Pressure in the Distribution System

Water pressure is an important factor to consider when planning a distribution system.  As a rule of thumb, the water pressure throughout the distribution system should be no less than 17 PSI.  In many high value districts, distribution lines are designed for a normal pressure of between 60 and 70 PSI. 

Low pressure in the mains can be a health hazard since the pressure in the pipes keeps contaminated water from entering the mains.  If the pressure in a pipe is too low or is negative, contaminants from nearby ditches, cross-connections, and poor quality house plumbing can be drawn into the water system.  

Low pressure in a pipe allows contaminated water to enter the pipe.

Investigations have proven that most water-borne disease outbreaks are the result of contamination of water after it is pumped into the distribution system.  To prevent contamination, an adequate chlorine residual must be maintained and the residual pressure should never be allowed to fall below 20 PSI.  The Virginia Department of Health issues an advisory for extra-precautionary measures during periods of low pressure or vacuum.   

So how is the pressure in the distribution system's pipes produced?  Most of the pressure is a result of static pressure built up in the storage reservoir.  Static pressure is a weight per unit area.  If water is pumped into the top of a storage tank, the weight of the water will build up a certain pressure on the bottom of the tank.  Then when water is allowed to flow out of the bottom of the tank and into the distribution system, the static pressure results in water pressure in the pipes. 

The height, or elevation, of the tank will determine the amount of static pressure on the water at the bottom of the tank.  To determine the static pressure, measure the elevation of the water surface at the top of the tank (E1) and the elevation of the pipe into which the water is flowing (E2). 

Diagram of a storage tank.

Then use the following equation:

Static Pressure = (E1 - E2 ) × 0.43 PSI/ft

Consider a tank which is sitting on the ground with the water flowing out of the tank into a
main at ground level (an elevation of 0 feet).  The surface of the water in the tank is 100 feet above the ground (an elevation of 100 feet.)  The static pressure would be:

Static Pressure = (100 ft - 0 ft) × 0.43 PSI/ft

Static Pressure = 100 ft × 0.43 PSI/ft
Static Pressure = 43 PSI

The water flowing out of the storage tank in the example above has a pressure of 43 PSI.  Once the water begins moving, the pressure becomes a dynamic pressure, also known as a residual pressure.  Dynamic pressure is equal to static pressure just as the water leaves the tank, but as the water moves through the pipe energy is lost and the residual pressure decreases.  The energy loss in the pipe is proportional to the square of the velocity of the water, so the faster the water moves through the pipe the more pressure it loses. 

The roughness of the inside of the pipe also influences the residual pressure in the pipe.  Rougher pipes have higher resistance factors (also known as roughness coefficients), which means that the water flowing through the rough pipes loses energy more quickly than it would if flowing through a smooth pipe.