Pumping Prescriptions
by Lev Nelik, Ph.D., P.E.
May 25, 2016

During the Pump School sessions that I host, I hear a variety of questions from many perspectives. Mechanics, more interested in the hands-on aspects of training, usually want to know better strategies for repairing, lubricating, aligning and installing equipment. Engineers, on the other hand, may have more interest in theoretical aspects of pumping systems, such as pump-to-system interaction, net positive suction head (NPSH) and affinity laws. Plant operators are often more interested in operating flow limitations and aspects of flow control, including plant transients and emergency situations.

But there is a common question I hear from all groups: "What is the most reliable pump we should use?" While this question may sound too general and perhaps impossible to answer because of a diverse range of applications, the answer is actually not impossible.

Types of Pumps

To begin, there are two main classes of pumps, according to the Hydraulic Institute (HI): centrifugal pumps and positive displacement pumps. The majority of pumps—perhaps 85 percent—are centrifugal pumps, and this is not likely to change in the foreseeable future. If you then separate the centrifugal pump world into "small" and "big" centrifugal pumps, most of these pumps (in terms of number of units) are small. If that is the case, we can include mainly single-stage end-suction and double-suction pumps, all typically under the 250-horsepower (hp) range. While this arbitrarily defined "small" group also covers small vertical turbine pumps, as well as some other specialty types, the single-stage overhung-impeller end-suction and double-suction between-bearing split-case pumps would be the majority.

A representative centrifugal pump most commonly used across industries would probably be a 25-hp end-suction pump, pumping 400 gallons per minute (gpm) against 50 pounds per square inch (psi) pressure. A typical centrifugal pump like this costs roughly $5,000 (including a motor, baseplate, coupling, etc.), typically operates 30 percent of time during the year, consumes about $5,000 of electricity, and may last 15 to 20 years, undergoing perhaps two or three repairs. These repairs include a change of bearings, seals or packing and an occasional shaft and/or impeller.

For a centrifugal pump like this, not much analysis is required, and as long as it runs well and fails rarely, everyone is happy. In these situations, discussions about efficiency, energy savings, best efficiency point (BEP) and flow analysis are not at the top of the priority list. Fortunately, given our assumptions above, the majority of pumps fall under this status quo. It is the other types of pumps that call for scratching heads and big bucks wasted. Large vertical turbine types at water plants, multistage boiler feeds and others may be fewer in numbers, but they are bigger in size, consume more energy and require much higher reliability—these pumps are in a world of their own.

In the world of the majority ("small" pumps), a few simple but common-sense features make the difference in longevity: short stubby shafts (L3/D4 ratio), packing as a default choice (mechanical seals only if absolutely necessary, such as when handling tough chemicals with no leaks allowed), no axial adjustments, and simplicity and robustness. These features will make pumps run well for a long time with little waste.

Over the years, many users have experienced the effects of not following these tips: They wrap the pumps in so much protection that it literally kills the equipment; they use multiple seals, fins and cooling coils; and they incorporate all sorts of sensors. They do all of this with good intentions to make their equipment more predictable and reliable, but they end up with a system that is much more expensive and less reliable because of so many extra "lines of defense"—the failure of even one of which usually precipitates the chain reaction that leads to the failure of the other lines of defense.

Overhung & Between-Bearings Designs

As a result, the two main candidates for the highest reliability in general applications are overhung and between-bearings designs. Of the two, a between-bearings type would typically be more reliable because the rotor is supported much more definitely on both sides of the impeller. This design helps resist the radial load, balances the axial load nearly completely and is easily repairable if needed. Plus, bearings can be replaced without having to uncouple them from the motor, the packing can be changed quickly and simply, and the impeller can be changed by simply lifting the top casing and replacing the rotating assembly with a stocked spare. The downside of this type of pump is a somewhat higher cost (perhaps 20 percent more over the comparable overhung type) and a somewhat larger footprint (perhaps 20 percent more space).

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