Pump Sizing for Efficient Irrigation

Pump Sizing for Efficient Irrigation

When it is time to irrigate a Hemp/Cannabis crop, many growers have asked if larger pumps are more efficient in their irrigation process. A good rule of thumb in selecting a proper pump size for your Hemp crop is that the total horsepower requirement will be 10% higher than the GPM needed to attain a pressure of 20 PSI at the farthest nozzle.

This will assure that the crop will receive the required GPM and pressure (PSI) needed throughout the irrigation system. For example, if you have 12 GPM needed to run your sprayer at 20 PSI, a pump with 14 GPM or more will properly size this application.

For irrigation systems requiring more than 100 psi, the pumps are often sized at 25% higher GPM than needed. For example, if you have 8 GPM required to run your system at 100 PSI, a pump with 10 GPM or more will properly size for this application.

Why Do You Need Correct Pump Sizing for Your Hemp Farm?

The proper sizing of irrigation pumps is necessary for several reasons:

1) GPM, not PSI, rates irrigation pumps. An undersized pump for the system will cause either the GPM to be reduced or the pressure (PSI) to drop. Both of these conditions can cause uneven spray coverage and water damage to your Hemp crop.

2) A pump oversized will cost more initially but could save money by not replacing a pump prematurely.

3) An adequately sized irrigation pump will last many years and provide many seasons of reliable service.

4) The "in-line" pressure losses due to friction in the lines can affect the actual GPM needed to run the pumps.

Although pumps supplied to meet the water needs of agricultural installations are usually sized by irrigators, this sizing process can be difficult without adequate knowledge or experience. That's why it's critical to understand how to size a pump for irrigation.

How Much Water Must the System Deliver?

Before selecting a pump for irrigation, you must first understand how much water the system needs to deliver. The average plant density of Hemp acreage is 2,500 plants per acre, and the total irrigation requirement includes:

1) The root zone (plant canopy)

2) Sidewalls and ditches

3) Roads and access ways

It is important to remember that proper irrigation management practices will enable an optimal water application distribution in the root zone. This may not be achieved using edge sprinklers, only at the plant canopy level.

Total system requirements are always determined by your average plant density per acre multiplied by your Root Zone Depth (RZD). Root Zone Depth is defined as the depth of water penetration to meet each plant's root zone requirement.

If the system is to be operated efficiently and economically, the irrigation map for each field must be determined. An irrigator should ensure that all portions of the field are supplied with water. Water is lost between the outside edge of the crop and the ditch or road. You can minimize this additional loss by using "in-line" pressure regulation devices (pressure regulators, pressure vacuum breakers).

Proper sizing of pipelines will also reduce this loss; the standard diameter of steel pipe is 4" and 6". A properly sized pressure regulator will ensure that the proper GPM is delivered to the edge of the field even with a reduction in PSI.

Lateral Lines, Mainlines and Sub-Mains

For an efficient water application distribution, it may be necessary to run several smaller diameter pipelines (lateral lines) from the mainline to deliver water to different field parts.

Pipe sizes for mainlines can be reduced to 90% of what was previously calculated because there is usually less friction in larger diameter pipelines. The same sizing process may determine mainline pipe size when applying water to multiple fields served by one common water source.

Factors for Pump Selection

The pump is the heart of any irrigation system. The pump must deliver the required GPM at the necessary pressure for efficient water application distribution. The need to operate pumps at their maximum efficiency throughout the irrigation season should be considered when selecting the type of pump.

One drawback to operating a centrifugal pump at high efficiency for extended periods is its tendency to cavitate. This can be overcome by using a submersible turbine pump to deliver 70-80% of its total capacity at low head pressure.

Once the pressure builds up, the turbine impeller begins generating power allowing it to move more water with less power. It's essential to consider these factors when deciding which pump is right for your system.

Pump sizing can be determined by:

PSI = Feet of pump head/2.31

The relationship between PSI and GPM remains constant regardless of the type of pump selected, but the efficiency of pumps varies.

Factors for Calculating the Horsepower for Pump Sizing

Let's assume you have an area requiring irrigation water to be delivered at 50 PSI and 10 GPM. This information should provide enough information to begin calculating your pump horsepower. All three factors should be recorded before starting the calculation process.

  1. Feet of Head: You have 25 feet of the head from the water source to the pump location in this scenario.
  2. Gallons Per Minute (GPM): You need 10 GPM of irrigation water delivered at 50 PSI.
  3. Pressure (PSI): Your pressure is 50 PSI which must be converted

The horsepower required to operate a pump can be calculated by first converting the head (ft) and the GPM (GPM) into gallons per minute, then multiplying those two numbers together and converting that number into horsepower (hp).

What is the Critical Flow Rate and How to Calculate It?

The critical flow rate is the minimum required to operate an irrigation system. It differs from the recommended flow rate, which generally provides a high enough flow for additional plants or emitters to be installed without compromising performance.

A significant fact to keep in mind is that a pump can only deliver its rated capacity or less. The maximum flow rate cannot be exceeded under any circumstances, so the largest-diameter pipe must be used. In addition, components such as valves and controls should be sized to accommodate the highest flow rate required by the system.

Critical Flow Rate Example: Drip Irrigation

For drip irrigation systems where each emitter represents one plant, the critical flow rate is the water requirements of all emitters. The critical flow rate for drip irrigation systems with eight emitters is 3 GPM (gallons per minute). The recommended flow rate for drip irrigation is higher than this and 4-5 GPM. A formula to determine the critical flow rate:

The critical pressure ratio is the difference in water pressure between where the pump is located and where it will be delivering water. If this figure is greater than ten psi, then a center pivot irrigation system might not be appropriate for that location.

If you are unsure whether your proposed application requires a critical flow rate calculation, contact the manufacturer of the equipment used or another irrigation company with experience in sizing pumps for this type of application.

Calculating the Total Dynamic Head

The total dynamic head is the pressure required to move irrigation water from its source to the point where it can be distributed. It represents an energy loss due to friction caused by flow through pipes, valves, and other equipment.

This figure is expressed in feet of lift or pounds per square inch (psi). The formula for calculating this number is also used to calculate the required horsepower of a pump, which is why it's called the total dynamic head. The formula is:

TDH= PSI + Elevation change + Suction lift – pressure boost + friction loss

TDH is the pressure to push water through a system. The suction lift requires the least energy, the pressure boost requires the most, and friction loss varies according to pipe size. It's important to remember that this equation only calculates static head loss, which does not include pump power consumption or gravity head.

One way to determine what size pump is needed is to identify the maximum amount of GPM required and compute this based on the design pressure and elevation change plus static pressure loss, which will result in an approximate horsepower requirement.

Once you know the horsepower of the pump, you can find one with a slightly higher HP and higher GPM output. This will reduce the number of runs required to irrigate your field and increase efficiency.

The head is one of two critical measurements used when sizing a pump for irrigation. Head represents the distance the water is raised. Pump head measurements are directly related to pressure requirements. The more head required, the greater the pumping height and the higher the pressure needed.

Head and GPM are two values that define the pump requirements for your irrigation system. For efficient operation, it's crucial to have a professional investigate all of the variables before buying equipment.

Buy the Right Irrigation Pump for Your Hemp Farming from FloraFlex!

Hemp / Cannabis irrigation is a specialized field, and every water source requires calculations specific to that area. With years of experience in the irrigation industry, Flora Flex understands your needs and is here to help. Contact us for help designing your FloraFlex Irrigation System!