Glossary of hydraulics for fire protection

A to Z of commonly used terms in fire sprinkler protection.




  • Area of operation: The maximum area over which it is assumed that the sprinkler will operate and is sometimes called the "assumed maximum area of operation"


  • Barometer: An instrument used for the measurement of atmospheric pressure

  • Bar: A unit of pressure 1 bar = 10000 pascal (Pa).  The bar is used throughout Europe to measure pressure in fire sprinkler systems.    
  • Booster pump: A fire pump used to boot the pressure of the existing water supply
  • Bourdon gauge: The most common device used to measure system pressures.



  •  Centrifugal pump: Modern industrial fire pumps are centrifugal pumps.  Pressure is added to the water by the centrifugal force created by a rotating wheel (impeller) or the rotating vanes of a turbine.
  • Certified shop test valve: Before the manufacturer ships a pump, it will be tested in the shop.  The results of this test will be plotted on graph paper.
  • Circulation relief valve: A small relief valve that opens up and provides enough water flow into and out of the pump to prevent it from overheating when operating at churn against a closed system.
  • Complex loop: A piping system that is sometimes called a "grid" and is characterised by one or more of the following:  more than one inflow point, more than one outflow point, and/ or more than two paths between inflow and outflow points.
  • Controller:  The electric control panel is used to switch the pump on and off and to control its operation.



  • Darcy-Weisbach:  Technique used to establish the pressure lost to friction in a piping system.

  • Design Density: The discharge density is given as mm/min and is the minimum amount of water needed over the design area (or sprinkler head) for each minute of operation. The density of discard required of any risk depends on the occupancies, classification of goods, and the design standard applied. 

  • Design Point: A point in the distribution pipe of a precalculated installation from where the pipework is sized by hydraulic calculation
  • Differential manometer: A device whose primary application is to reflect the differences in pressures between two points in a system.



  • Flow hydrant: The hydrant from which the water is discharged during a hydrant flow test.
  • Flow test:  Tests conducted to establish the capabilities of water supply systems are referred to as flow tests because they involve flowing fire hydrants.  The objective of a flow test is to establish the quantity ( gallons per minute) and pressures available at a specific location on a particular water supply system.
  • Fluid:  Any substance that can flow; a substance which has definite mass and volume at constant temperature and pressure but no definite shape; and with the inability to sustain shear stresses.
  • Fluid mechanics: In general terms of physics, the force causes motion.

  • Force: In general terms of Physics, force causes motion.





  • Hardy cross method: An interactive technique for solving complicated problems involving gridded water supply systems.
  • Hazen-Williams formula: An empirical formula for calculating friction loss in water systems that is the fire protection industry standard.  The Hazen-Williams formula must be used to comply with the most nationally recognised standards.
  • Head: Pressure expressed in units of Metre of water.
  • Horizontal split-case pump: A centrifugal pump with the impeller shaft installed horizontally and often referred to as a split-case pump.  This is because the case in which the shaft and impeller rotate is split in the middle and can be separated, exposing the shaft, bearings and impeller.
  • Hydraulics: The branch of fluid mechanics deals with the mechanical properties of liquids (in the text, water) and the application of these properties in engineering.
  • Hydrokinetics: A branch of hydraulics involving liquids (water) in motion, particularly in relation to forces created by or applied to the liquid in motion.
  • Hydrostatics: A branch of hydraulics dealing with the properties of liquids (water) at rest, particularly concerning pressures resulting from or applied to the static liquid.

  • Hydraulically most favourable area: Area of operation for which the water flow is at its maximum for a specified pressure, measured at the main control valve or pump set

  • Hydraulically most unfavourable area: Area of operation for which the system pressure, when measured at the main control valve or pump set, is required to be at its maximum to achieve the specified design density


  • Jockey pump: A jockey pump is a small-capacity, high-pressure pump that maintains constant pressure on the fire protection system.  A jockey pump often prevents the main pump from starting unnecessarily.



  • Kinematic viscosity:  The kinematic viscosity of a fluid is the ratio of its absolute viscosity (lb sec/ft2) to its mass density (lb sec2/ft4).
  • Kinetic energy: The energy which a body possesses because of its motion.

  • K-factor equation calculates the flow from a nozzle such as a fire sprinkler or water mist head.



  • Laminar flow:  A fluid is in the state of laminar flow if its Reynolds number is 2,100 or less; laminar flow is related to very low liquid velocities.
  • Liquid:  A fluid with a definite volume, unlike gases, which will expand to fill the vessel.



  • Maximum flow demand: The flow discharge from the sprinkler heads located at the hydraulically most favourable area when balanced to a fire pumps flow/pressure curve (QMAX point)

  • Moody diagram: A Diagram used with the Darcy-Weisbach friction loss computation technique to relate the Reynolds number, pipe size, and roughness to a friction factor.



  • Net pressure:  The net pressure is the pressure added to the system by the pump.



  • Orifice plate meter:  An orifice plate meter is a device used to measure water flow and is similar in principle to a Venturi meter.  The change of water velocity is accomplished by using a plate with an orifice smaller than the diameter of the pipe in which it is placed. 



  •  Pascal's law:  Principle 1, known as Pascal's law, points out that pressure acts in all directions, not simply downward.
  • Pascal: The SI unit for pressure is the pascal (Pa) equal to one Newton per square meter (N/m2).  For fire protection, this pressure measurement is small, so the unit Bar or kPa is used in most parts of the world.    

  • Piezometer tube: This device uses the heights of liquid columns to illustrate the pressures existing in hydraulic systems.
  • Pitot tube:  Common device used to measure velocity pressure and, thus, fluid velocity.  The pitot tube consists of a small diameter tube connected to a pressure gauge, usually about one-sixteenth inch in internal diameter.
  • Potential energy: Stored energy that can perform work once released. 
  • Pressure: is the force per unit area (symbol P).
  • Pressure head: This is a term used in fluid mechanics to represent the internal energy of a fluid due to the pressure exerted on its container. It may also be called static pressure head or simply static head.
  • PSI: In fire protection, pressure is most often dealt with in units of pounds per square inch (psi).



  • Relief valve: The relief valve is provided to open up and discharge water to a drain should the pressure become excessive.  This valve is located between the pump and the discharge check valve and is required with pumps driven by variable-speed drivers.
  • Residual pressure:  The pressure at the test hydrant while water is flowing.  It represents the pressure remaining in the system while the test water flows.
  • Reynolds number: is a dimensionless number that states if the flow is laminar or turbulent (Symbol Re).



  • Simple loop:  A loop with exactly one inflow point, one outflow point, and exactly two paths between the inflow and outflow points.
  • Specific gravity: A substance's specific gravity (Sg) may be defined generally as the ratio of the weight density of the substance to the weight density of another substance, usually water.
  • Static pressure:   The normal pressure existing on a system before the flow hydrant is opened.



  • Total energy:  The total energy (TE) at any point in a system might be defined as the sum of the potential and kinetic energy at that point.
  • Turbulent state:  Fluid flow is in the turbulent state at higher velocities where there is no definite pattern to the direction of the water particles.  Turbulent flow is reflected by a calculated Reynolds number of more than 2,100.



  • Venturi meter:  When coupled with a differential manometer, a venture meter may measure water velocity.  The device consists essentially of a pipe in which the cross-sectional area has been constricted.

  • Velocity: The water velocity in a pipe is the speed at which the water flows in m/s.



  • Water hammer:  Stopping any flowing stream too rapidly can cause a phenomenon called water hammer.  Water hammer is a violent increase in pressure which can be large enough to rupture the piping.