Can Process Flow Scheduling Help You?

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By Sam G. Taylor and Steven F. Bolander

The choice of a scheduling technique can hardly compare to life's great philosophical questions such as "Is there meaning to my existence?" Nevertheless if manufacturing is your game and scheduling your name, mundane questions like "Which scheduling technique is best?" may have a significant impact on your professional existence.

Process flow scheduling is a type of scheduling logic used in over 600 flow manufacturing plants. Like MRP and JIT, process flow scheduling (PFS) is not a specific software package but a general procedure for scheduling in certain manufacturing environments. Just as with MRP and JIT systems, there are many different ways of implementing PFS principles. Can PFS help you? Maybe. Maybe not. It depends on your situation.

Now, we are professors by trade and are quite fond of tests. So we have designed a short fitness test to determine if PFS can help you. You should recognize that failing the fitness test is more a reflection on PFS and your manufacturing environment than on you. Failure merely indicates that another scheduling technique will better fit your requirements. Like most really interesting tests, there are no right or wrong answers to our test. However, to help you grade the test we will present a few of our thoughts on each of the questions.

Just a word about our interests before we get under way. We are not in the software business. As professors we research, develop and promote new ideas. That is what this article is about -- our ideas for new scheduling systems framework.


The PFS Fitness Test
Take a moment and look over the fitness test in Table 1.

Table 1

Answer the questions you can right now. If a question needs clarification, wait and answer it after reading our discussion.

If you answered this question no, you may have more company than you think. An article in the Dec. 1, 1988, Datamation, "Post-MRP II: What Comes Next?" by M.J. Foley, indicated fewer than one in seven manufacturing firms were using MRP II. These results beg the question, "If so many firms are not using MRP, then just what are they using?" If you don't use MRP for plant scheduling, you may already use PFS principles. You may simply not be aware of this relatively new, integrative framework and the software that employs these scheduling principles. To find out, read on.

If you answered yes, you may still be a candidate for PFS. For this question, let us only consider those elements of an MRP II system that are used for scheduling. These are the modules for master scheduling, material requirements planning, capacity requirements planning and the "closed-loop" planning technique which links them together. If you have replaced these MRP II modules with another scheduling procedure, you too may already be using PFS principles.

This question assumes that you are one of the minority of manufacturers that are using MRP. If you are quite happy with your MRP system, then PFS is probably not for you. We humbly suggest that you quit reading this article now--unless your are just plain curious. PFS is only for those who are dissatisfied with MRP scheduling logic.

Assuming that you are on bad terms with MRP, let's examine why. Do you feel that MRP does not give you enough detail for scheduling plant operations? Do you have problems with the way MRP schedules capacity? Do the fixed lead times in an MRP system create problems for you? Do you hate opening and closing all those work orders? Do you feel that computer performance has changed significantly over the last two decades and that perhaps your scheduling technology should too? If you answered yes to several of these questions, you should consider replacing the scheduling modules in your MRP II system. PFS may be the answer. Then again, it may not. Read on.

Let's begin the discussion of this question by clarifying the definition of flow manufacturing. In flow manufacturing plants, materials flow through stages of production with relatively fixed routings. For example, in a chemical plant, materials may be processed in successive stages of separation, reaction and packaging. Flow manufacturers often use process flow charts to describe how products are made in their plants. In contrast, job shop manufacturers tend to specify routings for individual parts.

If you answered that you are a flow manufacturer, then you are a prime candidate for PFS. Currently PFS is used in flow manufacturing industries such as chemical, food processing, paper, brewing, high volume consumer products, pharmaceuticals, semiconductors, rubber and petroleum.

If you answered no, then we will assume that you are a job shop manufacturer that is having problems with MRP's scheduling logic. Although PFS cannot help in your manufacturing environment, you really should consider replacing or augmenting your MRP scheduling modules with a job shop-oriented, finite capacity scheduling system. You should note that finite scheduling systems vary widely. One group of finite capacity scheduling systems are PFS systems. Another group is for job shop manufacturers. As you examine finite scheduling packages, make sure you are looking at those systems designed for job shop manufacturers. Since PFS does not fit job shop manufacturers, you can go look for an appropriate article on finite capacity scheduling now. PFS is not going to help you.

At this point we are assuming that you are a flow manufacturer and do not want to use MRP scheduling logic. The question remains, do you really need a computer to help you develop schedules? If you answered no, we will assume that you have been quite successful with your JIT system and have the stable demands, easy product changeovers and excess capacity that JIT systems require. Or on the other hand, you may have a relatively simple manufacturing environment that just doesn't need a computer-based scheduling system. If you are doing quite well without computer assisted scheduling, then don't change-customers don't buy your schedules. You too can go on to another article now. PFS won't help you either.

If you answered yes, we will assume that you need efficient capacity utilization and forward visibility of future production, demand and inventories. You may also be in an environment where production sequences and product changeovers are an important consideration. If so, you are a good candidate for PFS. Read on.

At this point we are assuming that you are a flow manufacturer needing some type of computer-based scheduling system and recognize that MRP is an awfully poor fit. Will PFS help? Just what is PFS? To answer these questions, let's take a quick look at the three PFS principles given in Table 2. These principles are the foundation of all PFS systems.

Table 2

Calculations Guided by Process Structure
The first, and most important, principle concerns how schedules are developed. Process flow scheduling uses the process structure to guide scheduling calculations. In contrast, MRP uses the bill of material, i.e., the product structure, to guide its scheduling calculations.

Process flow scheduling uses some special terminology to describe processes in a way meaningful for scheduling.

