# Thread: Static Pressure vs Avail Flow

1. ## Static Pressure vs Avail Flow

I read the PSI VS. GPM thread... I have a question of the same general topic, but slightly different.

I understand that the general rule of thumb accord to IFSTA D/O is that avail water is:

0-10% Drop 3X amount of water being supplied
11-16% Drop 2X amount of water being supplied
17-24% Drop same amount being supplied
Over 25% Drop less than amount being supplied

Question 1:
What is the math used to determine the drop %?
I know that 100 static and then 80 residual is a 20% drop...
For Example: How would I figure the % drop for 60 static and then 43 residual? or 80 Static to 67 residual?

Question 2:
Our 1st due engine forward lays to the scene from the water source. Pre-connects are pulled and are charged right away. The D/O then gets off the panel and hooks the LDH to the intake and then gets back to the panel, closes take to pump and cracks tank fill to top off the tank, and runs off the water source.

Now, I have always read to determine the water avail, you have to note your intake pressure from the water source prior to opening lines and then notice the drop and use the rule of thumb. So, what do I do if I allready have my pre connects pulled and pumping prior to the inktake being charged up?

Any help would be greatly appreciated!!

2. Originally Posted by USAFfiredog
Question 1:
What is the math used to determine the drop %?
I know that 100 static and then 80 residual is a 20% drop...
For Example: How would I figure the % drop for 60 static and then 43 residual? or 80 Static to 67 residual?
(Static-Residual)/Static

(100-80)/100 - .2 or 20%
(60-43)/60 = .28 or 28%
(80-67)/80 = .16 or 16%

As for question 2 - another recent DPO grand & I are both of the opinion that you're just screwed on that one. I'll review my books when I get home later & confirm.

3. ## Question 2

A guide that has served me well over the years is this:
When the next line is charged, if you lose less than half your residual you should be able to pump another line of the same flow, as long as you don't drop below 20 on the intake gauge.
So, I'm pumping the lines that were pulled initially and supplied out of the tank off my supply line, tank is full, and got a residual of 70. I charge say a 2 1/2" line @ 325 gpm, and my residual drops to 50. Half of 70 is 35, and I dropped 20, so I should be able to get another 325 gpm, and stay above the minimum of 20 on the intake gauge. (50 - 20 is 30). Now you have to pay attention if you do take on that next line to make sure you don't get below 20. If I charge the next line and lose less than half the 50 residual, I should be able to take third 325 gpm line. Key is to know the flow of each line you add to the mix.

4. firedog: The compound gauge is showing the result of the static pressure (head) minus the friction loss caused by the flow of the water through the pipes and hose line. If the flow (value of F) causes a drop of 5 psi in the system, then a flow of 2F will cause 4 times that loss (like doubling the flow in a hose line) or a 20 psi drop. Suppose the starting static pressure (zero flow) was 50 psi. Then after flowing the first line (F) the pressure dropped to 45 psi. the percent drop would be (50 - 45)/50 = 10% or about 2 additional lines would be available. Testing this, since the friction loss increases with the square (1.87 power) of the flow, then (2 more plus the original) three (3) squared is 9 times the original (5psi) loss = 45 psi drop subtracted from the original 50 psi leaves a 5 psi incoming pressure. Remember these are estimates and are used to help with the decisions about water supply. Using the above example, where the first line was a 250 gpm nozzle, then an officer who calls for a 1,000 gpm master stream from this engine is going to be sorry, and will need to back down to 750 gpm. before the water system can keep up with the demand. If the original static was 100 psi.... (5% drop) ..You do the math and reply here. Can the officer supply a 1,000 gpm master stream and the original 2 1/2" hand line?

5. one way to denote the static is if you are aware of when a line is shut down to be move or advanced, you can see the static then. also look at the flow meter if you have one to ensure that the flow has stopped.

6. Does anyone use the Hydrant Free Flow Formula to determine expected GPM at the end of the supply hose? This has been part of our hydraulics program for years and every proby must learn and use it on their probationary exam, but I've yet to see it used in the field and question the true accuracy.

