Valve actuation glossary of terms
The following are valve actuation terms which are often referenced when discussing valves & valve actuators.
Actuator – A mechanical device used to operate a valve using electric, pneumatic or hydraulic means. Often used to automate or remote control of valve operations (opening/closing).
Electric Actuator – An electromechanical mechanism used to actuate (open, close or modulate) a valve. An electric motor and gear train is used to actuate the valve to alleviate the need for manually operated valves which require someone to be in attendance to adjust them.
Pneumatic Actuator – A pneumatic control valve actuator is an air operated mechanical device used to actuate (open, close or modulate) a valve. Air pressure is converted into mechanical force to operate the valve. The mechanical force output can be either rotational, as in actuation of a ball valve, or linear as commonly used in actuation of a diaphragm valve.
Double Acting Pneumatic Actuator – Any pneumatic actuator which uses air to drive the actuator output shaft in both the open and close direction. The air is delivered to one side of a piston drive or a diaphragm while the air contained on the opposing side is spent. Hence operating the system in forward and reverse open/close. Also referred to as “Air-to-Air” operation.
Spring Return Pneumatic Actuator – Any pneumatic actuator that contains a spring element with the capability of returning a valve/actuator system to its normal position (open or closed) upon loss of power (fail safe). As air moves the piston or diaphragm the spring is compressed. When the air supply is discontinued and exhausted, the spring extends and drives the piston or diaphragm in the opposite direction (open or closed).
Fail Open – A spring return in a pneumatic actuator which moves the valve to the open position upon loss of air pressure. May also be referred to as “Air-to-Close” or “Spring-to-Open” operation.
Fail Close – Spring return in a pneumatic actuator which moves the valve to the closed position upon loss of air pressure. May also be referred to as “Air-to-Open” or “Spring-to-Close” operation.
Fail Safe – A characteristic of a system that ensures that the unit will revert to a safe condition upon the loss of power (i.e. Spring Return closing valve under loss of power).
Air Supply Pressure – available air supply at the site to operate a pneumatic actuator
Service Temperature – maximum/minimum temperature of the media/material
Torque – Force required to operate the valve
Media – the type of fluid/material flowing through the valve
Maximum Line Pressure – the maximum media pressure for which the valve will need to close (shutoff)
On/Off service – when valve is being used to cease flow by moving from 100% open to 100% closed
Modulating Service – regulating the flows of media by varying the degree the valve is open or closed
Valve Mounting Bracket – A machined coupling and plate, spool, or length of rectangular tubing designed to adapt an actuator to a valve for mounting. Sometimes referred to as a Valve Mounting Kit.
Ball Valve – A quarter-turn valve which uses a round closure element (ball) with a hole in it centre to control flow through it. The valve is open when the ball’s hole is in line with the flow allowing fluid to flow through it. And closed when it is pivoted 90-degrees by the valve handle to close the hole and stop the flow.
Butterfly Valve – a valve to regulate or prevent flow the flow of a fluid. The closing mechanism is a flat disk that rotates and is either parallel or perpendicular to the flow. Unlike the ball valve, the disk is always positioned within the flow, so it induces a pressure drop, even when fully open.
Gate Valve – A gate valve, also known as a sluice valve, is a valve that opens by lifting a barrier (gate) out of the path of the fluid.
Globe Valve – A linear motion valve and are primarily designed to stop, start and regulate flow. The disk of a Globe valve can be totally removed from the flow path or it can completely close the flow path. Conventional Globe valves may be used for isolation and throttling services.
Diaphragm Valve – (or membrane valves) consists of a valve body with two or more ports, an elastomeric diaphragm, and a “weir or saddle” or seat upon which the diaphragm closes the valve. The valve body may be constructed from plastic, metal, wood or other materials depending on the intended use. They are operated by applying an external force to the diaphragm and are typically used for slurries (where other valve designs might clog) or in hygienic applications.
Double Block and Bleed Valves (DBBV) – a “single valve with two seating surfaces that, in the closed position, provides a seal against pressure from both ends of the valve, with a means of venting/bleeding the cavity between the seating surfaces.” This valve does not provide positive double isolation when only one side is under pressure.
