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Selecting The Right Pump

Posted by Robbie Bennett on

Here at Mid-South Ag. Equipment, we sell a lot of pumps; (seriously, it's a lot). So as you can imagine, we get a lot of questions from our customers.

"What kind of pump do I need for pumping molasses?"

"What is the difference between a Honda GC and GX engine?"

"What's the difference between a Cast Iron Pump & a Poly Pump?"

Just to name a few, there are a whole host of questions we get asked daily, so we thought it made sense to go ahead and make a post, a "buyer's guide" to sprayer pumps, if you will. We are going to cover a few topics in this post:

1. The most industry common pumps used & their specifications.

2. What these pumps can do & what they were built for.

3. How to pick the right pump for the right job.

4. A few frequently asked questions we get about pumps.

Positive Displacement vs. Non-Positive Displacement

Generally, most pumps can be put into 2 categories. Positive Displacement or Non-Positive Displacement. Roller, Diaphragm and Piston pumps are positive displacement. This means the flow from the pump is dependent on the pump speed. This positive flow is why all the positive displacement pump hook-ups must include a relief valve and bypass line between the pump outlet and the nozzle shut-off valve.

Centrifugal and turbine pumps are non-positive displacement. In these pumps, a rotating impeller creates a centrifugal force that feeds the liquid through the system instead of capturing and discharging a fixed volume per rotation like roller, piston or diaphragms would do. So if the outlet is closed, the impeller would simply continue to rotate harmlessly. So any special relief valves would not be required in centrifugal pump systems.

Now, when we're talking about pumps for sprayer applications, there are six (6) common sprayer pump types most always used:

* Diaphragm Pumps

* Centrifugal Pumps

* Roller Pumps

* Transfer Pumps

* Piston & Plunger Pumps

Let’s go into a little more detail about each one.

Diaphragm Pump
 (Positive Displacement)

This is a work horse of the agriculture industry, this pump is classified as a “Positive Displacement Pump”, which is a fancy way of saying it moves trapped liquid out of an area by some external force. Imagine if you had a bunch of water sitting in your hand, then you made a fist and squeezed really hard. The water would shoot out of your hand, that’s kind of a very basic idea of how a positive displacement works; except instead of your hand, it would be a piston or chamber pushing the liquid out, towards a discharge pipe. Make sense?

Now, most categories of diaphragm pumps are what we call two-chamber positive displacement pumps, and they are self-priming. Which means it does not have an external priming system because it can use the suction in the line by itself via a large reservoir around the pump casing. It will contain two (2) flexible diaphragms, parallel to each other on the sides of the pump. Between both of these diaphragms will be a chamber filled with compressed air.

Once the pump is powered, the compressed air will drive the liquid to the outlet of the pump, making this category of pump a very powerful use case for sprayer solutions and liquid flow transfer.

This is the typical flow of a self-priming diaphragm pump:

1.) The Pump is powered (energized)
2.) The pump will collect fluid in the casing.
3.) Air is suctioned in as fluid goes out.

Centrifugal & Transfer Pumps (Non-Positive Displacement)

In centrifugal pumps, spray chemicals and solutions enter through the center of a rotating impeller that’s drive at speeds up to 6000 RPM. The liquids are forced to the outer edge of the housing; the centrifugal force is what delivers the liquid to the nozzle. Usually a low to medium pressure range. (0-190 psi and a flow of 44 GPM) Because centrifugal pumps have minimum surface to wear and no valves, they are very durable, easy to maintain and well suited for pumping abrasive and corrosive materials.

Because centrifugal pumps operate at higher speeds, the PTO speed must be increased through a speedup gear drive, belt/pulley drive, gas engine drive or a high speed hydraulic motor. You can usually see these made of materials such as cast iron, polypropylene and stainless steel that stand up to a wide variety of agricultural chemicals.

Roller Pumps (Positive Displacement)

Roller Pumps are the number one all-around choice by farmers throughout the world. The rollers revolve inside the pump housing to force the spray solution through the outlet to the nozzle. Roller pumps hav ea low initial cost and are extremely versatile.

They operate efficiently at PTO speeds of 540 and 1000 RPM and have a wide pressure range of up to 300 PSI and flow rates of 2-62 GPM. Roller pumps are self-priming and easily adapt to PTO or gas engine drives. Specific seal, roller and casting materials can be selected for compatibility with certain herbicides, pesticides, fungicides and fertilizers.

Piston and Plunger Pumps (Positive Displacement)

Piston/Plunger pumps have a shaft, pistons or plungers and “inlet” and “outlet” poppet valves. The design of the pump converts the rotational drive into a oscillating vertical motion. On the down-stroke, the inlet valve opens, filling the chamber with solution. On the up-stroke, the outlet valve opens, and the piston forces the solution to the nozzle. Piston pumps deliver relatively low flow rate, less than 10 gpm (40 lpm), at pressures up to 1000 psi (69 bar). Plunger pumps are designed with ceramic plungers which can operate at higher pressures up to 3600 psi (248 bar) at flows up to 45 gpm (170 lpm).

