Hydraulic pumps, the mechanical heart of irrigation systems
From livestock wastewater treatment to biomass processing to phytosanitary applications and irrigation, hydraulic pumps are critical to a large number of agricultural operations. The different types of models and the jobs for which they are intended
The 2024 irrigation campaign is just around the corner, and this year, at least in the North-Center of Italy, it is expected to start on a fairly traditional schedule after years in which prolonged drought forced farmers to open spouts as early as late March to make spring sowing possible. There are many forms of irrigation, and they sometimes vary from territory to territory. However, from run-off to sub-irrigation, from sprinkling to precision fertigation, all have one element in common: the hydraulic pump. A device that recurs quite frequently in agriculture. As a matter of fact, we find it in enology and dairy farming, biogas establishments, pig farming, and various field activities, such as wastewater distribution, phytosanitary treatments, and, indeed, irrigation. In short, anywhere that more or less flu-id materials have to be handled. Fluidity is precisely one of the discriminating factors when choosing between the various pumps available on the market.
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It is a matter of the centrifuge
In these pages, we examine irrigation machines, focusing on an es-sential component such as the hydraulic pump. Having to move water, the viscosity of which is ex-tremely low, it is possible to use pumps with a rather straightforward design, such as centrifuges, re-serving more complex (and expensive) solutions for other areas. Such is the case with progressive cavity pumps, which are widely used in anaerobic digestion establishments, or volumetric lobe pumps, which are slowly gaining a foothold in the wastewater and digestate cycle. Irrigation, on the other hand, is currently bread for centrifugal pump implements that are efficient in operation and gen-erally inexpensive and bulky since they have an elementary structure. In fact, a centrifugal pump con-sists of a body, a shaft, and a propeller that moves fluid by taking it from the suction pipe and sending it to the discharge pipe. This, in their simplest form. However, centrifugal pumps can be of various types, depending on the type design, the number and shape of impellers, and their position relative to the flow.
Structure
In terms of design, a centrifugal pump can be horizontal or vertical. For irrigation, the former is almost exclusively used since the flow of water must be pumped horizontally. However, for draining from very deep wells or tanks, vertical pumps designed to transfer liquids from the bottom to the top may come in handy.
Impeller
The characteristics of the impeller (or impellers) depend on the material being handled and the level of efficiency sought. The open impeller -no more or less than a propeller- is less efficient than other solutions, but it does not clog and is therefore suitable for abrasive, very dense or debris-containing fluids. The semi-open impeller, more efficient than the previous one, is ideal for semi-abrasive and not excessively dense fluids. The closed impeller, consisting of two disks and a cylinder, inside which are the propeller blades, is specific for water and gas. However, for irrigation, given the ease with which water can contain sand, small stones or mud, open or semi-open impellers are usually employed. Their number varies essentially according to the required pressure and flow rate, ranging from single-impeller pumps to three-impeller pumps for higher flow rates. Multiple impellers placed in series, horizontally or vertically, form multistage centrifugal pumps, in which each stage amplifies the action of the preceding ones until extremely high pressures are reached, sufficient to push fluids to great heights and over long distances.
Orientation
Impellers may be placed on a shaft perpendicular or parallel to the liquid flow. In the former case, you have radial centrifugal pumps; in the latter, axial. The latter offer a greater flow rate than radial pumps and are more commonly used in irrigation, as well as semi-axial pumps, which push fluid in a direction intermediate between radial and axial.
Pressure, flow rate, and type of establishment
To say irrigation is not enough; there are many methods for implementing it, and they are so variable that they significantly affect the pump character-istics needed to achieve the best balance of efficiency, performance, and cost optimization. To sum-marize, the basic specifications to look at are three: pressure, flow rate, and head. Basically, pressure indicates the force with which the water is pushed out of the pump and conditions the distance to which the liquid can be delivered and the ability to sprinkle irrigation over large areas. It is measured in Bars. Head means the height to which the pumped fluid can be pushed vertically. It is convention-ally measured in meters and is directly proportional to pressure: the higher the pressure, the higher the head goes. In contrast, the flow rate is inversely proportional to the head and represents the amount of water that can move through a pump. That is why it is measured in liters per second (or minute) or cubic meters/hour. One factor to consider when choosing a pump is the size of the piping: with larg-er pipelines, the pump's flow rate increases, but the water pressure is reduced due to Poiseuille's law. For this reason, the performance of the pump used should be carefully calibrated to the characteristics of the irrigation establishment to be supplied.
Irrigation methods and specific demands.
A quick list helps to understand how much the working conditions of irrigation pumps can vary. The most common irrigation systems are sprinklers (sprin-klers or rotors) or drip (hose or micro-irrigation), either surface or underground, but pumps with piv-ots and rangers are also used in greenhouse irrigation, in draining water from ditches or ponds, and sometimes in flood irrigation. Another variable is the power system, ranging from the diesel engine (pump sets or motor pumps) to the tractor via power take-off or even the electric motor. Typically, the greatest demands in terms of pressure and head are with sprinklers, while drip irrigation needs ample flow but modest pressures. In the former case, 10 Bars can be reached, with transport lengths of up to 700 meters, while for hose systems, 3 to 5 Bars of pressure are sufficient, but with flow rates above 4 thousand liters per minute. The same applies to transfer pumps, that is, those used to draw water from reservoirs and transfer it to tanks or cisterns for transport. "For a good pump calibration, it is essential to take into account the establishment's operating parameters: type of irrigation, length to be covered, diameter of the pipes," explains Emanuele Mattiolo, from Padua, owner of an artisan company that makes pump units on a contract basis.