Double Isolation & Bleed Valve (DIB) – a “single valve with two seating surfaces, each of which, in the closed position, provides a seal against pressure from a single source, with a means of venting/bleeding the cavity between the seating surfaces. This feature can be provided in one or in both directions.”
Knife Gate Valve – for plastic solids and low-viscosity liquids, a specialty valve is used to cut through the media.
Pinch Valve – any valve with a flexible elastomer body that can be pinched closed, cutting off flow, using a mechanism or fluid pressure. This valve is often used in slurry and mining applications, as its operation is not affected by solid matter in the media.
Plug Valve – valves with cylindrical or conically tapered “plugs” which can be rotated inside the valve body to control flow through the valve. The plugs in plug valves have one or more hollow passageways going sideways through the plug, so that fluid can flow through the plug when the valve is open.
Control Valve – A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level.
Gearboxes – Used to ensure easier operation of larger valves, particularly ball valves.
Bevel Gearbox – There are two types of bevel gearboxes which include either straight or spiral teeth gears. Straight bevel gears have straight and tapered teeth and are used in applications requiring slow speeds. Spiral bevel gears have curved and oblique teeth and are used in applications requiring high-performance, high speed applications.
Bevel gears are typically constructed from cast iron, aluminum alloy or other steel materials but vary between manufacturers. Gears made from steel materials can be noisy when coming into contact with other gears and also make them prone to wear.
Worm Gearbox – Worm gears are able to withstand high shock loads. They are low in noise level and maintenance-free but are less efficient than other gear types. Worm gears can be used in right angle configuration. The worm gearbox configuration allows the worm to turn the gear with ease; however, the gear cannot turn the worm. The prevention of the gear to move the worm can be used as a braking system. When the worm gearbox is not active, it is held in a locked position.
Worm gears are typically constructed of aluminum, stainless steel and cast iron. The material used varies depending on the manufacturer.
Harmonic Gearing – also known as Strain wave gearing, is a special type of mechanical gear system that can improve certain characteristics compared to traditional gearing systems such as helical gears or planetary gears. The advantages include: no backlash, compactness and light weight, high gear ratios, reconfigurable ratios within a standard housing, good resolution and excellent repeatability when repositioning inertial loads, high torque capability, and coaxial input and output shafts. Strain wave gears are typically used for gear reduction but may also be used to increase rotational speed, or for differential gearing.
Cycle life – The duration for which an actuator will cycle without failure
Duty Cycle – The ratio of on time to off time, usually expressed as a percentage. – “time on” divided by” time off” x 100. Pneumatic actuators have a 100% Duty Cycle. Electric actuators are most commonly 25% Duty Cycle motors. To prevent overheating the electric motor, each 1-minute of operation requires 3-minutes of non-operation. Duty cycles greater than 25% are generally needed only when additional work loads are encountered, such as valve modulation or in actuation of larger butterfly valves.
Limit Switches – A set of electrical contacts activated mechanically at a pre-selected position Open, closed, 50% open etc). Electrical switches which may be applied to either electric or Pneumatic actuators to supply a signal that the valve cycle has been completed. In Electric actuators, the primary limit switches are used to control the open and closed position of the valve. All electric actuators have Open/Close limit switches.
When applied to manual or pneumatically actuated valves, it is most common to provide two switches in a NEMA rated enclosure. Each switch is activated by an adjustable tripping device driven by the actuator or valve system. Normally one switch is adjusted to trip in the open position and one is adjusted to trip in the closed position. Optional additional switches are available to perform other functions (i.e., pump start-up or shutdown).
Manual Override – Any mechanical device (wrench, handwheel or portable actuator) by which an automated valve may be manually operated. Typically engaged when the valve cannot be operated automatically by the mounted actuator.
Pilot Valve – A pilot valve is a small valve that controls a limited-flow control feed to a separate piloted valve. Typically, this valve controls a high pressure or high flow feed.
Positioner – A control accessory attached to a pneumatic valve actuator. The fundamental function of a positioner is to deliver pressurized air to the valve actuator to provide accurate, automatic modulating control of the valve between the open and closed positions by balancing the air supply to the actuator as determined by a varying input signal form an external instrument source.
