mutiple reference question

[elavergne]

I am attempting to create a friction loss calculator for fire hose, I am using a known formula that calculates the loss based on volume and diameter for pipe. the difference that I have with fire hose in lieu of pipe is that the hose diameter changes with the pressure drop.

I have 3 variables that i input, pressure, beginning diameter and length. however
as the water flows through the line the pressure changes in turn the diameter changes, I would like to set up my spreadsheet so that the initial variables inputed yield the correct diameter, and then reference back to the initial equation and recalculate based on the yeilded diameter, and recalculates,

I can determine the friction loss at 1 foot, in turn determine the diameter at 2 foot, but I wish the spreadsheet to work the calculation over the entire length.

I dont know if this is the right place to post this but I appreciate in advance any comments.

Eric

[royUK]

Welcome to the forum, please read the forum Rules & edit your Thread Title to a format laid down in the rules. thanks

[shg]

After you fix your thread title, you'll need a math model for the hose and nozzle before anyone can help you with an Excel implementation.

[elavergne]

Sorry Roy Uk how to I change the title? do I need to repost the entire thread?

Shg, what do you mean by the model of the hose and nozzle?

[shg]

Sorry Roy Uk how to I change the title?

Click the Edit button adajcent to your post, then click the Go Advanced button, then change the thread title.

what do you mean by the model of the hose and nozzle?

I'd guess you need to know the friction as a function of hose diameter and flow, the hose diameter as a function of pressure, and the nozzle pressure drop as a function of flow, and then create a differential equation that models the system. Then you would implement that as a difference equation in Excel. Not a trivial problem.

In the alternative, there may be (surely must be) tables you can use. You might look in Marks' Standard Handbook for Mechanical Engineers, a treasure trove of arcane information.

[elavergne]

Thanks Shg.

I am not considering the nozzle as it is my orifice. the hose is 12" diameter my flow is 11,000 gpm. If my beginning pressure is 175 psi. The beginning hose diameter is 14.026 inches, friction loss equates to 4.64 psi/100 foot or 0.046 psi/foot if the diameter was to remain constant.100 foot away the pressure will be 170.36 with a diameter of 13.98. However the hose diameter will decrease every foot due to the pressure decreasing.

What I am attempting to do is to have the program calculate the loss referencing the loss per foot which will be constantly decreasing diameter.

Hope I made it clear.

[shg]

The question is clear, the answer is not. I don't know how friction varies with diameter or how diameter varies with pressure, and you need to know those to solve the problem.

Once you do, you compute the friction, pressure, and diameter for some arbitrary delta-length, and then make the delta sufficiently small that the answer along the whole length doesn't change much.

Have you looked at the IFSTA Fire Steams Manual? It probably has everything you need at a much higher level.

[elavergne]

SHG,

You a fire fighter? I actually coauthored a couple of the manuals for IFSTA, and sit on the NFPA 11 foam committee.

This will not be found in IFSTA, I can compute friction loss for a fixed diameter over a 100 foot length, with the following formula (452*Q^1.85)/(125^1.85*D^4.87)

where:

Q = flow
D = diameter

My hose diameter is calculated with the following formula (D = 0.0094P + 12.381)

where:

P = pressure
D = diameter

Friction loss in small diameter hose is not a problem as the diameter expansion
is not that much, however on 12" diameter hose my differential is up to two inches.

If i could use the friction loss formula to provide the pressure for the diameter
formula in one foot increments then have the spreadsheet add the loss in each foot together for a total.

the first foot will have a higher pressure than the second foot, the second foot will have a higher pressure than the third foot.......

the first foot will have a larger diameter (because of the higher pressure) than the second foot, the second foot will have a larger diameter than the third foot.....

what i do not know how to have the formula reference the new diameter and plug it in the friction formula at each foot of the hose lay.

I can run the loss on each foot and then add them up but 1,000 foot run will cause me to run the calc. 1,000 times.

[shg]

You a fire fighter?

No, but I seem to do a lot of fire drills

I can run the loss on each foot and then add them up but 1,000 foot run will cause me to run the calc. 1,000 times.

Exactly -- maybe.

So do it every 16 feet and note the result.

Then do it every 8 feet and compare results. Big difference?

If so, do it every 4 feet and compare results. Big difference?

If so, do it every 2 feet and compare results. Big difference?

...

When the result stabilizes, it's good.

You do the formulas for a 1-foot hose and post the result, and I'll help you extend it. Please include all the formulas used.

[elavergne]

Shg, can I send you my spread sheet via email?

[shg]

Here's mine. Does it do what you want?

[elavergne]

Wow, yes that is perfect, it did exactly what I wanted it to do. I owe you dinner. I will study how the formulas reference back to itself. I was told many years ago "you can give a man a fish and feed him for a day or you can teach a man to fish and feed him for a lifetime".

Thank you

[shg]

What surprised me is that the varying diameter really doesn't have a lot of effect over 100 feet. Computing for the entire 100 feet gives a 4.64 psi drop; computed at 1-foot intervals gives a 4.68 psi drop. (The diameter only varies about 0.06").

Can you do a sanity check to verify that the result is in the range of reality?

[elavergne]

I agree, but the 100 foot lengths are just the standard, the scenario that I am working on has a total run of 2,000 feet. Diameter change of 1.38" and a pressure drop of 111 psi. utilizing the spreadsheet that you supplied. the standard method utilizing non expanding conduit equates to a pressure drop of 87.2 psi. difference of 23.8 psi.

Your calculations are accurate I will double check by empirically testing the hose at different pressures and measuring the diameter. From there I can simply adjust the coefficients.