Every plant wants to get more service life from site machinery. Since about 1990, quite a number of start-up consulting companies have formed to advise clients on equipment life extension. These companies use different approaches: some apply large-scale, computer-based statistical methods, while others blend traditional estimates with risk-based analysis. All of these approaches have merit, but none of them can provide all of the answers with high precision. The key ingredients of any useful endeavor include reviewing the assets past failure history, examining nondestructive testing (NDT) data, and upgrading the weakest link.
Failure history counts
Wherever failure history exists and the failures root causes were analyzed, authoritative answers on remaining service life are possible. The same can be said for thoroughly evaluating NDT data, which can provide focus to determine remaining life.
On stationary equipment and piping, wall thickness is of great importance. Loss of material decreases the allowable pressure rating. Corrosion and erosion can lower the safety of the equipment; thus, continued operation becomes risky. Thickness changes often occur at locations, such as elbows, where fluid flow changes direction. Changes in velocity such as at valves or near restrictions are of high interest. Some can be investigated with NDT methods, which certainly include X-ray imaging, among others. The extent of fluid-dependent corrosion can be estimated from coupons placed in piping and vessels.
Pumps. For pumps, failure history and past repair data must be matched with a thorough understanding of upgrade measures that have been taken by successful best-of-class organizations. Advanced lube application will probably be part of it, as will the extension of oil replacement intervals now possible by synthetic lubricants and advanced bearing housing protection measures.
To what extent superior bearings (ceramic hybrids) are of value must be determined on a pump-by-pump basis. Perhaps a set of angular bearings with unequal contact angles should be installed in your problem pumps. The symmetrical sets of angular contact bearings mentioned in the most widely used pump standard may not perform adequately. The extent that superior sealing technology (dual seals, as shown in Fig. 1) provides more value must be determined on a service-by-service basis. As a general rule, the industrys view about dual seals deserves to be reassessed. Sealing technology has made considerable progress in the past two decades. Virtually all present-day seals are cartridge-style configurations, and braided packing is being displaced by mechanical seals in the hydrocarbon processing industry (HPI), as well as in the power generation and mining industries.
Fig. 1. Dual mechanical seal in a
slurry pump. The space between the
sleeve and the inside diameter of the
two sets of seal faces is filled with a
pressurized barrier fluidusually
clean water. Source: AESSEAL Inc.,
Rockford, Tennessee, and
However, not all manufacturers of mechanical seals use the same acceptance test procedure for their products. A widely applied industry standard stipulates using air as a test gas for mechanical seal tightness. Of course, these seals are ultimately intended for safe containment of flammable, toxic or otherwise hazardous liquids. While the standards expectation is that leakage from these seals does not exceed 5.6 gm/hr, recent tests showed that merely following this easy testing routine can actually allow orders of magnitude more liquid to escape. It is, therefore, advisable to question seal vendors on the matter and to purchase only products that meet the purchasers safety and reliability requirements. We all want seals to leak no more than 6.5 gm/gr when first installed on pumps.
Lubricant application and standby bearing preservation are especially important in humid coastal climates and in dust-laden desert climates. Oil mist is the answer. The settling of foundations and pipe supports should be addressed. For steam turbines, the blade stresses and water quality must be compared with those units in successful long-running installations elsewhere.
In gearboxes, the remaining service life is largely examined by tooth loading (stresses on tooth face) and temperature measurements. In all instances, synthetic oils from the most experienced oil formulators will greatly extend gear life. Oil additives are everything. They drive both cost and service life. Oil cleanliness is equally important.
Certain warehouse spares (gears, electric motors, etc.) should be upgraded, if important. If doing so, it is likely to speed up re-commissioning after an unanticipated future shutdown.
For compressors, engineers should consider the mentioned points. Valve technology and piston velocity are important comparison-worthy parameters on reciprocating compressors. Onstream performance tracking and prior sealing technology are important for centrifugal compressors, etc. They determine seal system upgrade potential. Never overlook couplings and the work procedures used to attach couplings to shafts. They tell a lot about remaining run length.
Whether one ultimately receives life extension guidance from individual consultants or from billion-dollar consulting giants with applicable experience is of no consequence, as long as there is the one common thread: Determining where upgrades are possible. Upgrades are critical to imparting longer life to existing equipment, and they can often be accomplished at relatively low cost. Assessments of remaining life should include detailed advice on how to upgrade weak links, which implies:
- The expert authoritatively spells out recommended upgrade components
- Recommended upgrade procedures are explained
- Facilities recommended to do the upgrading are defined.
In short, the entity involved in advising you on equipment life extension must understand the feasibility of component upgrades. Component upgrading is one of the keys to life extension and deliverables that should be contractually agreed upon with the upgrade provider. Be sure that the consulting company youve asked to give advice on equipment life extension includes these deliverables. HP
|The author |
Heinz P. Bloch resides in Westminster, Colorado. His professional career commenced in 1962 and included long-term assignments as Exxon Chemicals regional machinery specialist for the US. He has authored over 520 publications, among them 18 comprehensive books on practical machinery management, failure analysis, failure avoidance, compressors, steam turbines, pumps, oil-mist lubrication and practical lubrication for industry. Mr. Bloch holds BS and MS degrees in mechanical engineering. He is an ASME Life Fellow and maintains registration as a Professional Engineer in New Jersey and Texas.