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A new correlation provides a rule of thumb for liquid flow calculations, yielding on-track preliminary estimates of flow rates.
It happens to most of us. We're standing in the control room or the corridor and a flow question comes up. "How much can we get through this 8-in. line?" Or, "How large of a pipe will we need to handle 500 gpm?" We don't have our graphs, or calculators, or computer programs with us. Sure, we can tell the team we'll check it out and let them know later, but it would be nice to be able to respond quickly with a reasonable answer.
The same type of problem can arise when the new process engineer Judy McJunior drops a piping and instrumentation diagram (P&ID) on your desk that call for a 4-in. Sch. 40 pipe to carry 250 gpm of cooling water. Is it adequate without redoing the calculations from scratch?
I have developed a new correlation that provides a good rule of thumb for liquid flow calculations. The correlation answers the above scenarios. I call it Jack's Cube:
The above equation applies for pipe sizes greater than 2 in. For pipe sizes 2 in. and smaller, the equation is similar:
These two equations are identical for a theoretical pipe diameter of 2.4 in.
Notice that the equations makes a direct correlation between the common units used for the respective terms: flowing quantity in gpm and pipe diameter in inches. No complications arise from converting velocity or pressure drop criteria to determine the proper pipe size.
Applying the equation to the first question above, we estimate a reasonable flow for an 8-in. pipe at 1.2(8 + 2)3, or 1,200 gpm.
For the second case, by remembering the cubes of 6, 8, and 10 as 216, 512, and 1,000, respectively, we conclude that since (1.2 x 8^swup 3^) is (512 + 20%) or about 600 gpm, a pipe diameter of (8 2) or 6 in. will appropriately handle the 500 gpm.
As for our colleague Judy McJunior, 1.2(4 + 2)^sup 3^ is (216 + 20%) or about 260 gpm - her pipe size is right on!
Caveats
The first caveat is that the rule applies to a typical liquid pipe with a normal supply...