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When choosing a valve to control water release from a reservoir, many factors must be taken into consideration. To make the best decision about the type of outlet valve to use, it is helpful to understand . . .

1. The Role of Today’s Outlet Valve
2. Factors that Influence the Type of Valve to Install
3. Three Common Outlet Valves in Use Today

THE ROLE OF TODAY’S OUTLET VALVE

Historically, outlet valves were installed near the base of a dam to provide a way to sluice sediment or release water in an emergency. Their size and type were largely dictated by the valve technology available at the time, and their mode of operation was predominantly wide open or fully closed. The modern outlet valve’s role has expanded to include facilitating fish migration, restoring downstream habitats and providing recreational flows such as those needed for whitewater rafting.

With droughts in the western U.S. occurring more frequently and for longer durations, outlet valves are under increased scrutiny as they may represent the only way to release water from the lower elevations of a drought-impacted reservoir. In many cases, releases need to be quantity- and time-specific, requiring the outlet valve to operate in a partially open position. Unfortunately, the vintage wedge gate valves installed in many existing outlets were not designed to operate in a partially open position against a high unbalanced head and would often cavitate. Because of this, new types of outlet valves were conceived in the mid-1900s, including several innovative designs by the U.S. Department of Interior’s Bureau of Reclamation.

FACTORS THAT INLFUENCE THE TYPE OF VALVE TO INSTALL

Factors that influence the type of outlet valve to use include maximum operating head, frequency of flow modulation, valve location, tailrace conditions, materials and types of construction, and method of actuation. The range of flow required typically does not factor into the type of valve to use, but can influence valve size. Larger valves often have a minimum open setting, below which the flow rate accuracy can vary. If low flow accuracy is important, bifurcating the outlet and providing a second, smaller trim valve to deliver the lower flows could be considered.

Knowing the maximum operating head is important because certain types of outlet valves are head-limited due to metal galling potential at high pressures. The frequency of modulation, or how often the valve position needs to be adjusted, is also important. In the majority of cases, the rate of change of flow is in units of weeks or months. Providing constant modulation (every few minutes) can eliminate certain types of valves for use as outlet valves.

Valve location within the outlet conduit, and relative to the tailrace elevation, is an important consideration when selecting an outlet valve. Most are designed to operate in free discharge mode where water discharges from the valve directly into the atmosphere. In some cases, the outlet valve is located in a conduit some distance upstream from the physical outlet. This type of installation also eliminates certain types of valves from consideration. Likewise, a partially submerged discharge can impart asymmetrical stresses on the valve, while a fully submerged one can lead to cavitation.

Tailrace conditions also can dictate the type of outlet valve to use. Certain types of outlet valves have highly energetic discharges that would quickly scour an erodible tailrace. These types of valves are best-suited for rock-lined tailraces or elevated discharge, where the energy is dissipated in air. Other outlet valves are designed to dissipate the discharge energy within the valve itself.

At a minimum, the parts of an outlet valve that are exposed to flowing water, the wetted parts, should be stainless steel construction. Parts that are not exposed to the water may be carbon steel coated with epoxy or other corrosion-resistant coating. In submerged or otherwise inaccessible environments, an all-stainless-steel valve may be warranted. It may also be more cost-effective to fabricate smaller valves from solid stainless due to the additional welding required to clad a wetted parts valve.

Outlet valves can be cast or fabricated. Larger valves are often fabricated due to limited large mold availability. One advantage of a fabricated valve is that it can be custom-designed to meet a specific operating pressure. This can appreciably reduce the valve weight and associated cost.

Finally, the method used to operate the outlet valve is a key consideration. Most outlet valves are electrically or hydraulically actuated. Electrically operated valves are typically limited to above-water locations, whereas a hydraulically actuated valve may be submerged if necessary.

THREE COMMON OUTLET VALVES IN USE TODAY

Three types of outlet valves in use today are:

• Fixed Cone (or Howell-Bunger) valve (FCV),
• Jet Flow Gate,
• Throttling Knife Gate

All three share the ability to minimize cavitation. However, each possesses unique features that should be carefully evaluated when specifying an outlet valve.

1. The Fixed Cone Valve

The FCV was conceived by Reclamation engineers Howell and Bunger in the 1930s. Its design has since been commercialized and adopted by a number of manufacturers worldwide. The FCV provides a way to release large volumes of high-pressure water from the end of a conduit, with the additional benefits of minimal operating force due to its balanced design (especially important for large, high-pressure applications) and minimal cavitation potential. If allowed to discharge radially into free air, an FCV is also an excellent energy-dissipating valve. However, because many tailraces are constrained due to nearby structures (e.g., powerhouses or substations), manufacturers began to supply hoods on FCVs to concentrate the discharge into a compact hollow jet. This solved the problem of overspray, but created increased energy of discharge. The latter led to the development of the bafflehooded FCV.

A fixed cone valve is an excellent energy dissipating valve if the discharge is unconstrained. A baffled hood can reduce the energy of discharge from a fixed cone valve by as much as 90%. The inset photo shows the discharge from a non-baffled hood, with the baffled hood visible on the valve on the right.

2. The Jet Flow Gate

The jet flow gate was also developed by Reclamation around the 1950s. The jet flow gate valve is essentially a slide gate (or in high head applications, a roller gate) that slides against lubricated bronze (or in some cases, greaseless composite) guides. Upstream of the gate, a floating, beveled bronze seat creates a jet discharge that is regulated by the gate movement. This reverse seat arrangement reduces the force of the gate moving against a fixed seat and allows for much higher operating pressures. Because it has a highly energetic discharge, it is ideally suited for installations where the tailrace is non-erodible.

The energetic discharge from a jet flow gate valve is ideally suited for non-erodible tailraces such as this rock-lined canyon in southern California.

 3. The Throttling Knife Gate

The throttling knife gate valve was developed by Hilton Valves in the 1970s as an economical, non-cavitating, low level outlet valve designed to operate at partial openings. This allows the valve to provide the variable, regulated flows in operating heads of less than 200 feet. This qualifies it for use at the majority of dams in the U.S. The throttling knife gate valve may be installed as a free discharge valve or, if necessary, upstream of the outlet discharge. In the latter case, a vent must be provided to prevent cavitation downstream of the gate.

A throttling knife gate valve is an economical low-level outlet valve for many projects.

Today’s modern outlet valve is required to do more than sluice sediment or release water in an emergency. There are several types of valves available, each with their own advantages and disadvantages. Careful investigation and evaluation can ensure the proper selection.