Descaling System Modification and 2011 Reliability Analysis

The author was responsible for the construction of the business case for this process upgrade to the scaling and continued to contribute to the project until final project sign off. The reliability analysis of the descaling system formed part of the business case for the project. The importance of being able to access accurate reliability information in this case was seen as further justification for the development of the TRAM method. The upgrade to the descaling system consisted of the removal of the motor- gearbox arrangement and the installation of a direct drive, motor to pump configuration. A variable speed drive (VSD) was installed and this now acts as the control for each motor-pump configuration. A reliability block diagram for this new system has been produced to reflect the modifications to this system and is shown in figure 8.6. The Hot Strip Mill operation required a sequential installation of each variable speed drive and its respective electric motor. This has been carried out since January 2011, with the final installation in March 2011. All new equipment was installed with no disruption to the Hot Strip Mill process.

There has been one recorded failure to date due to a faulty optical cable. The reliability testing regime has been extended to incorporate the Descaling system upgrade. This analysis is required to prove the revised reliability status of the upgraded descaling system. It is accepted that due to the short time since project installation there

is a limited amount of failure data available for analysis. However early indications are favourable, and the continual monitoring of this system will provide further verification of the systems reliability status.

Figure 8-6 Reduced Reliability Block Diagram Descaling System

The revised system has been operational for six months at August 2011, and the latest failure data has been used to calculate the reliability indices for the modified section of the descaling system. The reliability calculations are solely based on the pressurised water supply to the final section “Production Process”. This is considered as the water supply into the Seco water distribution valves (see Figure 8.5.) The descaling systems reliability indices over the period 2007 – 2010 have been compared to the upgraded descaling systems reliability indices in Table 8.5. The upgraded systems operation used the latest failure recording spreadsheet the, Mill Delay 2011 data sheet which was analysed using an updated TRAM method. For convenience sake it is prudent to focus on the areas of the descaling system which have been upgraded for this analysis.

Table 8-5 Comparison of Upgraded Descaling Systems Reliability Indices 2007-2010 Data IMTBF (Hours) 2007-2010 Data MTBF (Hours) 2011 data IMTBF (Hours) 2011 data MTBF (Hours)

Pump House 1357 1519 Pump

House 4032

4320 Accumulator 919 2184

Total 548 896 Total 4032 4320

It can be seen from Table 8.5 that the upgrade to the descaling system has considerably improved the reliability performance of the descaling system, with the MTBF values (bold type Table 8.5) rising from 992 hours for 2010 to 4320 hours for 2011. These figures must be reviewed with caution as the descaling system has been

VSD Motor Pump VSD Motor Pump VSD Motor Pump Pressure generation system Production Process

ramped into full operational mode and the full upgrade has not been in operation for enough time to collate meaningful failure data. However the continual monitoring of the descaling system through the TRAM method will allow the Tata engineers to correlate these findings at a later date.

The upgrade to the system was instigated to improve the descaling systems reliability and initial confirmation of this is reflected in Table 8.5. Other important consequences include the isolation of the accumulator which has decreased the amount of high pressure water maintained within the system to approximately 3000 litres which will improve system safety. In addition the energy usage required maintaining a large volume of water at a high pressure, plus the efficiency losses due to the gearbox and motor operation have been severely diminished. The upgraded system has reduced the cost of consumed energy by approximately 15% per month. It is believed that the more stable operating requirements offered by the upgraded system will remove the large fluctuations in operating pressures which will be reflected in reduced component wear. In addition the removal of water holding areas such as the accumulator should reduce the formation of rust and scale within the system. This will have a beneficial impact on nozzle performance and the corresponding descaling and product quality.

An improvement in product quality is an additional benefit that should be realised by the system upgrade. This system is expected to produce high volumes of water at the required pressure (up to 185 Bar) to the descaling headers. This produces a high- pressure water jet which is directed at the strip to remove scale from the surface of the metal. If insufficient volume or pressure of water is produced then the descaling operation will be partially successful, and may produce an inferior product. The reliance is then placed on downstream inspection to identify any abnormalities in the product. This is recognised by most modern manufacturing methodologies as the incorrect way to manufacture product with the latest production methods installing monitoring and failsafe methods to ensure that their systems work effectively.

The descaling system operates with two pumping elements in the normal operating mode. The system incorporates a third, redundant, pumping element which is built into the system to ensure effective operation if either of the two operational pumps fail. However this feature can mask inherent defects within the system and makes robust calculation of the descaling system’s reliability indices difficult. The fact that the

redundant system is normally designated for reconditioning during its redundant phase means that it will not be available for operation over a certain percentage of its redundancy period. This will mean that a one pumping element operation could occur with a corresponding effect on water pressure and flow which would affect product condition. It is appreciated that redundancy is incorporated into this system to ensure that the continuation of a pumped water supply is maintained, however the redundancy in this operation can allow systematic failures within the system to be covered over by the judicious use of the system’s “redundant” section. It could be hypothesised that this method of system operation makes full use of the system’s redundancy to ensure continuous production, but there may be additional effects on the systems performance which could be detrimental to the product’s quality.

When a failure impinges upon the systems operation, even for a short period, it could take water pressure and flow outside of the stipulated boundaries before the backup systems come into full operation. In effect the system cannot react quickly enough to accommodate all possible failure causes. When this occurs the steel material will be travelling through the mill stands at up to three metres per second. This means that if the pumped water drops outside the stipulated range for three seconds there could possibly be nine metres of steel of inferior quality produced within a 1000 metre steel coil. This production abnormality will be detected retrospectively with a possible re- examination of the coil being required. As can be imagined if defect or downgraded material is produced in sufficient quantities it raises the probability of defective material being supplied to the customer with possible quality ramifications on the steel manufacturing plant.

The use of the TRAM methodology for an in-depth analysis of the descaling system has assisted in identifying the most suitable new machinery for the process upgrade. This has led to the construction of a focused business case which has scoped robust criteria for asset purchase. This has shown that the TRAM methodology can improve the effectiveness of the asset purchasing system. The TRAM method will continue to confirm the upgrade’s progress by continually monitoring the systems reliability. The upgraded descaling system has a much improved reaction time through improved monitoring methods and tighter control of operational parameters. These features should improve product quality through minimising water pressure and flow

variability and providing advanced notice to operators regarding parameter deviation. In addition the initial use of the reliability analysis methodology in construction of the projects business case has ensured that a robust project proposal was made.

The preceding case file has shown that the judicious use of reliability analysis can support a business proposal to upgrade a process system and verify the upgraded systems performance. The TRAM method provides a long term monitoring method which will continue to monitor this system. The TRAM method is downwardly compatible with all the sub systems in this manufacturing process. It is recognised by the author that the current area classifications are not suitable for automatic retrieval of data relevant to subsystems. Therefore a full review of the data logging methods and area classifications is required to attain the most effective operation of the model.

The next chapter discusses the whole manufacturing scenario at Tata Steel – Port Talbot together with a method of integrating the TRAM method in to the software systems which are operational in this manufacturing plant. This is expanded onto the influences that reliability monitoring can have on the other operational control parameters used at Tata Steel.