Hydro Turbine and Hydro Turbine Governor Terminology

Case RMC

  • The following terms are used on this website, or they are commonly used in general discussions regarding Hydraulic Turbine Governor Systems.
  • This list of definitions is provided for reference only.
  • Note that some of the terms may have different meanings in other industries. We do not attempt to address such instances.
  • We welcome comments, questions, corrections, discussions and requests for additions.
  • Please use the form on the Contact Page to send your input to us.

Hydro Related Terms and Definitions:


Alternating Current.


A steel vessel or series of steel vessels used for hydraulic system energy storage and pressure equalization (i.e., pulsation dampening), whereby pressurized hydraulic oil is admitted, retained, and released as dictated by the varying system pressure. Accumulators are sized and selected to provide sufficient oil to Servomotors upon shutdown and loss of hydraulic pump. The main types of Accumulatorss used in Hydro Turbine Governor Control Systems are Bladder, Piston and Air-Over-Oil (aka – Pressure Tank).
Also see Bladder Accumulator, Piston Accumulator and Pressure Tank.

Accumulator Bank:

Several Accumulators Plumbed and Mounted Together.

Accumulator Pre-Charge Pressure:

Accumulator Pressure when there is no oil in the Accumulator. Pre-Charge is accomplished by admitting compressed nitrogen gas into the Accumulator before being placed into service. After being placed into service, the Pre-Charge Pressure can be measured by isolating the Accumulator from the rest of the Hydraulic Circuit and removing all of the oil from the Accumulator. If necessary, the Pre-Charge Pressure must be restored to the specified value before reconnecting the Accumulator to the rest of the Hydraulic Circuit.


Analog Input.


Also Known As.


American National Standards Institute.


Analog Output.


American Society of Mechanical Engineers.


ASME – Boiler and Pressure Vessel Code.

ASME B31.1:

ASME Code for Pressure Piping – Power Piping.

ASME B31.3:

ASME Code for Pressure Piping – Process Piping.


Rules for Construction of Pressure Vessels.


ASME Performance Test Code – Speed-Governing Systems for Hydraulic Turbine-Generator Units.

ASME Y14.5M:

Geometric Dimensioning and Tolerancing (GD&T). Formerly ANSI Y14.5M.


Units of pressure, equal to 100 kilopascals or approximately 14.5 psi.

Beta Ratio:

  • Beta Ratio is a measure of the Particle Capture Efficiency of a filter element (also known as the Filtration Ratio).
  • Per ISO 16889, if X = Particle Size:
    Oil Filter - Beta Ratio - X
  • For example, if 4,000 particles, 10 microns and larger, were counted upstream of the test filter (before) and 1,000 particles, 10 microns and larger, were counted downstream of the test filter (after), the β would be:
    Oil Filter - Beta Ratio - 10
  • The Particle Capture Efficiency is defined as follows:
    Oil Filter - Efficiency - X
  • So for our previous example:
    Oil Filter - Efficiency - 10
  • Therefore, the filter is 75% efficient in removing particles 10 microns and larger.
  • Contrast the Beta Ratio filtration rating system of ISO 16889 with the Oil Cleanliness Code of ISO 4406.
  • Also see Micron Rating – Oil Filter.

Bladder Accumulator:

See the figure below. A hydraulic Bladder Accumulator consists of a gas section (inside the bladder) and a fluid section (outside of the bladder). The flexible rubber bladder acts as a gas-proof barrier. The fluid around the bladder (hydraulic oil) is connected to the external hydraulic circuit. When the hydraulic pressure external to the Accumulator is higher than the pressure inside the accumulator, oil from the hydraulic circuit is forced into the accumulator, compressing the gas and thereby storing energy (in the compressed gas) and also storing a reserve volume of oil around the bladder. When the hydraulic pressure external to the accumulator drops below the pressure inside the accumulator, the compressed gas forces oil from the accumulator and the gas expands as the oil exits, until the pressure inside the accumulator and the pressure external to the accumulator are equalized. A Poppet Valve, normally held open by spring pressure, prevents the bladder from extruding out of the fluid port.
Also see Accumulator, Piston Accumulator and Pressure Tank.

Bladder Accumulator

Bladder Accumulator


  1. A Bushing is a Spool Valve component. It’s a hollow cylindrical hydraulic valve part with oil ports to receive system pressure and also ports to deliver the metered fluid. The internal diameter of the Bushing is sized to accommodate a metering Spool. The total flow area of the output metering ports is sized to deliver a given maximum flow for the conditions of a particular application.
  2. The Bushing is also sometimes referred to as a Sleeve.

Capacitor Bank:

Several Capacitor Bottles Plumbed and Mounted Together.

