Top 5 Things to Know to Properly Size a Pump
If you’ve ever talked to a pump technician or sales engineer about ordering the right pump for your application, you know they typically ask many questions. That’s because they see the pump not only as a device for moving fluids but as they main component in a network of equipment and product attributes that all have to work together to maintain flow throughout the processing system.
As you can imagine, using the wrong pump for your application can lead to processing slowdowns or shutdowns. Just as costly, the wrong pump can result in inconsistencies in product or the wrong product altogether. That’s why pump representatives ask for information about your application.
Pump experts at CSI consider five key factors for pump size in sanitary applications in food, dairy, beverage, and pharmaceutical applications. In this article, we explain why the factors are so important.
While even more factors must be considered, these are the top five:
- Differential Pressure
- Fluid Temperature
- Viscosity
- Flow Rate
- Density
Learning how the five key variables can affect flow helps in the selection, and having as much of the information with you when ordering will make the ordering process go smoothly.
The big picture
In processing systems, fluids are introduced into pumps under various conditions, with atmospheric pressure and fluid temperature, viscosity, and density all affecting the rate at which fluids make their way into the pump. At the point of entering the pump, several factors influence how the fluid continues on its way out of the pump.
One factor is pump design itself and its design-related operating parameters such as:
- Size
- Horsepower
- Speed
Another is the effect the pump design has on the fluid’s temperature, viscosity, and density. All of the factors combine to influence the pressure and flow rate of fluid as it enters and exits the pump.
While fluids move from one location to another in a processing system, they change under varying pressure, flow, and temperature conditions. To meet the needs of a wide range of liquid products and their flow properties — for example, the range from water to oils to honey —manufacturers have designed a range of pump solutions.
Having a full understanding of the fluid and the process is critical to successfully choosing the right pump technology and right pump size. Alfa Laval and CSI always live by the mantra “Right the first time.”
~ Russell Jones, Commercial Sales Manager - Pumps, Alfa Laval
1. What is the differential pressure?
Differential pressure is the difference between pressure at the pump inlet and pressure at the pump outlet. The amount of pressure available on the inlet side of the pump is called Net Positive Suction Head available (NPSHa) and is determined by several factors in your system.
Pumps vary in how much pressure they require on the inlet side of the pump to work properly, which we refer to as Net Positive Suction Head required (NPSHr).
That’s why your pump representative may have questions about the system components included before the pump, such as:
It’s because they all have an effect on NPSHa as the fluid attempts to make its way into the pump.
Once the fluid is in the pump, the pump’s mechanical action creates pressure to move fluids through the outlet side of the pump where the fluid now has to overcome pressure created by gravity and system components on that side of the system. The difference between inlet and outlet pressure is the pressure differential.
The importance of Net Positive Suction Head required (NPSHr)
All pumps have minimum pressure requirements at their inlets in order to work properly. The technical phrase for the minimum requirement is Net Positive Suction Head required (NPSHr).
The Net Positive Suction Head available at the pump inlet must always be greater than the Net Positive Suction Head required (NPSHr) to avoid cavitation.
Differential pressure calculation
Let’s say your pump is designed to generate 40 psi. With 3 psi of pressure on the fluid as it arrives at the pump, a pressure gauge at the inlet reads 3 psi. Since the pump is designed to create 40 psi of pressure, the discharge gauge reads 43 psi, and a differential pressure gauge reads 40 psi.
Pressure entering the pump at 25 psi and a discharge reading of 65 psi means a differential pressure of 40 psi. If installed, a differential pressure gauge measures pressure at the inlet and outlet to give you the resulting differential.
In centrifugal pumps, rotational speed of the impeller vanes is what adds pressure to the fluid. The rotational speed is constant. The impeller draws the fluid into the pump by reducing pressure at the impeller eye and then increases fluid pressure as the impeller spins.
If a change in the system on the discharge side of the pump causes flow to decrease — such as when you close a discharge valve — pressure increases at the pump discharge side because the impeller keeps working at a constant speed.
The result is excess velocity energy which is transformed into pressure energy. In addition to the system not operating efficiently, the added pressure can affect the life of seals.
A pressure drop before the pump causes a different kind of problem. Let’s say you have a filter installed in pipes on the inlet side of the pump. If the filter is clogged, the pressure moving the fluid through the pipe will fall in the pipe section between the filter and the pump.
At the pump inlet, then, the pressure will also be lower, and if the pressure available falls below the pressure the pump requires to work efficiently, cavitation occurs.
Cavitation is a gravelly sound created by small bubbles that form and burst and over time damage the impeller.
That’s why pump technicians ask about factors that affect differential pressure.
