Fertilizer dosing with electric actuated valves
Agriculture is the backbone of any economy. With the modernization of agriculture and development of new farming techniques, there are efficient and cost effective methods available to produce high quality crops that can cater to a larger population. Electric actuated valves play an important role to dose fertilizers efficiently. Choosing the right valve type and materials is crucial to limit corrosion and achieve high performance.
Use of fertilizers
Fertilizers, whether natural or man-made, are materials that are added to the soil to provide essential nutrients required for crop production. When crops are grown on tracts of land, the soil loses nutrients and is unable to provide more nutrients to next batch of crops. Hence fertilizers help to recharge the soil with essential nutrients. The three main nutrients provided by fertilizers are Nitrogen, Potassium and Phosphorus.
Fertilizer Induced Corrosion
The equipment used to deliver fertilizers is often prone to corrosion, depending on the nature of fertilizers used. These fertilizers tend to damage or attack various metals by reacting with them to produce substances such as ammonia or hydrogen sulphide. Acid fertilizer solutions with chloride tend to be very corrosive. Generally, a strong acid solution with PH below 3.5 is considered to be very corrosive.
The base of the used fertilizer can lead to some typical reactions for liquid fertilizers as below:
- Nitrogen based solutions such as ammonium nitrate and urea react slowly with steel and can have an accelerated reaction at welded joints, bolts, holes etc.
- Phosphate based solutions such as ammonium phosphate are less reactive and form a protective phosphate coat which protects metal from further attack by nitrogen based solutions.
Even when fertilizers are kept under dry conditions, they can absorb moisture from the atmosphere and become corrosive. Corrosion testing has shown that with nitrogen based fertilizers, carbon steel is susceptible to attack, whereas Aluminium, Chromium and Chromium nickel stainless steels are not attacked. In complete mix fertilizer solutions, only the chromium nickel stainless steels are not attacked. To an extent, the process of galvanising of steel helps in resisting corrosion.
The damage to metallic parts by fertilizer solutions can be minimized by better design of equipment and improved maintenance and cleaning.
Inhibitors can be added to fertilizers during usage such as thiourea, ammonia thiocyanate and Z- mercapto benzothiazole for mild steel exposed to ammonium nitrate or ammonia. Or use of polysulphates (sodium polyphosphate, dibasic ammonium phosphate) for aqueous ammonium nitrate, ammonium andurea.
The following materials are best suited for containing and handling of fertilizers. With respect to corrosion resistance and overall chemical properties, they can be ranked as below, in order of preference:
- 304 and 306 stainless steels
- Carbon steel
Stainless steels have a combination of strength, corrosion resistance, ease of cleaning and abrasion resistance. The 304 and 316 stainless steels are most commonly used in handling liquid fertilizers. The nickel content in stainless steel imparts toughness and corrosion resistance, and the chromium forms a thin oxide film, protecting the surface from corrosion attack. Although resistant to a wide range of chemicals, they are prone to stress corrosion cracking.
Aluminium and its alloys are generally corrosion resistant since they form a protective oxide layer on the metal surface. They however offer low corrosion resistance to chemicals that are capable of attacking this layer. They have low usefulness with mixed fertilizer solutions and are susceptible to attack by acids and caustic solutions. In the presence of chlorides, aluminium alloys can exhibit corrosion cracking. They offer best corrosion resistance in the pH range of 4.5 to 7.0.
Carbon steel is normally suited for handling anhydrous ammonia. It is susceptible to acids and acidic fertilisers. Many types of carbon steel are available with different carbon content and a wide range of properties. They however have limited corrosion resistance to fertilizer solutions.
Plastics such as PVC are used in low cost applications and since they are corrosion free, they are used to handle corrosive fertilizer solutions. But they have limited resistance to temperature, pressure and abrasion.
Brass offers moderate strength and toughness, and low corrosion resistance with respect to liquid fertilizer solutions consisting of Ammonium Nitrate and Ammonium Phosphate.
Other alloys such as Monel, Hastelloy, Inconel and alloys of tantalum, zirconium, titanium and nickel, offer excellent corrosion resistance, but are not widely used in fertilizer handling due to very high costs.
Usage of valves for fertilizer dosing
Fertilizer dosing valves are used to regulate the amount of fertilizer or acid to be added to the irrigation water. These can precisely deliver the required fertilizers and work as a part of a greater irrigation system. Many valve types can be used for this application. Traditional fertilizer dosing valves use under-pressure to create suction and flow with the help of the “Venturi Principle.”
There is a wide increase in demand for larger irrigation systems with the increase in agricultural production and greenhouse management. As irrigation systems get bigger, the amount of fertilizers that needs to be added to water increases. Since only a fixed fertilizer solution to water ratio of 1:100 is possible, there is a challenge to have higher and more efficient fertilizer delivery systems. This upsizing has caused traditional dosing valves to become restricting.
