How to Read a Pump Curve _ Intro to Pumps

1/7/2017

How to R ead a Pum p Cur ve | Intr o to Pumps

Intro To To Pumps Pumps

Home » Home » Centrifugal Pump Fundamentals Fundamentals » »

5. How to Read a Pump Curve Posted Po sted on 5 December, 2012 by 2012 by Jon Jon Penland Years Yea rs ago, making a pump selection meant sitting down with large printed catalogs and 멪ipping through them until you reached a pump curve that 嚬t a project’s hydraulic requirements. 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-멪o.. Another popular option for pump selection pump-멪o software is Intelliquip Intelliquip.. All of the curves in this article were we re generated by Pump-Flo’s web-based pump selection software. Pumps 101 Home https://ww w.i ntr otopum ps.com /r eadi ng- the- pump- cur ve/

1/7

1/7/2017

How to Read a Pump Curve | Intro to Pumps

What is a Pump Curve? 1. What is a Centrifugal Pump 2. Casings & Impellers

A pump curve provides a wealth of information regarding the performance capabilities of a pump. 3. The Main Parts of a Pump

Information is plotted on an x-y graph where the x-axis is measured in units of 멪ow and the y-axis is measured in units of head, power, and NPSHr. 4. Centrifugal Pump Designs

For the sake of example, today we’re going to look at a selection made for the following design 5. How to Read a Pump Curve

condition: 1,000 GPM at 100 ft. 6. What is NPSH

Here is one possible pump selection that might be a good 嚬t for that operating condition: 7. How to Make a Pump Selection 8. How to Size an Electric Motor 9. Variable Speed Operation 10. Selecting Materials of Construction 11. Getting the Installation Right

Composite Pump Performance Curve

Head & Flow The 嚬rst piece of information provided by a pump curve is t he 멪ow 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 Pumps 101 Home https://www.introtopumps.com/reading-the-pump-curve/

2/7

1/7/2017


performance the pump is capable of achieving with a minimum-trim impeller, and the performance 1. What is by a Centrifugal Pump impeller. provided the design-trim 2. Casings & Impellers The design-trim impeller is the impeller trim the pump selection software has selected as the closest

嚬t to the design condition provided. In this case, the design-trim is 16.31 25”, the max-trim is 17” and 3. The Main Parts of a Pump

the minimum-trim is 15”. We know this by looking at the left-hand side of the curve where these three numbers appear.

4. Centrifugal Pump Designs

Considering the design trim curve 5. How to Read a Pump Curve

we see that at zero 멪ow, also known as shuto굨 or dead-head, the pump 6. What is NPSH will develop about 130 ft of head. This is the heada the pump would 7. How to Make Pump Selection develop if it were operating against 8. How tovalve. Size an Electric Motor a closed 9. Variable Speed Operation Keep in mind that the actual

pressure measured between the 10. Selecting Materials of Construction

pump and the closed valve might

Pump Performance Curve

exceed this value because a pump 11. Getting the Installation Right

ADDS head to the liquid being pumped. In other words, if this pump were operating at shuto굨 with suction pressure of 20 feet the total head experienced at the pump discharge 멪ange would be 150 ft (20 ft + 130 ft). Considering the design trim impeller, we see that t he following things are true: Shuto굨 (a.k.a. zero-멪ow or dead-head) occurs at about 130 ft . The design condition falls very close to the pumps best-e倕ciency-point (BEP). Provided there is adequate NPSHa, the pump will operate down to approximately 60 ft of head and produce a 멪ow of approximately 1380 GPM at 60 ft. The maximum (17 ″) and minimum (15 ″) trim curves also tell us the possible conditions that the pump could be modi嚬ed to meet in the future by installing an impeller of a larger or smaller di굨erent trim.

Eciency In addition to head and 멪ow, most pump performance curves will also provide e倕ciency information. The e倕ciency 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. If a pump were 100% e倕cient then the input power required would be equal to the water horsepower being generated by the pump. However, since no pump is 1 00% e倕cient, every pump will require more input power than it will generate in water horsepower. In the case of this pump, the best-e倕ciency-point (BEP) falls at approximately 1075 GPM at 95 Ft, and e倕ciency at BEP is 84.1%. Pumps 101 Home https://www.introtopumps.com/reading-the-pump-curve/

3/7

1/7/2017


The Preferred Operating Range 1. What is a Centrifugal Pump

Special attention should be paid to the location of BEP relative to the operating condition. Pumps run 2. Casings & Impellers

best at or near BEP. For this reason the Hydraulic Institute (HI) has de嚬ned a pumps Preferred Operating Region (POR) as 멪ows from 70% to 120% of 멪ow at BEP for most centrifugal pumps. 3. The Main Parts of a Pump The POR for this pump stretches from approximately 750 GPM to 1290 GPM which is 70% to 120% of 4. Centrif ugal Pump Designs

1075 GPM.

5. How to Read a Pump Curve

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. For some pumps with high speci嚬c speed (Ns) impellers, t he 6. What is NPSH POR is smaller – 85% to 110% of BEP. You can read more about speci嚬c speed in our article about pump impellers. 7. Howcasings to Makeand a Pump Selection 8. How to Size an Electric Motor

The Allowable Operating Range 9. Variable Speed Operation

There is another region of operation that is de嚬ned by the pump manufacturer rather than by a standards organizations such as HI: the Allowable Operating Region (AOR). In our example curve, the 10. Selecting Materials of Construction AOR is indicated by the portion of the curve shaded in light yellow. This is the region that the pump 11. Getting the Installation Right comprises all of the points that this pump can operate at continuously. manufacturer has determined

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 con嚬nes of the AOR.

