Water pumps: the unrecognised mainstay of agriculture

An electric water pump as part of a solar pumping system.
 

Most people when they think of agriculture will picture a tractor. The humble water pump, however, chugging or humming away in a shed, uses as much or more energy and is likely to be just as essential to production. 

Pumps are used extensively in agriculture to move water from the water source, which could be a river, dam or bore, through pipes to either a point of usage or a storage facility, such as a water tank or an irrigation system.

In Australia, and in many developing nations, the cost of grid connection or lack of access to grid supplied electricity means that many farmers rely on diesel pumps. 

Electric pumps, however, are preferable for several reasons, including lower maintenance, greater potential for automated and remote control, and the ability to use solar power either as the only power source or as a supplementary power source for grid connected pumps. 

What ever the power source, the principles of pump selection and operation are the same.  Cost-efficient operation depends on selecting the right size and design of pump and maintaining it correctly. This article covers the principles of pump operation, the factors involved in selecting the right pump for the task and achieving energy efficient operation. 

Selecting a pump

The vast range of pumps available can be baffling as there are pumps of different types, capacities and physical sizes, and numerous pump product manufacturers. The first step in narrowing the field is to determine the pumping requirements of the intended application. This includes working out your daily water needs as well as gathering information about the water source, discharge point and the journey between the two. 

If not supported by other power sources, like the electricity grid or battery storage, the pump must be able to move enough water each minute during sunlight hours to meet your operation’s daily water requirements. The pump also needs enough power to move the required amount of water from the water source over the required distance and through the required vertical lift (the head). 

How a pump works 

A pump is defined as a system that is used to move liquid. It can be as simple as a spray bottle, or as complex as a human heart. Pumps move liquid by using energy to increase the amount of pressure in the system, shown in simplified form below. 

The principle of how a pump moves water.
 

Pumping Methods 

There are two types of pumping methods: dynamic pumps and positive displacement pumps. Dynamic pumps generally provide better efficiency and flexibility and are suitable for large pumping tasks, such as broad-acre irrigation. 

Dynamic pumps 

A dynamic pump uses a set of rotating blades called an impeller to increase or decrease the pressure in the system concept. By rotating specially crafted blades in either a clockwise or an anticlockwise direction, the flow of the liquid can be changed, thereby changing the pressure of that liquid. 

An electric fan illustrates this. Its specially-shaped blades allow it to change the flow of the surrounding air, forcing it in a specific direction. 

An electric fan works in a similar fashion to a dynamic pump.
 

A centrifugal pump is a type of dynamic pump most commonly used in solar water-pumping systems. 

Positive-displacement pumps 

Positive-displacement pumps operate by changing the volume in a closed system. As the volume is reduced, the pressure increases; as the volume is increased, the pressure decreases. These changes in pressure cause the fluid to be sucked into the system, then pushed out in the desired direction. 

To illustrate this concept, consider an open plastic bottle, held underwater and squeezed, thus reducing its volume. As pressure on the bottle is released, it increases in volume, decreasing the pressure and causing water to be sucked into the bottle. Then a lid is put on the bottle and it is taken out of the water. The sealed bottle is squeezed, reducing its volume and increasing the pressure on its contents. When the lid is opened this pressure is released, causing the water to be pushed out of the opening. 

Helical rotor pumps and diaphragm pumps are the most commonly used positive-displacement pumps in water-pumping applications (See Fig. 35 and Fig. 38). 

A diaphragm pump is an example of a positive-displacement pump
 
Key notes
  • The two pump types used most commonly in water-pumping applications are dynamic (such as centrifugal) pumps and positive displacement (e.g. helical rotor and diaphragm) pump

Pump technologies 

There are three main pumps used in rural water pumping applications: centrifugal, helical rotor and diaphragm. Each of these pump technologies has different pumping characteristics, which means they are suitable for different applications. 

Table 6: Pumping characteristics of the three main pump technologies.
 
 a) A centrifugal pump operation; b) a helical rotor pump operation; c) a diaphragm pump operation.
 
