Jim Elsey is a mechanical engineer who has focused on rotating equipment design and applications for the military and several large OEMs for 43 years in most industrial markets around the world. Elsey is an active member of the American Society of Mechanical Engineers, the National Association of Corrosion Engineers and the American Society for Metals. He is the general manager for Summit Pump Inc. and the principal of MaDDog Pump Consultants LLC. Elsey may be reached at firstname.lastname@example.org.
Impeller tip speed increases proportionally when the larger pump is required to deliver the same dynamic head as the smaller pump. The impeller in the larger pump at slow speed could approach twice the size of the smaller pump for the same head requirement. Impeller tip speed is a fundamental boundary for any pump, and industry best practice guidelines are based on the potential for erosion in correlation with the percentage of suspended solids in the pumped fluid.
Industry recommendations for maximum tip speed:
- Dirty water: 130 feet per second
- Medium slurries up to 25 percent solids and 200 micron solids: 115 feet per second
- Higher concentrations (slurry) and/or larger solids: 100 feet per second
- Elastomer impeller: 85 feet per second
For a given set of hydraulic conditions, the efficiency of the higher speed pump will more often be higher than the lower speed pump. While this consideration will also depend on where the pump will operate on its performance curve, it remains an important consideration. It is especially important for higher horsepower (hp) pumps as the duty cycle approaches 100 percent. Looking at graphs for specific speed (NS) ranges versus overall pump efficiency, the low end of the NS range is normally less efficient than the middle range. Selecting a lower speed pump can possibly place the pump in a lower NS range, which will be less efficient. The easiest way to determine this is simply to look at where each pump will operate on its curve and note the efficiency. Typically, the lower speed pump will be a few points less efficient, which may be an acceptable tradeoff.
Many of the initial designs for high speed pumps were based on the lower speed models that simply had the speed increased. Subsequently, they were frequently found unreliable. Because they were not designed to operate at higher speeds, this skewed reliability data and the common thinking that slower is better. Pumps that are initially designed (and properly maintained) to operate at the higher speeds are often found to be just as reliable as slower speed pumps from the previous design generations. The caveat I wish to illuminate is that they must also be operated and maintained at the higher standards required for valid reliability at the higher speeds such as balance, clearances, pipe strain and alignments.
As system head requirements increase at some point, the laws of physics will force you into higher speed pumps. Just as an alternate thought, I have witnessed several situations where an 1,800 rpm two-stage pump was actually the better option than a single-stage 3,600 rpm pump.
You can run a pump too slow and have issues with hydrodynamic stability or overheating the motor driver. I do not recommend operating most pumps below 600 rpm for any length of time unless the pump was designed for those conditions. Check with the original equipment manufacturer (OEM) in all cases.
When selecting a pump for any process application, look beyond the initial price. A general purchasing checklist for pumps would also include a serious consideration of the fluid properties and their effects, duty cycle, cost per kilowatt hour, hydraulic efficiency, specific speed (and suction-specific speed), NPSH margin, impeller tip speed, pump material, allowable forces, boundaries (pressure temperature, pH and lift), and where the pump will operate on its curve most of the time. Where the pump operates on its curve is a direct function of the system curve, and the system curve position and shape will be dynamic.
Finally, the comprehensive check list would cover the full life cycle cost of the pump. Does your staff have the skill set to perform best in class installations, precision alignments, precision adjustments, balancing, pipe strain mitigation and monitoring of critical performance parameters?