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Development and Implementation of Reliability centered Maintenance using Vibration Analysis: Experiences at Rourkela Steel PlantS.Parida, N.R.Kotu, M.M.Prasad
Research & Development Centre for Iron and Steel, Steel Authority of India Limited (SAIL),
Rourkela Centre, Rourkela 769 011, India
CMMS/CBMS Department, Rourkela Steel Plant, Steel Authority of India Limited (SAIL),
Rourkela - 769 011, India
In many steel mills worldwide, failures of critical equipment have frequently led to loss of production, high maintenance and operation costs. As the capital cost of modern machines is very high, the requirement of continuous and prolonged operation becomes inevitable, to sustain economic operation. There has been a trend towards adoption of various forms of Condition Based Maintenance Systems which contribute to better health of machines, reduced maintenance costs, efficient use of personnel and improved system efficiencies.
When a large number of machines are involved, a majority being rotating equipment, Vibration Analysis suits ideally the requirement of Condition Monitoring, inspite of relatively higher initial costs in terms of instruments. It is easy to record and transfer data, ensure accuracy of data for trend analysis and fault diagnosis. Further, its versatility and portability permit effective planning and implementation of monitoring schedules.
Rourkela Steel Plant (RSP) has a healthy maintenance practice right from its inception in 1959. Condition Based Monitoring through Vibration Analysis has been developed and is in use at RSP, since April '93, as a key technique for implementation of Predictive Maintenance Program. The coverage has grown over three folds from 40 to 140 critical equipment in the last three years. In addition, for the first time in SAIL plants, single plane in-situ balancing was introduced with tangible benefits in terms of preventing unplanned stoppages, man-hours saved on dismantling and re-assembly of equipment, and savings on cost of components. The number of major breakdowns prevented by timely diagnosis of faults has increased from a mere 5 to 70 cases, resulting in substantial financial savings.
The paper describes the efforts made during recent years, by Rourkela Steel Plant, Steel Authority of India Limited (SAIL), in close association with Research & Development Centre for Iron and Steel (RDCIS), SAIL, to implement Vibration Analysis and related methods as effective Non-Destructive Testing tools to implement Total Productive Maintenance Program in an integrated Iron & Steel works. The paper further discusses a few typical case studies briefly.
Every piece of equipment needs special care and attention, characteristic to it. The Coal Chemicals unit has Gas Boosters and Exhausters that handle coke oven gas, a highly inflammable commodity, whereas Sinter Plant Blowers and Waste Gas fans handle air containing highly abrasive sinter dust. The Turbo Alternators of Captive Power Plant require round the clock vigilance involving a variety of parameters. Seemingly innocuous Forced Draft & Induced Draft fans of the Reheating Furnaces also assume significance because of their criticality in application. Failure of these fans may lead to cut down of Hot Strip Mill/ Plate Mill production by 33 - 50 percent.
To maintain an efficiently operating unit and avoid failure of critical equipment, especially modern steel industry equipment, the focus has clearly shifted over the years, from Breakdown Maintenance, i.e. repairing the equipment after it malfunctions, to Preventive Maintenance, i.e. fixing the equipment according to planned maintenance schedule. The next trend was Computerised Maintenance Management Systems (CMMS), and the latest trend encompasses asset management and maintenance, supported by various methods of Condition Based Maintenance Systems (CBMS) and in-service inspection processes. CBMS or Predictive Maintenance methods are an extension of preventive maintenance and have been proved to minimise the cost of maintenance, improve operational safety and reduce the frequency and severity of in-service machine failures. The basic theory of condition monitoring is to know the deteriorating condition of a machine component, well in advance of a breakdown, for proactive maintenance.
There are many machinery parameters that can be measured, trended and analysed to detect imminent failure or onset of problems. Common among them are :
Additionally, operational characteristics such as flow rates, heat, pressure, tension, speed and so on can also be monitored to detect problems. In case of machine tools, product quality in terms of surface finish or dimensional tolerances is often an indication of problems. As all these techniques have value and merit, the application of any particular technique depends on the suitability and ease of implementation.
Vibration is a powerful information source for estimation of technical condition of equipment's, more so in the case of rotating equipment. Considering the vast array of equipment in a steel mill, majority being rotating machinery with either oil or grease lubrication systems or using anti-friction or journal bearings, vibration analysis ideally fits the requirement of predictive condition monitoring. Although the start-up costs are relatively higher due to investments in instruments, dedicated software and skill development of maintenance personnel, the accruing benefits offset these hindrances during implementation. The advantages include ease of operation for recording and transferring data, accuracy in handling data for diagnosis and applicability to a wide range of plant operations. The portability and versatility of instruments permits effective planning and scheduling of monitoring activities and if needed, planning of shutdowns for maintenance. According to survey data of American Society of Mechanical Engineers (ASME), upto 82 percent of malfunctions of the mechanical equipment can be detected with the help of vibro-monitoring and vibro-diagnostic methods.
