Pump Pressure Formulas

[Bill204 16]

Hi everyone,

I'm trying to do some pump ops math, and Im wondering if someone can help me out.

I need to calculate Gallons Per Minute, and Friction Loss. So both of those are easy, and here are the formulas:

GPM =

//NP = Nozzle Pressure

//d = Diameter of tip

30 * (d^2) * Squareroot(NP);

Friction Loss =

//C = coefficient of friction for hose diameter

// q = gallons per Minute

// L = Hose Length

c * ((q/100)^2) * (L/100);

Now here is the problem....

I have current pump discharge pressure, Tip Diameter, Hose Length / Diameter, coefficient of friction.

GPM needs Nozzle Pressure, and you can't get that without Friction Loss, and Friction Loss needs GPM

The only way I can think of doing it is to just say the friction loss is about 15psi / 50' of 1 3/4 and calculate GPM using that number, but I'm not sure if that is the most accurate.

If anyone wants to do some reverse pump ops math feel free to help!

Thanks

(I'm afraid I'm making a silly mistake, but I'm not quite sure)

[Guest alsfirefighter]

Hi everyone,

I'm trying to do some pump ops math, and Im wondering if someone can help me out.

I need to calculate Gallons Per Minute, and Friction Loss. So both of those are easy, and here are the formulas:

GPM =

//NP = Nozzle Pressure

//d = Diameter of tip

30 * (d^2) * Squareroot(NP)

Friction Loss =

//C = coefficient of friction for hose diameter

// q = gallons per Minute

// L = Hose Length

c * ((q/100)^2) * (L/100);

Now here is the problem....

I have current pump discharge pressure, Tip Diameter, Hose Length / Diameter, coefficient of friction.

GPM needs Nozzle Pressure, and you can't get that without Friction Loss, and Friction Loss needs GPM

The only way I can think of doing it is to just say the friction loss is about 15psi / 50' of 1 3/4 and calculate GPM using that number, but I'm not sure if that is the most accurate.

If anyone wants to do some reverse pump ops math feel free to help!

Thanks

(I'm afraid I'm making a silly mistake, but I'm not quite sure)

I'm not sure exactly by the way you laid it out what your looking for. But nozzle pressure has nothing to do with friction loss, they are 2 separate calculations. To do the GPM calculation, its the easiest with smooth bore nozzles being you have a given size diameter for the opening and want to ensure you are delivering the rated GPM for the diameter of the tip to ensure a quality stream. Combination fog nozzles have their set GPM ratings. You have to see if you have automatic nozzles (most common is 150 GPM for handline or master stream). If you have variable GPM you have to see what the ring is set at. Which I far too often see them set at 95...minimum for interior firefighting should be 150. There are other types out there but often they are for lower pressures..not GPM..don't get confused by that.

So for example for nozzle pressure for a smooth bore there are only 2 choices for nozzle pressure....50 psi when used on a handline...80 psi for master stream. What is the difference? Any device..key word is device..that flows 350 GPM or greater is considered a master stream. The following numbers will have slight variations depending on how you round off.

{For a 2" tip on a deck gun its rated GPM is: (30) x [(2)(2)] x (square root of 80) = (30) x (4) x (8.94)= 1072 GPM

If it were a 1 1/4" tip on a 2 1/2" line: (30) x [(1.25)(1.25)] x (square root of 50)= (30) x (1.56) x (7.07)= 330 GPM

When trying to get accurate numbers when dealing with hydraulic calculations (using true math, not street math) you have to use accurate numbers..you can't use things such as "just say the friction loss is about." Small variations that occur with how one rounds off is nominal. A difference of a few PSI or GPM's will have no real bearing.

Hope that helps.

Edited January 27, 2012 by alsfirefighter

[ny10570 745]

.

Edited January 27, 2012 by ny10570

[Bill204 16]

ALS, thanks, yea that helps... But lets say that you have 100' of 1 3/4 line with a smooth bore. By using field math your pump pressure would be 80psi, which gives you 50 psi at the tip... lets say that instead the pump discharge pressure is 50psi how would you figure out gpm?

Thanks again... This isn't as much for actually being on scene pumping, more for understanding how all te pump parts and pressures relate to each other.

[firemoose827 324]

.

Does this mean your speechless??

[Guest alsfirefighter]

ALS, thanks, yea that helps... But lets say that you have 100' of 1 3/4 line with a smooth bore. By using field math your pump pressure would be 80psi, which gives you 50 psi at the tip... lets say that instead the pump discharge pressure is 50psi how would you figure out gpm?

Thanks again... This isn't as much for actually being on scene pumping, more for understanding how all te pump parts and pressures relate to each other.

