Jeff Morrison, Assistant Director, Operator Training Committee of Ohio
Karen Mancl, Professor Food, Agricultural and Biological Engineering, The Ohio State University
Water system pressure ensures customers an adequate flow of water into their home or business. Pressure also provides adequate flow to fire fighting equipment. Even more importantly, water pressure protects the quality of water by making sure that any leaks in the system result in clean water leaking out, not contaminated water leaking in.
Water pressure in a community water system is measured in pounds per square inch (psi). Household water pressure should range from 50 to 60 pounds per square inch. A minimum of 20 pounds per square inch is required by law.
Water pressure will likely vary throughout the system. The highest points in the community and the most distant connections will have the lowest pressure. The lowest points in the community and those closest to the pressure source will have the highest water pressure. Water systems use booster pumps and pressure reducing valves to moderate pressure within the system.
Pumps at the water plant are the source of water pressure throughout the system. In some systems the well pumps provide the water pressure for the system. If pressure is reduced through the water treatment process, high-service pumps deliver water to the system at adequate pressure.
Water systems have three options to maintain pressure throughout the system. A community may use a combination of all three approaches to provide adequate water pressure to customers in different parts of their service area.
Tank and Booster Pump
The tank and booster pump combination is probably the most reliable method of supplying pressure to a water distribution system. When water demand causes the level in the tank to drop, the booster pump starts. The pump supplies the demand for water with excess backing up into the tank. When the tank reaches the upper limit, the pump shuts off.
By having the tank, constant pressure is put on the system by the height of the water in the tank. The tank provides a greater, more steady volume of water as well. During short power outages, the tank can provide pressure to the system. The Ohio Environmental Protection Agency requires that elevated storage be adequate to hold a one-day supply of water to the areas it serves without pumping.
The tank and booster pump combination requires a lot of space and is the most expensive to engineer and construct. The tank and booster pump system is suited for a large service area. Even if the initial service area is small, it is easy to expand.
Hydropneumatic System
The hydropneumatic system consists of a pressure vessel and a pressure pump. The pressure vessel contains water with a pressurized air space to provide the pressure for the system. With water demand, water flows from the vessel, increasing the air space as well as decreasing air pressure. This lower pressure signals the pump to start. The pump meets the demand with the excess volume backing up in the pressure vessel. This decreases the air space and increases the pressure once again. When the upper level is reached, the pump shuts off. The newer pressure vessels have a neoprene bladder to separate the air space from the water.
These systems are common in homes with an individual well system. They typically provide adequate pressure but low volume. They are ideal for a rural water system that has a limited service area that is elevated from the main line resulting in inadequate pressure. Hollows, cul-de-sacs, and small trailer parks utilize these to increase pressure. Some water systems have small hydropneumatic stations added as the system expands. These stations are relatively inexpensive to engineer and construct and are often available as package units.
Some of the problems with hydropneumatic stations are that while they provide adequate pressure, they provide limited volume. In some instances, with the nonbladder pressure vessels, air may be forced into the water lines if the demand for water exceeds pump capacity. Also, if the air space in the vessel is lost due to leakage (waterlogging), the pressure pump will stop and start too frequently, resulting in erratic pressure and low water volume. Also, pressure will not last long during power outages.
Variable Demand Pressure Pump
Variable demand pressure pump systems consist of a continuous-run pump with control values to divert flow not needed for demand around the pump and back into the pump intake. A similar system uses a variable speed motor reostatically controlled by pressure. These systems are utilized in much the same way as hydropneumatic systems. They can handle larger sections and can provide more volume. They require careful engineering, are rather expensive to purchase, but take up the least space.
Some of the problems encountered with these systems are their high operating cost due to the continuous-run feature, higher maintenance frequency, and the total and immediate loss of pressure in a power outage.
Comparing the Systems
 |
Merits | Limitations |
|---|---|---|
| Elevated tank with booster pump | Most reliable (stores large water volume) Least expensive to operate Can provide fire protection |
Most expensive to construct Requires two sites: one for pump/one large site for tank |
| Hydropneumatic systems | Lower construction costs Requires only one site |
Less reliable (stores small water volume) No fire protection |
| Variable demand pressure pump | Lowest construction cost Requires only one small site |
Least reliable (no water storage) High operating costs |