The main difference of construction is in a piston pump the sealing material moves with the piston while in a plunger pump the sealing material (u-cups) are stationary with only the plunger in motion. The replaceable piston cups can be of leather, fabric or Buna-N rubber, depending on the type of solution to be sprayed. They can be driven by 540 rpm PTO, gas engine, hydraulic or electric motor. Their low volume/ high pressure capability permits use in general spraying as well as task-oriented applications such as spraying fence rows and ditches, and hydrostatic testing. Plunger pumps are used primarily for cleaning operations.

Pump Drives

How a pump is to be driven is often a primary consideration in selecting the proper type of pump. If the power source has already been determined, the following chart may be of further help in selecting the type of pump that is best suited to your needs.

pump drive chart with RPM Rates

Flow & Pressure Requirements

Pump pressure required is often dependent on the application. Whether it be low pressure band spraying or high pressure tree spraying, it is the application that dictates what pressure is needed to get the right performance at the spray nozzle. Once you know what pressure is desired choose a pump with extra pressure capacity due to losses in pressure as it goes through the system components (strainer, valves, elbows, hose, etc.) out to the nozzle

flow and pressure chart for pumps |

Pump flow required is dependent on several factors. Application rate, width of boom or size of nozzle, speed of travel and agitation. To review your pump flow requirements follow through the calculations presented on the following pages. As with pressure you will want to choose a pump that has additional flow so that it meets your application needs over time as performance drops due to component wear.

flow rate chart for pumps

Determining Pump Flow and Pressure Requirements

Every pumping task has an optimum volume and pressure requirement. Determining that optimum (and selecting the pump that delivers it) is key to an efficient and economical spraying system operation.

Pressure requirements for agricultural pumps are dependent on both the material to be applied and application targets. Soil-applied herbicides generally require a relatively low pressure pump rating of 30-60 psi with foliar-applied herbicides at the top end of that range and slightly higher. Insecticides and fungicides can require higher pressure ratings of 100 to 500 psi. Pressure must be sufficient, in the case of heavy foliage field crops and orchard crops, to penetrate the leaf foliage. In the case of orchard crops, pressure must also be sufficient to carry material up and over as well as into the canopy.

A number of factors must be considered to properly determine the total flow you will need from your pump. They include:

  • Type of spray operation (broadcast, banding, low-level, etc.)
  • The chemical’s application rate, ground speed, boom width, hose length, tank agitation, etc.

The spray task is the first consideration in determining flow rate and pressure needs. The following formulas and calculations may help.

Calculating Agitation Requirements

The pump must produce enough flow for both the application rate and tank agitation requirements. Too little agitation will not keep the solution in proper suspension and too much agitation may cause foaming. Here are rule of thumb formulas for calculating how much additional pump flow you will need for agitation.

Reducing Agitation Flow Requirements

Agitation flow requirements can be reduced by using jet agitation in the tank. Jet agitators use a venturi design to multiply agitation output. Depending on the jet agitator model and pressure, one gallon per minute input can provide two to ten gallons per minute agitation output. If your sprayer is equipped with a jet agitator, consult the operator’s manual or documentation to find the output to input ratio and adjust your flow required for agitation accordingly.

Factor in an “Excess Flow” Requirement

It is wise to have some excess flow capacity so you will not end up with an undersized pump because actual operation conditions may cause changes in spray system performance (such as normal pump wear, operating at less than rated speeds, etc.). Hypro recommends you add an additional 20% to your calculated total pump flow requirement to compensate for these variables. Plumbing systems have a number of restrictions that will result in a pressure drop from the pump to the actual spray point. These must be taken into account and minimized.

Calculating Pump Flow for Broadcast Boom Sprayers

Chemical application is measured in gallons per acre (gpa) or litres per heactres (l/ha), whereas pump flow is stated in gallons per minute (gpm) or litres per minute (lpm). To calculate the pump flow required by a broadcast boom sprayer, multiply the application rate (from the chemical label) by the sprayer ground speed. Multiply the sum by the boom width on your sprayer. Then, divide that number by 495 for US units or by 600 for metric units. As a formula, it is written like this:

Calculating Pump Flow for Banding Sprayers

First, multiply the band width by the number of rows to determine the total width (w). Then, multiply the application rate (from the chemical label) by the ground speed. Multiply that result by the total width (w) calculated earlier, then divide the result by 5940 for US units or 60,000 for metric units. Here’s how the formula appears:

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