Minimum Continuous Stable Flow & the System Curve The example pump performance curve above has two more items which should be mentioned. First, the red line on the left hand side of the pump curve is the Minimum Continuous Stable Flow (MCSF) line. 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. Second, the blue curve beginning at 0 GPM and 0 Ft and extending through the design condition is the System Curve and represents the operation of the system in which the pump is being applied. System curve information should be provided by the Engineer designing the pump system. In the pump selection software, the system curve can be manipulated by manually entering data points, and is particularly useful when evaluating the variable-speed performance of a pump.

Net Positive Suction Head Required (NPSHr) The next part of the pump curve is the Net Positive Suction Head Required (NPSHr) curve. The NPSHr Pumps 101 Home

https://www.introtopumps.com/reading-the-pump-curve/

4/7

1/7/2017


curve provides information about

Net Positive Suction Head Required Curve

1. What is a Centrifugal Pumpof the the suction characteristics

pump at di굨erent 멪ows. The x-axis 2. Casings & Impellers

is still measured in 멪ow units, but the y-axis is now measured in feet of NPSHr. Each point along the curve identi嚬es the NPSHr required by the pump at that 멪ow to avoid cavitation issues that would be 3. The Main Parts of a Pump damaging to the pump and would have a negative impact on overall pump performance. 4. Centrifugal Pump Designs

Looking back at our example design 멪ow of 1,000 GPM we can see that this pump will require approximately ft of NPSHr 5. How to Read a7Pump Curve at that condition. A typical safety margin between Ne t Positive Suction Head Available (NPSHa) and required (NPSHr) is 5 ft. So in this case it would generally be 6. What is NPSH that this pump not be applied in applications where NPSHa at the design 멪ow of 1,000 recommended

GPM is less than approximately 12 ft. 7. How to Make a Pump Selection

Generally speaking NPSHr does not vary dramatically between variations in impeller trim which is why 8. to Size Electric Motor weHow do not seean separate curves for the minimum and maximum impeller trims. Those curves are

actually present, but they are overlaid by the design-trim NPSHr curve. 9. Variable Speed Operation

Horsepower Curve 10. Selecting Materials of Construction 11. the Installation Right curve TheGetting 嚬nal portion of the pump

is the power curve. Once again the x-axis is measured in units of 멪ow, but the y-axis is now measured in

Power Curve

power units. In this case the unit of measurement is horsepower, but in the case of an application measured in metric units, power is typically measured in kilowatts. This curve tells us how much power the pump will demand at any particular 멪ow point. This information is useful in ensuring the selected motor is suitably sized, and is also useful when calculating power consumption costs. At our design 멪ow of 1,000 GPM, we can see that power demand is approximately 30 HP and that power demand is greatest at approximately 1,300 GPM. Based on this information, if the pump were to be driven by an electric motor, most pump manufacturer’s would recommend that the next largest motor rating be used. In this case that would be a motor rated for 40 HP. Power demands vary considerably depending on the impeller trim. This is why separate power curves for the minimum and maximum impeller trims can also be se en in the power curve. This information is useful if the customer would like to size the drive unit to allow a future increase in capacity without requiring replacement of the drive unit. In that case, the customer could chose to size the drive unit for the maximum trim power curve, and at a later date the capacity of the pump could be increased by installing a maximum trim impeller without requiring replacement of the drive unit. In the case of our example pump curve, the motor required by the design trim impeller w ill be adequate to cover the power demanded by the maximum trim impeller, but this will not always be the case.

In Conclusion Pumps 101 Home https://www.introtopumps.com/reading-the-pump-curve/

5/7

1/7/2017


When combined, these three curves are called the composite pump curve, and they provide the 1. What is a Centrifugal Pump information we need to determine if a particular pump is a suitable selection for the hydraulic

requirements of an application. Understanding the information communicated by each part of the 2. Casings & Impellers

curve is critical to ensuring that the pump selected is a good 嚬t for the application’s hydraulics. 3. The Main Parts of a Pump ←

4. Common Pump Designs

6. What is NPSH? →

4. Centrifugal Pump Designs Updated 12/18/15 5. How to Read a Pump Curve 6. What is NPSH

More

7. How to Make a Pump Selection 8. How to Size an Electric Motor

Jon Penland

9. Variable Speed Operation

Jon is a pump industry veteran and freelance writer who has worked in

10. Selecting Materials of Construction

sales and application engineering for pump m anufacturers and

in the municipal water and wastewater markets. 11. Getting the Installationdistributors Right

←

→

Pumps 101 Home https://www.introtopumps.com/reading-the-pump-curve/

6/7

1/7/2017


1. What is a Centrifugal Pump 2. Casings & Impellers 3. The Main Parts of a Pump 4. Centrifugal Pump Designs 5. How to Read a Pump Curve 6. What is NPSH 7. How to Make a Pump Selection 8. How to Size an Electric Motor Contact Us 9. Variable Speed Operation Privacy Policy 10. Selecting About Us Materials of Construction We use Other Resources 11. Getting the Installation Right advertisements on this site. If you click on a link or banner on this page it may be an a倕liate or Adsense link for which we receive compensation.

Why SiteGround? Intro To Pumps © 2017

Pumps 101 Home https://www.introtopumps.com/reading-the-pump-curve/

7/7

How to Read a Pump Curve _ Intro to Pumps

Recommend Documents