2. What is the fluid’s temperature?
Because temperature-sensitive fluids change their flow properties with temperature changes, knowing processing temperatures helps with pump selection. Temperature is one variable that can affect product viscosity while it moves through pumps, pipes, heat exchangers, and other system components. High viscosity means higher resistance to flow; low viscosity means lower resistance to flow.
If you want to move water at 20 gallons per minute at room temperature, you can look at a pump curve to determine the pump size you need for any pressure situation within the pump’s range. However, complications must be accounted for when the viscosity of your product is not the same as viscosity of water or if it’s processed at temperatures higher or lower than room temperature.
Some food sauces, for example, have low viscosity when heated but thicken to high viscosity when cooled. Similarly, heating honey makes it flow faster than when it cools down to room temperature. That’s why it’s crucial to know product type, viscosity, and processing temperatures.
3. What is the fluid viscosity?
Dynamic viscosity is a measure of a fluid’s resistance to flow. We can imagine that water is less viscous — or resistant to flow — than corn syrup, so corn syrup has higher viscosity than water.
The good news is if you're not sure what your fluid viscosity is at its processing temperature, your processing expert can help. They can do rheology testing, which means you send in a product sample, they send that sample to a lab, and they provide you with the viscosity measurement from the lab.
Some liquids change viscosity when under stress or pressure, such as when they contact a rotating impeller inside a pump. Some liquids become less viscous (thinner) with increased force, while others become more viscous (thicker) with increased force. By comparison, other liquids, such as water, do not change their viscosity, no matter how much force is applied.
Positive displacement pumps deliver a constant flow of fluid at a given pump speed. When viscosity increases, however, resistance to flow increases, so to maintain system flow at higher viscosities, pumps require more horsepower.
4. What is the flow rate?
The pump you choose determines the flow rates you can achieve under the processing parameters of your system design. What is flow rate? Flow rate is the volume of fluid moved by the pump in a given time frame, as measured, for example, in gallons per minute or liters per second. A pump that moves 32 gallons of fluid every minute has a flow rate of 32 GPM.
How to calculate flow rate
Flow rate formula: Volume of fluid moved/duration of flow = flow rate.
- 200 liters moved in 2 minutes = 100 liters per minute flow rate
- 500 gallons moved in 6 minutes = 83.3 gallons per minute flow rate.
To determine flow rate when you don’t have a flowmeter built into your system, you can do a simple test.
- Use a 5-gallon bucket or another container whose volume you know.
- Using a timer, open the valve nearest the pump location until the bucket is full.
- Use the pump flowrate calculation: Divide the container volume by the length of time it took to fill up to get the flow rate. 5 gallons / 1 minute = 5 gallons per minute. 5 gallons /30 seconds (0.5 minutes) = 10 gallons per minute.
5. What is the fluid density?
Fluid density is a measure of a fluid’s weight by volume. More density in a fluid means more weight, given a certain volume and generally higher viscosity (resistance to flow). Fluid density of water is less dense than corn syrup, for example, so when you put equal volumes of water and corn syrup side by side, the corn syrup weighs more than the water. Density is why water floats on top of corn syrup — water is less dense than corn syrup. Knowing how the density of the product compares to water is important due to the fact that all pump curves are created.
The secret to pump selection? Paint a picture.
Because pumps are only one part of your processing system, selecting the right size is complicated by several system factors.
To help communicate your needs to a pump processing expert, create a sketch of your process system with the pump and system elements located just before and just after the pump.
The sketch and the five key factors will help you and your rep determine the best pump for your application.
A Guide to Choosing the Right Pump for Hygienic Applications
This guide is intended for engineers, production managers, or anyone concerned with proper pump selection for pharmaceutical, biotechnology, and other ultra-clean applications.
Next Steps
As you know, using the wrong pump for your application can lead to processing slowdowns or shutdowns. Just as costly, the wrong pump can result in inconsistencies in product or the wrong product altogether.
That’s why pump representatives ask for information about your application. Pump experts at CSI are ready to answer your questions and help ensure you choose the best pump for your application.
To speak with our pump experts, give us a call today at 417-831-1411 of click the link below!
ABOUT CSI
Central States Industrial Equipment (CSI) is a leader in distribution of hygienic pipe, valves, fittings, pumps, heat exchangers, and MRO supplies for hygienic industrial processors, with four distribution facilities across the U.S. CSI also provides detail design and execution for hygienic process systems in the food, dairy, beverage, pharmaceutical, biotechnology, and personal care industries. Specializing in process piping, system start-ups, and cleaning systems, CSI leverages technology, intellectual property, and industry expertise to deliver solutions to processing problems. More information can be found at www.csidesigns.com.