A pump adds kinetic energy to a liquid by accelerating the liquid with a revolving impeller. The amount of energy transferred to the liquid is related to the velocity at the tip of the impeller. The pump volute slows down the liquid, converting the kinetic energy to pressure energy at the pump discharge. “Head” is a measure of the kinetic energy that a pump creates.

Changing the speed (RPM) of a pump affects the flow, head and input power requirements of the pump. When the speed is changed, it affects the liquid flow through the pump by a proportion equal to the increase or decrease in speed. The pump head changes by the square of the proportion of speed change, while the input power changes by the cube of the proportion of speed change.

Cavitation can occur if the liquid vaporizes as it passes through the pump and then quickly turns back into a liquid. This will happen if the pressure of the pumped liquid drops below its vapor pressure. The formation and breakdown of bubbles by cavitation can cause damage to the pump and/or impeller. To prevent cavitation, the head entering the pump must be high enough to prevent the pressure from dropping below the vapor pressure.

The Net Positive Suction Head (NPSH) is the minimum amount of suction head needed for a pump to operate without cavitating. The amount of NPSH the pump requires to avoid cavitation is called NPSHR. The amount of NPSH available to the pump from the suction line is the NPSHA. When selecting a pump, it is important to check how much NPSH is required and be sure that the NPSH available is higher than that amount.

Pump Selection

1. First choose the style of pump required: submersible, centrifugal, self-priming centrifugal, axial flow, saltwater, magnet drive, sludge, etc.
   
2. Compare several pumps of the style selected for performance, suitability for service, cost, features, efficiency, availability and warranty.
   
3. To make performance comparisons, start by adding the actual lifting head (8') to the head loss of the piping (4') and the head losses due to devices like sand filters (20'). In this theoretical example, the total head is 32' or 14 psi. (Where suction head exists, be sure the pump can handle it without cavitation and include that in the lifting head.)
   
4. Compare the manufacturer’s performance curves by starting with the total head pressure and finding pumps that will provide the gallons per minute (gpm) required.
   
5. Compare the energy used by the pumps and select the most efficient, if efficiency is important (and it usually is in continuous duty applications).
   
6. An inlet filter or strainer may also be needed, along with a priming pot, check valve, union connections, ground fault interrupter (GFI), motor cover, control timer, flow switch (to protect other devices, if the pump is not pumping enough), etc.
   

Head: The amount of pressure a pump must work against during operation. Total head equals feet of vertical lift plus friction. The amount of head is an important value when sizing a pump correctly. One psi equals 27" of water.

Consider multiple pumps. If one pump moves 100 gpm, two of the same pumps together will move 200 gpm, three will move 300 gpm, and so on. If less than 300 gpm is ever needed, multiple smaller pumps will save electricity, as they can be individually turned on or off as needed. (Installing a check valve on each pump will prevent water from back flowing when that pump is not in use.) Also, multiple pumps may be preferred, as then only a portion of the total water flow would be lost when one pump fails. The cost of having a small pump on hand for back up is much less than a large one. For a “ready-to-go” back-up, extra pumps could be plumbed into the main line (put the pumps on separate circuit breakers) so that the reserve pump is ready when needed. Alternate the use of the pumps to keep them exercised.

Simplify the backup and spare parts inventory at your facility by using multiples of the same pump instead of several single-purpose pumps. Similar multiple redundancy can be used with air blowers, heaters, chillers, filters, etc.

Sound familiar?
Here is a note on energy efficiency from our 1981 aeration handbook and catalog: “... one kilowatt hour is equivalent to about two days of hard work by one man.” A man's labor for $0.04 per day - that's cheap!

As efficiency relates to aquaculture, pumping and aeration are the two biggest consumers of electricity. After feed costs and labor, electricity is probably the next highest overhead expense. Be careful when selecting a pump. Do not compare them by horse power alone. Often, a cheap pump has an undersized motor that must work very hard to do the job. This may be an appropriate pump selection for temporary or noncritical applications, but not where the lives of your animals are concerned. Often, pool type pumps, when used for low pressure aquaculture applications, keep the motor in a continuous overload condition.

Operating an undersized motor in the duty range of its service factor is acceptable from the pump manufacturers point of view, but not a fish farmer's point of view. It lowers the pump's cost (which looks good when you are comparing pumps), but increases energy consumption and operating temperature. Higher operating temperature shortens motor life.

We've painstakingly selected and tested all of our pumps for power consumption. We've illustrated ratings, specifications, and power consumption clearly. We use the term “aquaculture duty,” to indicate long term reliability and efficiency in humid, industrial applications.

Multiple Water Pumps Provide Redundancy
Bigger may not be better when it comes to pumping water. For example, to pump 300 gallons per minute (gpm) to a height of 20', you could use one large 300-gpm pump, two 150-gpm pumps, three 100-gpm pumps, four 75-gpm pumps, five 60-gpm pumps, etc. To determine which is best for your application, consider the following options.

Large pumps may only be available with 3-phase motors. If 3-phase power is not available, an expensive, power-robbing phase converter must be used, or multiple single-phase pumps. Even if one large pump can be used, another one must be available as a back up, if the pumping need is critical.