(As desired by the National Consumer Disputes Redresaal Commission (NCDRC), as a result of the case which was before its consideration, Consumer Protection Council, Rourkela vs. Indian Oil Corporation Ltd. & Ors., O.P. No. 224 of 2001, two Professors of Indian Institute of Technology, Kharagpur, (Dr. R.K.Saha, Professor & Head, Deptt. of Chemical Engineering and Prof. Subhabrata Ray, of Chemical Engineering Deptt.) undertook the study of the LPG (Indane) bottling plant, at Balasore, Orissa, in June 2003.  Their Report, with detailed statistical analysis and their inferences were made available to the NCDRC, in July 2003.  Since the Report was under the consideration of the Commission and the final order had not been passed, we also delayed publishing this informative report.  Readers can now enjoy reading the full content of the report. - Editor)
 

Report on capability of the carousel (LPG filling) machine and its accessories for filling correct weight of LPG in cylinders at the  Balasore LPG bottling plant of  M/s IOC Ltd. Marketing Division.

Executive Summary:

     The Plant was noted to normally fill these days around 6,000 cylinders in single shift operation and around 10,000 cylinders in two-shift operation.

     The capability of the carousel (LPG filling) machine and its accessories for filling correct weight of LPG in cylinders has been evaluated based on sampling of filled cylinders from the carousel machine and study of the operation of the plant, and study of the entire system.

     Analysis of the filled LPG quantity in these samples, drawn in presence of M/s IOC personnel and Mr. Vaidyanathan showed:

• The sample size was adequate and the statistics on the sample is closely expected to represent the day's production.
• The performance of the carousel with respect to the quantity of LPG filled-in cylinders shows large variation, around 11% of cylinders being under-filled (LPG wt. < 14.05 kg) and 42% of cylinders being over-filled (LPG wt. > 14.35 kg).
• At this level of performance of the carousel, the number of cylinders to be corrected for over-filling and under-filling is unduly large.  Hence the plant (carousel and its accessories) cannot bottle the correct weight of LPG (between 14.05 and 14.35 kg) at its normal production rates.

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Report on capability of the carousel (LPG filling) machine and its accessories for filling correct weight of LPG in cylinders at the Balasore LPG bottling plant of 
M/s IOC Ltd. Marketing Division.

Basis :

Target wt of LPG to be filled in cylinders  = 14.2 kg 
Lower specification limit of wt of LPG in cylinder (LSL)= 14.2 - 0.15 = 14.05 kg 
Upper specification limit of wt of LPG in cylinder (USL) = 14.2+0.15 = 14.35 kg 

     Filled LPG weight between the lower and upper limit is understood to be the correct filled weight. This weight is expected in cylinders leaving the ' carousel and the weight correction system for under-filled/overfilled cylinders'.
 

Steps

(1) Sampling
Random samples of LPG cylinders filled by the carousel machine were weighed in the check scales of the plant. These samples were from 'before the filled LPG weight correction facility'.  All scales in the plant including the ones on the carousal possessed valid certification from the appropriate authority. The check scales were verified by noting the scale readings after placing 30 kg weight on those. The raw data collected were (1) the tare weight of cylinders marked on the cylinder, (2) check scale reading.

The plant fills around 6000 to 10000 cylinders per day. For this range of production, the sample size code is 'M'. (ref.: IS 2500 code, table 1, inspection level v). from table IIA, M represents sample size of 315, under acceptance quality level of 0.01. the sample size of 305 is close to 315 and is adopted as 'acceptable'. The sampled data is included here in Annexure -1. Two samples were rejected as obvious outliers. The sample computations are therefore based on 303 samples.

The sampled data was processed to find the actual weight of LPG filled in the cylinders.   Mean and standard deviation of the LPG filled were computed. Frequency plot of the quantity of filled with the corresponding 'normal distribution' was plotted.

     It is seen that the distribution follows 'normal distribution' with fair closeness. This also signifies that the sample is a fair representation of the production for the day (population) and the mean and standard deviation of the sample and the population are expected to be close.
 

(2) Looking for presence of any bias in the filled quantity and the cylinder tare weight

A plot of the tare weight vs. the quantity of LPG filled for the sampled cylinders was done. The general trend is of filling quantity being above 14.2 kg, for all cylinder tare weights. No definite trend in the filled quantity variation with cylinder tare weight is observed.
 