Capacitor Bottle:

A steel vessel or series of steel vessels used exclusively for high pressure gas storage, used in conjunction with other accumulator vessels.


Flow – Cubic Feet per Second – generally water flow.

Charging Kit:

A high pressure hose, pressure gauge and valving assembly used to charge Bladder Accumulators and measure Accumulator Pre-Charge Pressure.


Flow – Cubic Meters per Second – generally water flow.

Control Ring :

  1. See figure below. The Control Ring is connected by linkages to all of the Wicket Gates. When the Control Ring is rotated by the Servomotors, each individual Wicket Gate is rotated on its own axis to vary the total water flow area available, thereby controlling the flow of water into the turbine.
  2. Also known as Shifting Ring and Operating Ring.

Wicket Gate Operating Mechanisms

Wicket Gate Operating Mechanisms


  1. See figure below. A Controlet is the same device as a Valve Servomotor, except Controlet is usually just used in reference to the Valve Servomotor of a Vertical Distributing Valve that has both the Opening and Closing Pistons located together on the same side of the Valve Spool.
  2. Sometimes Controlet is used to refer to just the case, or metal housing, of the Valve Servomotor, including the Hydraulic Porting.
  3. See also Valve Servomotor.

Valve Servomotor-Controlet

Valve Servomotor-Controlet


Direct Current.


A Deflector, is typically fitted at the discharge of each Impulse Turbine Nozzle and it’s function is to cut into the water stream exiting the Nozzle, preventing the water from reaching the Buckets of the Impulse Turbine Runner. The Deflector is a quick-response water control device, whereas the Needle Valve is a slower acting water control device.
See also Hydraulic Turbine – Pelton Style Turbine.


Discrete Input.

Digital Governor Control System:

A PLC based Digital Controller that includes Governor functions as well as other functions that may be considered Unit Control or Plant Control functions by traditional definitions of Hydro Turbine Governor Systems.

Directional Valve:

A Discrete-Acting Hydraulic Valve (i.e. bang-bang valve), commonly solenoid actuated. May also be manually operated by hand, pilot operated by oil pressure, or by various other methods. Used to operate an auxiliary actuator, or other device, by connecting it to system pressure and drain, as required by the particular application.

Distributing Valve:

  1. A Distributing Valve is in most cases a large Spool Valve with Valve Servomotor combination that is hydraulically pilot-operated by a smaller Hydraulic Valve. Control pressure from the smaller Hydraulic Valve, or Pilot Stage, acts on the Valve Servomotor in order to position the Main Spool Valve as required, so that it delivers the required oil flow to the Final Stage (Gate Servomotor or other Main Servomotor). In other words, the Second Stage provides Hydraulic Amplification between the Pilot Stage and the Final Stage Servomotor.
  2. A Hydraulic Valve, typically Two Stage, with the Second Stage being a Hydraulically Piloted and Infinitely Positionable Spool Valve. It works as a Hydraulic Flow Amplifier and is commonly used to position (by metering oil flow) main Hydro Turbine Control Devices (Wicket Gates, Runner Blades, Nozzles, Deflectors, etc.).
  3. Commonly used to indicate the just the second, or main, stage of a Two-Stage Spool Valve used for positioning the final Servomotor in Hydro Turbine Governor Systems, with provisions for limiting the travel distance of the Second Stage Spool (Limiting the Second Stage Spool travel also limits the flow rate of the metered fluid, thereby limiting the travel speed of the final Servomotor). Provisions for connecting to pressurized oil supply, drain and Servomotor control lines are also incorporated into the Distributing Valve Assembly. Distributing Valve is also a term used to refer to the entire valve assembly, including the First (Pilot) Stage, Second (Main) Stage, and any included Position Feedback or other devices.
  4. Common synonymous references include Relay Valve or Second-Stage Valve.


With respect to Hydraulic Distributing Valve operation, Dither is a small-amplitude, controlled oscillation of the Hydraulic Distributing Valve Spool. Dither is employed so the Distributing Valve Spool is continuously in motion. This ensures that it is always influenced by the effects of Dynamic Friction, rather than the more significant effects of Static Friction. Without Dither, the Control System could behave erratically. Dither is induced by superimposing a small sine wave signal on top of the Control Current Signal. The resulting Dither can be observed as an oscillation in the LVDT Position Feedback Signal.

Draft Tube:

A Draft Tube collects the water discharged by the Runner in Reaction-Type Turbines and delivers the water to the Tail Water, or Tail Race.

Duty Cycle – Governor Oil Pump:

The Governor Oil Pump Duty Cycle is the fraction of time that a Governor Oil Pump/Motor is in an active, or running, state relative to the entire Governor Oil Pump Cycle Time i.e. (On Time) ÷ (On Time + Off Time).

