Centrifugal pumps and diaphragm pumps or two of the most common types of pumps used on herbicide and pesticide sprayers. While the role of these two pumps is the same, they have very different characteristics that make them suited for very different types of spraying. So when used on a sprayer, what is the difference between a diaphragm pump and a centrifugal pump?
The difference between a diaphragm sprayer pump and a centrifugal sprayer pump is in the physical way they move liquid, the flow rate and pressure that they can produce, and the maintenance required for each one. Centrifugal pumps are generally able to produce higher flow rates and diaphragm pumps will produce higher pressure.
These differences vary depending on pump size, horsepower, materials of construction, and specific pump design. Read on and we will sort out the specific ways these pumps are different and how different factors affect their performance.
Working Principle
Diaphragm and centrifugal sprayer pumps belong to different pump “families”. Diaphragm pumps are in the positive displacement pump family and centrifugal sprayer pumps belong to the centrifugal pump family.
I cover the differences between these two pump families as well as how these pumps work in this article on the different types of sprayer pumps, The short explanation is a positive displacement pump moves a fixed amount of fluid by mechanical means, like a piston stroke or gear rotation. In the case of a diaphragm pump, a set of pistons move up and down inside a cylinder. The pistons have a diaphragm on each one. When the piston goes down it draws liquid in through a check valve, and when it goes up, it forces liquid out of another check valve.
A centrifugal pump moves a continuous flow of fluid as a shaft rotates and the impeller. The spinning impeller generates centrifugal force and “throws” liquid to the outside of the pump case and where it flows out of the discharge port. This creates a vacuum in the center of the pump drawing more liquid into it.
Why is this important? Well, if the discharge on a centrifugal pump is closed while the pump is still running, it can only reach a certain pressure. A positive displacement pump however will continue to push liquid via whatever mechanism that particular pump uses. If there is nowhere for the liquid to go the pressure can build until something breaks. For this reason, positive displacement pumps generally require a relief valve while centrifugal pumps do not.
Flow & Pressure
You may be asking “What’s with all these different pump types?”. These designs are intended to meet different needs. Centrifugal pumps are capable of producing much higher flow rates than diaphragm pumps driven by similar horsepower. Diaphragm pumps in general will develop much higher pressure than centrifugal pumps.
The volume of flow a centrifugal pump can generate is determined by the pump housing size, impeller size, rpm, and horsepower. The flow potential is almost unlimited if the pump is large enough and there is adequate horsepower. There are pumps to move 500, 1000, 2000 gallons, or even more.
In terms of sprayer pumps, they are usually made to move between 25-300 gallons. Diaphragm pumps made for sprayers range from about 1-100 gallons per minute. 7-15 GPM is very common for diaphragm pumps used on turf and tree sprayers.
When it comes to pressure, centrifugal sprayer pumps max out at about 150 psi. Diaphragm sprayer pumps range from about 300-700 psi. There are small 12-volt diaphragm pumps used on sprayers, these usually range from 40-100 psi. They typically produce flow rates of 1-5 GPM.
Maintenance
Eventually, a sprayer pump will need to be repaired or rebuilt. Centrifugal pumps require less maintenance and they generally have fewer parts to replace. The centrifugal pump has a much simpler design than diaphragm pumps, they will all consist of the same basic components. Impeller, seal, shaft, volute, a volute o-ring.
The main issue with a centrifugal pump is a bad seal. If the pump is leaking around the shaft, the seal can be replaced to repair the leak. If an impeller is worn over time, it can be replaced. Some centrifugal pumps will have a lubricated seal to keep them protected against running dry. Running a pump dry will ruin the seal.
Other than that, unless a centrifugal pump is transferring corrosive or abrasive materials, they generally do not require much maintenance. A diaphragm pump on the other hand has a lot of moving parts, different o-rings, etc.
Also, the pump manufacturers recommend that you replace the diaphragms periodically in order to keep the pump working correctly. The diaphragms will wear out over time especially if they are moving an abrasive or harsh chemical.
Advantages of a Centrifugal Sprayer Pump
- Higher flow rate: In general, a centrifugal pump is capable of delivering much higher flow rates than a diaphragm pump. This is why they are the choice for water trucks, nurse trailers, tanker truck unloading, and large agricultural sprayers.
- Lower maintenance requirements: Centrifugal pumps have fewer moving parts and do not require periodic maintenance as diaphragm pumps do. If operated correctly they can last a long time without issue.
- Can handle solids: Centrifugal pumps are capable of handling solids. Some centrifugal pumps are specially designed for large solids and debris, these are known as trash pumps.
Advantages of a Diaphragm Sprayer Pump
- Higher pressure capability: Diaphragm pumps are capable of generating much higher pressures than centrifugal pumps. They are the best choice for tree spraying.
- Can Move Thicker Liquids: Diaphragm pumps are capable of handling substances that are thick and viscous. Oils, molasses, fertilizers, and other liquids can be hard for a centrifugal pump to move efficiently.
Conclusion
To recap things, the main difference between centrifugal and diaphragm pumps for sprayers is their working principle. Centrifugal pumps are better for higher flow rates, while diaphragm pumps are better for higher-pressure applications and can handle thicker liquids. Centrifugal pumps require less maintenance. Hopefully, this helps you decide which one is right for your application. For more information, be sure to read this guide on sprayer pumps.
