The most critical criterion is the suitability of the material selected for the fluid to be pumped. This is particularly significant as corrosion and erosion directly affect maintenance, repair, and downtime costs. In specifying or purchasing a pump, it is important to make sure that all wetted components are made of proper material.
MATERIALS OF CONSTRUCTION
Metals have a fixed rate of corrosion in any given fluid. For many applications, the rate is so low that it is insignificant. However, where metallic corrosion is a factor resulting in excessive maintenance, expensive repair, pump failure, or product contamination, engineers tend to look closely at the selected materials. For many of these applications, stainless steel and exotic alloys are standard.
– ENGINEERED THERMOPLASTICS
Thermoplastics compete favourably in price with stainless steel, and thus, their chemical inertness has become a major reason to consider them. With installations requiring high alloys or exotic metals, thermoplastics offer significant savings in both initial equipment costs and upkeep.
There are many applications for which thermoplastic pumps are the only reasonable choice. These include:
- handling such corrosives as bromine and strong oxidising acids
- installations that cannot tolerate metallic contamination (eg: ultra-pure water, reagent grade chemicals, and pharmaceutical and electronics industry fluids
Another area where thermoplastic pumps are mandatory is the handling of waste streams with unknown chemical compositions or where composition fluctuates across the full pH spectrum.
The duty point of the application is the point on the pump curve at which the pump operates. It is a combination of flow rate and head for the application. The flow rate is how fast the liquid is being moved by the pump through the system.
The flow and head condition typically determine the pump and motor selection (taking the S.G. and viscosity into consideration).
SPECIFIC GRAVITY (S.G)
Specific gravity, more formally known as relative density, is a measure of the density of the substance in comparison to the density of water. This directly affects the head pressure and the motor power selection.
Viscosity is a measure of a fluid’s resistance to flow. It describes the internal friction of a moving fluid. A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion.
NET POSITIVE SUCTION HEAD (NPSH)
Checks including flow and pressure requirements to ensure the pump is not at risk of cavitation.
Metals and alloys tend to have a higher temperature tolerance than thermosets and thermoplastics, indeed the primary limitation on the use of thermoplastics is temperature.
Consider solids in the fluid are accounted for in the impeller design to avoid blockages, the mechanical and seal specification will be dependent on the solids type and flushed bearings will need to consider product or freshwater flush.
System configuration and arrangement, such as the number of pumps, duty/assist/standby, fixed and variable speed, and horizontal or vertical pumps.