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Statistical Process Control: Process and Qualit...

Statistical Process Control (SPC) provides a way to monitor chemical and other processes. We'll focus on continuous chemical processes and how the process and quality control departments utilize SPC. Process control engineers use SPC to monitor a process's stability, consistency and overall performance. Quality control engineers use SPC to see if the process is functioning within quality standards. In industry, these two departments work together to monitor a chemical process. SPC, in a classical sense, will not reveal much about the quality of the product. For example, a process may be operating very well and in a very stable manner...as far as the process engineer is concerned, everything is fine. However, if the process is currently 20% below the quality standard for A or top grade material, it would be difficult to say that the process is fine.

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To help introduce the basics of SPC, we'll assume that the variable being monitored is the specific gravity (SG) of n-hexane as it is being produced. We'll assume that the SG is measured four times per day at 0300, 0900, 1500, and 2100 by the plant's laboratory. Table 1 shows the results for a three day time period. A-grade industrial n-hexane must have a SG between 0.61 and 0.69. First, we'll see how the process engineer analyzes this data.

SPC Overview

In a continuous chemical process, two types of charts are commonly used: individual value or X-bar charts and moving range (MR) or R-bar charts. X-bar charts are used on a regular basis to monitor the process during a time of change. For example, an R-bar chart would be appropriate if you were changing the feeds to the process. The R-bar chart weights more recent data more heavily than historical data.

The chemical industry typically uses one of two types of process control. 3-sigma control specifies quality limits nearly equal to process limits. 6-sigma control specifies quality limits that are twice as large as control limits. We'll focus on the 3-sigma system.

With all of the different types of limits, it's easy to become confused. For our n-hexane process, we'll have 6 different limits we'll consider. Three UCL's (Upper Control Limits) and three LCL's (Lower Control Limits).

UCL (calculated) = statistical upper control limit
UCL (process) = pre-determined, acceptable process upper control limit
UCL (quality) = pre-determined, acceptable quality upper control limit

LCL (calculated) = statistical lower control limit
LCL (process) = pre-determined, acceptable process lower control limit
LCL (quality) = pre-determined, acceptable quality lower control limit

Table 1: n-hexane Specific Gravity Data

Date	Time	SG Result
2/25/99	0300	0.65
	0900	0.67
	1500	0.69
	2100	0.63
2/26/99	0300	0.64
	0900	0.65
	1500	0.63
	2100	0.68
2/27/99	0300	0.67
	0900	0.68
	1500	0.62
	2100	0.66
2/28/99	0300	0.62
	0900	0.65
	1500	0.63
	2100	0.66

The process limits are those which define boundaries of operation for the process or an acceptable operating value. The quality control limits are those used to "grade" material. The term "quality limits" will refer to the A grade or top grade material limits. You should realize that there are also B and C grades of materials that companies often sell as well. The limits of these other grades vary accordingly. Essentially, the farther away from specifications a product is, the lower the grade, and its value decreases sharply.

Typically in a 3-sigma system, the process limits are said to be "tighter" than the quality limits by 5-10%. This is done so that even if the process exceeds process limits by a small amount, it will still be within quality standards. However, the 6-sigma system dictates that the process limits be half of the quality limits. For example, if you had an upper quality control limit of 100, the upper process control limit in a 6-sigma system would be 50 while a 3-sigma system may have an upper process control limit of around 90. Basically, a 6-sigma system requires more strict (and sometimes unrealistic) control, depending on the process. This is why many chemical manufactures implement the 3-sigma system. Now that we've discussed the different types of limits and charts involved, let's see how our system is performing!