Flow Time Analysis Flashcards
Competitive Advantages of Shorter Flow Times:
- Shorter delivery response time
- Reduced inventory (by Littles law)
- More responsive to technological development and customer preferences
- Delaying production closer to the time of sales
- Moving from extremes of Make-to-Stock to Make-to-Order
Flow time is an integrative measure of overall process performance
Flow Time Measurement:
Direct Measurement
- Randomly sample flow units over an extended period of time.
- Measure the flow time for each flow unit from entry to exit.
- Compute the average of flow times
Indirect measurement
- Count the number of units processed over an extended period of time.
- Calculate the throughput: R = Total Units Processed / Total Time
- Count the number of units of inventory at random points during the time period and compute the average inventory I.
- Compute T = I/R
Process Flowchart:
- Process is a network of activities performed by resources.
- A flowchart is a graphical representation of the network structure of the process
Critical Path =
The sequence of project network activities which add up to the longest overall duration. This determines the shortest time possible to complete the project.
For complex processes with many paths, a more efficient approach is used for identifying the critical path(s).
The critical activities of a process are very important: a delay in completing them results in delay in completing the process.
Flow Time Reduction Levers:
To reduce the flow time we must shorten the length of every critical path.
Flow time is the sum of two components: Waiting time and Activity time.
These two components have different natures and the levers available for managing each are distinct.
The main levers for reducing waiting times are:
- Managing congestion (Utilization and Variability)
- Reducing Batch sizes
- Reducing Safety stock
- Synchronization of flows
Levers for Reducing Activity Times:
-
Restructuring the Critical Path
- Move work off the critical path to a noncritical activity
- Move work off the critical path to the outer loop
-
Work Smarter; Reduce Non-Value-Adding activities
- Value-adding activities increase the economic value of a flow unit from the perspective of the customer, e.g. performing surgery.
-
Non-value-adding activities do not directly increase the value of a flow unit, e.g. moving work or workers and setting up machines.
- Necessary: can be eliminated if the process is redesigned.
- Unnecessary: should be eliminated outright.
-
Work Faster
- Increase the speed of operation, work faster: Technology, Method, Training, Incentive.
-
Do It Right First Time
- Decreasing the amount of repeat work can often be achieved by process-improvement techniques such as statistical process control, method improvement and training
-
Modify Process Sequencing
- Modify the product sequencing - do the quickest thing first. Most processes involve a mix of products, characterized by different flow times for the various units of flow. If we give priority to flows unit that move through the process faster, the overall flow time of the process will decrease.
Flow Rate and Capacity Analysis:
-
Flow rate is the average number of flow units that flow through a (stable) process per unit of time:
- Observe the process over an extended period of time
- Measure the number of units processed over the selected period
- Compute the average number of units per unit of time
-
Capacity is the maximum sustainable throughput.
- Capacity can be measured by observing the process in periods of heavy congestion when the flow rate is limited by (and therefore equal to) capacity.
- It can also be measured by evaluating the resources deployed in the process
Resources =
- Activities are performed by capital and labour resources.
- Each activity may require one or more resources, and each resource may be allocated to one or more activities.
- A resource pool is a collection of interchangeable resources that can perform an identical set of activities.
- Each unit in a resource pool is called a resource unit
Cycle Time:
As illustrated in the previous example, after the first batch comes out (at time 40), one batch enters and exits the process every 20 min. This is called the cycle time of the process.
Process Cycle Time = 1 / (Process Effective Capacity)
Unit load and effective capacity of a resource unit:
- Unit load of a resource unit is the average amount of time required by the resource unit to process one flow unit.
- The effective capacity of a resource unit is the inverse of the unit load, i.e. the maximum sustainable flow rate through the resource unit if it were to be observed in isolation
- Effective capacity of a resource pool is the sum of effective capacities of all resource units in the pool.
Bottleneck =
The slowest resource pool of the process.
The effective capacity of a process is the effective capacity of its bottleneck
Resource Pooling =
Resource Pooling is combining separate resource pools into a single more flexible pool that is able to perform several activities, i.e. transforming specialized resources into general purpose resources.
Examples are cross-trained workers and general purpose machines.
It is a powerful operational concept that can significantly affect not only process flow rate and capacity but also flow time.
Takt Time:
In a synchronized operation: throughput = demand < Capacity
- Cycle time is related to capacity :
- cycle time = 1/capacity*
- Takt time is related to demand (or throughput):
- takt time = 1/demand*
For example suppose a process capacity is 15 items per hour. Then its cycle time 1/15 hour or 4 min. If the demand is 12 per hour for the same process, then takt time is 1/12 hour or 5 min.
Cycle Time ≤ Takt Time
Capacity ≥ Demand (throughput)
