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October 2006
 
  Exposed Curves
Why Keep a Good Thing Locked Up? 		 
   
 

Larry Bachus
I’ve always wondered why the pump performance curves are stored in a file and locked in a drawer in the engineer’s office. They really should be in front of the operators and mechanics that work with the pumps. I mean, would you hide the dash control panel of a car in the glove box or in the trunk? Of course not! The dashboard stares the driver in the face. The driver’s eyes must pass the dashboard to see out of the front windshield. DUH!! Most engineers will say they store the curves in a locked file cabinet so they won’t get lost or disappear. I agree that it’s important to keep valuable information in a secure place, but it’s just a piece of paper people.

The auto dashboard provides valuable information to the operator of a car. All cars — whether national or imported, economical or expensive — have dashboards with certain information on the gauges and warning lights. Maybe high-performance cars provide a few more gauges than warning lights. It’s the same with pumps.
Most all pump companies publish performance curves with certain information common to all pumps. Maybe high-performance pumps show a little more information on their performance curves. And, just as there is no fixed design or arrangement of the gauges and lights on the automobile control panel, there is no specific arrangement and location of the information on a pump curve.

Pump curves can be intimidating because sometimes the pump companies cram a lot of information onto the graph (Figure 1). (I believe this is the secret reason the engineers hide the curves in a locked drawer.) Imagine a dashboard with one gauge and five needles showing the car’s speed, the engine revolutions, the gasoline level, the water temperature and the battery voltage … all on one dial.
If you want to impress your friends by telling 100 funny jokes at a party, then you must know at least 100 funny jokes. Same thing with pump curves. In order to interpret many pump curves with different presentations of information, you must study and understand a lot of pump curves with different presentations of information. Pump performance curves are really quite easy to interpret.


Figure 1. Some pump curves provide an abundance of information.
Concisely, the curve indicates that the pump will discharge a certain volume of a liquid, at a certain pressure (head), at an indicated speed, while consuming a specific quantity of power (horsepower or kilowatts). Most pump companies publish their curves in two or three different formats. These formats are called:
  1. Tombstone curves
  2. Application-specific curves
  3. Family curves
Tombstone curves appear on most pump literature and brochures. They show the different pump models on one graph with the respective head and flow capabilities. When you determine the head and flow of a piping system that needs a pump, you can consult this graph and see all models made by the manufacturer that can meet the needs of your system.

An application-specific curve is developed for a specific liquid in a specific system. This curve normally accompanies a quote and purchase order. It shows a probable duty point based on customer supplied information. This curve proposes what the pump should do with the specific liquid, provided the customer-supplied information, piping layout, valves, fittings, and connections are correctly specified and installed into the system.


Figure 2. Family curves offer a complete performance picture of a pump.
Many people consider the family curve to be the most useful. It shows the complete performance picture of a pump model based on water at 70 F (20 C), and sea level with available impeller diameters. It shows head, flow, speed, efficiency, power consumption, energy requirements, and possibly other information like specific speed (Ns), and suction specific speed (Nss) (Figure 2).

The performance curve is actually four curves with five elements working and relating with each other on a common graph. Two elements, the head and flow appear on the same curve. This is called the H-Q curve.

Just as the car’s speedometer begins at 0-mph and rises to maximum speed, the H-Q curve begins at a point called shutoff head. A centrifugal pump can elevate a liquid in a vertical tube up to a point where the liquid’s weight is the same as the energy produced by the pump. When all of the pump’s energy is invested into elevation or head, at zero flow, this point is called the shutoff head.


Figure 3. The H-Q curve shows the flow and head of a pump.
As the pump’s energy is converted into flow (GPM), the head diminishes. As more and more energy is invested into flow volume, head continues to drop. The H-Q curve ends at the point where all the energy is invested into volume flow at zero elevation or head. This curve appears as a declining arc on the graph (Figure 3).

Another element is the power consumption. The units are horsepower or kilowatts. This curve begins at the power consumed at shutoff head (zero flow) and generally rises as flow increases. The power curve is a mostly linear curve meaning that more power is consumed as flow rises.

