There are many different types of pumps available on the market today, each designed to fulfil a specific function. These pumps can be divided into the following two broad categories:
- Positive Displacement Pumps: Create increasing pressure e.g. peristaltic, single diaphragm, double diaphragm, piston, progressive cavity
- Centrifugal Pumps: Create a specified head e.g. end suction, submersible, split horizontal, vertical long shaft pump.
The most common pumps utilised in the aquatics and aquaculture industries, and those supplied by TMC, belong to the second category. All pumps have been carefully selected for their suitability for both fresh and saltwater applications, along with tried and tested reliability and a long service life. Our range of plastic-bodied pumps has been extended to include units up to 12.5hp and flows up to 160m3/hr.
How Does a Centrifugal Pump Work?
As the motor driven impeller rotates, mechanical work is transferred to the water which is moved by the centrifugal force generated. The energy transferred is in the form of higher speed and higher pressure. As the water enters the impeller it is centrifuged out along the blades and then forced out into the pump housing. The job of the motor is now done as the water has the necessary energy.
The energy transferred to the water is in two forms:
• A static rise in pressure caused by centrifugal force.
• A dynamic rise in pressure caused by increased speed.
As we do not want a greater volume of water to leave the pump than is entering it, we want to convert the excess dynamic pressure into static pressure by reducing its speed (no energy is lost – it can only be converted to other forms). This can be achieved by the use of a spiral diffuser (a funnel like tube) and decentrally located impeller. This provides more and more room for the water flow to move away from the impeller. As the flow is constant through the pump it results in a reduction of speed and therefore an increase in static pressure and thus a head of fluid can be generated.
Guidelines for selecting the correct Pump
In order to ensure the correct flow rates for any application, it is essential that the correct pump is selected.Failing to spend some time selecting the correct pump can result in filtration components not operating correctly, desired flow rates not being achieved, or pumps operating outside of their designed range. The following simple steps will help you to select the correct pump:
- Determine the required flow rate. This will entail calculating the volume of the tank, and deciding on how many times an hour this needs to be turned over. For example, if you have a 1000 litre tank, and it requires a 4 times an hour turnover, then the required flow rate will be 4000 litres per hour (or 4m3/hr)
- Calculate the total expected head using the following simple equation: Total Head = VH + PH + FH
- Where VH = the Vertical Head, or the total height in metres that the water needs to be “lifted”
- and PH = the Pipe Pressure Head generated from the pipework. This figure will need to be determined by referring to Table 1 below (please note that the tables below are in feet, and values will need to be converted into metres by multiplying the end result by 0.3048). For example, a flow rate of 60 US gpm through a 2” pipe will generate 6.6ft of head per 100ft of pipe. If only 50ft of pipe were used, the total Pipe Pressure Head will be 3.3ft x 0.3048 = 1.00m head.
- and FH = the Fittings Pressure Head generated by the fittings used in the installation. This can be determined by utilising Table 2 below (please note that the tables below are in feet, and values will need to be converted into metres by multiplying the end result by 0.3048).For example, if 10 x 2” 90º Elbows were utilised for a flow rate of 60 US gpm, the resultant expected friction loss would be equivalent to 10 x 5.7 = 57ft of 2” pipe. From Table 1, 57ft of 2” pipe at this flow rate would be equivalent to 6.6/100 x 57 = 3.76ft of head loss. 3.76 x 0.3048 = 1.15m of head loss.
- Utilise the total expected head value to read off the performance curves for the pumps to correlate with the desired flow rate. Do not work outside of the optimum performance range indicated on the curves. A pump that is run outside of its optimum range can end up working too hard, will draw too many amps and will overheat and this will eventually result in permanent damage. In the Example Pump Performance Curve (on the right) this pump is required to run at between 12 and 6 metres head pressure, at which it will generate approximately 9.3m3/hr and 17.2m3/hr respectively
- If more than one pump is found to be suitable for the application, then refer to the power ratings. The P1 (or power in) is an indication of how many kilowatts the pump will draw. A lower value will correspond to lower running costs.
- Float Switches – are recommended as a failsafe so that the pumps are switched off in the event of a drain down. This will prevent any damage to the pump through running dry, and will prevent any "gas bubble disease" in livestock as a result of super-saturation of air in the water.
The following points should be taken into consideration when a pump is to be installed:
- Pumps will generally operate better if they are installed so that they have a flooded suction line (i.e. water enters the pump by gravity, thus eliminating suction lift).
- If two pumps are to be installed together, it is imperative that points 2 and 3 above are followed.
- Prior to commissioning a system, check that all solvent weld joints are in fact glued, and all unions have their associated o-rings and are hand tight.
- Air leaks should be checked for when commissioning a system. Air leaks can lead to super-saturation of gases in the water which is responsible for "gas bubble disease" in livestock. If it is not possible to test the total dissolved gases (TDG) of the water, a method of picking this up is by shining a torch into the pump lid and looking for small air bubbles. Refer to point 3 above if they are found to be present.
- Ensure that the suction line is situated in the deepest possible point of the sump to avoid any air entering the pump.
The following parts and fittings are recommended for any pump installation:
- Suction strainer (1) – to avoid any debris entering the pump which may cause physical damage or a restriction in the suction line. Valves – it is recommended that a valve is installed on both the suction (2) and the delivery side (3) of the pump so that it can be isolated and changed over without any water loss from the system.
- Non return/check valves (4) – placed on the discharge side of the pump below the isolation valve. This will prevent water loss from the system if a pump fails, and will prevent the pump running backwards if two pumps are plumbed into the same delivery line.
- Pressure switches (5) – should be installed on the delivery side of the pump before the non return, and can be wired into an alarm system to alert you to a pump failure.