Category: Centrifugal pump curve

Centrifugal pump curve

KGaA uses cookies to be able to optimally design and continually improve its web site. By continuing to browse the web site you consent to the use of cookies. The characteristic curve's shape is primarily determined by the pump type i. Secondary influences such as cavitationmanufacturing tolerancessize and physical properties of the fluid handled e. Curves plotted in percentage ratios referred to the best efficiency point ns increasing from left to right. In the case of centrifugal pumps with a low specific speed, the efficiency curve is relatively flat in the vertex, whereas the efficiency curve of high specific speed pumps is more pointed see High specific speed.

Evenat a given specific speed the characteristic curve's shape can still be influenced via the selection of an appropriate head coefficient. It is thus possible to match the pump's characteristic curve to the specific systems requirements. See Fig. The operating points at which pumps are usually not operated, are situated in the other three quadrants.

These include, for example, operation in turbine modethe pump's behaviour following a drive failure or start-up starting torque at reverse direction of rotation. A centrifugal pump's complete characteristics chart four-quadrant characteristic curves selection chart is primarily established on the basis of experiments and depends on the pump type.

Practical case: Operation of variable speed pumps in parallel

All parameters are specified as percentages of their design values including negative values in order to facilitate translating the results into diagrams for similar pumps. Search A-Z Suche nach. The presentation of parameters in a characteristic curve can provide Qualitative information See Fig. Pump Lexicon. Characteristic coefficient. Characteristic curves selection chart.Today this process is made much easier through the use of electronic pump curve catalogs.

One of the most well-known developers of electronic pump catalogs is Engineered Software and their pump selection software pump-flo. Another popular option for pump selection software is Intelliquip. A pump curve is a graphical representation of the performance characteristics of a pump. A pump curve provides a wealth of information regarding the performance capabilities of a pump.

Information is plotted on an x-y graph where the x-axis is measured in units of flow and the y-axis is measured in units of head, power, and NPSHr. The first piece of information provided by a pump curve is the flow that the pump will develop at any given operating head.

The curve that provides this information is called the pump performance curve. Some pump curves only provide a single pump performance curve, but most will provide the maximum performance the pump is capable of achieving with a full-trim impeller, the minimum performance the pump is capable of achieving with a minimum-trim impeller, and the performance provided by the design-trim impeller.

The design-trim impeller is the impeller trim the pump selection software has selected as the closest fit to the design condition provided. In this case, the design-trim is We know this by looking at the left-hand side of the curve where these three numbers appear. Considering the design trim curve we see that at zero flow, also known as shutoff or dead-head, the pump will develop about ft of head.

This is the head the pump would develop if it were operating against a closed valve. Keep in mind that the actual pressure measured between the pump and the closed valve might exceed this value because a pump ADDS head to the liquid being pumped.

In addition to head and flow, most pump performance curves will also provide efficiency information. The efficiency of a pump is the relationship between the input horsepower required to drive the pump at a given operating condition, and the water horsepower being created by the pump. Special attention should be paid to the location of BEP relative to the operating condition. Pumps run best at or near BEP. It is best to select a pump that will operate most of the time in the POR since this will have implications for pump life and power consumption.

You can read more about specific speed in our article about pump casings and impellers. There is another region of operation that is defined by the pump manufacturer rather than by a standards organizations such as HI: the Allowable Operating Region AOR.

In our example curve, the AOR is indicated by the portion of the curve shaded in light yellow. This is the region that the pump manufacturer has determined comprises all of the points that this pump can operate at continuously. While it is preferable to select pumps to operate within the POR, pumps should always be selected to operate within the AOR without exception. Very short-term operation outside of the AOR might be acceptable, but the pump manufacturer should be consulted before selecting a pump that will see intermittent operation outside the confines of the AOR.

Understanding pump curves #3: centrifugal pumps in parallel

This is the point beyond which the pump manufacturer has determined the pump should not be allowed to operate for any extended period of time.In fluids dynamics the term pump head is used to measure the kinetic energy which a pump creates.

Head is a measurement of the height of the incompressible fluid column the pump could create from the kinetic energy, that the pump gives to the liquid.

centrifugal pump curve

The head and flow rate determine the performance of a pump, which is graphically shown in the figure as the performance curve or pump characteristic curve. The main reason for using head instead of pressure to determine the performance of a centrifugal pump is that the height of the fluid column is not dependent on the specific gravity weight of the liquidwhile the pressure from a pump will change.

