Never Ever Use Averages for Bottleneck Detection of Cranes in Melt Shops!

“If the crane is 80% utilized, it will be the bottleneck!” How many times have I heard this? Yet every time, I find myself following up with a longer explanation about the dynamics of cranes and bottlenecks in a melt shop. Let me unfold this for you.

On a macro level, the major bottleneck of a melt shop could often be easily spotted. However, when assessed on a heat-basis, bottlenecks can shift significantly between the equipment depending on the steel grade recipe, product mix and overall plant utilization. So, in reality, there’s rarely ever ONE static bottleneck.

To understand this better, let’s classify bottlenecks as momentary and long-term. This distinction is crucial when talking about cranes.

Momentary Bottleneck: This is the machine that restricts production temporarily. It's recognized for its continuous activity over a given period, e.g., processing one heat after the other without idle time.

Long-term Bottleneck: This is identified by tallying which machine is the momentary bottleneck most frequently. The one most often acting as the momentary bottleneck emerges as the predominant long-term bottleneck which has the highest likelihood to constrain the overall production. The Long-term bottlenecks often define the maximum productivity of the melt shop for this production scenario.

So how does this apply to cranes?

It is rare for the crane to be a long-term bottleneck, but they can easily become momentary bottlenecks with significant influence on the overall production outcome

For a crane to be identified as a momentary bottleneck, we must look for periods of consecutive transport tasks without any idle time, e.g., five transport tasks after each other. In this period, the crane is 100% utilized, but this doesn’t necessarily imply negative consequences. However, if the 4th or 5th transport in this sequence was already due, this ladle transport will arrive late to the next station.

This can lead to various outcomes:

Best-case scenario: No major consequence if there are sufficient margins on transport time.

Moderate consequence: Delays in starting the next machine, with possibly resulting in changes in tundish weight, reduced casting speeds or shut-off of strands, clearly leading to productivity and quality losses.

Worst-case scenario: Sequence interruptions which could cause major productivity loss, compromised quality, and significantly increased operating costs.

So, a single momentary bottleneck situation of a crane can have drastic consequences for the production.

It's less about the crane's overall daily utilization and more about the number of consecutive transports it undertakes and if those cause a delay at the next machine.

A crane utilized 70% or 80% throughout the day doesn't necessarily indicate a problem. It just means they are very busy.

But if, in one situation, the crane causes a slowdown or, in more severe situations, sequence breaks, then the cranes are indeed limiting production and become the bottleneck.

So, to find out if your cranes are compromising your production

• look for periods of uninterrupted work,
• check if those transports caused a delay to the subsequent machine by comparing planned and actual-start.
• If there is no delay, move on, because this high utilization period did not cause a direct problem.
• If it does delay the production, see if this situation could have been avoided at all, by re-sorting the sequence of task performed by the cranes and talk with operators about this situation and possible improvement.
• If everything was executed optimally, the delay was inevitable, and the consequences to production are severe, then you've indeed identified the cranes as the bottleneck. Whether they were utilized at 50% or 80% on average is irrelevant. ??