Two months ago, we discussed how to determine where a centrifugal pump is operating on its efficiency curve [The Best Point,"July '03, p20]. We presented a method for obtaining and reducing test data so they could be plotted on a centrifugal pump curve.
As a rule of thumb, the preferred operating range is between 80 percent and 120 percent of the best efficiency point (BEP) flow rate.
This article examines what to do about a pump that's running outside this preferred operating range.
The sweet spot
Centrifugal pump operation is a lot like hitting a baseball. If you hit the ball with the bat's "sweet spot," everything is great,"energy transfer will be efficient, vibration will be low and the likelihood of damaging equipment will be minimal. End up too far in or too far out on the bat/curve, and problems are likely to occur.If you played ball, you'll remember early season games when the weather was cold and your hands were numb. A slightly mistimed swing caused you to hit the ball off the end of the bat. The vibration was like an electric shock. The same is true for centrifugal pump operation. The farther away you operate from the BEP, the more severe the problems, both mechanical and hydraulic. You wouldn't intentionally hit a baseball off the neck of the bat, so why do many of us run our pumps near or below the minimum flow rate? The key to attaining high pump availability and reliability is to make sure that the pump is suitable for the given application, and that it runs within its preferred operating range.
Worthwhile corrections
Energy savings is one of the most compelling reasons to correct operating point problems. In a well-defined product cost system, reduced energy consumption within the process can be directly assigned to reduce the unit cost of the product.To help determine the various costs of pumps through their lifecycles, consult the lifecycle cost calculator posted on the Hydraulic Institute's Web site (www.hydraulicinstitute.com). In general, the energy cost for any pump represents 90 percent of ownership cost over its useful life. Increasing pump efficiency by just a few percent can greatly reduce any facility's power costs. These cost reductions aren't theoretical. They're avoided expenses that flow right down to the bottom line.
A number of industrial studies show that pumps are running well below their BEPs. The Finnish Technical Research Center for Manufacturing Technology, for example, sampled 1,760 pumps at 20 industrial sites, and found average pump efficiency to be less than 40 percent. In addition, 10 percent of the pumps were found to be operating below 10 percent efficiency.
Another study by the U. S. Department of Energy (DOE) states that pumps in industry use 149 trillion kWH of energy annually. Extrapolating data from these two studies suggests that the U.S. industry can save $218 million per year for every percentage point increase in pump efficiency, based on an average energy cost of $0.06/ kWH.
In a quick tally of the BEP for 60 pump curves, as well as equipment from several manufacturers in a variety of industries, the average BEP for pumps in the sample was 68 percent with an average specific speed (Ns) of 837 in U.S. units. That 28 percent potential increase in efficiency compared with 40 percent in the Finnish study suggests that if we improved the average energy efficiency by half of the potential (14 percent), the U.S. economy would reduce its power requirements by 38.6 trillion kWH/yr or approximately $3.1 billion in annual energy savings. That represents the equivalent of 22.7 million barrels of imported oil.
The energy savings realized by moving a pump's operating point closer to its BEP represents only a fraction of the total power savings that are possible. Using variable-speed drives to match pump performance to system demand can cut power requirements substantially.
Improve process uptime
Another process metric that will improve with better pump operation is process uptime, an important factor in improving yields, quality and productivity. This comes from a reduction in the time and cost of repairs.In general, radial hydraulic forces on true volute pumps are minimized at the BEP and drastically decreased with reduced speed. By reducing hydraulic loads, shaft deflection is minimized, bearing loads are decreased and alignment of mechanical seals is maintained. These improvements lead to greatly reduced maintenance costs and increased equipment availability. The cost of the repair itself usually is trivial when compared to the lost revenue from the process outage. Proper pump operation will not eliminate pump failures, but it can greatly reduce frequency and severity.