Echelon Oil Pump Control, Auto:

A Duplex Oil Pump control scheme whereby the Oil Pumps automatically alternate the Lead Pump and Lag Pump duties. This is commonly done to even the wear over time on the two (2) Oil Pumps Assemblies.

Echelon Oil Pump Control, Lead-Lag:

A Duplex Oil Pump control scheme whereby pumps are staged to start at two different pressures, the Lead Pressure – higher pressure and Lag Pressure – lower pressure. The two (2) Governor Oil Pumps are generally turned off at the same pressure, the Maximum Normal System Pressure. The Lead Pump, which starts at the higher pressure is often sufficient to restore the Maximum Normal System Pressure by itself. The Lead Pump therefore assumes the majority of the operating time and wear. The Lag Pump is turned on at a lower pressure and it is needed less often than the Lead Pump, so it accumulates less operating time and wear.


Feedforward is a function that feeds the Process Setpoint directly through the Integrator Term of a PID Controller to allow Setpoint changes that are independent of the Controller Tuning.


See Pilot Control Manifold.


  1. Commonly Water Flow available to the Hydraulic Turbine. The dimensions are volume/time. Typical units are Cubic Feet per Second (CFS), or Cubic Meters per Second (CMS).
  2. Also commonly Oil Flow, as in Oil Pump Discharge Flow, or Maximum Servomotor Flow. Typical units are Gallons per Minute (gpm), Liters per Minute (lpm), Cubic Inches per Second, Cubic Centimeters per Second, etc.


The Hydro Turbine Gateshaft Wicket Gate Operating System is a design commonly seen on Hydro Turbine units produced primarily in the first half of the 20th century. The Control Ring of a Gateshaft-Design Francis Turbine is not connected directly to the Servomotor. The Wicket Gate Control Ring is rotated by links connected to a large Gateshaft. The Gateshaft reaches some distance, often to a different floor of the powerhouse, where it is rotationally actuated by a Servomotor, through a lever affixed to the Gateshaft.

Gateshaft Governor:

Hydro Turbine Governor Systems designed and produced to control Hydro Turbines with Gateshaft Wicket Gate Operating Systems.

Generator Rotor:

Rotating assembly of an Electric Generator, Alternator or Motor. See also Hydraulic Turbine Runner.


A Hydraulic Turbine Governor, or Hydro Turbine Governor, is a system that controls the turbine. The Governor Controls the rotational speed of the turbine and/or the electrical power load supplied by the turbine’s generator. The Hydro Turbine Governor Controls the flow of water supplied to the Hydro Turbine Runner, in order to control the unit’s speed and/or load.


Units of flow, gallons per minute. Typically used to quantify oil flow.

Guide Vane:

In some parts of the world, Guide Vane is a term used in place of the term Wicket Gate. Guide Vane is also occasionally used in conversation to mean Stay Vane.


Head is a measurement of the Difference in Elevation between the Head Water Reservoir and the Hydraulic Turbine – a distance dimension. Typical units are feet or meters.


The Hydraulic Pressure Supply System (HPSS), is comprised of the Hydraulic Power Unit (HPU), Pressure Tank(s), valves, and piping connecting the various parts of the Governing System.


The Hydraulic Power Unit (HPU), is comprised of the Oil Pumps, the means for driving them, the Oil Sump Tank, and the associated accessory devices.

Hydraulic Turbine:

The Hydraulic Turbine, or Hydro Turbine, is a rotating machine that extracts power from moving water. There are Two (2) Main Types of Hydraulic Turbines: Reaction Turbines and Impulse Turbines. There are a few different Styles of Hydraulic Turbines (Francis, Kaplan, Pelton). In general, each Style is either a Reaction-Type Turbine or an Impulse-Type Turbine.

Hydraulic Turbine – Reaction Type Turbine:

  • A Reaction-Type Turbine extracts power through the combined action of pressure and moving water. The Runner is placed directly in the water stream, with the water flowing over the blades (or buckets or vanes) rather than striking each one individually.
  • Reaction Turbines are generally used in applications with Lower Head and Higher Flows than applications suitable for Impulse Turbines.

Hydraulic Turbine – Impulse Type Turbine:

An Impulse-Type Turbine uses the momentum of flowing water to turn the Runner. Both the momentum exchange and the water discharge occur at atmospheric pressure. One (1) or more water streams are directed at Buckets (or similar features) on the rotating Runner (see figure below). The Impulse-Type Turbine is generally suitable for High Head, Low Flow applications.