GPM = 100 √((HP+EL-20)/ (HL/100) / CE )

Where:
HP = hydrant pressure
EL = elevation loss/gain
HL = hose length
CE = coefficient of hose dia

Example: Static pressure of hydrant is 80 psi, the engine is to be connected to the hydrant 300 feet away using 5” LDH. No elevation is considered.

HP=80
EL=0
HL=300
CE = .08

GPM = 100 √((80-20)/ (300/100) / .08)

GPM = 100 √((60/3) / .08)

GPM =100 √(20 / .08)

GPM = 100 √250

GPM = 100*15.81

GPM = 1581

7. Your equation is simply a re-arrangement of the friction loss equation, but ignores any pipeline losses from the water source to the hydrant. With large mains (12" and above) this is probably a valid assumption, however for 6" or 8" dead end maint, it will over estimate the flow available. Another place where trouble will occur is when multiple engines hook to the same system in one area. Don't know who devised this or put it on a promo test, but it should be reviewed by some very knowledgable people. The best way for you to understand this is to make copies of a water supply analysis graph (blank) and then plot two different hydrant and main systems.

8. ## Thanks, maybe the end is near (for this)

Originally Posted by KuhShise
Your equation is simply a re-arrangement of the friction loss equation, but ignores any pipeline losses from the water source to the hydrant. With large mains (12" and above) this is probably a valid assumption, however for 6" or 8" dead end maint, it will over estimate the flow available. Another place where trouble will occur is when multiple engines hook to the same system in one area. Don't know who devised this or put it on a promo test, but it should be reviewed by some very knowledgable people. The best way for you to understand this is to make copies of a water supply analysis graph (blank) and then plot two different hydrant and main systems.
A few of us had pondered the validity of the equation and how it stood the test of time, when no one can remember any use in the field. The fact is much of our water system is <12" mains so even when we don't consider multiple hydrants flowing it sounds like this might be worth removing. The origin of this equation is a mystery, I searched the web the other day as my new proby was preparing for his test, just to see where this came from and its validity, with zero results found. I suspect the formula may have been the work of a previous A/C years back, but he's since moved on to a six figure computer programming job. Thanks, KuhShise, I was hoping you'd chime in.

9. Originally Posted by RFDACM02
This has been part of our hydraulics program for years and every proby must learn and use it on their probationary exam, but I've yet to see it used in the field and question the true accuracy.

GPM = 100 √((HP+EL-20)/ (HL/100) / CE )
Show me the fireman that can do the SQRT of 250 in his head while getting a line in service and monitoring a radio or two. Formulas are good for getting folks to understand the 'whys' but don't translate well in the field.

10. Originally Posted by voyager9
Show me the fireman that can do the SQRT of 250 in his head while getting a line in service and monitoring a radio or two. Formulas are good for getting folks to understand the 'whys' but don't translate well in the field.
That has been the contention of this all along. Most of the Q formulas never make it to the street, but I'd rather have personnel that know the why as well, rather than just the how. Again the purpose of this particular formula saw no real use in the field.

In the same vein, how do operators determine engine pressure when supplying lines if they can't do math? Sure square roots are hard, but at 3 a.m. anything that doesn't end in 0 or 5 is hard for most people. We've found the old "rules of thumb" tend to be fairly far off reality when tested against actual coefficients and then tested against calibrated flowmeters. We post our actual friction loss for our required handline flows at the panel to eliminate guesswork or bad mathematics, but an operator must be able to understand when something is askew and from that determine why, hence knowing the math.

11. Originally Posted by RFDACM02
We've found the old "rules of thumb" tend to be fairly far off reality when tested against actual coefficients and then tested against calibrated flowmeters.
That's where in the field you sacrifice accuracy for speed.. as well as operators performing mental load shedding when surrounded by a chaotic scene. On the other hand, performing any analysis.. whether it is using a Rule of Thumb, rounded math, or real math is preferable to writing the pressure on the panel or only knowing enough to push the preset button. At a minimum we need operators to know when something doesn't make sense.. when what they are seeing does not match expectations. If they don't know the 'whys' then they have invalid/incorrect/incoherent expectations and won't act when something doesn't go right.

