Cement, Energy and Environment

under the scope of statistical control and assignable cause concerns the process control parameters. All variations due to assignable causes must be removed leaving the process parameters under the control of chance variations; otherwise the condition of normal distribution is not met. When a change in process condition occurs, it is observable by a reading on the control chart; if a corrective action is taken in time, the shift in process mean is prevented. Hence, the need for allowing 1.5 sigma shift can be avoided. If the shift is restricted to 0.'5 to 1.0 sigma, the same quality of product is ensured even with less than six– sigma. On the contrary, if the shift in the process mean remains uncontrolled and reaches 2.0 sigma, even under six sigma control, 32 defects per million will occur. Selection of sigma value for 3.4 defects per million when the shift from the mean is given as follows . Shift from Selected the Mean Value for .. ~ Control 0.0 Sigma 4.64 sigma distance 0.5- do- 5.0 sigma 1.0 -do- 5.5 sigma 1.5 -do- 6.0 sigma It means that in the event of zero process shift, 6-sigma control is not required, even 4.64 sigma will achieve the rate of defect of 3.4 ppm. When the specification requirements are relaxed for a product and the rate of defects 1s set at 32ppm, s1gma control can be as follows. 1 ~ 1 ,·akd''" 11 \l,lilliCIIilllCI.: It is suggested that the sigma value should be fixed in accordance with the customer/market demand, in the light of the rate of defect in ppm and the anticipated shift in the process from the mean in order to meet the level of quality committed for the product. This approach is applicable only to normal distribution of the characteristics, if it follows an exponential or poisson's distribution, the results are not reliable. Application of Six-Sigma to Maintenance: The concept of zero-defect in process quality control and zero breakdown in plant maintenance bears similarity as far as 6-sigma is concerned. Six– sigma approach inspires a comprehensive maintenance system of control, where the mach ine availability is improved and cost of repair goes down. Availability can be defined as the net running time of the machine divided by the gross time. In other words, the mean time between failures, MTBF (averaged from number of failures) is the average running time of the machine. Total time of the machine = MTBF - ' ' ' "...,,... '~"~ · r~ ,. (MTTR - average of repair times) + Lost time. Machine Availability = MTBF/ (MTBF + MTTR +Lost time) bhillit - ~ _,. l'l c'I.:Jllil,· \l.linl.:llalll'l' I t •'llditi<lll hd'c·d) Six-sigma improves machine availability time. Maintenance planning begins with categorizing the criticality of the plant equipment The criteria for determining the equipment is critical or not is based on the following. 0 Loss of production caused 0 Extent of downtime cost u Risk to safety 0 Cost of maintenance e.g. spares, manpower etc. 0 Life of machine 0 Miscellaneous others In view of the aforesaid the priority is decided in terms of normal, essential and vital. The configuration of the maintenance activity shown in Exhibit-4. Illustration on Condition Mon itoring : In the subject plant the critical equipment is condition monitored by recording vibration readings. The vibration velocity is a continuous variable parameter to predict the machine condition at any instant. It follows the statistical control as that of a normal distributton :r P. wh1cl1 IS symmetrical ''n ~llf it::l o::~HJt: VI dlt: IIIC:c.ll i. Ill u vibration monitonng study of the ktln fan . the observations of vibration velocity in mm/sec are tabulated as follows. IX -4.... ·