Impulse Turbine Runner

Impulse Turbine Runner

Hydraulic Turbine – Bulb Style Turbine:

A Bulb Style Turbine is a Reaction-Type and is essentially a Kaplan Style Turbine with a horizontal shaft. The generator and the shaft between the turbine and generator are housed in a steel case, which is submerged in the flowing water. The Runner is external to the steel case and typically located on the downstream end of the case. In this configuration Bulb Turbine resembles a submarine.

Hydraulic Turbine – Francis Style Turbine:

  • A Francis Style Turbine is a Reaction-Type that was developed by James B. Francis in Lowell, Massachusetts.
  • The Francis Style Turbine has a Runner with Fixed Vanes and only one (1) type of water control device, the Wicket Gate (See figures below). The water is introduced to the Turbine Runner at its outer diameter. The Turbine Runner is surrounded by Wicket Gates, which pivot on their long-axes to vary the flow area open into the Turbine Runner. Once the water reaches the Turbine Runner, it flows across the Fixed Vanes, radially inward toward the center of the Runner, where it is discharged.
  • Typically, a Penstock delivers the water to the Scroll Case which distributes the water evenly around the Turbine Runner at the input of the Wicket Gates. The Wicket Gates, which control the flow of water into the Turbine Runner, are rotated by a Wicket Gate Operating System which includes Servomotors, a Control Ring, and links and levers, as shown in the figure below. After exiting the center of the Turbine Runner, the water flows down into the Draft Tube, and then on to the Tail Race.

Wicket Gate Operating Mechanisms

Wicket Gate Operating Mechanisms

Francis Turbine Runner

Francis Turbine Runner

Hydraulic Turbine – Kaplan Style Turbine:

  • A Kaplan Style Turbine is a Reaction-Type Turbine developed by the Austrian professor Viktor Kaplan.
  • The Kaplan Style Turbine is a Propeller Style Turbine with rotating, or Variable Pitch, Runner Blades and is suited to Lower-Head applications where the Head Varies Significantly. The rotating-blade design allows the Kaplan Style Turbine to adapt to changing Head levels and operate continuously at the best possible efficiency.
  • The Kaplan Style Turbine has two (2) different types of water control devices, Wicket Gates and Runner Blades.

Hydraulic Turbine – Pelton Style Turbine:

  • The Pelton Style Turbine is an Impulse-Type Turbine, invented by Lester Allan Pelton and produced by the Pelton Water Wheel Company of San Francisco, CA.
  • The Pelton Style Turbine (aka – Pelton Wheel) uses one (1) or more Water Nozzles, with the flow impinging on several Buckets on the rotating Runner. Two (2) different types of water controlling devices are generally employed in the Pelton Turbine design. The two (2) different types are, Needle Valves in each Water Nozzle, which essentially control the flow of the water, and Deflectors, which determine how much of the water flow is allowed to impinge upon the Buckets.
  • Sometimes Pelton (Brand and Style) is used to refer to Impulse-Type Turbines in general.

Hydraulic Turbine – Propeller Style Turbine:

  • A Propeller Style Turbine is a Reaction-Type Turbine very similar in configuration and operation to a Francis Style Turbine, except the Runner has Blades much like a boat’s propeller, as the name suggests.
  • The Runner Blades can be fixed, or they can rotate (Variable Pitch). Typically if the Runner Blades are Fixed, the machine is referred to as a Propeller Style Turbine. If the Runner Blades Rotate, the turbine is generally referred to as a Kaplan Style Turbine.
  • The Propeller Style Turbine with Fixed Blades has only one (1) type of water control device, which is the Wicket Gate (same as Francis Style Turbine). The Fixed-Blade Propeller Style Turbine is suited to Lower Head than the Francis Style Turbine. The Head must be relatively constant in order for the Fixed-Blade Propeller Style Turbine to work well. For Lower-Head applications where the Head varies significantly, the Rotating-Blade Design of the Kaplan Style Turbine may be more suited.

Hydraulic Turbine – Pump Style Turbine:

  • A Pump Style Turbine is a Reaction-Type Turbine which is similar in configuration to the Francis Style Turbine. The Pump Turbine operates in both directions (it’s reversible). The Runner operates as a Pump and the Generator operates as a Motor.
  • During off-peak nighttime hours, when electrical power demand is lower and the cost is therefore lower, the generator operates as a motor, rotating in the opposite direction. This forces the turbine to rotate in the opposite direction also and function as a Water Pump. While the cost of electrical power is lower, water is pumped from the Tail-Water to the Head-Water Reservoir and Stored.
  • During peak daytime hours, when electrical power demand is higher and the price is therefore higher, the water is allowed to flow back down the Penstock and Generate Power in the traditional manner.
  • The Operating Cycle described above for the Pump Style Turbine is known as Pumped-Storage Hydroelectricity (PSH).