The head will also change as the volumetric flow rate through the pump is increased. When a centrifugal pump is operating at a constant angular velocityan increase in the system head back pressure on the flowing stream causes a reduction in the volumetric flow rate that the centrifugal pump can maintain. The relationship between the pump head and the volumetric flow rate Qthat a centrifugal pump can maintain, is dependent on various physical characteristics of the pump as:.

This relationship is very complicated and its analysis lies in extensive hydraulic testing of certain centrifugal pump. As can be seen from the picture below. First, we have to calculate the radial velocity of the flow at the outlet.

Centrifugal Pump Lexicon

From the velocity diagram the radial velocity is equal to we assume that the flow enters exactly normal to the impeller, so tangential component of velocity is zero :. Radial component of flow velocity determines how much the volume flow rate is entering the impeller. So when we know V r1 at inlet, we can determine the discharge of this pump according to following equation. Here b 1 means the blade width of the impeller at inlet. In order to calculate the water horsepower P w required, we have to determine the outlet tangential flow velocity V t2because it has been assumed that the inlet tangential velocity V t1 is equal to zero.

These principles apply regardless of the direction of the change in speed or in impeller diameter. It must be noted the Affinity laws give results that are approximate. There is a discrepancy between the real hydraulic values and calculated. This discrepancy is due to hydraulic efficiency changes. In order to increase the volumetric flow rate in a system or to compensate for large major or minor lossescentrifugal pumps are often used in parallel or in series.

Series operation of centrifugal pumps is used to overcome large system head lossor to gain large pressure increase when liquid is injected into very high pressure system e.

centrifugal pump curve

When a centrifugal pump is operated in a closed loop, the resulting discharge pressure will be simply the sum of the suction pressure and the pressure normally developed by the pump when operating at zero suction pressure. Therefore it is well suited for use as a booster pump when operated in series.Centrifugal pumps are frequently operated in parallel to achieve higher system flows, to enhance system flexibility or to provide greater pump redundancy and hence better system availability.

For very large projects, limited availability of sufficiently large pumps or limits on driver size or electrical starting current may dictate the use of multiple pumps. There are traps for the unwary. Simply running an additional pump in parallel may not provide the expected results. These preceding concepts are absolutely crucial to understanding many of the issues associated with operating pumps in parallel.

To begin, we will define parallel operation and outline how to predict system performance when two or more pumps are operated in parallel. We will discuss selection rules for parallel operation and then look at how different system characteristics react to parallel pump operation.

The limits of the centrifugal pump curve

This means that each pump will have the same differential head. The total flow is the sum of the individual pump flows. See Fig 1. Predicting performance To predict the system performance, a combined curve for all pumps can be produced by adding the individual flows from each pump curve at a common head. This is illustrated in Fig 2 below : As with any pumping system, the system will operate where the combined curve intersects the system curve. In this example, the Pump curves used are not identical and this highlights a likely serious operating problem.

This is highlighted in Fig 3. In this scenario, the pump with the lower head will be stalled, and assuming that a non return valve has been fitted, it will be running at closed valve with all the attendant risks and damage. Matching pump curves for parallel operation — selection rules The primary concern when running centrifugal pumps together in parallel is that pumps share the load safely at lower flows. This does not mean that pumps must have identical characteristics. Groups of low flow pumps and high flow pumps are often operated together in parallel to match variable system demand.

The aim is that all pumps have matching heads at low flow to prevent pumps being stalled or blocked in by the others. Be aware of minor performance variations that may change the suitability of otherwise well matched pumps. If one pump has had more use and is worn, its curve may no longer match the other pumps. Pumps using different drivers such as a steam turbine driven pump operating in parallel with an electric motor driven unit may operate at sufficiently different speeds to cause mismatch of pump curves at low flow.

Will parallel operation suit your system? This depends on what you are trying to achieve. If the aim is to increase and decrease system flow by operating one, two or more pumps in parallel to meet changing demand then it is important to understand the type and limitations of your system.

If your system is mostly friction resistance a steep system curve such as a closed loop circuit or a lengthy pipeline, then performance variation will be minimal. This may be a good or a bad thing depending on what you are trying to achieve. Flat system curves Refer Fig 5. When the system resistance is relatively flat not much frictionoperating additional pumps in parallel will produce a useful flow increase. Incorrectly, many operators expect that flow will double if two pumps are running in parallel.

