How are your operations doing on pump MTBF? How do you compare with the best of the best?
Examining pump repair records and MTBF (mean time between failures) is of great importance to responsible and conscientious pump users. In view of that fact, the preface to the 2006 Pump User’s Handbook (Ref. 1) alludes to “pump failure” statistics. For the sake of convenience, these failure statistics often are translated into MTBF (in this case, installed life before failure).
In an effort to avoid arguments on statistics, during the early 2000s, many best-practices plants simply took all their installed pumps and divided that number by the number of repair incidents per year. For a well-managed and reasonably reliability-focused U.S. refinery with 1200 installed pumps and 156 repair incidents in one year, the MTBF would be (1200/156)=7.7 years. The refinery would count as a repair incident the replacement of parts—any parts—regardless of cost. In this case, a drain plug worth $1.70 or an impeller costing $5000 would show up the same way on the MTBF statistics. Only the replacement of lube oil would not be counted as a repair.
A best-practices plant’s total repair cost for pumps would include all direct labor, materials, indirect labor and over-head, administration cost, the cost of labor to procure parts, even the prorated cost of pump-related re incidents. There exist many references to the stated average cost of pump repairs: $10,287 in 1984 and $12,000 in 2008. We believe this indicates, in relative terms, a repair cost reduction, because a 2008 dollar bought considerably less than the 1984 dollar. It also can be reasoned that predictive maintenance and similar monitoring have led to a trend toward reduced failure severity.
Using the same bare-bones measurement strategy—and from published data and observations made in the course of performing maintenance effectiveness studies and reliability audits in the late 1990s and early 2000s—the mean time between failures (installed life before failure) of Table I have been estimated.
Other studies of pump statistics In early 2005, Gordon Buck, John Crane’s chief engineer for Field Operations in Baton Rouge, LA, examined the repair records for a number of renery and chemical plants to obtain meaningful reliability data for centrifugal pumps. A total of 15 operating plants having nearly 15,000 pumps were included in the survey reflected in Table II. The smallest of these plants had about 100 pumps; several plants had over 2000. All facilities were located in the United States. In addition, all plants had some type of pump reliability program in progress. Some of these programs could be considered as “new,” others as “renewed” and still others as “established.” Many of these plants—but not all—had an alliance arrangement with John Crane. In some cases, the alliance contract included having a John Crane technician or engineer on-site to coordinate various aspects of the program.
Not all plants are refineries, however, and different results can be expected elsewhere. In chemical plants, pumps have traditionally been “throw-away” items as chemical attack can result in limited life. Things have improved in recent years, but the somewhat restricted space available in “old” DIN and ASME-standardized stuffing boxes places limits on the type of seal that can be fitted. Unless the pump user upgrades the seal chamber, only the more compact and simple versions can be accommodated. Without this upgrading, lifetimes in chemical installations are generally believed to be around 50 to 60% of the refinery values.
Target pump and component lifetimes Based on the lifetime levels being achieved in practice in 2000 and combined with the known “best practice” as outlined in available reference texts, the target component lives of Table III are recommended and should be considered readily achievable.
It should again be emphasized that many plants are achieving these levels of installed lives. The actual operating life of a seal would be approximately one-half of its installed life. Nevertheless, to reach these pump lives, the pump components themselves must be operating at the highest levels. An unsuitable seal with a lifetime of one month or less will have a catastrophic effect on pump MTBF, as would an under-performing coupling or bearing.
Upgrades can reduce maintenance costs Many “standard” ANSI and ISO-compliant pumps were designed decades ago—when frequent repairs were accepted and plant maintenance departments were loaded with personnel. Unless selectively upgraded, a “standard” pump population will not allow 21st century facilities to reach their true reliability and profitability potentials.
We are intending primarily to provide pump failure statistics on relatively inexpensive ANSI and ISO pumps, as well as API-compliant (“traditional”) refinery centrifugal pumps. But, this article also should remind the reader of important issues that cannot be overlooked in attempting to extend pump life. Pump sealing, advanced bearing housing protection and bearing lubrication topics are of primary importance (and the author would be pleased to alert you to specifics at either his or another relevant Website).
It goes without saying that unscheduled maintenance often is one of the most significant costs of ownership, and failures of mechanical seals and bearings are among the major causes. Therefore, in recent years, improved pump designs and the management of mechanical reliability issues have led to significant increases in the pump MTBF rates for many process plants. Back in the 1970s and early 1980s, it was not unusual to find plant MTBF rates of six months to 12 months. Today, most reliability-focused plants have pump MTBF rates that track the tabulations found in Tables I through III. Important improvements have been achieved by paying proper attention to pump components with the highest failure rates.
Buried in Table I is information from a plant with more than 2000 installed pumps—in average sizes around 30 hp—that presently enjoys an MTBF in excess of nine years. This, as one might suspect, is a facility that looks at mechanical and process interactions. The reliability professionals at this plant fully understand that pumps are part of a system and that the system must be correctly designed, installed and operated if consistently high reliability is to be achieved. It should be pointed out that plants of this type are conducting periodic pump reliability reviews.
Reliability reviews start before purchase The best time for the first reliability review is before the time of purchase. This subject is given thorough treatment in Ref. 2. It is rather obvious that individuals with reliability engineering backgrounds and acute awareness of how and why pumps fail are best equipped to conduct such reviews. This implies that these contributors should have an involvement in the initial pump selection process. Individually—or as a team—those involved should consider the possible impact of a number of issues, including the ones mentioned above. They merit close attention and are again summarized for emphasis:
- Keep in mind the potential value of selecting pumps that cost more initially, but last much longer between repairs. The MTBF of a better pump may be one to four years longer than that of its non-upgraded counterpart.
- Consider that published average values of avoided pump failures range from $2600 to $12,000. This does not include lost opportunity costs.
- One pump fire occurs per 1000 failures. Having fewer pump failures means having fewer destructive pump fires.
Remember that there are several critically important applications where buying on price alone is almost certain to ultimately cause costly failures. Included are the following:
- Applications with insufficient NPSH or low NPSH margin ratios [Ref. 3];
- High- or very-high-suction energy services;
- High specific-speed pumps [Ref. 4];
- Feed and product pumps without which the plant will not run;
- High-pressure and high-discharge energy pumps;
- Vertical turbine deep-well pumps.
At a minimum, then, these are the six services where spending time and effort for pre-purchase reliability reviews makes much economic sense. Such diligent reviews concentrate on typical problems encountered with centrifugal pumps; an attempt is made to eliminate these problems before the pump ever reaches the field. Among the most important problems that the reviews seek to avoid are:
- Pumps not meeting stated efficiency;
- Lack of dimensional interchangeability;
- Vendor’s sales and/or coordination personnel being reassigned;
- Seal problems and compromises in materials, flush plans, flush supplies, etc.;
- Casting voids (repair procedures, maximum allowable pressures, metallurgy, etc.);
- Lube application or bearing problems [Ref. 5];
- Alignment, lack of registration fit (rabbetting), base plate weakness, grout holes too small, base plates without mounting pads, ignorance of the merits of pre-grouted base plates;
- Documentation: manuals and drawings shipped too late;
- Pumps that will not perform well when operating away from best efficiency point, i.e. prone to encounter internal recirculation [Ref. 6].
Perform your own projected MTBF calculations Although not perfect from a mathematician’s perspective, simplified calculations will give an indication of the extent to which improving one or two key pump components can improve overall pump MTBF [Ref. 7]. Say, for example, there’s agreement that the mechanical seal is the pump component with the shortest life, followed by the bearings, coupling, shaft and sometimes impeller, in that order. The anticipated mean time between failures (operating MTBF) of a complete pump assembly can be approximated by summing the individual MTBF rates of the individual components, using the following formula:
1/MTBF = [(1/L1)2 + (1/L2)2 + (1/L3)2 + (1/L4)2]0.5Eq. (1)
In a 1980s study, the problem of mechanical seal life was investigated. An assessment was made of probable failure avoidance that would result if shaft defections could be reduced. It was decided that limiting shaft defection at the seal face to a maximum of 0.001″ (0.025 mm) probably would increase seal life by 10%. It was similarly judged that a sizable increase in seal housing dimensions to allow the installation of the newest seal configurations would more than double the MTBF of seals. By means of such analyses, all of the components under consideration for upgrading were examined, the life estimates collected and the latter used in MTBF calculations.
In Equation (1), LLL and L represent the life, in years, 1,2,34 of the component subject to failure. Using applicable data collected by a large petrochemical company in the 1980s, mean times between failures and estimated values for a reliability-upgraded pump were calculated. The results are presented in Table IV. As an example, a standard construction ANSI B73.1 pump with a mechanical seal MTBF of 1.2, bearing MTBF of 3.0, coupling MTBF of 4.0 and shaft MTBR value of 15.0, resulted in a total pump MTBF of 1.07 years (actual operating hours). By upgrading the seal and bearings, the estimated achievable pump MTBF (actual operating hours) can be improved by 80%, to 1.93 years.
Table IV thus shows the influence of selectively upgrading either bearings or seals or both on the overall pump MTBF. Clearly, choosing a 2.4 year MTBF seal and a six-year MTBF bearing (easily achieved by preventing lube oil contamination via superior bearing housing seals), had a major impact on increasing the pump MTBF and—assuming the upgrade cost is reasonable—may well be the best choice.
As has been noted, a typical pump failure, based on actual year 2002 reports, costs $5000 on average. This includes costs for material, parts, labor and overhead. Let us now assume that the MTBF for a particular pump is 12 months and that it could be extended to 18 months. This would result in a cost avoidance of $2500/yr—which is greater than the premium one would pay for the reliability-upgraded centrifugal pump.
In addition, the probability of reduced power cost would, in many cases, further improve the payback. Recall also that the proper hydraulic selection of a pump can have a further marked positive impact on the life and operating efficiency of a pump. Audits of two large U.S. plants identified seemingly small pump and pumping system efficiency gains that added up to power-cost savings of many hundreds of thousands of dollars per year. Thus, the primary advantages of reliability-upgraded process pumps are extended operating life, higher operating efficiency and lower operating and maintenance costs.
Table IV provides a quick means of approximating the annual pump repair frequency based on the total (installed life) MTBF. Equation (1) and Table IV also can be used to determine potential savings from upgrades and should shape the pump user’s strategies. MT
Contributing editor Heinz Bloch is the author of 17 comprehensive textbooks and over 340 other publications on machinery reliability and lubrication. He can be contacted at: firstname.lastname@example.org
- Bloch, Heinz P., and Allan R. Budris, Pump User’s Handbook, Fairmont Press, Lilburn, GA 30047, ISBN 0-88173-517-5, 2nd Edition, 2006.
- Bloch, Heinz P., Improving Machinery Reliability, Gulf Publishing Company, Houston, TX, 3rd Edition, 1998.
- Karassik, Igor J., “So, You Are Short On NPSH?” presented at Pacific Energy Association Pump Workshop, Ventura, CA, March 1979.
- Ingram, J.H.,”Pump Reliability— Where Do You Start,” presented at ASME Petroleum Mechanical Engineering Workshop and Conference, Dallas, TX, September 13-15, 1981.
- Bloch, H.P., “Optimized Lubrication of Antifriction Bearing for Centrifugal Pumps,” ASLE, Paper No. 78-AM-1D-1, presented in Dearborn, MI, April 17, 1978.
- McQueen, R., “Minimum Flow Requirements for Centrifugal Pumps,” Pump World, 1980, Volume 6, Number 2, pp. 10-15.
- Bloch, Heinz P., and Don Johnson, “Downtime Prompts Upgrading of Centrifugal Pumps,” Chemical Engineering, November 25, 1985.