Hydraulic Turbine Runner:

The Hydraulic Turbine Runner is the rotating component of Hydraulic Turbine which extracts mechanical power from moving water. See figures below showing a Francis Style Runner and an Impulse-Type Runner.

Francis Turbine Runner

Francis Turbine Runner

Impulse Turbine Runner

Impulse Turbine Runner

Hydro Turbine:

  • Hydro Turbine is short for Hydraulic Turbine.
  • Sometimes “Hydro Turbine” is preferred over “Hydraulic Turbine” so as to avoid confusion regarding the two (2) related, but different, uses of the word “Hydraulic”.
  • For Example, Hydraulic Turbines (Water Turbines) are operated by Hydraulic Systems (Pressurized Oil Systems).

Hydroelectric Turbine:

A Hydroelectric Turbine is a Hydraulic Turbine used in an Electrical Power Generation application.


Inside Diameter.


International Electrotechnical Commission (IEC) – International Standards and Conformity Assessment for all Electrical, Electronic and Related Technologies.

IEC 60308:

Hydraulic Turbines – Testing of Control Systems.

IEC 61362:

Guide to specification of Hydraulic Turbine Governing Systems.


Institute of Electrical and Electronics Engineers.

IEEE Std 1207:

IEEE Guide for the Application of Turbine Governing Systems for Hydroelectric Generating Units.

IEEE Std 125:

IEEE Recommended Practice for Preparation of Equipment Specifications for Speed-Governing of Hydraulic Turbines Intended to Drive Electric Generators.

IEEE Std C37.2:

IEEE Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.

Inlet Valve:

  • Known as Turbine Inlet Valve, Main Inlet Valve, or Turbine Shutoff Valve it is typically a water isolation valve at the transition between the Penstock and the Scroll Case.
  • The Inlet Valve allows the turbine to be dewatered without draining the Penstock.
  • The Inlet Valve is commonly a Spherical Valve, but other types of valves are also used.


  • Input/Output (IO) refers to a hardware based Input or Output point related to the Governor Digital Controller.
  • For example, an Analog IO Point refers to the Analog Outputs or Analog Inputs installed in the Governor Digital Controller.


International Standards Organization.

ISO 4401:

ISO Standard: Hydraulic Fluid PowerFour-Port Directional Control ValvesMounting Surfaces.


Units of flow, liters per minute. Typically used to quantify oil flow.


  • An LVDT is a Linear Variable Differential Transformer, which is a Linear Position Feedback Device.
  • An LVDT is commonly used to provide Position Feedback for the Distributing Valve Spool, or Plunger.
  • In other words, LVDTs are commonly used to provide the Linear Position Feedback Signal for the Second (2nd) Stage of Hydro Turbine Governor Distributing Valves.


Milliamperes (units of electrical current).


Micron is a unit of length equal to 1*(10)^(-6) meter, or 1 Micrometer.

Micron Rating – Oil Filter:

  • Oil Filter Elements typically have a Micron Rating. If an oil filter has a 10 Micron Rating, it generally means that the filter element is capable of removing particles of that are sized 10 Micron and Larger.
  • However, a 10 Micron Rating Does Not Mean that All particles that are sized 10 Micron and Larger Will be Removed by the oil filter.
  • Refer to Beta Ratio and Oil Cleanliness Code.


  • An MLDT is a Magnetostrictive Linear Displacement Transducer, which is a Linear Position Feedback Device.
  • An MLDT is commonly used to provide Position Feedback for the Servomotor, or Final Stage in Hydro Turbine Governor Control Systems.


A Magnetic Pickup (MPU) is a type of Speed Sensor which is installed facing the outside diameter of a rotating gear made of Ferromagnetic Material. As each gear tooth passes the MPU tip, it cuts through the magnetic field and this creates an AC voltage sine wave in the coil of the MPU. The sine wave magnitude and frequency are directly related to the speed of the rotating gear. The frequency of this sine wave is an accurate indicator of the machine’s rotational speed.

Nozzle-Needle Valve:

  • The Nozzle-Needle Valve is a water flow controlling device of an Impulse-Type Turbine. One (1) or more Nozzle/Needle Valve assemblies are generally employed in Impulse-Type Turbine designs.
  • The Nozzle focuses and directs the water flow so that it impinges on the Buckets of the Impulse Turbine Runner.
  • The Needle Valve adjusts the flow of the water as required. Sometimes the Needle Valve is also referred to as a Spear Valve.
  • Also see Pelton Style Turbine.


Operations and Maintenance.


Outside Diameter.


Original Equipment Manufacturer.

