Four Friction Loss & Six Fluid Principles

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Fluid Principles explained:

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The speed with which a fluid travels through hose or pipe is developed by pressure upon that fluid. The speed at which this fluid travels is often referred to as velocity. It is important to identify the type of pressure because the word pressure in connection with fluids has a very broad meaning. There are six basic principles that determine the action of pressure upon fluids.

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FP #1:

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Fluid pressure is perpendicular to any surface on which it acts. This principle is illustrated by a vessel having flat sides and containing water. The pressure exerted by the weight of the water is perpendicular to the walls of the container. If this pressure is exerted in any other direction, as indicated by the slanting arrows, the water would start moving downward along the sides and rising in the center.

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FP #2:

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Fluid pressure at a point in a fluid at rest is the same intensity in all directions. 'To put it another way, fluid pressure at a point in a fluid at rest has no direction.

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FP #3:

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Pressure applied to a confined fluid from without is transmitted equally in all directions. This principle is illustrated by a hollow sphere to which a water pump is attached. A series of gauges is set into the sphere around its circumference. When the sphere is filled with water and pressure is applied by the pump, all gauges will register the same pressure. (This is true if they are on the same grade line with no change in elevation.)

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FP #4:

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The pressure of a liquid in an open vessel is proportional to its depth This principle is illustrated by three vertical containers, each 1 square inch in cross-sectional area. The depth of the water is 1 foot in the first container. 2 feet in the second, and 3 feet in the third container. The pressure at the bottom of the second container is twice that of the first, and the pressure at the bottom of the third container is three times that of the first. Thus, the pressure of a liquid in an open container is proportional to its depth.

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FP #5:

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The pressure of a liquid in an open vessel is proportional to the density of the liquid. This principle is illustrated by two containers. One container holds mercury 1 inch deep, the other holds water 13.5 inches deep, yet the pressure at the bottom of each container is approximately the same. Thus, mercury is 13.55 times denser than water. Therefore, the pressure of a liquid in an open vessel is proportional to the density of the liquid.

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FP #6:

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The pressure of a liquid on the bottom of a vessel is independent of the shape of the vessel. This principle is illustrated by showing water in several different shaped containers, each having the same cross-sectional area at the bottom and the same height. The pressure is the same in each container.

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Friction Loss Principles explained:

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The concept of, pressure loss due to friction in a hose or piping system, play an important part. The common term for pressure loss due to friction is simply friction loss. The fire service definition of friction loss is that part of the total pressure lost while forcing water through pipe, fittings, fire hose, and adapters. In fire hose, friction loss is caused by the following:

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cont. FL Principles explained:

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The rougher the inner surface of the pipe (commonly referred to as the coefficient of friction), the more friction loss that occurs. Friction loss can be measured by inserting in-line gauges in hose or pipe. The difference in the residual pressures between gauges when water is flowing is the friction loss.

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FL #1:

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If all other conditions are the same, friction loss varies directly with the length of hose or pipe.

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FL #2:

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When hoses are the same size, friction loss varies approximately with the square of the increase in the velocity of the flow. This principle points out that friction loss develops much faster than the change in velocity.

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FL #3:

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For the same discharge, friction loss varies inversely as the fifth power of the diameter of the hose. This principle readily proves the advantage of larger hose and can be illustrated by one hose that is 2 ½” inches in diameter and another that is 3"

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FL #4:

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For a given flow velocity, friction loss is approximately the same, regardless of the pressure on the water. This principle explains why friction loss is the same when hoses or pipes at different sizes flow the same amount of water.