Key notes 
  • A centrifugal pump can deliver higher flow at a lower head. These pumps are most efficient when run at their rated operating voltage. 
  • A helical rotor pump can deliver lower flow at a higher head. This type of pump is efficient at a range of operating voltages, making it a pump type well suited to solar PV-powered pumping. 
  • A diaphragm pump can deliver lower flow at a lower head. These pumps are generally not damaged by solids or by dry running. 

Pumping installation types: surface and submersible pumps 

There are two types of water pumping installation: surface pumps and submersible pumps (Fig. 37). 

  • Surface pumps: This type of pump is mounted on the ground above the water level, such as on a dam wall. The pump draws the water from the water source, then pushes the water to the point of storage or usage. 
  • Submersible pumps: This pump type is installed below the source water level, such as in a dam or a bore. The pump pushes water directly from the source to the point of storage or usage. 

There is a limit to the maximum distance between a surface pump and its water source: it is more difficult for a pump to draw water upwards than for a pump to push the water is has drawn uphill. The practical suction lift limit is about 25 feet (7.62 metres) but pump manufacturers should provide a suction lift limit figure for each pump model. This limitation needs to be considered when designing a solar water pumping system. 

a) A surface pump needs to draw water out from the dam and then push it up to the delivery point; b) a submersible pump only needs to push water from the pump to the delivery point.
 

Pumps that are exposed to the elements need to be protected: this applies mainly to pumps installed in rivers or dams, rather than to bore pumps. A surface pump could be protected easily with a shelter as this type of pump is usually mounted on the ground. Protecting a submersible pump may be more challenging as these are located in water, preferably in the middle of the source where the water is deepest. Some manufacturers provide floating pontoons that are designed to protect submersible pumps in these situations. 

Key notes 
  • A surface pump is installed on the ground next to the water supply. There is a limit to how high uphill a surface pump can be from the water supply (ior how far a pump of this type can draw water upwards). 
  • A submersible pump is installed below the water level or floating on top of the water (in a dam). 

Pump design parameters: flow and head 

There are three key parameters of a pumping system: flow, head and power. Flow refers to the rate at which water can be pumped through the system; the head refers to the amount of resistance to the water movement (primarily vertical height); and the power is the amount of energy required to meet a certain flow rate and head combination. In general, a high head means the pump may deliver only a low flow rate, and a low head means the pump may deliver a high flow rate. The parameters of flow and head are explored further in the following sections. 

Flow rate 

The flow rate is the amount of water pumped in a certain time period. The flow rates able to be met by a pump are usually quoted in L/min or m./day (where 1m. = 1,000L). 

In a solar pumping system, the anticipated flow rates of the pump must be estimated according to the variability of the site’s solar radiation levels: 

  •  If a minimum daily flow rate is required every day, regardless of the available sun hours, the system design would need to increase the required daily flow rate to compensate for reduced pumping performance on sun-reduced days. The calculations for adjusting the daily flow rate are given in the design section. 
  • The hourly flow rate should take into consideration that there is a limited number of sun hours per day to pump the water. The estimated flow rate also needs to take into consideration that the levels of solar radiation vary from hour to hour. For example, for a north-facing array, more water will be pumped in the middle of the day when the solar radiation levels are higher compared to the morning or afternoon when the solar radiation levels are lower. The calculations for estimating an hourly flow rate are given in the design section. 

The flow rate is a critical figure in the design of a solar pumping system and it must be accurately determined to optimise the pumping system design. 

Total dynamic head 

The total dynamic head (TDH) represents the sum of the resistances experienced by the pumped water (Fig. 38). The required system’s TDH is a specification that the pump must meet in order to move the required amount of water. The TDH includes both the height (elevation loss) through which the water needs to be lifted (static head) and the friction of the water running through the pipes (dynamic head). 

  • Static head: This is the vertical distance that the water travels. In a submersible pump, it would be the height difference between the water pump and the water destination. In a surface pump, it would be the height difference between the top of the water source and the water destination. 
a) the static head of a submersible pump; b) the static head of a surface pump.
 
