Frikkie Koegelenberg Pr Eng and Gert Conradie Pr Eng

Pumps, valves, filters, sensors and mixers also form an integral part of a fertigation system. This month we discuss the different types of pumps and all the secondary equipment.

Electrically driven pumps

Electrically driven pumps use electric energy to drive a pump mechanism by means of an electric motor. The pump mechanism can take various forms. Electrically driven pumps can be divided into centrifugal pumps and positive replacement pumps, for example hydraulic screw, diaphragm, reciprocating and gear pumps.

•Centrifugal pumps

An additional centrifugal pump can be used to inject fertilisers into the irrigation pipeline. Although the use of the centrifugal pump is very economical, there are certain requirements to comply with. The centrifugal pump used for this purpose must be able to deliver a higher pressure than the irrigation pump at the injection point. Back-flow can take place freely through the centrifugal pump, therefore additional equipment, for example a non-return valve, must be installed to prevent back-flow. To obtain sufficient pressure, a multi stadia centrifugal pump can be used.

•Screw pumps

Screw pumps can apply fertilisers at higher pressures than centrifugal pumps. The same benefits and precautions apply as with centrifugal pumps. Screw pumps are generally more expensive than centrifugal pumps.

•Diaphragm pumps

Injectors mostly used in Southern Africa are diaphragm pumps driven by electric motors or solenoids. An adjustable screw on the shaft from the motor is used to change the injection rate. The setting mechanism must be simple, accurate and dependable, as the flow must remain accurate at a specific setting for a long period.

Piston pumps

Piston pumps are available with single or double cylinders, with a wide range of injection rates.

Peristaltic pumps

Although peristaltic pumps are very expensive, they can be used for fertigation. Peristaltic pumps are very accurate and have few moving parts, which mean low maintenance. Only the rubber tube and the pump are in contact with the fertilisers.

Gear pumps

Gear pumps are also positive displacing pumps. Their efficiency is influenced by the gear shape and the number of gears used.

Hydraulic (water-driven) pumps

Water-driven pumps use the available operating pressure in the irrigation system. Two types of water-driven pumps are available: turbine and piston (or diaphragm) driven pumps. The turbine types use the pressure in the system, but do not waste water. A pressure loss does however occur, and this must be considered in the design.

Piston or diaphragm pumps use a small amount of the water, but cause no pressure loss. The volume of water usually wasted in the process is about two to three times the volume of ferti­lisers applied. The injection rate can be varied by a change in water pressure and volume, and also by limiting the outlet of the system with a small open­ing. A piston-driven pump that does not use water is also available.

It is important to determine the mini­mum and maximum operating pres­sure at which the water-driven pump still functions effectively and can still deliver the required injection rates.

Table 1 on the next page was published in the previous edition to give a sum­mary of the injectors, but gives a good summary of the pumps discussed as well.

Other equipment

Ensure that the use of all equipment relevant to fertigation is safe for both man and nature. Valves, connections and pipework must therefore be of good quality, the correct size and pres­sure strength to handle fertilisers over a long time.

Electrical interlock

The irrigation pump and injectors must be electrically interlocked. This means that if one of the two stops, the other will also switch off. The benefit of this

is that over-injection of fertilisers cannot take place and if the irrigation pump should stop operating, the injec­tors will stop, and vice versa.

Flow sensor

A flow sensor must be installed just before the connection at the irrigation pipeline. If a leakage of some kind then occurs and the flow sensor registers flow, the injection pump will automati­cally switch off the irrigation pump. Flow sensors that transmit electronic impulses are available and they can be used to measure the injection rate. The impulse from the flow sensor can then be used to keep the injection rate constant by means of a programmable control system.

Fertiliser injection line check valve

A non-return valve must be installed in the pipe from the fertiliser tank to prevent back-flow of water from the irrigation system to the fertiliser tank. The valve’s mechanism must be able to prevent the gravitational pressure from the fertiliser tank from opening it. Only the increased pressure from the injec­tors must be able to open the valve.

The same task can be performed by a solenoid valve which is normally closed. If no injection is done, or when a power cut is experienced, the valve will close automatically and water flow in any direction will be prevented.

Manual valve

A normal hand valve must be installed at the outlet of the fertiliser tank for repair work to the injectors when necessary, without the fertiliser tank running empty.

Strainer on suction pipe of chemical injector

The purpose of the strainer is to pre­vent particles and solid fertilisers from being sucked in. The residue can cause blockages, inaccurate injection rates or leakages.

Air valve

An air valve must be installed just after the injection pump. It can be used to bleed air from the system during a cali­bration, or to relief the system pressure before some system components are removed for service or replacement.

Fertiliser tanks

Fertigation requires tanks for the transport and storage of fertilisers. Fertiliser tanks are manufactured from medium density polyethylene, which is resistant to most agricultural fertilis­ers. They are also resistant to ultra­violet rays and can therefore remain in the field throughout the year.