Some curves show power consumption with a different presentation. Rather than showing horsepower or kilowatts, family curves will indicate that up to X-GPM flow, you’ll need a 10-Hp motor. Up to Y-GPM flow, you’ll need to install a 15-Hp motor. And up to Z-GPM flow, you’ll need to use a 20-Hp motor. This is what I like about the family curves.

Efficiency is another curve on the graph. Some curves present efficiency as a rising and falling arc like a camel’s hump. Efficiency is zero at both extremes of the H-Q curve. Between the extremes is a point of best efficiency.

Other curves show the efficiencies as concentric arcs or ellipses on the H-Q curve. This presentation is convenient when mating the pump curve with the system curve. It indicates that the pump is X-percent efficient between A and B flow. Efficiency means that the majority of the energy is directed toward generating head and flow and little energy is wasted as heat, vibration, distortion, and noise.

Fixed velocity pumps will indicate the speed on the curve. Variable speed pumps will show performance at different selected speeds.

The gasoline gauge on a car indicates if the car has an adequate supply of energy (gasoline) to travel across town or across the state. Likewise, the NPSHr (net positive suction head required) element indicates the pump’s requirement. There must be sufficient energy in the fluid at the suction nozzle for the pump to convert the energy into head and flow. If the energy is inadequate, then the pump’s discharge head and flow will be likewise inadequate. If the liquid is leaving the pump faster than it can come into the pump, then the pump is starved. The requirement rises with flow. The requirement rises even faster at higher flows with poor efficiency.

The curve is the control panel of the pump. All operators, mechanics, supervisors, engineers, and anyone involved with the pump should understand the curve and it’s elements and how they relate.

With the curve we can fully understand the differential-pressure gauge readings on the pump. The differential gauge readings will determine if the pump is operating at, or away, from its best efficiency zone. You wouldn’t buy or drive a car without a dashboard. Why would you buy and use a pump without gauges or the curve?

We spend a lot of time on curve interpretation in my lectures. This is the secret to extending the life of your pumps. There is no magic bullet, or battery operated digital device that can replace knowledge.

Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a member of ASME and lectures in both English and Spanish. He can be reached at larry@bachusinc.com or
615 361-7295.

www.bachusinc.com

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Larry Bachus (a.k.a. "Pump Guy") demonstrates the principles of NPSHr vs. NPSHa at his Aug. 18-20 Pump Guy Seminar in the Chicago area.

Larry Bachus (a.k.a. "Pump Guy"), a regular contributor to Flow Control magazine and a widely recognized expert on pumping technology, recently presented his Pump Guy Seminar in the Chicago area to an eager crowd of pump users. Here's what some of the attendees had to say about this training event:
  • “I attended your seminar this week in Chicago, and it’s already paying for
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    taught me about concentric reducers on suction lines and horizontal elbows on split case pumps, which will be quite helpful on several plant system designs I’m currently working on.”
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FOR MORE INFORMATION & TO REGISTER FOR THE PUMP GUY SEMINAR, CLICK HERE.

KEY SEMINAR TOPICS INCLUDE:
 • Basic Pump Principles
 • NPSH
 • Cavitation
 • The Affinity Laws
 • Work & Efficiency
 • Pump Classification
 • Pump Curves
 • System Curves
 • Shaft Deflection
 • Pump-Motor Alignment
 • Bearings
 • Pump Packing
 • Mechanical Seals
 • Pump Piping

For a sampling of Larry's latest "PUMP GUY" columns from Flow Control magazine, see:
 •   "Cheat Sheets: Energy, Work & Power"
 •  "Cheat Sheets: Unwritten Pump Rules"
•   "Cheat Sheets: The Affinity Laws"
FOR MORE INFORMATION & TO REGISTER FOR THE PUMP GUY SEMINAR, CLICK HERE.

 If you have any questions about the PUMP GUY SEMINAR or need help registering, please contact Matt Migliore at 610.828.1711 or Matt@GrandViewMedia.com.

  
     
   

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