How exactly is it 80 psi PDP? You didn't give me all the info to be able to quantify your numbers. I'm assuming you're saying that's with a 150 GPM discharge which that number would be pretty accurate...but what is the tip size and its actual rating? For a standard 15/16" opening I think its rated GPM is like 185 GPM..that's by memory as I don't have time to punch the numbers right now.

As far as the your PDP is 50 psi what is the GPM...you would need to use a pitot gauge at the nozzle to get what the number is..but I would actually say who cares...you're not going to get anything remote out of it being the tip needs 50 to work right. One of the reasons why its not discussed the way you put it...what is the GPM with a PDP of 50 is you don't want to work it out that way..you want to deliver the appropriate rated capacity.

If you don't mind me asking..is this one of those they are telling you what to pump at or marking the gauges with the put the needle here technique and you believe they aren't right? If that's the case...good for you for trying to ensure the safety of your people.

Edited January 27, 2012 by alsfirefighter

[Bill204 16]

How exactly is it 80 psi PDP? You didn't give me all the info to be able to quantify your numbers. I'm assuming you're saying that's with a 150 GPM discharge which that number would be pretty accurate...but what is the tip size and its actual rating? For a standard 15/16" opening I think its rated GPM is like 185 GPM..that's by memory as I don't have time to punch the numbers right now.

Yes

As far as the your PDP is 50 psi what is the GPM...you would need to use a pitot gauge at the nozzle to get what the number is..but I would actually say who cares...you're not going to get anything remote out of it being the tip needs 50 to work right. One of the reasons why its not discussed the way you put it...what is the GPM with a PDP of 50 is you don't want to work it out that way..you want to deliver the appropriate rated capacity.

Right, I know you shouldn't pump that way, I was just wondering if there was an equation to figure it out.

And as for you're last question, no it actually has nothing to do with physically pumping, I was just curious about the pressures themselves and how it all relates. Luckily no one is saying put the needle here and you'll be set.

Thanks again for all the help

[efd184 59]

Add more or less when asked by the guy on the line lol

[wraftery 1,275]

Without adding any more formulas to confuse the issue:

1. Every length of hose will have a slightly different friction loss. It has to do with the nooks and crannies that cause turbulence within that individual length of hose.

2. The friction loss tables and even the formulas are estimates or averages of many, many lengths of that size hose.

3. Everything you do has a bearing on friction loss. If you lay out your hose in a zig-zag manner, sinuosity (I love that word) comes into play and adds friction loss to the equation.

4. If you were to calculate engine pressure for 50psi at the tip by any means, (formulas, hand method, chart method, etc.) it would be a rare occasion if you got exactly 50psi at the tip using a pitot tube. In fact, the pitot reading itself is not that accurate because it has to be in the center of the strean, at the proper distance out from the nozzle, and so on. Not to mention that you jiggle when you try to hold the pitot in the right spot.

My suggestion for you was told to me a long time ago and is still valid today, but I had to add a couple of words for the newer electronic throttles:

Charge the line, then slowly throttle up until the nozzleman leaves the ground. Then turn your throttle back a half turn (For electronic throttles you give the 'Lower' button two quick taps). If you can't actually see the nozzleman, this operation can also be done audibly."

[ny10570 745]

Does this mean your speechless??

Never. I just know a much better post when I see one, and that was ALS's

[Guest alsfirefighter]

Yes

Right, I know you shouldn't pump that way, I was just wondering if there was an equation to figure it out.

And as for you're last question, no it actually has nothing to do with physically pumping, I was just curious about the pressures themselves and how it all relates. Luckily no one is saying put the needle here and you'll be set.

Thanks again for all the help

Bill, I've never done it in a reverse formula but I'm sure there has to be something out there. Otherwise like I pointed out the only way I've seen and done is with a pitot tube. And while not completely accurate its generally close enough and I've always done it where you don't physically hold it, it was with a kit that held the pitot and the handline was secured in place.

The best thing for anyone to do is get the coefficients from the manufacturer of the hose they use. Textbooks and curriculums use the numbers they get as said as a median of group of hose for educational purposes. And yes every length will have a different friction loss but unless it has severe memory in it or deposits its often not anything that is earth shattering number wise.

This is very simple really...if they ask for more give them more..if they ask for less and you know you are flowing it at what it should be..tell them you knocked it back a bit...I never actually do. Flow the lines to what they should be. If something goes down and with today's apparatus being as computer run as they are...it will know what RPM's you were at, etc. I'm not going to have on my conscious or my plate that I wasn't giving the right flow if something happens.

Thanks NY I appreciate the comment. I have you a +1 for it.