A Typical Week In The Life Of - Day 2

Yesterday we conducted a flow test and obtained the following results:

Static Pressure: 54 psi
Residual Pressure: 48 psi
Rate of Flow: 1,113 gpm

An analysis of our water flow test is done using N^1.85 graph paper.

Once plotted we can determine what water pressure will be available at any flow. For our project we can determine we will have approximately 52 psi available if your sprinkler produces a demand of 600 gpm. If we had a system that produced 1,400 gpm demand the city water supply would have 45 psi available.

The more water required the less pressure is available.

Yesterday we talked about how static pressure alone wasn't an indicator of the quality of an available water water supply.

Consider a few years ago I had a flow test that produced the following results:
Static Pressure: 115 psi Residual Pressure: 24 psi Pitot Pressure: 16 psi

(As you will see this supply sucks).

Using a coefficent of 0.90 what would be the calculated rate of flow if we obtained a pitot pressure of 16 psi?
Where:

d=Diameter in inches of discharge orifice
p=Pitot pressure in psi

Answer: 746*0.90=671 gallons per minute.

Static Pressure: 115 psi Residual Pressure: 24 psi
Rate of Discharge: 671 gallons per minute.

Plotting the test summary we obtain this graph:

By plotting two points on a curve (static pressure and residual pressure @ flow) we can determine what pressure will be available in the line at any flow.

When analyzing flow test results you want to keep in mind flow test results can vary hour by hour. The results you obtained today probably won't match the results you get tomorrow so when laying out a system make sure to leave yourself some "safety factor". I always attempt to give myself 10 psi.

Static pressure on a gravity flow municipality indicates the height of water level in the water tank. Water weighs 0.433 pounds per foot and we can use this to determine the height of water above our static pressure test gauge.

With a static pressure of 54 psi the water level is 54/.433=124.7 feet above the test gauge.

With a static pressure of 115 psi the water level is 115/.433=265.6 feet above the test gauge.

Caution is something you should always have.

Consider the water tower in the photo.

In large metropolitan areas the technician generally doesn't need to
be all that concerned because elevated tanks are generally much larger and there are multiple elevated tanks.

But in smaller towns and villages the technician needs to be aware of not just the pressures available but the total water supply available as well.

From the two flow tests:

Test #1
Static Pressure: 54 psi
Residual Pressure: 48 psi
Rate of Flow: 1,113 gpm

Test #2
Static Pressure: 115 psi
Residual Pressure: 24 psi
Rate of Flow: 671 gpm

Which of the two flow test results do you suppose offers the "best" pressures as far as capable of producing the system with the smallest diameter pipes therefore the most economical system?

The answer is "it depends".

Let's superimpose the results of the second test on the graph with the first test.

The first thing we notice is that at 550 gpm 53 psi pressure is available at both tests.
We're going to concern ourselves with five occupancies as far as fire sprinklers are concerned.

Light Hazard Occupancies requires the least demanding fire sprinkler design. Light Hazard Occupancies include such occupancies as hospitals, nursing homes and office buildings. As can be determined from NFPA #13 the density for light hazard occupancies is .10 gpm over 1,500 sq. ft. with an additional 100 gpm for hose stream.

We can expect Light Hazard Occupancies to require not less than 150 gpm for sprinkler plus 100 gpm for hose allowance for a total water requirement of not less than 250 gpm.

Ordinary Hazard Group I Occupancies include those occupancies where storage does not exceed 8' in height and combustibility of product is low such as a light bulb factory or food processing plant.

The minimum density for an Ordinary Hazard Group I Occupancy is .15 gpm over 1,500 sq. ft. plus an additional 250 gpm for hose stream demand.

We can expect Ordinary Hazard Group I Occupancies to require not less than 225 gpm for sprinkler plus 250 gpm for hose allowance for a total water requirement of around 475 gpm.

Ordinary Hazard Group II Occupancies include those occupancies where storage does not exceed 12' in height and combustibility of product is moderate such as a shopping centers, drug stores, most furniture stores, shopping malls, machine shops and most factories.

The minimum density for an Ordinary Hazard Group II Occupancy is .20 gpm over 1,500 sq. ft. plus an additional 250 gpm for hose stream demand.

We can expect Ordinary Hazard Group II Occupancies to require not less than 300 gpm for sprinkler plus 250 gpm for hose allowance for a total water requirement of not less than 550 gpm.

Extra Hazard Group I Occupancies include those occupancies where storage does not exceed 12' in height and combustibility of product is greater such as die casting, metal extruding, plywood and particle board manufacturing. Printing (using inks having flash points below 100°F), rubber reclaiming, compounding, drying, milling, vulcanizing and saw mills.

The minimum density for an Extra Hazard Group I Occupancy is .30 gpm over 2,500 sq. ft. plus an additional 500 gpm for hose stream demand.

We can expect Extra Hazard Group I Occupancies to require not less than 750 gpm for sprinkler plus 500 gpm for hose allowance for a total water requirement of not less than 1,250 gpm.

And finally there are Extra Hazard Group II Occupancies which include those occupancies where storage does not exceed 12' in height and combustibility of product is high such as asphalt saturating, flammable liquids spraying, flow coating, manufactured home or modular building assemblies (where finished enclosure is present and has combustible interiors), open oil quenching, plastics processing, solvent cleaning, varnish and paint dipping.

The minimum density for an Extra Hazard Group I Occupancy is .40 gpm over 2,500 sq. ft. plus an additional 500 gpm for hose stream demand.

We can expect Extra Hazard Group II Occupancies to require not less than 1,000 gpm for sprinkler plus 500 gpm for hose allowance for a total water requirement of not less than 1,500 gpm.

You might be asking "What exactly is 'hose stream demand'?"

Sprinkler systems are designed to deliver a minimum quantity of water over the fire but what happens when the fire department rolls up, attaches hose lines to hydrants and starts spraying water on the fire?

They "rob water" from the overall system and hose stream allowances insures the system will continue to function as it should in spite of the disappearing water.

One can design most any sprinkler system given 20 to 30 psi but with these low pressures heads will have to be spaced closer together (more spinklers=more money) and pipes feeding sprinklers will have to be larger (bigger pipes=more money). While it is possible to design a system to work on 20 psi, assuming the building is not to high, the costs can get stupidly high.

The cost of running 4" pipe could run $4.00 per linear foot while the cost of running 8" pipe can easily exceed $30.00 per linar foot. Doesn't sound like much until you consider you could be easily dealing with a thousand feet of pipe. Lot of difference between $4,000 and $30,000.

Tomorrow we'll start hydraulic calculations so you can see this all start to go together.