It's Monday morning and we get a call from a developer 20 miles away who wants a price for having a sprinkler system installed at a new 7,000 sq. ft. drug store they are looking at building.
We don't know what the cost would be because pipe size is all dependent on the water pressure available. Larger pipe always costs more money not just in terms of material cost but labor to install as well. For example 6" pipe costs three times as much to purchase but can easily require twice the labor to install.
Before we can do anything we need to run a flow test.
Checking with the city we determine there's an 8" city water main running down the street in front of the proposed building and what we need now is two hydrants in front ideally "straddling" the property as shown above.
We're are going to conduct a flow test on "Hydrant A" using both "Hydrant A" and "Hydrant B".
The tools we need are a water pressure gauge with 2 1/2" NSHT (National Standard Hose Thread) adapter and a pitot tube which are shown in the photo to the right.
About the gauges. In my opinion the accuracy of the flow test we are about to conduct is the most important part of laying out any sprinkler system. We will want to use good quality gauges that have been lab certified for accuracy sometime in the preceding twelve months.
During my career I've made some mistakes; I've run into beams I didn't know were going to be there and I've missed heating ducts I didn't know were there and had to reroute pipe to go around obstacles but these mistakes are easily corrected in the field.
If you do this work you will make the same mistakes but these aren't really any big deal. Mistakes like this typically cost anywhere from a couple hundred to maybe even one or two thousand dollars but while nobody likes to lose money they'll happen.
The one mistake that has the potential to cost a lot of money, sometimes tens of thousands or even hundreds of thousands of dollars, is not using an accurate, up to date flow test in the design of your system. Being a certified layout technician carries a lot of responsibility and "missing it" is your mistake.
All the pipe sizes you use in designing your system will be based upon this flow test. If it is wrong your entire system is wrong and most likely a mistake like this will cost money to fix. Sometimes a lot of money.
You are also going to want a "Flow Test Summary" where you can document the flow test. I've included a copy of the flow test summary report I use to the left.
I do not carry a hydrant wrench, used to open and close hydrants, because if something breaks I don't want to be the one responsible. What I do is call either the water or fire department so they could have someone there to witness the results and operate the hydrants. If the hydrant breaks I want them to be the ones to break it.
Not to mention if you open a hydrant someone will notice and unauthorized operation of hydrants is against the law in all the municipalities I've done work in.
It is not good to spend an afternoon in jail.
After we get there the we'll connect the pressure gauge to our test hydrant, this is "Hydrant A", which is the upstream hydrant to obtain our static and residual pressures. Once connected we'll have someone from the fire department fully open the hydrant so we can obtain our static pressure.
The static pressure is going to look something like this... on this test we see a static pressure of 54 psi.
Static pressure is the pressure available in the line without water flowing. While most municipal water systems typically have from 50 to 80 psi available I've seen static pressures as low as 22 psi (almost always unusable) to as high as 240 psi as I found once in Akron, Ohio.
Now we need to flow "Hydrant B".
To flow the hydrant we'll have the water department remove the cap from the 2 1/2" hydrant opening then fully open the hydrant getting something like this.
I'm sure you've seen this before and now you know they're most likely conducting a flow test. Look for the guy holding a clipboard.
When the hydrant is fully open we're going to get two readings.
The first reading we'll get is the "residual pressure" or the pressure that is in the line with the downstream hydrant fully discharging.
On a gravity system, a gravity system is a system employing water storage tanks, the residual pressure will always be less than the static pressure.
With the hydrant fully open we expect to see a drop and we're right. Reading the gauge at "Hydrant A" we note the pressure has dropped 6 psi from 54 psi to 48 psi.
The residual pressure available for this flow test is 48 psi.
I've seen all kinds of residual pressure drops. I've seen those with static pressures of 60 psi drop to 58 or 59 psi and I've seen them drop from 60 psi to 12 psi.
You got to take your readings and you got to be accurate. Never guess
because if you guess it will come back to bite you.
I've always enjoyed conducting flow tests. They're different.
The last reading we need is the quantity of water being discharged from the open hydrant and we determine this by using the pitot tube to measure the force of water being expelled.
To obtain pitot pressure we place the opening in the center of the stream with the tip half of a diameter away from the hydrant butt. In the case of a 2 1/2" hydrant butt we would want to hold our tip 1 1/4" away from the opening as shown in the photo.
It's hard to see in this photo but our pitot pressure was 44 psi.
The formula for theoretical discharges through circular orifices is:
d=Diameter in inches of discharge orifice
p=Pitot pressure in psi
Commit this formula to memory because you will use it on every flow test you conduct.
The diameter of our outlet is 2.5 inches and our pitot pressure was 44 psi.
The theoretical discharge from the 2.5 inch hydrant butt with a pitot pressure of 44 psi is 1,237 gpm.
Wait, we're not done yet.
But the formula is for theoretical discharge from a perfect circular orifice and 2 1/2" open hydrant butts are not perfect. I guess this just goes to show nobody's butt is perfect. :)
We need to multiply the theoretical discharge by a coefficient of discharge which is recognized as 0.90 for relatively new hydrants which is most hydrants less than 60 years old.
Looking at all that water coming out of the hydrant butt we viewing something real close to 1,113 gallons per minute.
Some insurance underwriters require a discharge coefficient of 0.80 so if this project was a Factory Mutual risk we would have to use a flow of 0.80*1,237=990 gpm.
For 90% of the projects you do you will be using a coefficient of 0.90.