VSDs to the rescue
Variable-speed drive technology is the most effective method for matching centrifugal pump output to varying system demands. The most popular form of these is the variable-frequency drive (VFD), which allows the user to control the pump's speed, matching such parameters as flow or discharge pressure to meet process needs. These drives have become increasingly compact, reliable and affordable. Systems with large changes in demand and low static heads, i.e., where most of the head is frictional, benefit most from using motor drives.Pump speed can be varied to meet system demand, so the power demand for the pump changes with the cube of the speed change:
BHP2 = BHP1 x (N2/N1)3
Where:
BHP1 =Known Power (hp)
BHP2 =Unknown Power (hp)
N1 = Known Speed (rpm)
N2 = Desired Speed (rpm)
A pump operating at 50 percent of the known speed only requires 12.5 percent of the power required at the known speed. This can represent a huge power savings for your company.
But not always
Speed control is not right for every application. Systems with high ratios of static-to-friction head or virtually constant operating conditions may not see significant energy savings using variable-speed drives. Variable-speed operation can also cause significant mechanical problems, such as resonance, improper coolant flow and increased wear in journal surfaces from off-design operation. Consult the manufacturer or a competent consultant if you have questions regarding the proper application of variable-speed technology for your application.Need more flow?
You need to increase the flow rate of the pump, but the valves are wide open. Now what? The solution might be to replace some or all of the discharge pipe with larger diameter pipe. Look at the size of the piping throughout the system. There might be only a short section of the pipe that is undersized. A rough but controversial rule of thumb for the maximum flow rate in a specific pipe size is to limit the fluid velocity in the pipe to 20 ft./sec. or less for pipe diameters smaller than four inches, and 30 ft./sec. or less for larger pipe. Some fluids have a minimum level of velocity required to keep particles in suspension or prevent scaling.Various impeller diameter trims also can create a better fit between pump and system characteristics. The system should be modeled for different flow rates to plot the system curve. Plotting can be done by hand, on a simple spreadsheet program or by using commercial commercially available software. Plotting the system curve on a copy of the pump performance curve will determine the benefits of the impeller trim and the correct trim diameter. It's never good practice to buy a full-size impeller trim with a new pump. An impeller size approximately 80 percent to 85 percent of the full trim leaves room for the capacity increase in a couple of years or "wiggle room" for sizing mistakes.
Still off the curve?
What if you find that your pump is not operating on the performance curve? Double check the pump's operating speed. An air bubble might be blocking the suction eye. Highly aerated pumpage or a vortex in the suction tank can cause air ingestion and reduce flow rate.Another problem might be insufficient Net Positive Suction Head Available (NPSHA). If a pump is severely cavitating, it will perform off the pump curve, but might not sound like it is cavitating at all. If, after testing, you find insufficient Net Positive Suction Head Required (NPSHR), there are a few steps that can be taken to increase it.
Throttle the discharge valve back to the minimum flowrate in the preferred operating range. This will decrease the NPSHR and increase the NPSHA.
Raise the level in the suction tank to increase the NPSHA.
Slow the pump speed to reduce the NPSHR with the square of the speed change.
Reduce the temperature of the fluid to reduce its vapor pressure.
Make piping changes to reduce the friction losses.
If you have sufficient NPSHA and all of the above conditions have been checked, it's probably time to take the pump apart. Check the pump impeller for problems such as plugged impeller passages.
Summary
Improve the efficiency of pumps by ensuring that they operate in the preferred operating region. Further, increase energy efficiency with variable-speed drives to greatly reduce the cost of ownership. Substantial savings in energy and maintenance costs still are low-hanging, but unpicked, fruit for many facilities. These savings require only small investment, but yield a large return.Efficiency and innovation are critical to maintain the competitiveness of manufacturing and process industries. Making these simple changes to pump operation can help these savings flow directly to your operation's bottom line. You'll help keep your facility and your company in the black. CP
Greg Case is president of MechTronix Engineering, Elk Grove, Calif. The company specializes in rotating equipment design, testing, condition monitoring, analysis and education. Contact him at [email protected].
