I found that exact website for Hunter's Curve and attempted it myself (also without success). See attached for my attempt.
Interestingly, our numbers for binomial and normal flow rates match...but they're both obviously wrong when compared to the tables. I would love to find out what I'm doing incorrectly here, as having an excel equation could replace many cumbersome "table+interpolation" values in our master design spreadsheets.
Equipment that discharges either continuously or semi-continuously, such as pumps and ejectors, is assigned two fixture units for each gallon per minute (gpm) of discharge rate.That conversation rate was assigned in order to accommodate not only the waste that is being discharged, but the air that will be pushed in front of it as well. This type of discharge is very disruptive to a gravity drainage system and if not accommodated for properly can result in a waster closet being turned into a bidet at a very un-opportune time, or result in the discharge stopping the gravity flow upstream leading to stoppages.As far as converting to a GPM value, I believe Andrew Peter is more on a correct path. The building drainage system is designed to flow at no more than 1/2 full, so in using his example of 4 inch cast iron at 1/8 inch slope, 55 GPM is a reasonable number. A 4 inch building drain run at 1/8 inch slope is able to accommodate 180 DFU. If you convert that using the 1 GPM = 2 DFU then you end up with 90 GPM. The building just will not flow like that. Even at peak demand it will never be at a full flow condition.I believe it is reasonable to provide the GPM value based on the size of the building drain using Manning's Formula for Uniform Flow, you will be providing them a number for the maximum possible discharge possible for the building drain installed even if it isn't carrying the maximum DFU allowable. Public sewers are almost always flowing at a 1/2 full condition and in many cases will have surcharges during peak hours throughout the day, using the maximum flow rate possible based on the building drain size should help jurisdictions properly size their sewer mains and avoid surcharging conditions as much as possible. If on the other hand you provide a number based on full flow conditions, this could inadvertently lead to oversizing which is never a good idea in a gravity drainage situation when you consider the fixtures are using less and less water. Eventually that could lead to a situation where "there isn't enough water in the pond to float the boat", which again will lead to stoppages.
The conventional method of designing a sanitary drainage system is based on drainage fixture unit (dfu) load values. The fixture unit approach takes into consideration the probability of load on a drainage system. The dfu is an arbitrary loading factor assigned to each fixture relative to its impact on the drainage system. The dfu values are determined based on:
•Average rate of water discharge by a fixture;
•Duration of a single operation; and
•Frequency of use or interval between each operation.
Because dfu values have a built-in probability factor, they cannot be directly translated into flow rates or discharge rates. A dfu is not the same as a water supply fixture unit (wsfu) in Table E103.3(2), Appendix E.
In our State, Maryland, peak flow is estimated per the plumbing code. Maryland has a guide for predicting GPD, based on the 10 States Code.
See link below:
Michael J. Purtell, PE, CPD, LEED AP, CxA
Gipe Associates, Inc.
MECHANICAL / ELECTRICAL / PLUMBING
1220 East Joppa Road, Suite 223
Building A, Radio Park
Towson, Maryland 21286
Going BEYOND the Expected