Hydrant Flow Rate Estimators:
First it is true that the static pressure (non-flowing) on a hydrant is NOT a good indicator of the water available from a hydrant. Imagine a ¼” copper tube coming from a reservoir with a water elevation of 231 feet above the test location. The gauge will read 100 psi, but the instant you open a valve the pressure will drop so low as to be nearly impossible to measure because all or almost all of the pressure will be lost due to friction loss in the tubing. Actually for a 10,000 ft. long tube, the flow would be about 0.1562 gallons per minute even though the static pressure indicated was 100 psi. before the valve was opened.
The static pressure on a hydrant, however is a valuable starting point for making an estimate of the total water flow available from that particular hydrant. The suggested method for taking advantage of these indicators is as follows:
1. Note the static pressure on the hydrant before water is flowing and make a note of that pressure.
2. Get your first large line in operation properly supplied at a known flow rate. Determine the flowing pressure from the hydrant with this amount of water being discharged and compare it to the original (static) pressure.
3. Apply the following rules. A 5% or less drop in pressure = 3 additional lines of that size. A 10% or less drop in pressure = 2 additional lines of that size. A 25 % or less drop in pressure = 1 additional line of that size before the hydrant would reach an ineffective pressure.
Let us use your example in the above passage. Assuming that the static pressure was 50 psi before you started supplying the master stream device, then 50 psi – 40 psi / 50 psi = 20% drop so you should have at least another 1100 gpm available from the hydrant.
On the other hand, if the static pressure was 100psi before we started, then 100psi – 40psi / 100 = a 60% drop, and while there will be some additional water available, it will be considerable less than the 1100 gpm you are currently flowing.
The reason that these rules of thumb work as estimators is because the friction loss (drop in hydrant pressure) must follow the Hazen-Williams formula where friction loss is proportional to the 1.87 power (nearly the square) of the flow rate. If we have a flow of ½ the capacity of the line the friction will be ¼ (1/2 squared) of the static pressure. 1/3 of the flow will have a drop of 1/9 (1/3 squared) or about 10% drop and ¼ of the available flow will only need 1/16 (6.25%) of the pressure to deliver the water to the plug.
Your question concerning adding another hand line with a 125 gpm nozzle can be answered by calculating a “k” value for the hydrant (0.496) and then using 1225 gpm to recalculate the loss in pressure (it would drop to 25.6 psi) after the hand line is supplied. The addition of 125 gpm cost you 14.4 psi of hydrant pressure. Because the factor controlling the friction loss is a square function, even small changes in flow (about 10%) caused a drop of 36% in the pressure (40 down to 25.6).
It is well to remember that these estimates work when only one hydrant is being utilized, but fall apart when multiple engines are working on the same grid system. Down to the 20 psi minimum pressure, for every gallon one engine tries to take from the system, another engine tied to the same system will try to compensate for the drop by throtteling up to compensate for the loss.