The New York State Department of Health standard indicates that normal pressure in a water distribution system should be approximately 60 to 80 pounds per square inch (psi) and not less than 35 psi while maintaining a minimum pressure of 20 psi under all conditions of flow.
OCWA attempts to operate and maintain its system within these parameters whenever possible. If our entire system was flat, this would be a relatively easy task. All our storage tanks would be the same elevation and all our customers would have approximately the same pressure at their tap.
However, the significantly varying topography in Central New York causes higher and lower pressures. The result is the need to have pumps, tanks and pressure reducing valves to raise, maintain and lower the pressure of the water we supply to our 500,000 consumers accordingly.
Hydraulics is the scientific concept which governs water pressure. It is the study of water under pressure and the methods of moving and raising it. Important terms associated with this concept as they relate to the pressure of OCWA water include:
Flow rate – amount of water that flows through a pipe during a certain time period, usually measured in gallons per day (gpd)
Velocity – average speed at which water moves through a pipe, usually measured in feet per second (fps)
Pressure (dynamic) – force of water flowing through a system, normally measured in pounds per square inch (psi)
Head – another way to express pressure; height to which water would rise if it were not contained, normally measured in feet (a column of water 2.31 feet will exert a pressure of 1 psi); there are three types of head: pressure head, velocity head and elevation head
Hydraulic Gradient – slope or steepness of various head elevations in a water system which indicates the quantity or rate of flow in the system; large leaks in a system may be detected readily by checking the hydraulic gradient; typically the smaller the pipe, the greater the hydraulic gradient
Friction loss – loss in energy due to friction between water and pipe; pipes can be rough or smooth; the greater the friction, the greater the loss of head or drop of the hydraulic gradient
OCWA’s five-county water system is a combination of gravity fed and pumped service areas. The Authority operates 47 pump stations and over 100 individual pumps, ranging in pumping capacity from 200 gallons per minute to 26,000 gallons per minute. Each pump station works to maintain a level in one or more associated water storage facilities.
OCWA maintains and operates 60 water storage facilities, which include elevated tanks, ground tanks, standpipes and reservoirs. These facilities range in storage capacity from 150,000 gallons to 30 million gallons, and are necessary to maintain pressure and meet short term peak demands, such as firefighting, major industry cycles and summer time residential demands.
The other important component of OCWA’s water system which regulates pressure is our series of pressure reducing valves (PRVfs). We operate over 85 valves, both electronically and manually, to reduce the pressure of water flowing from higher elevations to lower ones. The size of these valves ranges from 4 inches to 20 inches.
A pressure zone is an area of the OCWA system which is generally supplied water from a tank and pump station. The boundaries of our pressure zones are controlled through normally closed control valves, or through PRVs. As illustrated on the below map, there are 70 unique pressure zones in OCWA’s five-county water system due to the many hills and elevation changes in our region.
How OCWA Checks and Maintains Proper Pressure
OCWA employs a sophisticated electronic monitoring and operations system called Supervisory Control and Data Acquisition (SCADA) to monitor and maintain proper system water pressure. Hundreds of sensors are deployed throughout our system to continuously monitor water pressure. Automated alarms will sound if pressure either drops too low or raises too high.
If pressure drops too low, whether caused by a fire or major leak, we can open PRVs or increase pump flow to stabilize the situation. Conversely, if the pressure in one part of our system suddenly spikes, mostly likely due to a malfunctioning PRV or pump, we can quickly isolate the problem and fix it.
In areas where water pressures exceed 80 psi despite our best efforts, customers are responsible for installing small residential PRVs in their homes, and to periodically check/maintain these devices. Failure to do so may result in water damage and/or damaged water fixtures. When required for meter installation, residential PRVs are to be installed either in a meter pit or within the house just before the meter. Customers should check the requirements within their municipality, but some require that a licensed plumber complete the installation.