This cannot happen because, when added together, the combined pump curve intersects the system curve at a higher head due to increased frictional resistance and hence each pump will be operating at a higher head and lower individual flow than when operating alone. Steep system curves Refer Fig 6. When the system curve is steep mostly friction this steep increase in friction head as system flow increases, means that each pump will be running at a significantly higher head and the individual pump flows will be much less than in single operation.

Overall, the change in flow will be small when additional pumps are switched on. This may be frustrating if you want to increase or vary system flow by switching pumps on and off.A typical centrifugal pump performance curve.

The centrifugal pump curve has high and low flow limits, which can cause significant mechanical damage to the pump if not avoided. At the low flow end of the curve, flow recirculation can damage a pump, while at the high flow end, excessive NPSH REQUIREDhorsepower and choke flow can result in mechanical damage to impellers, casing, shaft, bearings and seals.

Each of these factors is discussed below. As we examine these factors we can see that oversizing a centrifugal pump will result in low flows through the impeller. A portion of the flow will reverse itself and set up turbulence as it reenters. The abrupt change I direction and very high acceleration can result in cavitation on the back side of the impeller vane. Oversizing an impeller can significantly affect performance and mechanical reliability.

Pumps are designed to operate at minimum radial thrust loads at their best efficiency point. Low flow operation results in high radial loads, which can cause premature bearing failures unless bearings are selected to accept these higher loads in an anticipation of operation at low flows. Pressure surges and flashing of the liquid can also occur at low flows. This can cause loading and unloading of the mechanical seal faces, which can result in a high temperature rise through the pump, because the amount of energy absorbed by the liquid is low compared to the absorbed by friction losses.

The relationship can also be used to determine the approximate flow rate of any centrifugal pump, by measuring the pipe temperature rise. Referring to the particular pump shop test curve for the calculated efficiency will allow the approximate pump flow rate to be determined. Note: This approach assumes the pump is in new condition. A worn pump will reduce the flow to a greater extent. Selecting a pump to operate to the far right of the best efficiency point can result in potential problems as listed below.

Forgot Password? View Cart Checkout. Share Tweet. William Forsthoffer. Auxiliaries When rotary pumps can replace centrifugals.If you are new to pumps and fluid processing, reading a pump curve can be a daunting, confusing task.

Then, just when you think you understand curves, you realize that different types of pumps centrifugal, positive displacement, air operated diaphragm In this post, we'll break down the anatomy of a centrifugal pump curve. A centrifugal pump imparts energy on a liquid, and based on the system, has flow and head characteristics.

The amount of required pressure the pump must overcome dictates where the performance point will be on the curve and how much flow is produced. As pressure increases, the flow decreases moving your performance point to the left of the curve. As pressure decreases, the performance point runs out to the right of the curve and flow increases. Below are descriptions of the basic parts of a performance curve. See examples as they relate to the performance curve provided below.

The title box provides information about the pump model, size, speed, and other identifying criteria specific to the pump. If checking the performance of an existing pump, confirm that you are matching the pump to the associated curve. To start your selection, identify the amount of flow required from the pump. For this example, we have chosen gpm. The horizontal axis of the curve indicates flow.

You will also need to know the total head the pump needs to overcome at the specified flow.

centrifugal pump curve

For this example, we will use ft. The vertical axis indicates head. Follow ft across the curve intersects your flow line which indicates your performance point. To accommodate different performance points, centrifugal pumps allow for trimmed impellers. Reducing impeller size limits the pump to your specific performance requirement.

The curve lists the impeller diameters on the left side of the curve and the performance for each trim across as a bold line.

centrifugal pump curve

Centrifugal pumps can also be limited by variable speed, which is the ideal means of control when several performance points are required by a single pump and not achieved by a single trim without system modification. Variable speed curves will be covered in a later post. Now that you have your performance point, we can determine the amount of horsepower required. Horsepower is indicated across the curve as a dotted line in this case at a downward angle.

Our performance point is between the 10hp and 15 hp lines, we estimate this selection to require 12 hp. Net positive suction head required is important for proper pump operation.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 will 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:. We look forward to serving you!


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