Oil Cleanliness Code:

  • Per ISO 4406, Oil Cleanliness Levels are defined by three (3) numbers. The numbers represent and indicate the following particle sizes and quantities present in a one (1) milliliter (1 ml) sample of oil:
  1. The quantity of four (4) Micron and Larger Particles
  2. The quantity of six (6) Micron and Larger Particles.
  3. And the quantity of 14 Micron and Larger Particles.

ISO 18/16/13 Cleanliness Level – Per ISO 4406

ISO 18/16/13 Cleanliness Level – Per ISO 4406

Oil Filter – In-Line:

A filter arranged such that the full flow of an oil line is routed through the filter. Commonly used in Governor Oil Pump discharge lines, but can be used on other oil lines.

Oil Filter – Kidney Loop:

  • A Low Pressure Filtration System which removes oil from the reservoir, filters it, and returns it to the reservoir, independent of the main Hydraulic Circuit.
  • A filter can also be placed in the reservoir return line of a Pump Unloader Valve to accomplish this type of Oil Filtration.

Operating Ring:

Same as Control Ring.


A Penstock is a pipeline or conduit which transports water from the Head Water Reservoir to a Hydraulic Turbine. For High-Head Hydro Turbines, with remote reservoirs, the Penstocks can be quite lengthy. For Hydro Turbines installed in Hydroelectric Dams, the length of the Penstocks can be relatively short.


  • PID = Proportional, Integral and Derivative.
  • PID is a type of Control Algorithm commonly used in Hydro Turbine Governor Control Systems.

Pilot Control Manifold:

  • A Pilot Control Manifold is a Pilot Valve, or Pilot Stage, which typically operates a Larger Valve using Hydraulic Pilot Pressure. In this case the larger valve is a Hydro Turbine Governor Distributing Valve.
  • The assembly containing the Pilot Valve is commonly referred to as a Pilot Control Manifold, or just Pilot Manifold.
  • A Pilot Control Manifold is a Hydraulic Assembly typically consisting of a Manifold Block, an Electrohydraulic Proportional Valve (or similar device), Solenoid-Actuated Shutdown Circuit, and Test Connections, used to Hydraulically Pilot-Operate the Second Stage (Main Stage) of a Hydro Turbine Governor Distributing Valve.
  • Commonly, the Proportional Valve (Pilot Valve) receives an electrical current (or voltage) command signal as an input and provides hydraulic fluid flow as an output. This output flow serves to position the Second Stage of the Distributing Valve, which in-turn provides an Amplified Hydraulic Fluid Flow to control and position the Final Stage (Servomotor).
  • The same physical assembly can function as a Pilot Control Manifold and also function as a Servo Control Manifold.

Pilot Manifold:

See Pilot Control Manifold.

Pilot Stage:

See Pilot Control Manifold.

Pilot Valve:

  • A Pilot Valve is typically a smaller valve which operates a larger valve, or other device, by applying Hydraulic Pressure, in order to position or otherwise operate the larger device.
  • See Pilot Control Manifold.

Piston Accumulator:

  • See figure below.
  • A Piston Accumulator is a Cylinder equipped with a Piston to act as a barrier which separates the two (2) working fluids, Hydraulic Oil and Nitrogen Gas.
  • Also see Accumulator, Bladder Accumulator and Pressure Tank.

Piston Accumulator

Piston Accumulator


PLC = Programmable Logic Controller.


Same as Spool.

Positive Fixed Displacement Pump:

A Positive Fixed Displacement Pump is a Hydraulic Pump that causes a fluid to move by trapping a fixed (not variable) volume of fluid on the low pressure (suction) side, compressing the trapped volume and expelling it at high pressure at the Pump Outlet (Pump Discharge). Volumetric Flow increases in proportion to shaft speed (and corresponding Gear, Lobe, Screw, or Vane rotation). The Pump Outlet Pressure is dictated by activity in the Hydraulic Circuit that is receiving the Pump’s Discharge.

Pressure Tank:

  • In the context of Hydro Turbine Governor Hydraulic Systems, a Pressure Tank is an Air-Over-Oil Accumulator with no barrier between the oil and the air.
  • The Pressure Tank is often times a custom, special design, optimized to service a particular Hydro Turbine size and design type.
  • A Head of Compressed Air is maintained above the oil. Air lost from the system through leakage, dissolution into the oil, or by other means is commonly replenished, as required, by an Air Compressor.
  • Also see Accumulator, Bladder Accumulator and Piston Accumulator.

Proximity Probe:

A Proximity Probe is a sensor that is used to detect the presence or absence of ferromagnetic materials. This type of probe is commonly used to determine the position of a mechanism component and also for Creep Detection and Speed Sensing.


Units of pressure, pounds per square inch.

