BHP2 = BHP1 x (N2/N1)3
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 alwaysSpeed 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.
SummaryImprove 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 firstname.lastname@example.org.