Why doesn’t static head include the horizontal distance of the pipe? 
Static head doesn’t include the horizontal distance of the pipe. This is because it only takes a very small amount of energy to move water horizontally, and energy is only needed to overcome the friction in the pipes (part of the dynamic head calculation). On the other hand, it takes a lot of energy to move water vertically against the force of gravity. 

 

Dynamic head represents the friction losses in the pipes. The main contributors to this parameter are the velocity of the water (flow rate), the diameter of the pipe, the length of the pipe and the pipe material. The dynamic head should also allow for the effect of inline piping accessories such as filters, valves, elbows and inlet pipes. 

The dynamic head represents the pipe friction.
 

The total dynamic head is equal to the sum of the static head and the dynamic head. 

Total dynamic head = static head + dynamic head

The relationship between static head, dynamic head and total dynamic head is shown below. 

The relationship between static head (green), dynamic head (blue) and total dynamic head (red). Dynamic head increases as flow rate (velocity of water) increases but static head is the same regardless of flow rate. The total dynamic head is equal to the sum of these.
 
Key notes 
  • The total dynamic head is a core parameter for sizing the pump. 
  • The total dynamic head represents all of the resistances experienced by the pumped water, both the height through which the water needs to be lifted (static head) and the friction experienced by the water as it runs through the pipes (dynamic head). 

Pump priming 

Pump priming refers to filling the pump with water before starting to remove any air. This is important for pumps that cannot operate if there is air inside them. Positive displacement pumps (helical rotor and diaphragm types) do not need priming as they naturally remove any air inside when they start, and are not damaged by having air inside them. Dynamic (centrifugal) pumps cannot operate with any air inside them and so need some form of priming. 

Most dynamic pumps available for water pumping are self-priming. These pumps need to have their casings filled manually with water when they are initially installed. After that, this type of pump can use the casing water to remove any water that’s inside the pump mechanism each times it starts up. A dynamic pump would also need re-priming if the pump’s casing water was emptiedfor maintenance. 

When starting up any pump, the manufacturer’s instructions should always be followed. 

Key notes 
  • Helical rotor and diaphragm pumps are naturally self-priming. 
  • A self-priming centrifugal pump must be filled with water (primed) when it is first installed, but can self-prime after that. 
  • A centrifugal pump that is not self-priming must be filled with water every time it is started. 

Pump issues: cavitation 

One of the principal causes of damage to a pumping system is cavitation: the implosion of air bubbles in a liquid. Cavitation can have such force that it tears apart metals or ages pump materials prematurely. 

Air bubbles in pumping applications are caused by water boiling when subjected to low pressures. This occurs because at low pressures, water boils at a lower temperature than normal, and boiling water releases air bubbles. As water moves from a low-pressure area to a high-pressure area, these air bubbles can implode, damaging the pump. 

To prevent cavitation, it is important to ensure that the flow rate of the pump will remain on its prescribed curve, as given in the specification sheet. This will prevent excessive pressure drops in the water that is being pumped, minimising the risk of cavitation. 

Key notes 
  • Running a pump at a too-high flow rate could cause it to be damaged by cavitation. 
  • Cavitation can be prevented by following the flow rate specifications of the pump. 

Pump maintenance 

Regular servicing and maintenance of pumps is essential to ensure that the system performs as required for its designed lifetime. Pump manufacturers may advertise their products as ‘low maintenance’ or ‘no maintenance'.  Regardless, the system owner can perform certain maintenance actions to ensure the pumping system is working optimally. 

  • Listen to the water pipes: Listen to the flow of water through the pipes at an accessible point in the system. Sounds of uncharacteristic rushing water could mean increased flow or pressure in the system due to blockages. 
  • Listen to the pump and motor: Rattling or rumbling in the system that does not normally occur could indicate a damaged pump or motor. If you detect such sounds in your system, contact the manufacturer immediately. 
  • Check the float switch: Regularly check the system’s float switch to ensure it is working as per the manufacturer’s documentation. 
  • Other manufacturer maintenance requirements: You should have been provided with an operation and maintenance guide for the pumping system. Always read the installation guide. 

The manufacturer may allow the system owner to replace certain parts without voiding the warranty. Be sure to read the operation and maintenance guides for the system before making such replacements. 

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