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. Pumps are efficiently sized 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. This may confuse us. Our Pump Buying Guide is designed to assist you in determining what pump is best suited for your system. Pump selection is based on the following factors, type of pump, flow rate (GPM) and total dynamic head (measured in feet). Additionally, horsepower, the location and power source are important factors in choosing the type of pump you need. 


What type of pump do I need?

There are so many different types of pumps, for so many different applications, it can seem overwhelming. Centrifugal pumps are most common in irrigation and water features. 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 submersible pond/sump pumps, 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 on your part. This guide will help you understand the parts of the process and the calculations required to come up with 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 certain time frame. This will be 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 systems and 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, (i.e. 100 button drippers) and multiply by the emitters’ flow rate (i.e. 1 GPH).

 

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


To convert this to GPM just 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 then multiply by the flow rate(s) and total. 


Some examples of different emitting devices and calculated flow rates:

Emitter Type

Optimal PSI

Flow Rate per Emitter

# of Emitters

Total Flow Rate

Button Dripper, PC #3558

15

1 GPH

100

100 GPH

Drip Tape, P1 12” spacing #2055

10

0.25 GPH

500

125 GPH

Micro-Sprinkler, SpinRite #4854

25

8.6 GPH

9

77.4 GPH

Pop-Up Spray, 360° head #2643

30

3.7 GPM

6

22.2 GPM

Sprinkler Rotor, #2 nozzle #2380

45

2 GPM

6

12 GPM


2. System Pressure Requirements

Once you have determined the flow rate needed, the next variable to determine will be the pressure (PSI) your system requires. In an irrigation system, this means you will need to determine the 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 Irrigation - 15 - 30 PSI

Spray or rotary sprinkler heads - 30 - 45 PSI

Sprinkler Rotors - 40 - 60 PSI 


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


Elevation

Elevation changes in your design play into pump selection, as well. If the pump sits above the water level it is pumping from, you must consider suction lift into the calculation. Suction lift is defined as the vertical distance between the water level and the pump inlet. Most irrigation pumps are designed to push not pull water and are 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 not possible to measure vertical distance (A to C) then you can measure the static pressure in PSI, with a pressure gauge, and convert to feet by multiplying PSI x 2.31 to get feet of head (vertical distance). To do this you would need to install a pressure gauge on the bottom end of your supply line, fill the line with water from the top end and measure with gauge. Then convert.

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 then simply use Pythagorean theorem, a² + b² = c² where c is 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. Truth is, in fact, the opposite. You must push water through a smaller pipe faster to maintain the outflow needed, which in turn 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 the emitter.


Elevation

Suction Lift - 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 manufacturer’s specification charts.


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

The pressure required at the end of the lines 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 manufacturers 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


Pump Horsepower

Horsepower is what drives the motor to operate the pump. More horsepower means more volume (flow) and more pressure (PSI), although it should not be used as the means of selecting the proper pump size. While oversizing is a common error and can be very inefficient, you can expect a larger system will require a larger pump with more horsepower. Most pumps are designed to push water, so when they have to lift water also, horsepower and impeller size (and shape) may play a role in pump selection. It is likely this has already been considered in the manufacturing of the pump rated to meet your system needs. 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 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 1 if you are pumping water)


Actual power required is called brake horsepower (BHP) which is what would be required to meet your horsepower needs. Pump manufacturers should have 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 AC. Taking this into account, you must have a power source nearby.