Pump Wisdom. Robert X. Perez
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1.11 Pump manufacturers usually plot only the NPSHr trend associated with the lowermost curve. At that time a head drop or pressure fluctuation of 3% exists and cavitation damage is often experienced.
Source: Taylor [7].
Note, again, that recirculation differs from cavitation, a term which essentially describes vapor bubbles that collapse. Cavitation damage is often caused by low net positive suction head available (NPSHa). Such cavitation‐related damage starts on the low‐pressure side and proceeds to the high‐pressure side. An impeller requires a certain net positive suction head; this NPSHr is simply the pressure needed at the impeller inlet (or eye) for relatively vapor‐free flow.
What We Have Learned
Understanding the concepts of N s and N ss will assist in specifying better pumps. In addition to fluid properties pump life is influenced by throughput. Just as an automobile transmission is designed to work best at particular speeds or optimum gear ratios, pumps have desirable flow ranges. Deviating from optimum flow will influence failure risk and life expectancy.
References
1 1 SKF USA, Inc.; “Bearings in Centrifugal Pumps”, Kulpsville, PA, Publication 100‐955, Version 4/2008; excerpted or adapted by permission of the copyright holder.
2 2 Emile Egger & Cie.; “Operating Manual”, Salt Lake City, Utah, and Cressier, NE, Switzerland, 2008.
3 3 ITT/Goulds Pump Corporation; “Installation and Maintenance Manual for Model 3196 ANSI Pump”, Seneca Falls, NY, 1990.
4 4 Mitsubishi Heavy Industries, Ltd.; “Publication HD30‐04060”, Tokyo, Japan, and New York, NY.
5 5 Fraser, Warren H.; “Flow recirculation in centrifugal pumps,” Proceedings of the Texas A&M University Turbomachinery Symposium, Houston, TX, 1981, pp. 95–100.
6 6 Ingram, James H.; “Pump reliability–where do you start", presented at ASME Petroleum Mechanical Engineering Workshop and Conference, Dallas, TX, September 13–15, 1981.
7 7 Taylor, Irving; “The Most Persistent Pump‐Application Problems for Petroleum and Power Engineers”, ASME Publication 77‐Pet‐5 (Energy Technology Conference and Exhibit, Houston, Texas, September 18–22, 1977).
8 8 ANSI/HI9.6.3‐1997; “Allowable Operating Region”, Hydraulic Institute, Parsippany, New Jersey, 2008.
9 9 Bloch, Heinz P., and Alan Budris; “Pump User's Handbook”, 3rd Edition, Fairmont Press, Lilburn, GA 30047, 2010 (ISBN 0‐88173‐517‐5).
2 Pump Selection and Industry Standards
Anybody can buy a cheap pump, but you want to buy a better pump. The term “better pumps” describes fluid movers that are well designed beyond just hydraulic efficiency and modern metallurgy. Better pumps are ones that avoid risk areas in the mechanical portion commonly called the drive‐end. That is the part of process pumps that has been neglected most often and where cost cutting should cause the greatest concern.
Deviations from best available technology increase the failure risk. As three or four or more deviations combine, a failure is very likely to occur. An analogy could be drawn from an incident involving two automobiles, with one driving behind the other. When the trailing vehicle travelled at (i) an excessive speed, with (ii) worn tires, on (iii) a wet road, and (iv) followed the leading car too closely, a rear‐end collision resulted. Had there just been any three of the four violations, the event might be recalled as one of the many “near miss” incidents. Had there been any two of the four, it would serve no purpose to tell the story in the first place.
Too much cost‐cutting by pump manufacturers and purchasers will negatively affect the drive‐ends of process pumps. Flawed drive‐end components are therefore among the main contributors to elusive repeat failures that often plague pumps – essentially very simple machines. Drive‐end flaws deserve to be addressed with urgency, and this short chapter will introduce the reader to more details that follow later in Chapters 5, 8 and 10.
Why Insist on Better Pumps
Well‐informed reliability professionals will be reluctant to accept pumps that incorporate the drive‐end shown in Figure 2.1. The short overview of reasons is that reliability‐focused professionals take seriously their obligation to consider the actual, lifetime‐related and not just short‐term, cost of ownership. They have learned long ago that price is what one pays, and value is what one gets.
Figure 2.1 A typical bearing housing with several potentially costly vulnerabilities.
Anyway, while at first glance the reader might see nothing wrong, Figure 2.1 contains clues as to why many pumps fail relatively frequently and sometimes quite randomly. It shows areas of vulnerability that must be recognized and eliminated. The best time to eliminate flaws is in the specification process. A number of important vulnerabilities, deviations from best available technology or just plain risk areas exist in that illustration:
Oil rings are used to lift oil from the sump into the bearings;
The back‐to‐back oriented thrust bearings are not located in a cartridge;
Bearing housing protector seals are missing from this picture;
Although the bottom of the housing bore (at the radial bearing) shows the desired passage, the same type of oil return or pressure equalization passage is not shown near the 6 o'clock position of the thrust bearing;
There is uncertainty as to the type or style of constant level lubricator that will be supplied. Unless specified, the pump manufacturer will almost certainly provide the least expensive constant level lubricator configuration. Putting it another way: The best lubricators are rarely found on newly sold pumps.
Each of these issues merits further explanation and will be discussed in Chapters 5 through 9. Recall also that our considerations are confined to lubrication issues on process pumps with liquid oil‐lubricated rolling element bearings. The great majority of process pumps used worldwide belong to this lubrication and bearing category. Small pumps with grease‐lubricated bearings and large pumps with sleeve bearings and circulating pressure‐lube systems are not discussed in this text.
ANSI and ISO vs. API Pumps
ANSI stands for American National Standards Institute; ISO is the International Standards Institute, and API is the American Petroleum Institute. In general, ANSI and ISO pumps comply with dimensional standards; the measurement conventions are inches and millimeters, respectively. ANSI pumps will have the same principal dimensions regardless of manufacturer, as will ISO‐compliant pumps in their respective size groups. Principal dimensions, for the sake of this overview include, but are certainly not limited to, the distance from base mounting surfaces to the shaft centerline, or to the pump suction and discharge flange faces.
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