What Imperial Unit Conventions are Used in Fire Protection?

MeyerFire University | G401.03

By Franck Orset

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• Notes Page

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TRANSCRIPT

Today we’ll define the units which are the most commonly used in the Imperial System around the fire protection industry. 

Note that sometimes, the unit in the Imperial System and the Metric System is the same. This is the case, for example, for the rotational speed of a pump, expressed in rounds per minute. 

For most generic measurements, 

• Temperature in degrees Fahrenheit. This comes up with sprinkler ratings, heat detector ratings, smoke control design, and much more.
  
  
• Spacing and distances, generally in feet and inches. This is for pipe lengths, height of storage, separation distance between racks, height of building, etc.
  
  
• Area of application, generally in square foot. This is often expressed with sprinkler or smoke detector spacing, area of a room for egress calculations, and much more.
  
  
• Slope, such as the slope of a roof or slope of pipe, is expressed in inches per foot, such as ½” per 10-ft for pipe slope or in inch per inch, such as a 4 in 12 roof slope. 

For fire pumps,
 

• Flow is generally expressed in US gallons per minute, or gpm. Be aware that the US gallon is not a pure Imperial unit, as there is an Imperial gallon, still used in Canada and Great Britain, for example. Roughly, one Imperial gallon equals 1.2 US gallons. Flow through a single sprinkler could be as low as 7.4 gpm, compared to the demand of an ESFR system which could easily be 2,000.
  
  
• Pressure is generally expressed in pounds per square inch, or psi. It can also be expressed in terms of feet of water column, with 1 foot of water column corresponding to 0.43 psi (under normal temperature and pressure conditions). This is also referred to as “head” or “feet of water”. In a pump sizing chart, for example, a fire pump with a rated pressure of 231 feet of head would be equivalent to about 100 psi.
  
  
• Rotational speed is expressed in rotations per minute, or rpm. This is the number of rotations the pump impeller travels during one minute. Rotational speed for pumps is often around 1770 or 3650 rpm.
  
  
• Power associated with the fire pump driver is generally expressed in horsepower (hp). This is a critical item for electrical engineers when determining electric feeds and backup power supplies for electric fire pumps. Very small NFPA 13D pumps might be under 10 horespower, while light commercial horizontal split-case fire pumps could be 100 horsepower. The larger end of fire pumps can get as large at 600 horesepower.
  
  
• Current and Voltage for electric drives are expressed in Amperes (A) and Volts (V). These values are often taken as readings with project acceptance and again at testing intervals. 

For water supplies,

• Volume for the water tanks is generally expressed in gallons. Again, be careful with the distinction between US gallons and Imperial gallons.
  
  
• Duration for the water supply is generally expressed in minutes or hours. Hours are generally used for long durations. 

For sprinkler systems,

• K-factor in gpm per square root of psi (gpm/√psi), as the K factor is defined by the formula Q =K √P, with Q a flow in gpm, and P a pressure in psi. K-factors range from 2.0 to 28, with nominal intervals defined in NFPA 13. The most common sprinklers in production today are K5.6.
  
  
• Thread and orifice sizes are in inches. Threads typically come in ½, ¾, 1, and 1 ¼ inch sizes. Orifices are measured in fractions, which align with k-factors. A k-factor of 2.8 has an orifice size of 3/8”, a 4.2 has orifice of 7/16”, a K5.6 has an orifice size of ½”, and so on. This comes from the table titled Nominal Sprinkler Orifice Sizes in NFPA 13.
  
  
• Design density in gpm per square foot (gpm/ft2). A common density for a Light Hazard sprinkler system is 0.10 gpm/sqft out of NFPA 13.
  
  
• Hydraulic demand in gpm for the flow and psi for the pressure. The requirement for a standpipe hose connection, for example, typically requires 500 gpm at 100 psi at the top of the most remote standpipe.
  
  
• Pipe diameter size, generally in inches. Pipe diameters get as small as quarter-inch. From there, increasing in size, we typically see ½”, ¾”, 1-inch, 1-1/4”, 1-1/2”, 2”, 2-1/2”, 3”, 4”, 6”, 8”, and 10”. Sprinkler system pipe normally will range from 1-inch to 6-inch for most light commercial projects. Storage and industrial projects can get into the higher pipe sizes, and small auxiliary functions like outlets for gauges will use pipe sizes less than an inch.
  
  
• Speed for the velocity of water inside the pipes, in feet per second. There used to be more attention paid to the speed of water within pipes, but with the use hydraulic calculations today, the speed of the water is less a concern because friction loss is essentially self-limiting. We’ll cover that in more detail down the road. 

For fire alarm systems,

• Light intensity is in Candelas, which is used for the light intensity coming from a strobe. Common candela ratings at 15, 30, 75, 90, 110, 115, and 177. 
  
  
• RTI, or the Response Time Index for measuring response of heat detectors, is in square of feet by square of seconds.
  
  
• Sound intensity, used for measuring horn and speaker volume, is measured in decibels. A common requirement is a sound rating for horns to be 15 decibels above the ambient sound condition. Of course plenty of other nuance requirements apply there.
  
  
• Illumination, which is a measurement of a light intensity over an area, is measured in footcandles. Yes, that’s footcandles. This is almost like a density of light on a surface. It’s used when doing performance-based calculations for light intensity, and also on the emergency lighting side of electrical lighting design. 

For gas protection systems,

• Volume, for the application of the system or room size, expressed in cubic foot
  
  
• Weight, for the quantity of gas, expressed in pounds
  
  
• Duration, for the application or delivery time, generally expressed in minutes.

For smoke control and performance-based designs,

• Air flow is measured in cubic feet per minute, or CFM. This is a volumetric measurement of movement.
  
  
• The speed of gas movement can also be expressed as feet per second, which considers only the speed of air movement and not the volumetric flow. This is important for applications such as make up air speed or the speed required to prevent smoke spread through certain thresholds.
  
  
• Particle density is measured in parts per million.
  
  
• Heat release rate, or the amount of heat that is projecting from a fire source, can be measured as Kilowatts, Megawatts, or BTU per hour.
  
  
• Energy delivered by a fire, generally expressed in BTU, or British Thermal Unit. A Btu is the quantity of heat required to raise the temperature of 1 lb of water 1°F at a pressure of 1 atmosphere and temperature of 60°F
  
  
• Power delivered by a fire, generally expressed in BTU per hour
  
  
• Heat flux, generally expressed in BTU per hour and per square foot. 

To end with a little history. 

Where does the difference between US gallon and Imperial gallon come from? 

Before the 19th century, the definition of the gallon was depending on the country where the measurement was made and what was measured. 

In the 19th century, two definitions became popular: the wine gallon defined as 231 cubic inches and the beer gallon defined as 282 cubic inches. This last value was further redefined as 277.42 cubic inches and was based on the volume of 10 pounds of water at 62 degrees Fahrenheit. This unit measurement was adopted by Great Britain in 1824.  

But at that time, the US was using the wine gallons which had been defined as the volume of a cylinder with a diameter of 7 inches and a length of 6 inches. It never changed since then. 

The difference in the units is coming from the fact that British preferred drinking beer while American preferred drinking wine. 

For Franck Orset, I’m Jeff Kelm, this is MeyerFire University