Some of the equations are very poorly written. I assume that the equals sign belongs outside the square root symbol, not inside.

Here is the data I used:
D = 54" (your number, and the correct units to use in the empirical equation for Y)
K = 0.616 (Table 3.0, for D > 24")
N = 5.666 (Table 4.0, for liquids)
SG = 0.998 (your number)
DP = 33.7 in_H2O (your number, and probably the correct units to use in the empirical equation for DP)

For liquid flow, I get: Y = K * N * D^2 * SQRT(1/SG) = 0.616 * 5.666 * (54^2) * SQRT(1/0.998) = 10,187.78

For flow rate, I get: Q = Y * SQRT(DP) = 10,187.78 * SQRT(33.7) = 59,141.8 gpm

So, within rounding error, and assuming DP is in inches of water, I agree with your result. What I don't see on this page (unless I am blind) is an indication that the units for DP actually are inches of water instead of something else. It's probably inches of water, but I can't prove it.

That being said, I suspect that the person who programmed the SCADA typo'd one or more of the constants. I created a little solver routine on my SwissMicros DM42 calculator (an enhanced clone of the Hewlett-Packard's wonderful HP-42S) and tried to find an obvious typo. Unfortunately, all I found were non-obvious typos. For example, for Q = 55,555.56 gpm, K must be 0.579 -OR- N must be 5.322 -OR- or D must be 52.337" -OR or SG must be 1.131. Of these possibilities, I would say N is most likely candidate, but still not very likely. I did find that for SG = 1.131, the inverse is 0.884, so maybe it was a combination of typo and not inverting the SG. Or, maybe somebody forgot to take a square root.

I conclude that your math is correct and the SCADA scaling is incorrect.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

Thank you for the help. I didn't even realize the DP units were not given. I was assuming inches of water but your correct, it could be something else.

for a more modern annubar style measurement the d/p is 29 inWC

so you appear to be right on the money unit-wise and otherwise...

Correction

At 85 MGD 33.12 inWC
At 80 MGD 29.27 inWC