Precast Consulting Service
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Process Controls

This is an article written for the non-technical general reader about a study led by Bill Ray.  It illustrates the importance of process controls.  In addition to the steps below, to achieve the results in this article required an observation design, a mathematical thermal balance of the steam heat to the forms, calibration of the heat ramp in the SureCure, control of concrete mix temperature, and a lot of training.
 

Form Heat Ramp
By Bill Ray

What the heck is a form heat ramp?  Well it is an engineering term for how fast the form and the concrete in it heats up.  It is the key to controlling cure of the product.

Here is what happened.  We now have the newest version of SureCure, the computer automated control system for form heat.  It was working properly.  But, we were facing widely varying cure times.  This can play havoc with schedules and caused a lot of wasted time.  The cure was undependable. A corrective action team was formed headed by the author.   The objective was to get the heat system under control and produce consistent results.    The first task of the group was to discover the root cause.  As it turned out, there were several.

The first root cause, and the easiest to find, was that the two forms had different heating systems.   Although they looked much the same, Form 2 has half again the heating capacity of Form 1.

More difficult to diagnose were leaks in the hot air circulation.  A “smoke bomb” was set off.  It showed there were leaks everywhere in one form. The form was taken down, air leaks welded or caulked, and returned to service.

Low Breaks
The steam system had interactive faults.  When one valve was open, pressure was reduced to another valve, thereby reducing overall efficiency.  Steam system bottlenecks were diagnosed.  This troubleshooting required around the clock monitoring because the system was operating automatically most of the time.

Measuring the product temperature itself was a problem.  We were all surprised to find that temperature probes relatively close together could read a difference of 20° or more.  Concrete is not a particularly good conductor of heat.  These measurement problems made it harder to understand what was really happening.  A permanent thermocouple probe system was developed and installed.  It is more reliable.  It also saves QC inspectors the time required to install probes in each module cast.

Finally, the casting crew had problems operating the system.  They have to interrupt the form pre-heat cycle to cast the product.  Then the heat cycle must be interrupted to retrieve insert pins.  In each case, it was hard to tell if the system was properly restarted.  One solution was a “kill switch” that automatically restarts the system.  Another solution was operation indicator lights that clearly show when different elements of the system are operating.   Each casting crew member can take pride in learning the elements of this sophisticated system.

The hard work is paying off!  The casting crew continues to set records and is consistently making their plan.  To do that, they can now rely on a dependable “heat ramp.”

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