Since a high volume of fertilizer solution needs to flow through these valves, higher injector suction is needed to attain required flow. Due to this higher flow capacity, the pressure loss in the valve needs to be raised exponentially to the flow. This can sometimes cause cavitation. Cavitation is a depressurization of the liquid to the point where it becomes gaseous.
With the advent of new electric controlled valves, higher dosing capacities and increased control options are possible.
Types of valves used for Fertilizer Dosing
The simplest and most commonly used valve for fertilizer dosing is a needle valve. These provide very accurate flow control by means of a needle controlling the orifice opening. The disadvantage is that the outflow is limited due to the size of the orifice. Larger needle valves are not feasible due to size constraints. Hence there is a need to have maximum injection volume with high accuracy.
Needle valves regulate with high accuracy.
Angle seat valves and Globe valves
These valves are used in greenhouse irrigation systems and field crop irrigation. They generally have a spherical body and consist of a plug connected to a spring-loaded diaphragm. They are generally used to regulate flow in a pipeline. An electric globe valve consists of a solenoid valve which can be electrically actuated to achieve the required flow. They are built using durable and corrosion free materials. The inlet and outlet ports can be in a straight line or at an angle to each other.
They can operate at a wide range of flow rates, from near zero to maximal flow as per capacity.
Angle seat valves and Globe valves are robust and suitable for high volume flows.
Electric ball valves
Electric ball valves are simple and reliable valves used to control flow in fertilizer dosing systems. They have an electric controlled ball which can be opened or closed based on the flow of electricity. These are more cost efficient compared to diaphragm valves. They are typically installed upstream from the bypass line to control flow to the venturi injectors used to supply fertilizer solutions to irrigation water. Flurocopolymer lined ball valves are normally corrosion resistant to various fertilizers.
Electric ball valves are reliable and robust and usually are used as an open/closed valve.
Solenoid control valves
2-way solenoid valves are used to control the flow of irrigation water and dosing or mixing of fertilizers. These valves are controlled electrically. When the solenoid coil is energized, the valve is opened and when the coil is de-energized, the valve is closed. The solenoid pilot valve introduces control water to the diaphragm chamber and subsequently to the outlet port during operation. In a 2-way valve, the flow is just switched ON or OFF.
Solenoid valves are electrically actuated valves. They are suitable for clean media and have a short response time.
3-way solenoid valves have the same working principle as a 2-way solenoid valves, but have 3 ports where the flow is switched between two outlet ports. Multiple solenoid valves can be placed together on a manifold.
3-way solenoid valves have 3 connection ports and can have different circuit functions.
Solenoid valves offer fast and safe switching, high reliability, long service life, low power usage and modular design. These valves have a diaphragm separating the electrics from the media due to the possible corrosion effect from fertilizers.
Pressure controlled valves
Pressure control valves are used to control the inlet pressure of irrigation water to a more controllable outlet pressure. These valves are typically used in combination with venturi injectors which supply fertilizers to the irrigation water. These injectors are installed in bypass from the main line and the pressure reducing valve provides the required pressure differential to operate the venturi injector.
Pressure control valves can also produce the required pressure differential to operate fertilizer tanks. These tanks mix water with fertilizer using irrigation water pressure.
Seals and Seal Materials
Various seals are used in fertilizer dosing valves such as PTFE (Polytetrafluoroethylene), NBR (BUNA-N), EPDM (Ethyl-Propylene), FKM, etc. These materials offer high corrosion resistance along with other properties.
PTFE (commonly known as Teflon) seals are non-reactive and most chemically stable compared to all plastics. They have excellent thermal and electrical insulation properties. Owing to their low co-efficient of friction, they are suitable for many low torque applications.
NBR is a polymeric material having good resistance to water and hydraulic fluids. It has a good tensile and compressive strength and abrasion resistance.
EPDM offers very good chemical resistance to various acids and weak alkaline solutions. It has good abrasion, tear and heat resistance and the ability to withstand various weather conditions. It however has poor resistance to concentrated acids and strong alkaline solutions.
FKM, popularly known as Viton is one of the most popular seals used in fertilizer dosing valves. It is chemically compatible with various concentration solutions and is suitable for use across a wide temperature range. It can be used with various mineral acids and salt solutions. It also has excellent heat resistance.
Fertilizer induced corrosion presents a real challenge in the selection of different valves for dosing applications. In order to choose a corrosion resistant valve for a specific fertilizer solution, there is a variety of choice of materials and of construction:
- 304 and 316 stainless steel
- Carbon steels
The availability of various types of valves facilitates fertilizer dosing for high yield producing crops. Electric operated valves increase efficiency and lead to long term cost savings.
Seals, including Viton, PTFE, NBR, EPDM and FKM, are available to ensure compatibility with harshest agricultural chemicals.
Based on specific requirements, the correct valve type, valve- and sealing material must be selected to enhance the quality of dosing.