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May 2006
 
  Vicious Pump Bites Operations Budget
Interpreting Friction & Velocity Losses 		 
  By Larry Bachus  
 

Larry Bachus
We have pumps in our everyday lives that provide good service with practically no maintenance. Think about the water pump on the radiator of your car. It has bearings and a mechanical seal. When was the last time you had to change the bearings on that pump? How much clean purge water do you inject to lubricate and clean the mechanical seal? When did you last change the mechanical seal on the radiator water pump?

Think about your air conditioner compressor. A compressor is a pump that is adapted to handle air or gas. When did you last grease the bearings on the compressor? When did you last add a charge of Freon to the AC compressor? I guess that means the seal doesn’t leak, right?

Too many industrial pumps go through bearings and leaking seals like a newborn baby through diapers. As a pump consultant, I talk with maintenance engineers who play musical chairs with their mechanical seal and bearing suppliers. So, what’s the problem with industrial pumps?

It turns out that many industrial pumps are poorly mated to their systems. In my last column, I explained the relatively simple procedure to specify a pump into a system. Perhaps you remember the four elements that are added together to determine system head. Hs + Hp + Hf + Hv = Total Head.
In pump terminology, the system head is called the total dynamic head or TDH. The four elements represent the elevation differential (Hs), the pressure differential (Hp), the friction losses (Hf), and the velocity losses (Hv) in the system.

There is nothing complicated about the Hs and Hp. But friction and velocity losses give engineers ulcers. These losses are energy that divert the pump away from its mission.


Scale deposits consume energy.
Friction energy is between the fluid and the internal surfaces of the pipes and fittings. Friction may go from insignificant to detrimentally high in a matter of seconds or over the life of the piping system.

Velocity losses are energy too. This energy increases in the short term when a control valve is throttled and the passageway narrows through the valve. In the long term, straight runs of pipe tend to form scale deposits, which decrease the pipe diameter. A four-inch pipe will eventually become a three-inch pipe. Again, the fluid velocity must increase to compensate for the decreased orifice.

This friction and velocity energy must be computed and designed into the pump so that it will overcome the energy resistance in the system and still meet the pressure and elevation needs of the system.

Friction and velocity energies are derived from charts showing constants (K values per 100-ft of pipe). The charts are easy to understand. Why is this energy so difficult to control?


Clean and descaled pipes conserve energy.
To begin, the K values are averages and don’t necessarily represent the values of the pipes, valves, and fittings installed in a particular application. Pipe is made in runs, or batches, and there are variances from one run to the next. There are variances from one pipe plant to another pipe plant.

Engineers who design new pumping systems are like Christopher Columbus. They must start with a fickle idea and end up with something that works. Columbus had an idea that the world was round, but didn’t really know where he was going. At least the astronauts can look at the moon and see where they want to go.

Friction and velocity losses go ballistic once a new plant is commissioned. Control valves open and close. Filters clog. Pipe forms scale. Solids settle, forming dams inside the pipe. The pump is starved with increased resistance in the suction piping. The pump migrates on its curve when friction and velocity resistance rises in the discharge piping. Pump maintenance and downtime goes up as the pump moves away from its design zone.

The affinity laws say friction and velocity losses are quadrupled as the fluid velocity doubles. (I’ll consider the affinity laws in more detail in a future column.) These losses divert the pump’s energy away from its ultimate mission (i.e., moving the liquid).

Rarely does the operator or engineer realize the friction and velocity heads are out of control. Rather, the perception is that the pump is vibrating, or consuming high horsepower, or generating heat, or not able to fill that tank over there, or eating seals and bearings.

Most process managers will try to solve the problem by purchasing a vibration analyzer, thermographic imager, or some CMMS (computerized maintenance management system) program. What they really need is a competent instrumentation technician who can install a flowmeter and pressure gauges on the pump and then interpret the information these instruments provide. For example, if you use a standard centrifugal pump to move 200 GPM of ambient water in a system with 38 ft. of four-inch pipe, while elevating the water 30 ft. and into a tank pressurized at six PSIG, then your pump’s pressure gauges should indicate 19 to 20 PSI differential. If the differential pressure is 22 PSI or higher, then too much energy is invested in friction and velocity heads and THIS PUMP IS SICK. Likewise, if the differential pressure is less than 17 PSI, THIS PUMP IS SICK. That’s all there is to it.

Adequate gauges and proper interpretation are crucial. If a standard pump is going to elevate cold water 30 ft., the pump must generate 13 PSI. The pressurized discharge tank adds another six PSI to the pump’s requirements. And 200 GPM through 38 ft. of four-inch pipe should consume only about 0.5 PSI in friction and velocity energy. That’s why the differential pressure should oscillate between 19 and 20 PSI (13 PSI + 6 PSI + 0.5 PSI) across the pump.

Look people, when a newborn baby cries, the baby is telling its parents exactly what the baby wants or needs. But, most new parents don’t know how to interpret the baby’s crying. Parents must learn the basics before taking the baby to a psychiatrist to determine the problem. It’s the same with pumps. Learn the basics first.

That’s why I started this column five months ago by saying that the instrumentation technician is pump reliability’s best friend. Once the basics are mastered and the pumps are running numerous years without problems, then the expensive high-tech gadgets can extend your pump life even more.

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

Get Hands-On Training at the
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Oct. 21-23 - Houston, TX

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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
    itself. Your seminar
    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.”
  • “Just a brief note to say ‘thank you’ with regards to the Pump Guy Seminar. I thought the seminar was very informative and entertaining as well. I wish to thank you for all of your efforts with the seminar on behalf of the attendees. A good job well done!"
  • “The course was everything I expected. I needed this information 30 years ago, but it’s never too late.”
  • “This course holds tremendous value for anyone involved in the design, operation, maintenance, or purchasing of pump systems.”
  • “The information I’ve learned from this seminar will most definitely help my understanding of pump issues at work.”
  • “For my line of work, this seminar was dead on. It met my needs fully. Best money my company has ever spent for a training course.”
  • “The monetary price for your shared knowledge and ability to bring pump design back to the basics was worth every penny. Thank you for making your seminar attendees look good with our colleagues.”
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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