Relay Valve:

  • Name used for the Distributing Valve on early Woodward Hydro Turbine Governor Systems, such as the Gateshaft Governors and the Woodward Type “A” Actuator Governors.
  • See Distributing Valve.

Relief Valve:

A Relief Valve is a Pressure-Activated Valve, used to limit the pressure in a system or Pressure Vessel in order to prevent system components from being exposed to pressures in excess of their design limits. When the Relief Valve Set Pressure is reached, excess pressure is relieved by the valve opening, which allows the pressurized fluid (oil) to exit the system and return to the reservoir (or to the atmosphere in the case of an air relief valve). This causes the pressure in the system to drop until the Relief Valve Reseating Pressure is reached, at which point the Relief Valve will close once again.

Runner Blade:

A Runner Blade is Blade on the Runner of a Propeller Style Turbine. Most commonly Runner Blade is used to denote the rotating, Variable Pitch Blades, on the Hub of a Kaplan Style Turbine.

Scroll Case:

  • A Scroll Case makes the transition from the Penstock to the Wicket Gates. The Scroll Case distributes the water evenly around the turbine at the input of the Wicket Gate system. The cross sectional diameter of the Scroll Case water passage decreases as it wraps around the turbine, giving it an appearance similar to the shell of a snail.
  • Scroll Case is also known as Spiral Case

Second-Stage Valve:

See Distributing Valve.


Servo is commonly used as a abbreviated term for Servomotor.

Servo Control Manifold:

  • A Servo Control Manifold is a Single-Stage Electrohydraulic Control Valve assembly that operates the Final Stage Servomotor directly, without the aid of an Intermediate Amplifying Stage (no Second Stage).
  • A Servo Control Manifold commonly consists of a Proportional Valve (or similar device), for metering the fluid, with additional devices for system shutdown and safety. The Servo Control Manifold may also include Flow Control Valves, or similar devices, in order to limit and set the Maximum Servomotor Stroke Rate (set minimum stroke time).
  • The Proportional Valve of the Servo Control Manifold receives an electrical current (or voltage) command signal as an input and provides hydraulic fluid flow as an output. This output flow serves to position the Final Stage Servomotor directly.
  • The Servo Control Manifold has essentially the same function as a “Distributing Valve”, but the Servo Control Manifold has no Intermediate Amplifying Stage.
  • Similar in configuration to a Pilot Control Manifold, except the Servo Control Manifold operates the Final Stage Servomotor directly, while the Pilot Control Manifold operates an Intermediate Amplifying Stage, which in-turn operates the Final Stage Servomotor.
  • The same physical assembly can function as a Pilot Control Manifold and also function as a Servo Control Manifold.


  • In the context of Hydro Turbine Governor Systems, a Servomotor is an actuator, or Hydraulic Cylinder, used to operate Hydro Turbine Wicket Gates, Runner Blades, Needles, Deflectors or other types of water control devices, depending on the requirements of the particular application.
  • Servomotors are commonly used in pairs, attached to a single Control Ring in a Push-Pull Arrangement, to actuate Wicket Gates. Most other water control devices are actuated by a single Servomotor.
  • Typically a Servomotor has a Position Feedback Device and can be positioned at any point along its stroke.

Shifting Ring:

Same as Control Ring.

Single-Stage Valve:

  • A Single-Stage Valve is an Electrohydraulic (or other type) Control Valve with only One (1) Stage, and with No Amplifying Stage.
  • See Servo Control Manifold.


See Bushing.

Spear Valve:

See Nozzle-Needle Valve.

Speed Signal Generator (SSG):

Hydro Turbine unit speed feedback device.

Spiral Case:

See Scroll Case.


  • A Spool is Hydraulic Valve component, located inside the Bushing of a Spool Valve, which is a Fluid Metering Device.
  • The Spool moves axially, as commanded, in the Bushing, thereby directing oil flow through the output ports of the surrounding Bushing or Sleeve.
  • The movement of the Spool can be controlled by Pilot Oil Pressure, Electrical Actuation or by other means.
  • The Spool is also commonly referred to as a Plunger, and less commonly as a Piston.

Spool Valve:

  • See figure below. A Spool Valve has as its main components a Bushing and a Spool. A Spool Valve, can be a Fluid Metering Valve (Distributing Valve), a Directional Valve, or possibly other types of valves.
  • The Spool slides back and forth inside the Bushing, to block and uncover Ports in the Bushing, thereby Metering the Pressurized Fluid, admitted into the center of the Spool, out to the receiving, or driven, device (Servomotor or other device). Sealing is accomplished by a close machine fit between the Spool and Bushing.
  • The Main Stages of most Distributing Valves used in Hydro Turbine Governor systems are Spool Valves. The Proportional Valves, used for Pilot Stages of many Distributing Valves, also utilize the Spool Valve design for Metering Pilot Fluid.