|Fig 1: Flow chart for implementation of PMP based on vibration analysis|
After successful completion and implementation of UNDP aided CMMS project, Phase-II of the same project was envisaged with CBMS as an important component. It started as an extension of CMMS in April 1993, and is nearly seven years old. In fact, vibration monitoring is a key technique through which the Predictive Maintenance Programme (PMP) is being done at Rourkela Steel Plant (Fig. 1). There are as many as 100 departments, who are maintaining various machines under them. After an initial survey and detailed discussion with all the departments, 40 critical machines were chosen and at present more than 140 critical machines are being monitored under this programme.
STEPS IN IMPLEMENTING PREDICTIVE MAINTENANCE
The logical steps in implementing a PMP are
Detection involves measuring and trending vibration levels at marked locations on each machine included in the programme on a regularly scheduled basis. The objective is to reveal significant increases in a machine's vibration level to warn of developing problems. Analysis helps to pinpoint specific machinery problems by revealing their unique vibration characteristics. Corrective action is taken after specific problem has been detected and identified by planning and scheduling all activities to ensure that machine downtime is kept to the absolute minimum (Fig. 1).
Critical machines of steel plant like Gas booster, Exhauster, Turbo-Alternators, etc. are continuously monitored by use of permanently mounted vibration transducers. By current standards, breakdown maintenance has been totally eliminated, and in no case it is more than 10% of the maintenance actions.
|Fig 2: Schematic setup of GCP ID Fan #1, SMS-II|
The normal assumption of failure was unbalance due to deposits on the impeller. This led to high vibration amplitudes (Table I and Fig. 3) and consequently resulted in breakdowns. After every break down, the machine was inspected to identify the primary damages as well as secondary damages. The cost involved for such failures were on account of,
|Table 1: Machine vibration & spike energy readings before (17th May '99) and after (21st May '99) repairs|
Fig 3: Machine health trend report
Towards the end of 1998, a condition based maintenance programme was implemented in the shop. In the first phase, the fan was stopped before the vibration velocity exceeded 8.00 mm/s and sequential inspection and cleaning were taken up. It was found that almost at an interval of 40 days, the impeller was taken up for cleaning which resulted in breakdowns, leading to stoppage of nearly 14 hours. By this method, uncertain pattern of breakdown was totally eliminated.
In the second phase of the implementation, critical review of the behavioral pattern of the entire system was taken up as part of Dynamic Predictive Maintenance. A problem diagnostic study revealed that apart from the influence of unbalance caused by the form of deposits, there are also other major inaccuracies in the system. Detailed frequency analysis, phase analysis, coastline down pattern and orbit analysis was taken up on this equipment to have a very precise diagnosis of the system behaviour.
The following major defects were identified to be present in the equipment -
|Fig 4: Schematic setup of WG Fan #1, SP-II|
The vibration readings taken before repair showed operation beyond alarm level (Table II and Fig. 5). Subsequently,
detailed spectrum analysis revealed
After opening the bearing housings, it was observed that the sleeve bearing on fan non-drive end was burnt and fan drive end bearing condition had also deteriorated. Two sets of white metal sleeve bearings were replaced. After boxing up of fan side bearings, proper alignment was done between motor and fan. The fan was then rotated and found to be dynamically unbalanced. IRD 885 Data Analyser/ Balancer was used to do in-situ balancing and the post repair vibration levels were well within the alarm level (Table II and Fig. 5). This sort of selective repair action is possible only because of vibration monitoring and accurate assessment of source of trouble.
Table 2: Vibration readings before repair (22nd October '98) and after repair & in-situ balancing (23rd October '98)
|Fig 5 : Machine health trend report|
|Table 3: Vibration readings after in-situ balancing (3rd July '99)|
Trending of vibration amplitude data at scheduled intervals indicated normal operation of the fan till 30th June '99. On this day, the fan had tripped twice and diagnosis by CBMS techniques indicated that the fan impeller was unbalanced. It was dynamically balanced at 1500 rpm, and the vibration level reduced from 14.53mm/s to 2.94mm/s. The vibration recorded just after in-situ balancing of the fan on 3rd July '99 is shown at Table III. Since then, the machine is running smoothly and uninterruptedly.
|Fig 6:Growth through CBMS activities|
|Fig 7: Equipment health condition at RSP|
As a corollary, there has been vast improvement in the health of equipment
(Fig. 7) available for production processes in accordance with ISO 2372. There has been an evident shift in the percentage of equipment from category F towards category A, with a majority of equipment under category B and C. This signifies a direct correlation with improvement in reliability of equipment, since CBMS activities incorporate a reliability-based philosophy, such that it concentrates on mean time to symptom, and not failure.
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