12. Since firedog didn't take a crack at the question posed in post #4, does anyone else want to try?

"If the original static was 100 psi.... (5% drop) ..You do the math and reply here. Can the officer supply a 1,000 gpm master stream and the original 2 1/2" hand line?"

13. Real life answer to scenario 2.

Forget the formulas. Keep the handlines charged, since people are depending on them. Slowly open the master stream valve while watching the vacuum gauge. (With a side mount you could even rest a foot on the hose to make sure it stays rigid but I guess that is frowned upon nowadays.) Get as much as you can out of it without sacrificing the people on the handlines. If you can't get satisfactory flow on the master stream with the existing setup, tell the OIC.

Since you can't manufacture water on scene, do the best you can with what you have. Just don't let down the manned lines where people could be injured if they lost water.

Eventually, someone with a white hat will have to decide which is most important.

Then, when it is all over, go back to the station and use all the formulas you want.

14. Originally Posted by firepundit
Real life answer to scenario 2.

Forget the formulas. Keep the handlines charged, since people are depending on them. Slowly open the master stream valve while watching the vacuum gauge. (With a side mount you could even rest a foot on the hose to make sure it stays rigid but I guess that is frowned upon nowadays.) Get as much as you can out of it without sacrificing the people on the handlines. If you can't get satisfactory flow on the master stream with the existing setup, tell the OIC.

Since you can't manufacture water on scene, do the best you can with what you have. Just don't let down the manned lines where people could be injured if they lost water.

Eventually, someone with a white hat will have to decide which is most important.

Then, when it is all over, go back to the station and use all the formulas you want.

Whilie i agree that on the fire ground I am not going to break out a calculator to figure the formulas out. That is not to say that I dont use a grease pen from time to time. However by understanding the why (the formulas) and combining that with experience pumping (fire ground and the pump pit) I can tell you yes or no if i have the water before i start pumping. This is based off of the method that you discribed by the environmental observations and a working knowledge of the formulas. You dont count on you hand every time you add 4 + 5 + 6 do you?

I can introduce you to more then a few BCs that are going to be pretty upset if they see water flowing out of the master stream and then it shuting off due to not having enough water. However they will not be upset if they call for it and the pump operator replies sorry chief I dont have the water. They expect professional pump operators not lever pullers.

15. Originally Posted by KuhShise
Since firedog didn't take a crack at the question posed in post #4, does anyone else want to try?

"If the original static was 100 psi.... (5% drop) ..You do the math and reply here. Can the officer supply a 1,000 gpm master stream and the original 2 1/2" hand line?"
With out doing the actual calculations. Based off of my SWAG (see above post) I am going to say no you cannot supply the 1000 GPM master stream and the 2 1/2. I feel fairly certain you could supply a 750 GPM master stream and the 2 1/2 or the 1000 GPM master stream by itself.

I get that from saying the 5% drop from static with a flow of 250 GPM; Gives me the ablity to safely pump 3 more lines at 250 GPM or a total of 750 more GPM. Which results in a total avalible GPM flow of 1000 GPM. This assumes 20 PSI residual.

Now that is not to say i might not cheat alittle on the fire ground and drop below that 20 PSI residual to get the 1000 GPM. If that is the case I am going to be watching that pump like a hawk, to prevent any problems.

16. RFD21: Nice job. You might have reached almost 800 additional gallons, so the correct choice for a solid bore nozzle would have been a 1 3/4" tip. If firepundit had chosen to try to supply a 2" tip, the reach would have been much less than satisfactory at only 45 psi of nozzle pressure and a reach of around 60 feet. With a 1 3/4" the flow would have been the same with 80 psi nozzle pressure and a reach of about 85 feet. Depending upon the relationship between master stream placement and the seat of fire, the results would have been entirely different. Your choice (without wind) of a 750 gpm combination, would reach about 95 feet, but would be prone to having the stream break-up in wind or heavy fire induced drafts. Choosing a 1 1/2" tip or a 1 3/8" tip would have restricted the delivery volume and perhaps compromised the operation until the fuel was reduced to the exringuishing capability of the master stream.

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