(3) Computation based on sampled data

Sample size  =  303 cylinders

Filled wt of LPG (mean, xbar) = 14.3206 kg

Standard deviation of filled wt of LPG (s ) = 0.2667 kg

Mean overfilling (+)/under-filling (-) = 0.1206 kg
 
 

No. of under-filled cylinders (LPG wt < 14.05 = 14.2 - 0.150 kg)	34
No. of overfilled cylinders (LPG wt > 14.35 = 14.2 + 0.150 kg)	130
No. of ok cylinders (14.05 kg => LPG wt <= 14.35 kg)	139
Total no. of cylinders sampled (n)	303

 

(4) Preliminary inference from the sampled data analysis 

The number of cylinders under-filled and overfilled in the 303 samples observed correspond to 11.22% (=100 x 34/303 %) and 42.90% (=100 x 130/303 %) respectively. These are the percentages of the cylinders which should be passing through the filled LPG bottle correction scales / system. We can say that about out of ten cylinders filled, one has to be corrected for under-filling, and four for overfilling. These proportions are 'uncommonly high'. Weight correction of this large fraction of cylinders from carousel is possible only at a lower throughput of the filling plant. The carousel performance is therefore found to be unsatisfactory as it has a large variability in the quantity of LPG filled in the cylinders.
 

(5) Estimation of limits on the cylinders filled for the day @ 95% confidence (LNTL95, UNTL95)
(Ref: Chapter 4, Principles of Quality Control, Jerry Banks, John Wiley and Sons, NY 1989)

The range limits within which the average quantity of LPG filled per cylinder (for all cylinders filled in that day) by the carousel is expected to lie with 95% confidence are estimated.

LNTL95 = xbar - (Za/2) x s / Ö n = 14.3206 - 1.96 x 0.2667/Ö 303
            = 14.3206 - 0.0300 = 14.2906 kg

UNTL95 = xbar + (Za/2) x s / Ö n = 14.3206 + 1.96 x 0.2667/Ö 303
             = 14.3206 + 0.0300 = 14.3506 kg

It is seen that the two limits are close and this signifies that the sample represents the day's filling quite well. The mean and standard deviation for the sampled cylinders therefore shall represent the same for all cylinders filled in that day.
 

(6) Process Capability Ratio (PCR)
(Ref: Chapter 9, Principles of Quality Control, Jerry Banks, John Wiley and Sons, NY 1989)

Process capability ratio (PCR) aids in evaluation of processes with respect to their specification limits, and is defined as:
PCR = (USL - LSL)/(6 x s ) : When both upper and lower limits exist
PCR = (USL - m )/(3 x s ) : When only upper limit exists
PCR = (m - LSL)/(3 x s ) : When only lower limit exists
     Where, USL and LSL are upper and lower specification limits, m and s are the mean and standard deviation respectively.

Recommended minimum values for existing processes are:
(From: Table 9.1, Principles of Quality Control, Jerry Banks, John Wiley and Sons, NY 1989)
 

Process Type	2-sided specification	One sided specification
Existing Process	1.33	1.25

In this case, USL = 14.35 kg, and LSL = 14.05 kg, s = 0.2667 kg

Hence, PCR (2 sided) = (14.35 - 14.05)/(6x0.2667) = 0.187 << 1.33 limit

PCR (if only LSL exists) = (14.3206 - 14.05)/(3 x 0.2667) = 0.3382 << 1.25 limit

PCR (if only USL exists) = (14.3500 - 14.3206)/(3x0.2667) = 0.0367<<1.25 limit

The PCR values are substantially away from the corresponding minimum required. This denotes that a large fraction of the filled cylinders are overfilled and under-filled from the carousel. Operation of the filled weight correction system is essential to correct for this deviation. Expectedly about 11% and 42% of the cylinders have to pass through the filled LPG weight correction system. Thus 50% of the cylinders filled in the carousel passing through the weight correction system shall constrain the throughput of the plant.  In other words, the present condition of the carousel and its working system is not capable of delivering the correct weight of filled LPG (14.2 +/- 0.15 kg), at the normal bottling rate of the plant.
 

Conclusions

Based on the typical operation of the plant observed and analysis of the results of the samples drawn the following conclusions are made:

1. While the filling machine and the system are found to be the ones commonly used for LPG bottling all over the world, the carousel and its accessories were found to be showing large 'variability' with respect to the quantity of LPG filled in each cylinder. 
2. At this variability observed the carousel and its accessories is not capable of delivering the correct filled weight of LPG (14.2 +/- 0.15 kg) in the cylinders while the plant runs at normal filling/production rate.