World Grain - March 2019 - 72

MILLING OPERATIONS

Maintaining
mill balance
Step charts and cumulative ash analysis are valuable tools
by Jeff Gwirtz

In my previous article in the November 2018 issue of World
Grain, the procedure to develop and analyze granulation
curves was presented as part of a series of articles to discuss
in greater detail the development and utilization of the three
tools mentioned in Mark Fowler's article, "Balancing the
mill," in the February 2012 issue of World Grain.
John Wingfield, in the "Dictionary of Milling Terms" published by the International Association of Operative Millers in
1989, defines mill balance as "the proper distribution of stock
to the various parts of the milling system, as determined by
the flow sheet, to ensure proper loading of equipment and high
milling efficiencies."
Wingfield suggests that the distribution or balance of the
ground stock must be kept within the parameters of the system
design to maximize mill performance as part of the balancing
act measures. The development of a step chart for a mill is
a valuable exercise in developing an understanding of both
the mill flow sheet and balance. A weigh off and analysis of
all mill streams identified in a commercial mill step chart is
seldom conducted due to time and resource constraints. More
often, a weigh-off and analysis of only the flour streams is
conducted to develop a cumulative ash curve.
STEP CHART DEVELOPMENT
A step chart is a method of tabulating the flow rate, percentage of total stock, and the quality of stock going to (destination) and from (origin) each operation in a mill flow for the
purpose of analyzing load and quality balance throughout
the entire mill.
Before rates can be measured and percentages calculated,
the source and destination of product streams must be properly identified. This identification step alone can provide a
better understanding of mill flow and balance for the new mill
72

Book 1.indb 72

operator. In simplest terms, it lets the operator know what
downstream systems are impacted by a process. Properly developed, the step chart provides the sequence to be followed
for a weigh-off and sampling to ensure product flow rates and
qualities are measured without having been disturbed or put
out of balance by previous disruptions of material flow or
machine performance. Flow rate measurement and samples
should be taken from the end of the mill forward or from the
lower left toward the upper right of the step chart.
Coupled with granulation profile data and sifting efficiency, the step chart can be used to estimate loads in the mill.
Such an estimate could be useful in large mills where measurements might be difficult or inaccurate due to the large
flow rate, number of locations to be sampled, weighing and
timing errors. Extremely light or heavy mill loads might call
into question the need or value of a designed separation when
coupled with product quality information such as granulation, density and ash. Product rates and quality can be used
to evaluate and understand equipment utilization, especially
roll, sifter and purifier surface. In addition, the load information and quality information collected is useful in identifying
material handling requirements for either bucket elevators or
pneumatic conveying systems.
A process for generating a step chart using Excel is provided as an alternative method to using graph paper. The step
chart begins with a vertical list of all process system names
along the left-hand side of the sheet representing the stock's
origin. A horizontal column of the same systems names in
identical order is to be placed above the vertical column using the Copy, Paste Special, Transpose function. Identify the
destination of stock from each origin by placing an "X" in the
appropriate column indicating that stock flows from B1 to B2,
1C, 3C, and R1 as shown in Figure I (top of page 74).
March 2019 / World Grain / www.World-Grain.com

3/6/2019 12:31:24 PM

http://www.World-Grain.com

World Grain - March 2019

Table of Contents for the Digital Edition of World Grain - March 2019