Sustainability strategies
"In the same way," adds the young mechanic, "it is important to calibrate the engine in relation to the size of the pump and the performance required of it. In other words, a balance is sought between the characteristics of the establishment being powered and the tools to power it: hydraulic pump on the one hand, the motor that drives it on the other. Both efficiency and cost depend on this, and not least, the environmental sustainability of the irrigation cycle. "In the case of the engine, size, number of cylinders, and power output affect not only the pump performance but also the fuel consumption. Using a powerplant that is too small results in higher diesel use and earlier wear and tear, as the engine is always working at maximum speed and overheating. As a rule, a static irrigation motor should run at between 1,400 and 1,500 rpm. In that case, and if everything is calibrat-ed in the right way, consumption drops to 8 liters per hour. From this value up to, say, 10 l/h, it is within a good range of efficiency. Above 10 liters per hour, there is probably something wrong." It should then be noted that lower consumption obviously means lower emissions and, thus, less air pol-lution. On the subject of emissions, it should be noted that the regulations are controversial: wheeled pump units, like sprinkler units equipped with their own engine, must comply with Stage V require-ments and thus are practically obliged to adopt engines with Scr (Selective Catalytic Reduction using urea additive) exhaust abatement system, whereas stationary engines, thus without wheels and draw-bar, are stuck at Stage III parameters. However, decarbonization is beginning to be discussed in this area as well. The most promising avenue is electrification, a viable solution when dealing with sta-tionary pumping points, i.e., typically those that draw from wells or reservoirs of various kinds. Funds for this purpose are provided in the NRP. It is much more difficult, not to say impossible, to electrify mobile irrigation systems, for which clean engines, such as hydrogen engines, are being rea-soned about instead. Of course, still to come. One aspect that could be considered marginal but none-theless has its own relevance, including for the peace of mind of the farmer and those living in rural areas, is the noise pollution produced by pump units powered by thermal engines. These are, in fact, systems that operate 24 hours a day, at a time - summer - when people live outdoors or at least with windows open, particularly during the night hours. Reducing noise emissions from the engines, there-fore, becomes a priority, both so as not to inconvenience the inhabitants of the treated areas and to avoid the predictable protests from the latter. Thus, diesel engines have begun to be shielded with soundproof hoods composed of sound-absorbing panels of varying thickness (and cost). The results are good: good shielding reduces noise to levels between 70 and 80 decibels (about the same noise as driving a state-of-the-art tractor). When it comes to noise (and consumption), there are also patent-covered solutions on the market that involve a heat exchanger upstream of the pump on the water in-take pipe. It is used to cool the engine coolant, eliminating the radiator and fan, which produce noise and draw up to eight percent of the power.
Technologies 4.0
Cooling and insulation systems, dual tanks for diesel and urea, and the latest gen-eration of motor pumps are decidedly different from the outdated and noisy ones of past decades. This is because technology, including digital technology, has now colonized even this niche of agricultural mechanization, improving its efficiency, environmental impact and comfort of use. For example, elec-tric winches or hydraulic movements are available for the draft, i.e., the proboscis that fits into the irri-gation canal or reservoir, and in some cases, also for the stationary feet. However, it is in the man-agement of the pump that the greatest strides have been made. Control is delegated to an electronic control unit, which can have different levels of sophistication and automation, ranging from simple text message alarms in the event of a malfunction to systems that keep a memory of the irrigations car-ried out and can also be programmed remotely, to start or stop the delivery or change the hourly flow rate. Besides, even in the world of irrigation, Agriculture 4.0 has steadily entered the world of irriga-tion, and solutions such as variable metering are becoming increasingly common.
Maintenance at a minimum
A machine that must run 24 hours a day in a season when time is scarce, and downtime is not con-ceivable unless the delicate irrigation schedule is compromised must forcibly require extraordinarily little maintenance. Centrifugal pumps, widely majority in this field, have precisely the virtue of sim-plicity. At the beginning of the season, they must be prepared to clean properly, check the seals, and change the oil in the gears. Once this is done, it is enough to intervene with the replacement of the stuffing box or mechanical seal if any leakage from the pump body or pressure loss is noticed and with the periodic cleaning of the impeller in case working with water polluted by sand or algae. This is particularly so if a semi-closed impeller is fitted, whereas the open impeller should not get dirty but may experience premature wear due to the abrasiveness of sand and must be replaced if a loss of flow rate is detected.
Italian job
Agricultural mechanics is now completely globalized, but irrigation remains a purely Italian affair, at least in its power systems. The very large proportion of centrifugal pumps used in irrigation is, in fact, produced in Italy, while in other areas, see wastewater management and purification, there has been a penetration of foreign brands, mainly German. In the centrifuge world, on the other hand, whether due to the simplicity of construction or the need to adapt to requirements that are often more regional than national, Italian brands predominate.