Spool Valve

Simple Spool Valve

Standard Cubic Foot (SCF):

Standard Cubic Foot (SCF) is a Mass-Based Unit of Compressible Gas Measure expressed in terms of Gas Volume at Standard Atmospheric Pressure and Temperature Conditions.

Standard Cubic Foot per Second (SCFS):

Standard Cubic Foot per Second (SCFS) is a unit used to quantify Compressible Gas Flow.

Stay Vane:

Stay Vanes are Fixed Flow-Directing Vanes between the Wicket Gates and Turbine Runner of a Reaction-Type Turbine. Stay Vanes are also often structurally important to the Scroll Case design.

Turbine Shutoff Valve:

See Inlet Valve.

Unloader Valve – Pump:

  • A Pump Unloader Valve is a Hydraulic Device which allows the Oil Pump to Start and Stop in an Unloaded Condition. This means the Pump Discharge Flow is Not Forced into the Main Pressure System while the motor/pump starts, and accelerates to normal operating speed, and also as the motor/pump coasts down to a stop after being switched off. Rather, the oil is allowed to return directly to the reservoir while running in the Unloaded Condition.
  • Starting and Stopping in the Loaded Condition would be much more stressful for the equipment, and in many cases cannot even occur because of physical limitations.
  • Also, refer to Oil Filter – Kidney Loop. Some systems are equipped with Oil Filters in the Unloader Return Lines for Kidney Loop Filtering action. In these systems the motor/pump is not actually turned off when additional oil is not needed in the pressure system, but rather Allowed to Run Continuously in the Unloaded Condition, Filtering the Return Oil, until the pressure system needs oil once again. Then Pump Loading is initiated and the Pump Cycle is repeated.

Valve Servomotor:

  • See figure below. A Valve Servomotor is similar in operation to a Final Stage Servomotor, except this device is acted upon by hydraulic fluid from a Pilot Valve in order to move and position the Spool (Plunger) of a Distributing Valve (Second Stage).
  • The Valve Servomotor is sometimes physically integrated into a single assembly (as in the figure below) with the Second Stage Spool (Plunger).
  • A Valve Servomotor can be an Equal Area Servo or a Differential Servo. An Equal Area Servo has Equal Areas for the pressure to act against on opposing sides. A Differential Servo has Unequal Areas for the pressure to act against on the opposing sides.
  • Also see Controlet.

Valve Servomotor-Controlet

Valve Servomotor-Controlet

Valve Servomotor/Controlet – Equal Area:

See figure below. An Equal-Area Valve Servomotor has Equal Areas for the Pilot Pressure to act against on opposing sides.

Equal-Area Valve Servomotor-Controlet

Equal-Area Valve Servomotor-Controlet

Valve Servomotor/Controlet – Type-A:

See figure below. A Type-A Valve Servomotor is a Differential-Area Valve Servomotor having a Larger Area on the “A”, or Opening, Side for the Pilot Pressure to act against and a Smaller Area on the “B”, or Closing, Side.

Type-A Valve Servomotor-Controlet

Type-A Valve Servomotor-Controlet

Valve Servomotor/Controlet – Type-B:

See figure below. A Type-B Valve Servomotor is a Differential-Area Valve Servomotor having a Larger Area on the “B”, or Closing, Side for the Pilot Pressure to act against and a Smaller Area on the “A”, or Opening, Side.

Type-B Valve Servomotor-Controlet

Type-B Valve Servomotor-Controlet

Wicket Gate:

  • See figure below. The Wicket Gate is a device that controls the flow of water into the Turbine Runner. A Wicket Gate has an air-foil shaped cross-section and it pivots on its long-axis to increase or decrease the area through which water must pass to reach the Runner.
  • A Reaction Type Hydro Turbine Runner is surrounded by Wicket Gates, which pivot in unison to vary the total flow area open into the Runner.
  • Typically, a Penstock delivers the water to the Scroll Case which distributes the water evenly around the turbine at the input of the Wicket Gates.
  • The Wicket Gates are rotated by an Operating System which includes Servomotors, a Control Ring, and links and levers, as shown in the figure below.
  • See also Hydraulic Turbine – Francis Style Turbine.

Wicket Gate Operating Mechanisms

Wicket Gate Operating Mechanisms


A Zero Velocity Pickup (ZVPU) is a Proximity-Type Probe that uses an Active Sensor to detect the presence or absence of ferromagnetic material. This type of probe is commonly used for Detecting Dead Stop, Creep Detection, and for Speed Sensing.

Links to Other Hydro Turbine and Hydro Turbine Governor Terminology, and Related Information: