Next to electric motors, centrifugal pumps are the most prevalent modern industrial machines. However, the fact that some U.S. oil refineries, on average, must repair their centrifugal pumps every three years while other refineries enjoy average repair intervals of 10 years is a matter of record. The purpose of this article is to briefly highlight how best-of-class refineries and other petrochemical companies have accomplished such obviously significant failure reductions, and how other pump users can obtain the same results.
Premises and rules
Facilities that reduce failure incidents and rule out certain pump failures accept three well-founded premises and adhere to a few fundamental rules.
Premise No. 1: Repeat failures occur only for one of two possible, fundamental reasons: Either the root cause of the failure hasn’t yet been found, or the root cause is known but remedial action isn’t pursued.
Premise No. 2: Given their simplicity, most pump repair frequencies are excessive. Therefore, many pumps can be improved for reduced failure risk and extended runs, and any repair event presents an opportunity to upgrade and reduce failure risk.
Premise No. 3: Improvements are possible and have been thoroughly documented. Predictive maintenance is valuable; it indicates that a failure will occur unless the user shuts down and repairs the pump. If cost-effective upgrading is feasible and extends component life, one can reduce the frequency and cost of certain predictive maintenance activities. Whenever cost-effective upgrading is feasible and avoids failures, it adds more value than continuing the predictive maintenance cycle.
Parts replacement: Not good enough
Why “doing the same thing we’ve always done” isn’t an acceptable approach is best explained by an example. For the sake of illustration, consider example case 1 — a centrifugal pump that develops a problem. It’s taken to the repair shop and dismantled. The technician finds a defective bearing and replaces it. The pump is reassembled, reinstalled and restarted. The technician might not realize that the failed bearing had been weakened by water in the lube oil. If nothing is being done to exclude water, the repair cycle will simply roll onward and downward.
Cost-effective remedial action through upgrading would involve installing a dual-face magnetic bearing housing protector seal (Figure 1), as was done in a municipal water pump (Figure 2). Before this modification, bearings had lasted only about two months. With the seals, the bearings have been in service for more than two years and exhibit no sign of distress.
Figure 1: Dual-Face Magnetic Bearing Housing Seal (Source: AESSEAL plc, Rotherham/UK, and Knoxville/Tennessee)
Figure 2: Vertically-oriented Axially Split Centrifugal Pump in Municipal Water Service. Installing a Dual-Face Magnetic Bearing Housing Seal Prevented Future Water Contamination (Source: AESSEAL plc, Rotherham/UK, and Knoxville/Tennessee)
Structured failure analysis
There are literally hundreds of examples of a properly structured root-cause failure analysis and upgrade program yielding immediate and measurable payback. Fortunately, such programs are deceptively simple and quickly implemented by anyone who wishes to do so. We call it the FRETT approach because it recognizes that, without exception, the basic agents of machinery component and part failure mechanisms are always force, reactive environment, time or temperature1. These basic failure mechanism agents may combine to hasten component degradation.
Isolate the basic agents involved through a process of elimination. Start by thoroughly examining the failed part and determining from its appearance whether the damage occurred while the pump was operating or while standing still. Suppose the technician had observed corrosion on the bearing. If it had occurred in operation, the corrosion would be uniform. Non-uniform corrosion would suggest that it had happened at standstill.
In any event, keeping water away from the bearing is a sound remedy. Magnetic bearing housing protector seals are ideal for the flooded lubrication in vertical pumps. Non-contacting labyrinth seals, not lip seals, (Figure 3) are best for the majority of horizontal centrifugal pumps.
Shaft-Contacting Lip Seal, Top, vs. Non-Contacting Dynamic O-Ring Seal, Bottom. (Source: AESSEAL, plc, Rotherham/UK and Knoxville, Tennessee)
Consider a case in which an unduly hot or noisy bearing exhibited the load pattern shown in Figure 4. From understanding the published load-speed-life relationship and knowing that failure had occurred after a relatively short time, one might easily rule out the first “T” ingredient — time. Absent evidence of discoloration and carbon formation, one could rule out the second “T” — temperature. If no corrosion was evident, a reactive environment could be ruled out. You’d be left to pursue “F” — force. A continuous wear pattern rules out installation damage from hammer blows.