Rotary Actuator – An actuator that produces a rotary motion or torque for a rotary valve.
Scotch Yoke – A reciprocating motion mechanism, converting the linear motion of a slider into rotational motion, or vice versa. The linear motion is defined by a pin or roller element trapped within a channel in the rotary element. Commonly used in control valve actuators in high-pressure oil and gas pipelines. Also known as a slotted link mechanism.
Solenoid Valve – An electro-magnetically operated valve with a movable element that switches flow as a method of controlling actuator position. Double-acting actuators require a four-way solenoid, while the spring-return actuators require a three-way solenoid to achieve the proper supply-exhaust air flow patterns. Solenoid valves are available in various NEMA ratings.
Transducer – A device that senses a process parameter such as temperature or pressure and converts this information to an electrical or pneumatic signal for use in control systems.
NEMA Rating – A code established for electrical components and enclosures by the National Electric Manufacturers Association. The most commonly used NEMA ratings for electric actuators and electrical accessory enclosures are NEMA 4 and NEMA 4X.
NEMA 1: General-purpose. Protects against dust, light, and indirect splashing but is not dust-tight; primarily prevents contact with live parts; used indoors and under normal atmospheric conditions.
NEMA 2: Drip-tight. Similar to Type 1 but with addition of drip shields; used where condensation may be severe (as in cooling and laundry rooms).
NEMA 3: Weather-resistant. Protects against falling dirt and windblown dust, against weather hazards such as rain, sleet and snow, and is undamaged by the formation of ice. Used outdoors on ship docks, in construction work, and in tunnels and subways.
NEMA 4: Watertight. Used outdoors on ship docks, in dairies, in wastewater treatment plants and breweries
NEMA 4X: Watertight and Dust tight – same as NEMA 4 with addition of corrosion resistance (X).
NEMA 5: Dust-tight. Provided with gaskets or equivalent to exclude dust; used in steel mills and cement plants.
NEMA 7: Certified and labelled for use in areas with specific hazardous conditions: for indoor use in Class I, Groups A, B, C, and D environments as defined in NFPA standards such as the NEC.
NEMA 9: Certified and labelled for use in areas with specific hazardous conditions: for indoor and outdoor use in locations classified as Class II, Groups E, F, or G as defined in NFPA standards such as the NEC.
Safety Integrity Level (SIL) – a system used to quantify and qualify the requirements for a Safety Instrumented System (SIS) to maintain or achieve the safety state. There are 4 SIL levels. With SIL 4 the most dependable and SIL 1 the least. The higher the perceived associated risk, the higher the performance required of the safety system and therefore the higher the SIL rating.
Industry standards have been set to assist operators with qualifying safety performance requirements for hazardous operation.
- IEC 61508 Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems
- IEC 61511 Safety Instrumented Systems for the Process Industry Sector
There are two basic elements associated with this measure:
- Hardware safety integrity: which is typically based upon probabilistic analysis of random hardware failures normally estimated via probability of failure on demand (PFD).
- Systematic safety integrity: due to the diversity of causes of failures this can be hard to estimate as failures that may affect both hardware and software may be introduced during the specification, design, implementation, operational and modification phase of a system.
Safety Instrumented Function (SIF) – a set of equipment implemented as part of an overall risk reduction strategy which is intended to eliminate the likelihood hazards related system function failures. The safety function is put in place to:
- Automatically place an industrial system to a safe state when specified process conditions are violated.
- Maintain/Restart a process in a safe manner when specified process conditions allow (permissive functions); or
- Actively alleviate the effects of an industrial hazard.
A SIF is comprised of sensor(s), logic solver(s), and final element(s). These elements are in place to detect imminent accidents, decide upon necessary actions for the detected accident, and then execute the required actions to bring the process to a safe state as required by the safety integrity level (SIL) of the function.
Safety Instrumented Systems (SIS) – A safety instrumented system consists of an engineered set of hardware and software controls which are especially used on critical process systems with the goal of dramatically reducing the risk of accidents