Pump Head vs Discharge Pressure (Easy explanation and examples)
Jun 28, · Differential Head is the pressure differential created by the pump under a given set of conditions with a reference fluid and is independent of the absolute pressure of the environment in which the pump operates. Head is the height to which a pump can raise water straight up. Water creates pressure or resistance, at predictable rates, so we can calculate head as the differential pressure that a pump has to overcome in order to raise the water. Common units are feet of head and pounds per square inch.
Centrifugal Pump curves show 8 important factors that are critical in selecting the right pump for your application. The Pump curve also displays the Pump model, the Pump size and number of stages for multi-stage pumps.
The Pump curve shows the Flowrate range on the horizontal scale. The vertical scale on the main curve is always differential head. Since pressure is relative to the process fluid density, virtually all pump curves display this metric in a unit of length FT or Meters.
This makes the pump curve applicable to most liquid types. Exceptions are liquids with solids or high viscosity. Most centrifugal pumps are characterized by an elliptical head curve which is at a maximum at zero flow Shut Off and a minimum at Run Out. Once the Rated Flow is plotted vertically, and the Rated Head plotted horizontally, where the two points meet, is Rated Duty Point, which is usually shown as a triangle.
Pump curves are drawn showing the performance of the Rated Impeller Diameter. Some curves are drawn to show the performance of several Impeller Diameters from the Maximum Impeller to the Minimum Impeller. Centrifugal pump impellers can be machined to larger or smaller diameters to alter the performance to best match the application. The Rated Diameter Impeller is the actual size of the impeller installed in the pump.
NPSHR is another important value that is plotted on most pump curves. NPSHr how to remove pubic hair for women at home generally increase exponentially as flow rate is increased.
Now you have a general understanding of how to read a centrifugal pump performance curve. Stay tuned for our upcoming training blogs that explore more aspects of pump application, design and maintenance.
Below we identify these factors on this Curve. Share this:. Pump Hold Down Fasteners April 14, We look forward to serving you!
Jul 11, · TOTAL DIFFERENTIAL HEAD (THD)=System Head = total discharge head - total suction head, i.e. H = h d - h s h d = h sd + h pd + h fd h s = h ss + h ps = h fs where: h d = total discharge head h s = total suction head h sd = discharge static head h ss = suction static head h pd = discharge surface pressure h ps = suction surface pressure h fd = discharge friction head h fs = suction friction head. Total dynamic head (sometimes called differential or generated head) is a measure of the energy imparted to the liquid by the pump, and is equal to the algebraic difference between the total discharge head and the total suction head. Total dynamic head, where suction lift exists, is the sum of the total discharge head and total suction lift. Sep 24, · Differential Head, often referred to as “Head” or “Total Dynamic Head”, is a universal method of quantifying the pressure the pump is capable of developing. Since pressure is relative to the process fluid density, virtually all pump curves display this metric in a unit of length (FT or Meters).
How to read a pump performance curve remains a topic of great interest across the food , dairy , beverage , and pharmaceutical processing industries , so in this post we provide important information on two of our most popular styles — Centrifugal and Positive Displacement.
Also called a pump selection curve, pump efficiency curve, or pump performance curve, a pump curve chart gives you the information you need to determine a pump's ability to produce flow under the conditions that affect pump performance. Reading pump curves accurately helps you choose the right pump based on application variables such as:. The curve will show you if the pump you have selected will perform in that application.
Centrifugal pump curves are useful because they show pump performance metrics based on head pressure produced by the pump and water-flow through the pump.
Flow rates depend on pump speed, impeller diameter, and head. Head is the height to which a pump can raise water straight up. Water creates pressure or resistance, at predictable rates, so we can calculate head as the differential pressure that a pump has to overcome in order to raise the water. Common units are feet of head and pounds per square inch. A pump curve calculator might offer different units such as Bar or meters of head. As Figure 1 illustrates, every 2.
Flow is the volume of water a pump can move at a given pressure. Flow is indicated on the horizontal axis in units like gallons per minute, or gallons per hour, as shown in Figure 2. While pump curves help you select the right pump for the job, you first have to know the total dynamic head for the application. Learn more about centrifugal pumps and key calculations. Let's say you want to know the flow rate you can achieve from the pump in Figure 3 at 60 Hz when the design pressure is 80 PSI.
In this case, the curve shows that the pump can achieve a flow rate of gallons per hour at 80 PSI of discharge pressure. Because some centrifugal pumps operate across a range of horsepower, their curves will include additional information.
Figure 4, for example, features a pump that can operate from 2 to 10 horsepower depending on desired performance. Impeller size is another variable for meeting performance requirements.
The curve above shows impeller trim sizes, at the right end of each curve, ranging from a minimum of 4. Reducing impeller size enables you to limit the pump to specific performance requirements. The curve above shows maximum pump performance with a full-trim impeller, minimum pump performance with a minimum-trim impeller, and performance delivered by the design-trim impeller, or the impeller trim closest to the design condition.
Impellers are typically trimmed 0. Impeller size is also a factor when handling shear sensitive liquids, or liquids that change viscosity when under pressure. NPSHr is the minimum amount of pressure required on the suction side of the pump to avoid cavitation , or the introduction of air into the fluid stream.
