Predictive Maintenance Methods

Predictive maintenance (PdM) or condition based maintenance (CBM) is to monitor predefine parameters over the time and predict it’s failure time to make corrective repair to restore its normality.

This is the last and final stage of preventive maintenance which aims to optimize maintenance period and costs by monitoring the trends of equipment deterioration trends.

It is possible to sustain zero breakdowns through TBM itself but there may chance to replace some spares before their life ends which means over and costly maintenance. So, By increasing scope of CBM we can optimize maintenance cost and time.

It targeted at parts that have had serious breakdown in pasts and or parts that cause costly damage (Total downtime costs) when they fails and due to high costs of spares it is not economical to consider it under TBM.


Pre-requisites for PdM
: rushing to PdM diagnostic tools or service yields no results

a) Identify accelerated deterioration factors and implement thorough restoration and improvement through JH

b) Un expected failures can be reduced. Consider in-depth Time Base Maintenance (PM / TBM) in order to find the deterioration pattern

c) Sufficient data to compare with ideal.

d) Failures patterns are not frequent and a costly affair for both to afford reactive maintenance and keeping spare.

e) Decide control value for each deterioration pattern

Prediction Methods

1. Measuring electrical motor currents, temperature, Insulation level: Monitoring electrical motors current and temperature trends tell you about Mechanical side problems, aging of motors.

2. Thermal imaging – It is a non-contact temperature capturing device which collects temperature data from multiple points and store in a 2D profile. Some thermal imagers can compare present profile with base profile and help to detect the problem area. Excellent and versatile method of predicting problems. Commonly use for .

i) Measuring bearing temp. of large motors

ii) Identify leaks

iii) Loose connection in electrical circuits.

iv) Hot-spots in electronics circuits

3. Motor currents wave form – these spectra will tell us bout jerk loads. As example, If we want to check belt conveyor motor then we have to monitor coefficient of friction between belt and pully which is decreasing with time. If slippage is occurring then produced torque also decreasing which in proportion with load current. So, We can detect slippage by analyzing wave form of load current).

4. Vibration Analysis : Vibration data. gives performance evaluation for Rotating machinery (identifying potential defects such as bearing failure, gearing faults, drive belt problems, bent or misaligned shafts, coupling faults and driven element faults. Structural failures such as poor mounting, resonances, and feedback from other machines

Popular method is shock pulse method (SPM). It measures two value. One is carpet value and another is Maximum value..

5. Engine oil analysis (Used oil analysis and ware particle analysis). Used oil analysis determines the condition of the lubricant itself, determines the quality of the lubricant, and checks its suitability for continued use. Wear particle analysis determines the mechanical condition of machine components that are lubricated. Through wear particle analysis, you can identify the composition of the solid material present and evaluate particle type, size, concentration, distribution, and morphology.

But again, reliable predictive maintenance equipments are precise but costly too. So, it need to be use cautiously to determine diagnostic process and frequency to optimize maintenance time and cost. (One of the powerful PdM paradigms is not to repair equipment too early or too late

Another difficulty is to gather data for perfect ideal one to compare. So, prerequisites must be fulfilled.

Scope of PdM in Indian scenario:

In Indian scenario, PM (TBM) is still a popular method over PdM which is not so popular because

1.      Less knowledge on technical know how on PdM analysis methods and so, less confidence on these technologies.

2.      For in-house PdM it needs a big investment for purchasing hardware which in not feasible in most of the cases.

3.      For operating and analyzing it needs a specifically trained and experienced personnel which is not available.

4.      In European country, there are many private agencies who offer predictive maintenance services with up to date equipments. But here due to less requirement and awareness there is no such strong customer base and so, no such frequent services available. (Shock Pulse Method Services available for compressor and generators but other sources of services still I don’t know )

5. Less awareness on energy audits and survey in general industries

Standards

Reference these standards(curtsy :Fluke website) when creating safe standard work procedures for PdM program.

International Electrical and Electronic Engineers standard IEEE-90 describes the elements of an EPM program, including safety, while 1584â„¢-2002 provides a guide for arc flash hazard calculations and ANSI/IEE C2-81 National Electrical Safety Code governs heavy industrial installations.

MTS-2001 Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems from the International Electrical Testing Association (NETA), www.netaworld.org, contains set of step-by-step standard procedures for inspecting, testing and evaluating system components and appendices that recommends maintenance intervals for various components.

Standard NFPA 70B Recommended Practice for Electrical Equipment Maintenance from The National Fire Protection Association provides a PdM program overview, as well as an appendix with sample tests and record forms. For each component of the electrical system, it describes how you should inspect and test each sub-element. For example, in the section on rotating machinery it covers stator and rotor windings, then goes into brushes, collector rings, commutators, and then bearings and lubrication.

NFPA 70B also includes a section on test methods, including insulation testing, transformer turns-ratio testing, circuit breaker testing and ground impedance testing, and power quality issues such as harmonics, transients, unbalance, sags and swells.

Also refer to NFPA 70E Standard for Electrical Safety in the Workplace for safety training, procedures, personal protective gear (PPE) requirements, and lockout/tagout procedures.

Occupational Safety and Health Administration OSHA 29 CFR, 1910 Subpart S Electrical and Subpart I Personal Protective Equipment Safety standards for electrical systems, safe work practices, maintenance requirements.

ISO 6781 International Standards Organization (ISO) (American National Standards Institute) discusses thermal insulation, qualitative detection of thermal irregularities in building envelopes, and infrared methodology.

ASTM International http://www.astm.org ASTM E 1934, 1213, 1311, 1316, and 1256 Standard guide for examining electrical and mechanical equipment with infrared thermography, plus additional thermography references in ASTM 1060 and 1153.

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