Dosing pumps 

Here we examine the variable volume dosing pumps.

Option 1:

Is the use of a centrifugal dosing pump in conjunction with a flow meter. The centrifugal pump doses out of the tank into the mainline.

Although this option as a unit is slightly cheaper than the other two options, it has quite a number of operating costs and added problems. First off the unit is very inaccurate. Pressure variation influences water flow and thus volume dosed. Any variation in mainline pressure and the pump doses far outside the allowable parameters. The same applies in variation in water flow. Should there be a build-up on the flow meter
ball, it will influence the weight of the ball and thus the reading and the accuracy.

Furthermore the operating cost of the unit is high. The flow meter and pump impellor are both sensitive to fertiliser, poisons, chlorine and other chemicals. Unavoidable wear and tear will lead to untimely and expensive replacement and repair costs. Always when you need it least. Additional cost should also allow for labour cost as the unit needs to be under constant surveillance.

Option 2

Is a piston pump. In circumstances where operating pressures exceed 8 Bar this is the cost-effective option. An electric motor drives an eccentric shaft which in turn drives the piston. The piston lies inside a pump head where the fluid is suctioned into from the tank and dosed out of into the mainline. The piston forms a vacuum inside a seal which facilitates the operation.

The advantages of this system is that is doses constantly independent of outside influences i.e. pressure variations or flow variations. Whether pressure varies at the point of dosing or over the irrigated area, the pump will dose the volume as calibrated and set by the user. On this unit it is possible to vary the volume the client wants to dose, with the help of an adjustment knob, variable speed drive or signal receiver.

Disadvantages of this pump are that is needs some follow-up labour. The piston needs to be cleaned at the end of every dosing cycle. This is to prevent crystallisation as caused by chemicals. Crystals scratch the piston and seals which prevent the vacuum from forming and then the pump cannot dose. It also allows chemicals to leak past the piston and into the gearbox causing expensive replacement of the gearbox. This can happen with any roughness which might get in to the pump head such as sand or leaves. Should the piston or seals be replaced t is important that the supplier does this so as to ensure correct fitting.

It is also important to note that the piston must always dose in fluid as when heat forms from friction it expands and gets “stuck” in the head.

The electrical piston pump is recommended for industrial dosing processes where the fluids are void of any impurities which can scratch it.

Option 3

Is an electrically driven diaphragm pump such as the I-Feeder dosing pump. This pump works on the same principle as the piston pump except here the eccentric shaft drives a diaphragm. Disadvantages of these units are that their maximum operating pressure is 8 Bar.

The diaphragm pump is the most reliable and rugged option. Ideal for agriculture as the diaphragm is not sensitive to crystals or other impurities. The diaphragm is easy to replace en maintenance can be done in the field by the farmer. Day to day maintenance is not necessary as crystallisation has no influence on the diaphragm. This unit is also uninfluenced by pressure and water flow variation. No minimum pressure is needed for the accurate dosing either.

Variation is dosing is achieved with a control dial calibrated in percentages or units. Furthermore a variable speed drive is available. It is a lower capital expenditure than a piston pump.

When choosing between dosing pump suppliers one has to be aware of the following points. Some dosing pump manufacturers do not use off-the-shelf electric motors. Should the motor need to be replaced due to age or overheating he has to buy it from the pump supplier which often add unnecessary extras he client does not need.

Some suppliers also do not have the support of agents with the much needed technical knowledge on the ground. The client can end up with a much more expensive unit than needed.

It is important to note how simple a diaphragm pump’s installation is. On the suction side there is a suction pipe from the tank/ dosing plant to an inline filter to the inlet valve of the pump. On the dosing side a pipe is coupled to a one-way valve and then doses into the mainline. Additional labour required is once-off calibration and an electrician to supply electrical power.

Advantages of the I-Feeder Diaphragm pumps:

• Our pumps are available with diaphragm or piston heads.
• For our over 40 different volume capabilities only 2 gearbox ratios exist.
  This has the implication that for volumes of 10 l/hr up to 4800 l/hr there
  are only 2 sets of gears, 2 pump head sizes, 2 valve sizes and 2
  diaphragm sizes. Never will spares not be available.
• Above mentioned also means that should one find that the volume dosed
  is inadequate in time due to expansion or other, the pump volume can be
  upgraded to a higher volume by way of either a new pump head, additional
  gearbox or gear ratio adjustment.
• Our motors are separate units and thus can be replaced by an off-the-shelf
  unit should it be necessary.
• Our pumps are positive displacement pumps. They are thus unaffected by
  outside influences such as pressure or flow variation in the mainline.
• Our pumps will not be affected should their liquid-ends run dry for any
  period of time.
• The pumps are supplied complete will all items necessary for installation.
  Included are chemical inline filter, one-way/ anti-siphon valve,
  quick-coupling, Direct-on-line starter with overload and piping and fittings.

Choice Of Dosing Pump:

1: What is the volume of the product being dosed?

The volume of the fluid that needs to be dosed must be supplied by the chemical
agent.

Herewith the common sum for calculating capacity:

Sum:

1. Area: Total Hectare
2. Time to complete circle
3. Kg per Hectare which needs to be applied
4. Litre: Kg divided by sg of product

Hectare divided by time: For example 20 ha/14 hours

Answer: 1.43 hectare per hour.

Suggestion: Example 250 kg/ha

Litres: 250kg divided by 1.25(sg)

Answer: 200 l/ha

Application: 200 l/ha x 1.43ha/hr

Answer: 286 litres per hour.

Pump capacity: 300 litre /hour

2: What is the maximum operating pressure of the system?

The maximum operating pressure must be supplied by the client to ensure the correct pump choice. Should operating pressure exceed pump pressure the diaphragm and gearbox will be damaged. This value is very important.

Should the system have a very high pressure the client can dose at the suction side of their water pump. In this instance the water pump assists the dosing pump in injecting the chemicals into these high pressure circumstances. This also takes working pressure off the diaphragm ensuring long life of the unit. It is important that an anti-siphon/ one-way valve is applied when above mentioned installation takes place.

3: What is the product being dosed?

The product dosed determines the fluid-end materials. As standard the pump is suited for all liquid fertilisers, organic fertilisers, low concentrations chlorine, most poisons and some acids.

Options available for fluid-end materials include Ertalite, SUS, Teflon and others.

4: Electricity

Single phase or Three phase. Both are available. Single phase is more expensive though.