NPSHr is determined by the pump. NPSHa, with "a" standing for available , is determined by the process piping. Good pump efficiency means that a pump is not wasting energy in order to maintain its performance point. When selecting a pump and motor combination, consider not only the total current demand but future demand to ensure your selection has the capacity to meet changing requirements. To that end, s izing the pump for performance variables rather than peak efficiency is a common practice.
For example, while the middle of the pump efficiency curve is generally where a pump is operating at maximum efficiency in terms of pressure and flow rate , moving right on the curve above shows an increase in horsepower needed to maintain a flow rate as head increases. For example, 2 hp is required for a flow rate of 40 gpm with 80 feet of head, but maintaining 40 gpm of flow at feet of head would require a 3 hp motor.
You can audit pumping systems using pump performance characteristics. Once you determine the best efficiency point BEP for your application, you can make adjustments to improve overall system efficiency, such as adding a variable frequency drive VFD and changing the diameter of the pump impeller.
Controlling flow rate by adjusting pump speed via VFD instead of pressure valves can result in better efficiency and greater energy savings. When using pumps in parallel, you can increase flow rate at the same rate of head.
Finally, variable speed pump curves show flow rates at various RPM, as shown in Figure 6. A positive displacement PD pump produces the same flow at a given speed in revolutions per minute--RPM no matter what the discharge pressure. Positive displacement pump curves give you the information you need to determine a pump's ability to produce flow under the conditions that affect pump performance. A Positive Displacement Pump Curve answers several important questions during the pump selection process:.
Remember that some performance curve calculators might include units such as liters per minute LPM , so check calculation units when using calculators. 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.
Low viscosity also affects pump performance in the form of slip. Slip is the internal recirculation of low viscosity fluid from the discharge side of the pump back to the suction side of the pump. In Fig. With changes in viscosity and pressure, slip correction indicates that flow capacity drops from a high of about 7 GPM to a low of about 3. If slip is not a factor, use the 0 PSI line to determine flow rate. Because PD pumps generate flow to transport relatively high viscosity fluids, PD pump selection requires analysis of three key influences on fluid transfer:.
By common sense alone, we can imagine that water is less viscous, or resistant to flow, than corn syrup, so corn syrup has a higher viscosity than water. We measure internal resistance to flow as absolute viscosity also referred to as dynamic viscosity. As the following comparison shows, differences in viscosity vary dramatically by fluid:. Water is less dense than corn syrup, for example, so if you put equal volumes of water and corn syrup side by side, the corn syrup would weigh more than the water.
Also, due to the differences in density between water and corn syrup, water would float on top of the corn syrup if combined. The following comparison shows the difference in density between water and corn syrup in kilograms per cubic meter:.
Shear-sensitive liquids change viscosity when under stress, such as when they are hit by an impeller inside a pump. Some liquids become less viscous with increased force called shear thinning , while others become more viscous with increased force called shear thickening.
The viscosity of shear-sensitive substances through a process line does change, however. When you size a PD pump it will be important to select the correct brake horsepower. Brake horsepower BHP is the power the pump requires to overcome the discharge pressure.
To properly analyze brake horsepower, you must look at work horsepower versus viscous horsepower. Work horsepower WHP is the horsepower required for the selected PD pump to achieve the desired flow rate considering the anticipated pressure drop from system components.
WHP is sometimes called external horsepower. Recall the required RPM was a result of flow required coupled with slip correction, if any. Maintaining pump capacity at various viscosities requires meeting horsepower minimums, as shown in Fig. There is a certain minimum horsepower requirement to force the rotating parts of the pump to turn, considering the viscosity of the fluid in the pump. VHP is sometimes called internal horsepower. As you processor, you need a pump that transfers product safely and efficiently from point A to point B.
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Centrifugal Pump Curve. Reading pump curves accurately helps you choose the right pump based on application variables such as: Head water pressure Flow the volume of liquid you have to move in a given time period As you will see, you can also use pumps in parallel to increase flow.
What is Head? Fig 1. Every 2. A basic pump performance curve for centrifugal pumps show it's performance range. In this curve, head is measured in PSI; flow is measured in gallons per hour.
What is Total Dynamic Head? Total Dynamic Head TDH is the amount of head or pressure on the suction side of the pump also called static lift , plus the total of 1 height that a fluid is to be pumped plus 2 friction loss caused by internal pipe roughness or corrosion. Static Height is the maximum height reached by the pipe on the discharge side of the pump.
Friction Loss or Head Loss are the losses due to friction in the pipe at a given flow rate. In this pump performance curve, he pump can generate 80 PSI of discharge pressure at a flow rate of gallons per hour. Pump curve charts indicate flow rates on the horizontal axis and pressure on the vertical axis. Reading Centrifugal Pump Curves that Contain Additional Information Because some centrifugal pumps operate across a range of horsepower, their curves will include additional information.
Impeller Trim Size Impeller size is another variable for meeting performance requirements. Without enough net positive suction, the pump will cavitate, which affects performance and pump life. Efficiency and Performance Variables Good pump efficiency means that a pump is not wasting energy in order to maintain its performance point.