So for our flow test we obtained the following results:
Static Pressure: 54 psi
Residual Pressure: 48 psi
Rate of Flow: 1,113 gpm
Using a coefficent of 0.90 what would be the calculated rate of flow if we obtained a pitot pressure of 20 psi?
Answer: 834*0.90=750 gallons per minute.
Using a coefficent of 0.90 what would be the calculated rate of flow if we obtained a pitot pressure of 40 psi? (It isn't double).
Answer: 1,179*0.90=1,061 gallons per minute.
Doubling the pressure does NOT double the discharge.
How accurate are the numbers? I wouldn't get all bunged up over a few gallons. 1,055 gpm is just as valid as 1,065 gpm but if truth be known it's probably within a few percentage points.
Professional Engineers, these are people with the initials PE after their names, are might cringe hearing me say this but we're getting awfully close to doing real engineering but it is important to recognize we are not engineers. We are highly specialized technicians.
Ok, now that's over and we'll talk a bit about what we did during the ride back to the office. It's getting close to lunch time and I'll buy, you listen.
Static pressure by itself doesn't mean anything to us. If you told me the line in front of a building had 140 psi of pressure I couldn't tell you for certain if that would be adequate to supply a fire sprinkler any more than if you told me the line had 45 psi of pressure. You can't tell from this one number.
I got an idea 140 psi would be a great water supply but I wouldn't stake my life and reputation on it until I conducted a flow test. While the guess might be good there's to much money and liability attached to go around foolishly guessing.
Sprinkler systems are designed using the "density area" method. The density for a shopping mall, grocery or drug store is .20 gpm over the most remote 1,500 sq. ft. plus 250 gpm for hose stream. Using the density area method the theoretical minimum amount of water a sprinkler system would have to have would be .20*1,500=300 gpm for sprinklers plus an additional 250 gpm for fire department use for a total of 550 gpm. This is the theoretical minimum and I can tell you now it will be more, probably 10% to 20% more, but it can not be less than 550 gpm.
What the density area means is we have to be able to prove the sprinkler system we install is capable of discharging a minimum of .20 gpm per square foot over an area of 1,500 square feet. We are not interested in what it will actually do but what we must show is it will do at least the minimum... anything over is just gravy.
This brings up an interesting point, does the size of the building have anything to do with the total amount of water required?
The answer is no.
Here's the deal, 99% of sprinklers don't go off like you see in the movies, each individual sprinkler head is individually activated by heat. Figure one shows a Viking VK100 1/2" standard response sprinker rated at 200 Deg. F. The green liquid in the bulb indicates temperature rating.
On the vast majorit of systems pipe connected to the sprinkler is full of water under pressure all ready to go. The only thing stopping water from being discharged is the glass bulb that holds the sprinkler seat (seal) in place. Once this glass bulb breaks water will flow instantly.
Sprinkler heads can easily be activated by holding a heat source to the glass bulb. In Fig. 2 I am going to break the glass bulb by applying heat from a common cigarette lighter.
It doesn't take much, it happens fast and if connected to a sprinkler system water is instant. Lots and lots of water, it isn't like the movies.
In just a few short seconds the liquid in the bulb expands breaking the bulb which releases the seat allowing water to flow.
Sprinkler heads are a one time operation deal. Once they operate you have to replace.
Ever wonder how much water one will put out? That depends on the water pressure.
p=pressure in psi
q=gallons per minute.
In the case of the VK100 1/2" sprinkler the k=factor=5.6.
If a sprinkler is supplied with 100 psi it will discharge 5.6*100^.5 or 56.o gallons per minute. 56.0 gpm is a lot of water and to put it into perspective it will fill a 55 gallon drum in less than a minute.
If supplied with 50 psi the 1/2" VK100 sprinkler will discharge 5.6*50^.5 or 39.6 gallons per minute. Just because you double the pressure does not mean you double the discharge.
People often ask about high challenge fires, will sprinklers put out a plastics fire or a pile of styrofoam cups 20 feet high?
Yes, they will if properly designed. We have heads with k-factors of 25.0 with a maximum allowable spacing of 100 sq. ft. per sprinkler. You find these kinds of sprinklers in warehouses where you have high challenge fires such as foam rubber warehouses.
With something like a foam rubber mattress you will most likely have a large fire pump capable of pumping 2,000 to 3,000 gallons per minute at 125 psi.
From a single open k-25 sprinkler head:
q=25.0*125^.5 or 279 gpm from one sprinkler head. This sprinkler will fill a 55 gallon drum in under 12 seconds. That's a lot of water.
Imagine the amount of water if you had 12 of these sprinklers going off.
You put any fire under Niagra Falls and it will go out.
Sorry, kind of got sidetracked there.
With the 1,500 sq. ft. area of operation the idea is to either control or extinguish the fire before it grows out of that area.
If the entire building is on fire what's there to save?
A small 1,500 sq. ft. building will require a minimum of 550 gallons per minute.
A large one million square foot building will require the same amount of water.... 550 gallons per minute. In each case the area of operation is the same.
While 550 is the theoretical minimum it will be more... probably 600 gpm or somewhere around there.
When flowing water through a pipe the more water you flow the less pressure you have available. We saw that in our flow test; without water flowing we had 54 psi available but with 1,113 gpm flowing we had only 48 psi.
The reason we did this flow test was to determine how much pressure we have available to design our system to at 600 gpm or whatever our end demand will be.
Enough for one day. We'll talk about flow test evaluation tomorrow.
And here you thought high school algebra was a waste of time.