Hydro Review: Today’s Outlet Valve: Not Your Grandfather’s Sluice Gate

Outlet valve

When choosing a valve to control water release from a reservoir, many factors must be taken into consideration. This article discusses the variety of valves available, consideration for their operation, and advantages and disadvantages based on application.

By Doug Hartsock, P.E.

They go by many names: outlet, low level outlet, sluice outlet, pressure relief outlet, river outlet, bypass outlet, diversion outlet, emergency outlet. But these conduits that release water from a reservoir have one thing in common: a valve for controlling the flow of water.

The 2021 U.S. Hydropower Market Report published by the U.S. Department of Energy forecasts 281 hydro facilities will need to apply for new Federal Energy Regulatory Commission operating licenses between 2020 and 2029. Another 217 projects are in various stages of permitting or design and are projected to go online in the next 10 years. Among the improvements often stipulated by new or renewed operating licenses is the installation or replacement of outlet valves capable of releasing water at specific rates and at specific times to address concerns such as fish migration, habitat restoration and public recreation.

Even if a dam is not included in the nearly 500 projects mentioned above, based on the average U.S. hydro facility age of 63 years,1 chances are good that the original outlet valve is nearing the end of its useful life and will need to be replaced.

To make the best decision about the type of outlet valve to use, it is helpful to understand the role of today’s outlet valve, factors that influence the type of valve to install and three common outlet valves in use today.

The ever-evolving roles of the outlet valve

Historically, outlet valves were installed near the base of a dam (or sometimes in a diversion tunnel around the 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 or kayaking. 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, the above releases are quantity- and time-specific, requiring the outlet valve to operate in a partially open position to provide the necessary flow regime. Unfortunately, the vintage cast iron wedge gate valve installed in many existing outlets was not designed to operate in a partially open position against a high unbalanced head. Difficulties associated with operating gate valves in partially open positions on dam outlets contributed to the understanding of the air-water relationship in hydraulic valves, in particular the detrimental effects of cavitation. As a result of this understanding, 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.

Outlet valve selection

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 it can influence the valve size. Larger valves often have a minimum percent open setting, below which the flow rate accuracy can vary. If low-flow accuracy is important, consideration might be given to bifurcating the outlet and providing a second, smaller trim valve to deliver the lower flows.

Minimum operating head and frequency of operation

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 is also important to consider, i.e., how often will the valve position need to be adjusted? In the majority of cases, the rate of change of flow is in units of weeks or months. Needing to provide constant modulation, e.g., every few minutes, can eliminate certain types of valves for use as outlet valves.

Valve location

Valve location within the outlet conduit, and also relative to the tailrace elevation, is one of the more important factors to consider when selecting an outlet valve. Most are designed to operate in free discharge mode, i.e., 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.

Overlooking the outlet valve location relative to the tailwater elevation can result in significant operational issues. A partially submerged discharge can impart asymmetrical stresses on the valve, while a fully submerged one can lead to cavitation. The latter can be addressed by ensuring the valve is adequately aspirated, and the former by designing the valve robust enough to withstand the stresses.

Tailrace conditions

Interestingly, 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. 

Materials and types of construction

At a minimum, the parts of an outlet valve that are exposed to flowing water should be stainless steel construction. The industry term for this type of valve is a wetted parts valve. The surfaces of a wetted parts valve that are not exposed to the water may be carbon steel coated with an 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 are typically of cast or fabricated metal construction. Larger valves are often fabricated from standard steel shapes due to limited large mold availability. One advantage of a fabricated valve is, unlike many cast valves, it can be custom-designed to meet a specific operating pressure. On large valves, this can appreciably reduce the valve weight and associated cost.

Method of actuation

Finally, the method used to operate the outlet valve is a key consideration. Although manual actuation is possible, 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. To protect the environment, fish-friendly and non-petroleum-based hydraulic fluids are commonly used.

Other valve actuation requirements to consider are stroke speed and emergency actuation. If necessary, a hydraulic actuator can actuate a valve very quickly (although in most cases, slower actuation is desirable to avoid pressure transients). An accumulator filled with pressurized hydraulic fluid can also provide a failsafe means for operating the valve. If the valve is electrically actuated, specifying a direct current motor allows the use of a battery bank to actuate the valve during a power outage.

Three common types of outlet valves

Three types of outlet valves in use today are the fixed cone (or Howell-Bunger) valve (FCV), jet flow gate valve, and throttling knife gate valve. All three share the ability to minimize the formation of cavitation. However, each possesses unique features that should be carefully evaluated when specifying them for use in an outlet.

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.

A fixed cone valve is an excellent energy dissipating valve if the discharge is unconstrained. (credit: Wikipedia Commons)

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 one problem, overspray, but created another one: increased energy of discharge. The latter problem has been thoroughly studied at the Water Research Lab at Utah State University, resulting in development of the baffle-hooded FCV.

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. (credit: DeZURIK, Inc.)

Jet flow gate valve

The jet flow gate valve was also developed by Reclamation around the 1950s as a replacement for failing needle valves. Hoover Dam is a high-profile example of a project where jet flow gate valves have replaced the needle valves.

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 (see Figure 1). Upstream of the gate, a floating, beveled bronze seat creates a jet discharge that is regulated by the gate movement. This novel reverse seat arrangement reduces the force of the gate moving against a fixed seat in a conventional knife gate valve design and allows for much higher operating pressures. The jet flow gate valve is designed to be a non-cavitating valve.

Figure 1. The gate-to-seat arrangement in a jet flow gate valve is reversed from that of a conventional knife gate valve. (credit: DeZURIK, Inc.)

The jet flow gate valve has a highly energetic discharge. Because of this, it is ideally suited for installation at projects 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. (credit: Shimmick Construction)

Throttling knife gate valve

The throttling knife gate valve was developed by Hilton Valves in the 1970s as an economical, non-cavitating, low-level outlet valve. The primary difference between it and a conventional knife gate valve is its ability to operate at partial openings. This allows the valve to provide the variable, regulated flows required by many projects today.

Because the stainless steel gate slides against a metal seat, the throttling knife gate valve is limited to operating heads of less than 200 feet. However, this qualifies it for use at the majority of dams in the U.S.

A throttling knife gate valve is an economical low-level outlet valve for many projects. (credit: Justin Smith)

The throttling knife gate valve may be installed as a free discharge valve (see Figure 2) or, if necessary, upstream of the outlet discharge (see Figure 3). In the latter case, a vent must be provided to prevent cavitation downstream of the gate.

Figure 2. The discharge spool on a throttling knife gate valve operating in free discharge mode should be no greater than one port diameter in length to ensure air can reach the cavitation zone. (credit: DeZURIK, Inc.)
Figure 3. If installed upstream of the outlet discharge, a vent needs to be installed in the discharge pipe immediately downstream of the valve. (credit: DeZURIK, Inc.)


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

Doug Hartsock is an application engineer specializing in Hilton brand valves for hydropower applications for DeZURIK, Inc. Hartsock earned his bachelor’s in mechanical engineering from California Polytechnic State University-San Luis Obispo and has spent the past 20 years working in the hydropower industry.

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