impact on the selection of what elastomeric materials or liquids to be used.

Flow Characteristics. For the selection of the right pump, it is important to consider and to determine the type of the flow. This is especially important for the rotary lobe pumps. Under some circumstances the fluid will appear to flow like a layer in a smooth and regular way.

Velocity. The velocity of the fluid is described as the distance the fluid travels per unit of time. The fluid velocity can be very important especially when using rotary lobe pumps for pumping out slurries and other fluids that contain solids.

Vapour Pressure. The vapour pressure of a particular fluid is described as the pressure at which the fluid that is being pumped is going to change into vapour, and it is presented as absolute pressure. Each fluid is characterized by its own vapour pressure and temperature relationship.

Fluids Containing Solids. When the fluid is being pumped it is essential to determine if the fluid contains any solids, and if contains solids to determine the size and the concentration. The lobe pumps require special attention when abrasive solids are being pumped. Determining the size of the solids is very important, and the pump inlet needs to be large enough for the solids to travel without any problems.

Performance Data

Capacity. The capacity or the flow rate is the volume of the fluid that moves through certain area per time unit. For different fluids, the most usual units for capacity are: liters per hour l/h, cubic meters per hour m3/h, UK or US gallons per minute gall/min. For the mass of the fluid the most common units for capacity are: kilogram per hour kg/h, tonne per hour t/h, pounds per hour lb/h.

Pressure. The pressure is described as the force per unit area. In the universal SI system, the unit used for pressure is the Newton, or shortly N, and the area is presented in square meters. Therefore the pressure is presented by units of Newton per square meter, N/m2. This unit is known as Pascal. Different types of pressures can be identified: atmospheric pressure, gauge pressure, absolute pressure, vacuum, inlet and outlet pressure, differential pressure.

Net Positive Suction Head (NPSH). The inlet side of the pump need to enable one smooth flow of the liquid, so that the liquid can enter the pump at the required pressure so that cavitation can be avoided. This is known as the Net Positive Suction Head or shortly known as NPSH. The pump manufacturers usually provide data about the net positive suction head that is required for proper pump operation. When selecting and choosing a pump it is very important the net positive suction head in the system to be bigger than the net positive suction head which is required by the pump.

For more information on lobe pumps click here...