Pump Buying Guide

Choosing the right pump for the job encompasses multiple variables. Unfortunately, you cannot just pick a pump by the horsepower it produces or the gallons per minute or hour it claims to put out. Off the shelf pumps may work for your water fountain or garden hose end sprinkler but not necessarily for an irrigation system. A pump is sized for efficiency according to the water flow and water pressure your project requires. As consumers, when we want to purchase a water pump, we often say "we need a pump with 45 PSI and 28 GPM flow", but only see specifications noting maximum feet of head and GPM flow. Which causes confusion. Our Pump Buying Guide is designed to assist you in determining what pump is best suited for your system. Pump selection includes the following factors, type of pump, system flow rate (GPM), and total dynamic head (measured in feet). Additionally, horsepower, location, and power source should be considered when choosing the type of pump you need. 

What type of pump do I need?

There are so many different types for the various applications it can seem overwhelming. Centrifugal pumps are most common in irrigation. Centrifugal pumps move or push the water with an impeller. If you are unsure what type of pump you need, talking to a pump professional is probably the first step in selecting a pump that best fits your application. Here at Drip Depot, we carry the LP centrifugal pumps, several submersible pumps and pond/sump pumps. All of which are useful in several applications, such as water features, hot tub/spa drainage, water transfer, dewatering, and even drip irrigation. Our supplier is always happy to help. You may visit Munro Pump at http://www.munropump.com.

Sizing the Pump

Selecting the most efficient pump for your application requires some research and calculating. This guide will help you understand the parts of the process and the calculations necessary for the minimum specifications for a pump to fit your application. In the end, this will allow you to use the pump manufacturers pump curve charts to select the most efficient pump for your system.

1. Flow Rate

The first variable to consider is the maximum flow rate your system. Flow is the volume of water moving through your system in a specific time frame. The result is the gallons per hour (GPH) or gallons per minute (GPM) your system, or largest zone, will require at one time. Drip irrigation is most often noted in gallons per hour (GPH), while other larger or high pressure systems are noted in gallons per minute (GPM). To calculate flow rate you will want to add up the number of emitting devices in your system, e.g., 100 button drippers, and multiply by the emitters’ flow rate, e.g., 1 GPH.

100 drippers x 1 GPH each = 100 GPH required in the system.

To convert this to GPM, divide by 60. The minimum flow rate of the pump should be equal to or greater than the maximum flow rate of your system. Follow the same idea for a higher pressure lawn sprinkler system, count the total number of sprinkler heads in the largest zone and multiply by the flow rate(s) and total.

Some examples of different emitting devices and calculated flow rates:

2. System Pressure Requirements

Once you have determined the flow rate, the next step is determining the pressure (PSI) your system requires. For irrigation, this means you will need to figure out the system pressure required at the end of the lines in the largest zone per the optimal rating of your emitting device. 

Examples of pressure requirements in irrigation systems are:

Drip/Micro Irrigation:  10 - 30 PSI

Spray or rotary sprinkler heads:  30 - 45 PSI

Sprinkler Rotors:  40 - 60 PSI 

Additional factors in your irrigation system that will affect pump selection include suction lift, elevation change, and friction loss.

Elevation

If the pump sits above the water level it is pumping from, the suction lift is considered in the calculation. The suction lift is the vertical distance between the water level and the pump inlet. Most irrigation pumps are designed to push, not pull, water and are often not designed to lift water more than 25 feet. Furthermore, if the area you are taking the water to is higher than the location of the pump, you must calculate the vertical distance from the pump inlet to the highest point in your system. 

If it is impossible to measure vertical distance (A to C), then you can measure the static pressure in PSI with a pressure gauge, and convert it to feet by multiplying the PSI x 2.31 to get feet of head (vertical distance). You will need to install a pressure gauge on the bottom end of your supply line, fill the supply line with water from the top end and measure with a gauge. Then convert using the formula mentioned earlier.

Alternative methods will involve thinking back to your high school mathematics days. You can figure out the side dimensions of a triangle when you have at least one side measurement and one angle degree (not the right angle) using trigonometry - sine, cosine, and tangent formulas. Or, if you know two side dimensions use the Pythagorean theorem, a² + b² = c², where c is the hypotenuse (longest side).

Friction Loss

In many systems, friction loss will also be a part of the equation. As water moves through the pipes and fittings in your irrigation system, friction loss occurs, which reduces pressure. Most manufacturers of tubing and pipe will have a friction loss chart to help with this. Choosing the optimal pipe size is relevant to an efficiently operating system. A common misconception is a smaller pipe size will increase pressure. The truth is, in fact, the opposite. You must push water through a smaller pipe faster to maintain the outflow needed, which increases pressure loss. In fluid motion it is best to keep velocity to no more than five feet per second. 

Here is a sample of a Friction Loss Table:

Let’s lay out the formula in easy-to-follow steps.

Total GPM (gallons per minute) __________gpm

Add up all the emitters in your system or largest zone multiplied by the flow rate of each emitter.

Elevation

Suction Lift - the vertical distance between water level and pump inlet. __________ft

If using a submersible pump this will be zero.

Elevation Change - the vertical distance between the pump inlet and the highest point in your system. __________ft

Friction Loss (in feet) __________ft

Here is a link to our Friction Loss Calculator for our poly tubing. For PVC or iron pipe see the manufacturer’s specification charts.

Convert to head in feet using the following equation: PSI x 2.31 = Head in Feet (or Feet of Head).

Required PSI (pounds per square inch) converted to head in feet __________ ft

The pressure required by the emitters or watering devices in your system or largest zone. Convert to head in feet using the following equation: 

PSI x 2.31 = Head in Feet (or Feet of Head).

Total Dynamic Head (TDH)__________ft

Add up the feet measurements.

You can now use the GPM and the TDH to map your pump requirements to the manufacturer's pump curve performance chart for the closest pump to meet the needs of your irrigation system, water feature, or other application.

Pump Curve Chart for Munro Pond Pumps

For a full overview of ALL of the pumps we carry, here is a link to our pump chart: https://dripdepot.freshdesk.com/a/solutions/articles/11000097949

Pump Horsepower

Horsepower is what drives the motor to operate the pump. More horsepower means more volume (flow) and pressure (PSI), although it is not what you use to select the proper pump size. While oversizing is not recommended, you can expect a larger system to require a larger pump with more horsepower. Most pumps are designed to push water, so when lifting water also, horsepower and impeller size (and shape) may play a role in pump selection. For situations where this must be considered some helpful formulas are noted below.

WHP = (TDH×Q×SG) / 3960 

• Water horsepower (WHP) = minimum power required to run the water pump
• Total Dynamic Head (TDH) = Vertical distance liquid travels (in feet) + friction loss from pipe (in feet)
• Q = flow rate of liquid in gallons per minute (GPM)
• SG = specific gravity of liquid (this equals one (1) if you are pumping water)

The actual power required is called brake horsepower (BHP) which is the requirement to meet your horsepower needs. Pump manufacturers should list a pump efficiency rating which is generally somewhere between 85% - 50%.

BHP = WHP / Pump Efficiency

Power Source

Since a pump generally requires a motor to operate it, you must have a source of power that matches the pump; AC (electrical), DC (battery), gas/fuel or possibly solar. The pumps we currently sell, at Drip Depot, are electrically powered, single phase 110V or 220V AC. Taking this into account, you must have a power source nearby.