Gestión de Mantenimiento

A. SOFRONAS, CONSULTING ENGINEER

Case 84: Remaining service life of plant equipment

can be determined

For new plant machines and equipment, the life expec- tancy is determined by the original equipment manufacturer (OEM). Yet, different analysis techniques are often required in a failure analysis. In a processing facility, the reliability or project engineer may be asked if damaged equipment can be safely operated until a controlled shutdown is possible. This requested time is usually days, not months. In the real world, careers are not enhanced by mandating immediate equipment or unit shutdowns without supporting data.

Solutions. Engineers have the analytical tools available to make decisions based on an educated and thorough analysis. Here are some examples:1

Bearings. The service life of new ball and roller bearings

can be determined by the loads, speeds, lubrication method, cleanliness and other factors. The number of cycles that the bearings will endure before evidence of metal fatigue occurs determines the length of service (life). For new designs, the engineer will consider the L10 life or the life that 90% of the bearings will exceed before failure. Exceeding the L10 life will usually not result in a failure. For a failure analysis, engineers should be interested in knowing under what conditions fail- ures are most likely to occur. A 90% chance of a failure occurs at about 14 times the L10 life.2

Shafts, rotors, pressure vessels and structures. With

the various loads and stresses, material properties, surface fin- ishes and stress concentrations known, the fatigue life in axial, bending, shear and torsion can be determined in cycles. Us- ing certain techniques (e.g., the Palmgren-Miner linear dam- age rule), the life reduction due to excessive periodic loading can also be determined. A metal “remembers” when it has been overstressed, as its remaining service life will have been perma- nently reduced. The damage rule will approximate how much.

Steel parts with cracks. When cracks are present, tradi-

tional fatigue calculation methods may not be applicable. This is because 90% of the fatigue life of a part is exhausted by the time a crack develops. Other techniques, such as fracture me- chanics, must be applied. When the crack size is known along with the material type and the stresses opening and closing the crack, the growth rate can be determined.

A life assessment can then be done by assuming that a very small crack is present and then determining how fast it will grow, or if it will grow at all. When the part is a brittle material, meaning certain low-ductility materials, the crack growth can be fast (such as 7,000 ft/sec). This crack should never be al- lowed to materialize! Growth of such cracks is dangerous, and it is difficult to monitor as these cracks are unstable.

Use this rule with brittle materials: In critical equipment constructed of old brittle material that has cracks, shut it down and repair it or assess by using fracture mechanics tech- niques. Even if the equipment has run successfully over the years, one major upset or shock loading could cause cracks to grow rapidly.

Part wear. Wear equations make it possible to determine

the wear rate of two materials sliding together dry or with lubri- cation between them. The wear of gear teeth, extruder barrels/ screws or the wear of a rotating shaft in a bore are examples in which service life calculations in cycles are possible.

Gear life due to pitting, bending, wear and scoring fail- ures. Load calculations will allow service life assessments on

new gears with different lubricants, loads, speeds and tooth profiles. Service life will be in cycles or the probability of fail- ure. Pitted gears will not provide useful life assessments, but a sensitivity analysis may show the root cause for the pitting.

Creep life. The service life of furnace tubes and other com-

ponents under high temperature can exhibit creep—an elonga- tion due to failure under a constant stress. The stress could be in a pressurized tube in a high-temperature environment. The fur- nace tube material can thin out and rupture at any given time. This service time can be calculated and estimated in hours.

Best advice. All service life calculations are approximations since the data are scattered, and probabilities are used. Users must understand that the equipment service life could be longer or shorter. However, if the life calculations indicate that a three- year life is remaining and that only a day run is required, then this is valuable data to use in the decision-making process.

NOTE

Case 83 was published in HP in March. For past cases, please visit

HydrocarbonProcessing.com.

LITERATURE CITED

1 _{Sofronas, A., Analytical troubleshooting of process machinery and pressure vessels,}

John Wiley & Sons, 2006.

2 _{Palmgren, A., Ball and roller bearing engineering, Burbank and Co., pg. 74, 1959.}

TONY SOFRONAS, D. Eng, P.E., was worldwide lead mechanical engineer for ExxonMobil Chemicals before retiring. He now owns Engineered Products, which provides consulting and engineering seminars on machinery and pressure vessels. Dr. Sofronas has authored two engineering books and numerous technical articles on analytical methods.

Hydrocarbon Processing | MAY 201537

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GREGORY A. DOLAN has held a variety of senior management positions with the Methanol Institute (MI) over the past 17 years. He is CEO of the global methanol industry trade association. Mr. Dolan manages MI’s offices in Washington, DC, and in Singapore and Brussels, while directing international governmental relations, media relations, public education and outreach efforts. Mr. Dolan came to MI after spending a decade in a variety of public information positions in New York State, including the Department of Environmental Conservation, the Department of Transportation, and the Energy Research and Development Authority. Mr. Dolan holds a BA degree in political science from Boston University, and did extensive post- graduate work in political communication at the State University of New York-Albany.

GREGORY DOLAN, CEO