Figure 1

Figure 1 shows a process train that is composed of process units, clusters and stages. A process unit is a collection of process equipment that shares the same schedule.Thus, a bank of heat exchangers, a reactor and some distillation towers may comprise a single process unit because they are all scheduled as a single unit. Every process unit must have a schedule and every schedule must have a process unit.

Process clusters consist of one or more process units grouped together for various scheduling purposes. Process units within a cluster have different schedules; however, these schedules are closely linked to each other. Take for example the finishing cluster shown in Figure 1. The mixing unit produces red, blue and green products. These feed a packaging process unit which, in turn, fills large and small containers. Note that there is no inventory between the mixing and packaging operations. Thus, when the mixing unit is producing red product, the packaging unit must also be processing red product. However, since the packaging schedule must specify size as well as color, the schedules for the mixing and packaging units are slightly different. Although the schedules are different, they are closely coupled because of the lack of inventory between the two process units.

Process stages are groups of process clusters that are decoupled from preceding and succeeding stages by inventories. Thus, for the process train shown in Figure 1, the finishing cluster schedule in stage 2 is decoupled from the reactor cluster schedule in stage 1 by intermediate inventories. Finally we have process trains. Process trains are sequences of process stages which can be scheduled more or less independently from other process trains within the same plant.

Process flow scheduling uses this structure of process units, clusters, stages and trains to build schedules. The procedure generally begins by scheduling the most difficult process clusters first and then moves on to schedule the other process clusters. The cluster schedules are often constrained by guidelines such as production sequences, run lengths and target minimum and maximum levels for each inventory. The exact scheduling details vary widely from one situation to the next, but the underlying concept of using the process structure to guide scheduling calculations is central to all PFS systems.


Cluster Scheduling Alternatives
The second PFS principle concerns the two alternatives for scheduling process clusters: processor dominated scheduling (PDS) and material dominated scheduling (MDS). Many flow manufacturers have well defined capacity limits for process clusters and develop finite capacity schedules for these clusters before checking inventories. Figure 2 illustrates this procedure.

Figure 2

The other cluster scheduling alternative is material dominated scheduling. MDS first develops a material plan for a cluster and then checks the processor's capacity. This procedure is similar to MRP, but there is one very important difference. An MRP system first develops a material plan for all clusters in the plant and then checks inventories. In contrast, a MDS technique checks both material and capacity for a cluster before scheduling other process clusters in the plant.


Train Scheduling Alternatives
The third PFS principle concerns the way in which process trains are scheduled. Figure 3 illustrates each of the three alternatives. Many flow manufacturers use a reverse-flow scheduling procedure. This procedure first schedules the final stage in the process structure, and then successively schedules the earlier stages.

Figure 3

Reverse-flow scheduling tends to be driven by customer demands and backward schedules through the process structure to raw material requirements.

Forward-flow scheduling is not as common as the other two alternatives. Here scheduling begins with the first stage in the process flow and proceeds forward through the process structure until the final stage is scheduled. Forward flow scheduling is most common in operations that are driven by the arrival of key raw materials. This situation occurs in some food processing operations that cannot store agricultural products before their processing.

Mixed-flow scheduling is quite common. Mixed-flow scheduling may focus on a critical process cluster that occurs at an internal stage within the process flow. Scheduling begins with this bottleneck operation and then proceeds forward and backward through the process structure.

Now comes the critical question. Do these three principles seem to make more sense for scheduling your plant than other scheduling frameworks? If so, read on.


For Those Who Passed
What should you do if you passed the PFS fitness test? Well, again, the answer depends a lot on your individual circumstances. If you think you passed the fitness test but are still somewhat confused about PFS, we are not surprised. Our explanation of PFS was quite brief and perhaps you should read more in the APICS book. (See the sidebar for details.)

If you passed the fitness test but are not happy with your current system, perhaps you should consider a change. If you are using MRP, you really should take a look at switching to a PFS system. On the other hand, maybe you are using a spreadsheet-based system which employs PFS principles, but the system is quite difficult to maintain. If you are in this category, you may want to consider one of the commercial PFS packages.

If you are already using PFS and are happy with your system, then not much action is warranted. On the other hand, most systems can be improved and you may want to consider a variety of situation specific enhancements to your PFS system.

Process flow scheduling provides a framework for integrating the scheduling practices of many flow manufacturing companies. PFS is a fundamentally different way of approaching scheduling than either MRP or JIT. Process flow scheduling is not appropriate for all manufacturing. However, for those of you who passed the fitness test, PFS provides an appropriate framework for your scheduling activities.


Further Reading
"Process Flow Scheduling," By Sam G. Taylor and Steven F. Bolander, 168 pp./1994. Available Exclusively from APICS. Call APICS Customer Service at (800) 444-2742, Ask for Stock #01294.


Process Flow Scheduling Vendors
AVYX, Inc., 23 Inverness Way East, Suite 100, Englewood, CO 80112-5708. Contact: Bryan Scott
Scheduling System: Advanced Scheduling Environment

Chesapeake Decision Sciences, Inc., 200 South Street, New Providence, NJ 07974. Contact: Walt Beadling;
Scheduling System: MIMI

Fygir Logistic Information Systems, 25 Burlington Mall Road, Suite 300, Burlington, MA 01803. 270-0683; .
Scheduling System: GRIP

Manugistics, Inc., 2115 East Jefferson Street, Rockville, MD 20852. Contact: Arlene Gerst;
Scheduling Systems: Manugistics Finite Capacity Scheduler, Manugistics Constrained Production Planning.

Numetrix, Ltd., 655 Bay Street, Suite 1200, Toronto, Ontario, Canada M5G 2K4. Contact: Veerle Schengili;