The sizes and numbers of tanks required depend on the following factors:

1. Is the tank used only for storage of fertiliser?
2. Must the tank be stationary, or must it be moved with a full load of fertiliser?
3. Will liquid fertiliser be used, or will the farmer make his own ferti­liser solution from water soluble fertiliser?

•Must the fertiliser in the tank be stirred?

•What is the pH, specific weight and temperature of the fertiliser?

•What is the volume of fertiliser needed over a certain period, injecting quantities and intervals between injections?

•If liquid fertiliser will be used, how often can the supplier deliver and what is the minimum load that can be transported at a time?

Green or blue tanks are internationally standardised as water tanks, therefore, other colours such as white natural polyethylene, reddish-brown or black tanks are used for fertilisers. The fertiliser supplier also influences the colour of the tank. Black inner walls are recommended, because some iron compounds can precipitate as a result of ultraviolet rays. Algae growth is not a problem, because the concentration of fertiliser mixtures is usually too high for algae growth.

Storage tanks are usually round and upright, available in different diameters and height combinations. The bottom is flat, with an outlet slightly above the bottom. If the fertilisers remain on the bottom, it will dry out and crystallise with time. The crystals usually dissolve again with the next injection. The tank should be scrubbed after each season. Hollows in the bottom, that are difficult to scrub, can cause the crystals to collect there and tanks with hollows should therefore be avoided. By using a 90° bend, bent downwards from the outlet, the liquid remaining in the tank is limited to the minimum. Transport tanks are usually horizontal, with a special base that lies on the truck’s cargo bed. Any sized indicator on a tank is very handy.

Pollution must be kept to the minimum when tanks leak or break, especially acid tanks. Foundations must therefore be firm and fertilisers must be drained away from the water source and collected if possible. This can be done by placing the tanks in a dam built for this purpose. Fertilisers can also be recovered this way. Tanks last longer if they are not standing empty for long periods and must be filled with clean water if necessary. The site where the tanks are standing must preferably be fenced in if they are not standing in a dam.

When buying a tank, the fertilisers to be used must be known, because the SW (specific weight) of especially fertiliser mixtures vary a lot and can be up to 1,7. The higher the SW of fertiliser mixtures, the stronger the tank required. When fertiliser mixtures are made, mixing must be done before the tank is completely filled (50 to 75% of the tank volume), because the reactions between some mixtures can lead to an increase in volume.

Agitators

Fertilisers are supplied in liquid concentrate or a water-soluble dry form. If a liquid concentrate, which does not easily precipitate, is used, an agitator is not necessary. When water soluble solid fertiliser is mixed, or when the possibility exists that the fertiliser can precipitate, an agitator is required.

Some fertiliser mixtures also crystallise at low temperatures and this can be prevented by using an agitator. Mechanically driven agitators, mixing by recirculating the concentrate through a pump, or bubbling air can be used. Mechanically driven agitators must be manufactured from materials such as stainless steel, which is resistant to highly corrosive fertiliser mixtures. Mechanical agitators are available in the trade and the choice (size, type of blade, angle, speed, et cetera) depends on the volume and dimensions of the tank, the SW of the liquid, the viscosity of the liquid, variable liquid level and electrical energy available.

Suppliers of mechanically driven agitators can usually help with the choice. For tanks up to 5 000 ℓ, it is recommended that an agitator is installed in the tank at a 15° angle (Nel, 1999). On tanks larger than 10 000 ℓ, it may be necessary to install static mixing fins inside the tank. If mixing is done by means of a recirculating pump or aeration, it can be accepted as a rule of thumb that the tank’s volume must be circulated about 5 times per hour.

Static in-line mixer

It is important to obtain good mixing of irrigation water and fertilisers in the pipeline during application. This can be done by ensuring that there are enough bends in the pipeline before the first emitter, or by installing a static in-line mixer.

Fresh water supply valve

Fresh water must be available for washing and flushing/rinsing of tanks and injectors after injection.

Filters

A very high level of component accuracy is required in an injection pump, and its operation can be adversely affected by any wear or blockage caused by residue of fertilisers from the fertiliser tank. It is therefore recommended that a filter of 200 microns is installed on the suction side of the injection pump, preferably at the outlet of the fertiliser tank. If injecting to the main filters of the irrigation system takes place, a separate filter must be installed at the irrigation blocks to prevent impurities from reaching the system.

Equipment for mixing of fertilisers

It is often necessary to mix fertiliser concentrate with water or to mix different fertiliser combinations in liquid form to obtain the correct concentrations for injection. It is good to have mixing equipment to make the task easier. A centrifugal pump that circulates the contents of the fertiliser tank while mixing takes place, will ease the process. The centrifugal pump must deliver a high flow rate at a relatively low pressure, and must displace the fertiliser tank volume at least once every two minutes. A funnel with its inlet mounted on the suction side of this circulation pump can also be used to mix the concentrate into the water.

Visit www.arc.agric.za for more information. Next month we shall discuss electronic control and safety measures.