SLP is a systematic approach to layout planning that was developed by Richard Muther and Associates.
The steps of SLP are shown in Figure 3.1. As seen from the figure, one has to collect all data related to the current and forecasted production. Sometimes, it may be possible that 3 to 5 types of products account for 70 to 80 % of the total sales volume. The balance 20 to 30% can be grouped in such a way that only a few product groups need to be considered. Each product group and its respective volume for a projected horizon [Turner et al] should be listed. The projected horizon depends on how frequently the product or market changes, but a projection for each product for the next 5 years is sufficient. Any other information relevant to the layout should be included in the general comments.
The basic input data or information needed for making layout can be remembered by the following letters:
P — Product and type characteristic of the material needed for this Q — Quantity of each type of product
R — Route i.e. sequence of operation & machines needed for completing these operations
Layout Planning 39 S — Supporting activities like moving the material from one work place or machine to
another and maintenance etc.
T — Timing as to how many times in the year and how quickly the products are to be made.
1. Having collected all the data, one can go for Step 1 of the SLP procedure called the preparation of the process charts. This chart depicts the flow of material graphically through the plant. If the products are few, one can make separate operations process chart for each. But, if there are many products, a multi-product process chart may be used. An ‘operation chart’ shows only the operations and inspection.
But a ‘flow process chart’ shows operations, inspections, transportations, delays and storages. These charts are thoroughly discussed under the topic ‘Work Study’ in any Industrial Engineering Book.
Input data and activities
1. Flow of materials 2. Activity
relationship
3. String diagram
4. Space
requirements 5. Space available
6. Space relationship diagram
7. Modifying considerations
8. Practical limitations
9. Develop layout alternatives
10. Evaluation
Figure 3.1. Systematic Layout Planning Procedure.
40 A Modern Approach to Operations Management In some cases, for example a job shop, it will be difficult to represent all the flows with a few charts. So, one can go for a ‘From-To Chart’ in such situations. This chart shows the number of trips from one area to another area and is based on historical data or proposed production. The trips can be attached suitable weights depending on production volume or the degree of difficulty. Table 3.1 shows a ‘From-To Chart’ for an office situation. The number indicates the number of trips made by the person from one place to another. Based on these charts (flow chart, and From-To Chart), one can construct the layouts. But, sometimes, these charts are not enough. There may be some areas where the product flow is non-existent, and some in which the flow sequence differs for each of the many products. In such cases, one has to go for ‘activity relationship diagram’.
Table 3.1. From-To Chart of an Office
From\To Chairman Secretary Computer center Staff room — Total
Chairman — 8 5 3
Secretary 20 — 2 4
Computer center 10 5 — 2
Staff room 8 10 25 —
— — — — — — —
Total
In short, the following tools are used in the layout preparation phase:
• Graphic and schematic analysis: Perhaps the most common layout planning tools are tem- plates—two dimensional cutouts of equipment drawn to scale.
• Operation Process Chart (OPC): operations, and inspections only
• Flow Process Chart (FPC): operations, inspections, transports, delays, and storage.
• Multiple-product Chart (MPC):
• From-To Chart (FTC):
2. An ‘activity relationship diagram (ARD)’ shows the desired closeness of departments and areas within the plant. It reflects the fact that not all important relationships can be shown by
Table 3.2. A set of closeness ratings for ARD
Letter Closeness
A Absolute necessary
E Especially important
I Important
O Ordinary closeness O K
U Unimportant
X Not desirable
product flows. Table 3.2 shows a set of closeness ratings proposed by Muther [Turner et al]. For any paired combination, an A rating indicates that it is absolutely necessary to locate the two areas adjacent to each other. On the other hand, an X rating shows that keeping two areas adjacent to each other is not
Layout Planning 41 desirable. For example, a machining center and the conference room can be straightaway given an X rating to avoid their being placed together. To decide about the closeness ratings, it is a good idea to involve all the stake-holders in future layout. They can be asked to give ratings and finally an average closeness rating can be decided.
3. Step 3 consists of using the information generated in Steps 1 and 2 to prepare a string diagram showing near optimal placement of the facilities without considering the space constraints.
The placement is done by trial and error. Usually, the areas with an A closeness are shown first and are connected with 4 straight lines, then E with 3 straight lines, and so on. When an activity has to be close to several other areas, it can be stretched out or distorted. The areas may be moved around and interchanged until a final acceptable arrangement is obtained. It is helpful to imagine the straight lines as stretched rubber bands and the jagged lines as coiled springs representing varying attraction and repulsion forces. So, an A rating would imply 4 rubber bands pulling the areas together while an I rating would imply only 2 rubber bands [Turner et al].
Many diagrams and arrangements will have to be made before a good layout is obtained. Normally, two or more alternatives are developed. Space will have to be added and some modifications made, but the overall picture should not change much. Thus, step 3 is supposed to be the most creative and important one.
4. Step 4 may be called the ‘adjustment step’. Here the adjustment must be made for space needs as related to space availability; so, the space requirements have to be determined. This can be done through calculations, adjustments of past areas, intuition or estimates.
5. Once these space requirements are known, it is necessary to consider the space available. In some cases, since the layout must fit into the existing buildings, the space available is highly restricted.
In other cases, the capital budget is the main restriction, and, therefore, the space availability may be less restricted. In any case, one has to balance the space requirements and the space availability before going to step 6.