2.1 Distribution Valves

Distribution Valves

Quick Answer: Valves are essential components of a water distribution system, providing control over flow, pressure, and direction. Understanding the different types of valves, such as gate, butterfly, check, and air release valves, is crucial for effective system operation and maintenance. A regular exercising program is mandatory to ensure reliability.

Valves are the control mechanisms of the water distribution system. They are installed to isolate sections of the system for repairs, regulate flow and pressure, prevent backflow, and release entrapped air. Without valves, a single main break would require shutting down the entire distribution system.

Gate Valves

Gate valves are the workhorses of water distribution. They are primarily used as isolation valves, meaning they are designed to be either completely open or completely closed. Operating a gate valve in a partially open position (throttling) can cause the gate to vibrate and chatter, leading to premature wear and failure.

Resilient Wedge vs. Slide Gate

There are two main types of gate valves found in distribution systems:

1. Slide Gate (Double-Disc) Valves: These older style valves use two metal discs that are wedged against metal seats when closed. Over time, debris can accumulate in the seat pocket at the bottom of the valve, preventing it from closing completely.
2. Resilient Wedge Valves: The modern standard. The gate is coated in a resilient elastomer (like rubber). When closed, the rubber compresses against the smooth interior of the valve body to create a tight seal. Because there is no pocket at the bottom for debris to collect, they are much less prone to leakage and are the preferred choice for new installations.

Butterfly Valves

Butterfly valves use a disc that rotates on a shaft to control flow. When fully open, the disc is parallel to the flow; when closed, it is perpendicular.

• Advantages: They are generally easier to operate than large gate valves, require fewer turns to open/close (typically a quarter turn, though geared operators increase the number of turns), and are less susceptible to wear from throttling.
• Applications: They are increasingly replacing gate valves on larger diameter mains (typically 16 inches and larger) because they are more compact and easier to operate under high pressure.

Check Valves

Check valves are automatic directional valves designed to allow water flow in only one direction.

• Function: They prevent reverse flow (backflow) which could contaminate the system or drain reservoirs.
• Locations: Commonly found on the discharge side of pumps to prevent water from flowing backward through the pump when it turns off, and at connections between different pressure zones.

Air Release and Vacuum Valves

Air is a hidden enemy in water distribution. It enters the system through pumps, leaking joints (under negative pressure), or during draining and filling operations.

• Air Release Valves: Installed at the highest points in the distribution system. Air naturally rises to these high points, where it can accumulate and restrict water flow (air binding). Air release valves automatically vent this trapped air.
• Air/Vacuum Valves (Combination Valves): These serve a dual purpose. They release large volumes of air when a main is being filled. More importantly, when a main is being drained (intentionally or due to a break), they allow air to enter the pipe. If air couldn't enter, a vacuum would form, which could cause the pipe to collapse or draw in contaminants from the surrounding soil.

Blow-Off Valves

Blow-off valves are specialized valves installed at low points in the system and at dead-end mains.

• Purpose: They are used to flush the system. At low points, they help remove sediment and debris that settles out of the water. At dead ends, they are crucial for flushing out stagnant water to maintain chlorine residuals and water quality.

Valve Keys and Boxes

Most distribution valves are buried underground. Access is provided through a valve box—a vertical cast-iron pipe extending from the valve to the street surface.

• Valve Key: To operate a buried valve, an operator uses a valve key (a long-handled wrench with a slotted head) that reaches down through the valve box to engage the 2-inch operating nut on top of the valve.

Valve Exercising Programs

A valve that is never operated will eventually freeze in place due to corrosion or mineral buildup, rendering it useless during a main break.

• Frequency: A proactive maintenance program requires exercising every valve in the system on a regular schedule. Critical valves (e.g., near hospitals, major transmission mains) should be exercised annually. Other valves should be exercised at least every 3 to 5 years.
• Direction: Operators must know the opening direction. While most valves open by turning to the left (counter-clockwise), some older systems have right-hand opening valves. Turning a valve the wrong way can snap the stem.
• Number of Turns: The general rule of thumb for gate valves is approximately 3 turns per inch of valve diameter, plus two or three. (For example, a 6-inch valve takes about 20-21 turns). Counting turns helps verify if the valve is fully open or closed, or if it is stuck partially open.

By implementing a strict valve exercising program, utilities ensure that valves will function when needed, significantly reducing water loss and property damage during emergencies.