For the purpose of this article, I will limit the discussion to an end suction, single stage, centrifugal pump with ball bearings and oil lubrication. Further, we’ll consider the difference between 1,800 and 3,600 rpm units. (Variable speed will be considered in a future article.)
I will initiate the subject by stating there is no 100 percent correct answer 100 percent of the time. As an experienced pump person, my main goal in any installation has always been to design for maximum reliability. This myopic focus is admittedly my own paradigm based on conservative engineering training, my naval submarine service, decades as a provider of pumps, acquired pump knowledge, and mentoring to power plants and refineries.
Benefits of a Slower Speed Pump
It is an industry accepted principle, supported by several studies, that pump wear is at least directly proportional to pump speed. A common quantitative formula is that pump wear is proportional to the cube of the pump speed, which simply translates to a factor of eight. The higher the percentage of solids in the pumped fluid, the more this axiom is true. An oversimplification, perhaps, but it is unequivocally the reason why slurry pumps are big and slow in lieu of small and fast.
Two of the biggest destroyers of centrifugal pumps are bearing and mechanical seal failure. Many of these failures can be attributed to the phenomena of shaft deflection. In this case, shaft deflection is created by unbalanced hydraulic radial forces on the impeller as a result of operating the pump away from its best efficiency point (BEP). Deflections are simply a shaft bending that occurs twice per one revolution. At 3,600 rpm a shaft will deflect 7,200 times a minute, while the 1,800 rpm shaft will deflect only 3,600 times per minute. An operating pump shaft can also exhibit a phenomena called “whip,” which manifests as another sort of deflection, but the root cause in this instance is impeller imbalance. As a side note, understand that broken pump shafts are commonly due to cyclic stress.
In the case of shaft deflection, the shaft is not permanently bent, but it does deflect in dynamic operation. If you were to disassemble the pump and check the static shaft, it would measure as straight. These deleterious deflections are assuredly the principle reason for most mechanical seal failures. And while contaminated lubrication contributes more to bearing failure than shaft deflection, the deflections do bring undesired forces and fatigue to the bearings. In the formula for calculating the expected life span of ball bearings, the operating speed is a main factor. Bearing life expectation L-10 is inversely proportional to speed.
One of the more common reasons for using lower speed pumps is the net positive suction head required (NPSHr) factor. An 1,800 rpm pump will require substantially less NPSH for the same hydraulic conditions as the 3,600 rpm machine. Normally the requirement will approach half of the high speed pump value and often more. The higher the NPSH margin (available NPSH as compared to required NPSH), the better the pump reliability will be. Inadequate NPSH leads to cavitation, which creates impeller erosion (imbalance), hydraulic pulsations and mechanical vibrations.
Pump noise will be substantially less on lower speed pumps. Published noise levels are based on log scales, and the major contributor in most cases is the motor, not the pump (mostly due to the cooling fan). Actual noise levels in the field will be different than published data due to in situ geometries and unique ambient conditions. This is based on a properly selected pump operating within 25 percent of BEP.
As pump casing sizes get bigger in diameter, most manufacturers incorporate a dual volute design somewhere in the neighborhood of 12 to 14 inches. The dual volute design significantly reduces radial thrust. While a dual volute pump costs more to manufacture, the consequential reduction in radial thrust will contribute greatly to reliability.
Negative of a Slower Speed Pump
For a given head and flow condition, an 1,800 rpm pump can approach twice the size of the 3,600 rpm pump. In this era, where modern business methods dictate minimizing floor space, the (potentially) more reliable pump will have to compete with the smaller footprint pump.
Ostensibly, pumps are really just material (usually metal) manufactured in the geometry of a useful machine and sold by the pound. A pump that is twice the size can easily cost twice as much. When the decision making process for pump acquisition is solely based on initial dollars spent as opposed to lifetime operating costs, the higher speed pump almost always looks like a winner. However, the lower speed pump should be considered if you factor in operating maintenance budget and overall plant reliability for the next 20 years.
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.
General Comments
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.
Summary
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?