ARCA Regler GmbH/ARCApedia
Acceptance testing

The testing defined in the regulations to guarantee safety, function and quality of the valve.

Acoustic conversion level

Coefficient for the fraction of the power loss of the control valve that is converted into sound. For a subcritical flow, the acoustic degree of conversion is 10-6 to 10-4, depending on the trim and valve shape.


American National Standards Institute American standards institute that promotes and administers American industrial standards. www.ansi.org


European directive for equipment for correct use in areas at risk of explosions. ATEX guidelines for the valve industry published by the VDMA: Download

Bellows seal

Hermetic sealing on the valve stem against hazardous media by means of a metal or plastic bellows.


Diversion, auxiliary path. A bypass solution is suitable where a measurement directly in the medium is not possible.


Computer Aided Design Computer-aided design and construction, e.g. the graphically interactive generation of a workpiece representation as well as the execution of all calculations and checks required in connection with it. www.cad.de


Computer Aided Engineering Computer-aided planning, design, development and project planning (computer-aided engineering work in the broadest sense. The resulting data goes online directly to the following areas, e.g. CAM).


Computer Aided Manufacturing Computer-aided manufacturing (production in computer-automated manufacturing systems)


Sudden appearance of the gaseous phase (escape of dissolved gas or formation of the vapour phase) in a fluid due to a spontaneous pressure drop. If there are places in fluid streams where the static pressure falls below the temperature-dependent vapour pressure of the fluid, the fluid evaporates. The result is a vapour-filled cavity (cavern, cavitation). The caverns implode with increasing pressure; compression of the vapour results in very high temperatures and pressures.


These describe the ratio between valve position and opening cross-section prescribed by the shape of the control plug. Linear or equal percentage characteristics are customary.

Choked flow

Beyond a certain pressure difference ratio x = (p1-p2)/p1 the mass flow through a valve can no longer rise by means of further reduction of the downstream pressure p2. Sonic velocity then occurs at the narrowest throttling cross-section.

Choked Flow

See choked flow.


Cleaning In Place
“CIP” refers to a (cleaning) process in which the cleaning agents and disinfectants can be circulated into the cleaning loop without having to dismantle the equipment and devices of the plant. Used in particular in food, chemical and pharmaceutical plants.


A compressor is used to compress gases, i.e. the volume of the gas is reduced by compression. With compressors, a distinction is made between two functional principles: turbocompressors (turbo machines) and piston compressors (displacement machines). The former are used with large volumetric flows and the latter with low throughputs. Piston compressors operate according to the displacement principle. They operate cyclically and have low volumetric flows and high pressure ratios.


According to DIN 19226, control or controlling is a process in which a variable, the variable to be controlled (control variable) is continuously measured, compared with another variable (guide variable) and influenced depending on the result of the comparison in the sense of equalisation with the guide variable. The resulting sequence of action takes place in a closed circle, the so-called control loop.

Critical differential pressure ratio

see choked flow.

Cryogenic temperature valve

Special valve for liquid gases at temperatures near absolute zero (-273 °C).

Declaration TR TS 00053 for actuators 811/812/813/MA

Process-related: Barrier to a material flow that acts selectively for different substances. Polymers are mainly used, but also ceramics, metal, glass, etc. Diaphragms must be characterised by good selectivity, high permeate performance (flow), mechanical strength, chemical resistance, etc.

Diaphragm actuator

Pneumatic control actuator in which the pressure chamber is sealed off by a diaphragm. The diaphragm allows (particularly when compared with a piston actuator) a frictionless stroke movement and thus a very uniform actuation movement.


Deutsches Institut für Normung, registered office: Berlin. www.din.de

Downstream pressure increase

For the purposes of sound reduction the exit pressure of the valve is increased by the use of fixed throttling stages (perforated discs and perforated cages) downstream of the valve. Lower pressure differences thus occur within the valve and, in the case of gases, reduced velocities.


European standard.

Explosion protection

In technological plants and areas of the chemical and petrochemical industry where gases, vapours, mists or dusts can form explosive mixtures with air, special precautions must be taken to ensure explosion protection. This is done, among other things, by the use of explosion-proof electrotechnical equipment. Various types of ignition protection are defined for this in corresponding guidelines, such as pressurised encapsulation, potting or intrinsic safety.


Fieldbus Foundation The Fieldbus Foundation is a trading company consisting worldwide of 350 leading companies in the field of process and manufacturing automation. Their main objective is to create an open and neutral environment for the development of a single international and interoperable fieldbus. www.fieldbus.org


A fieldbus is a method that makes it possible to establish digital communication between field devices, including control devices. A variety of fieldbus systems are available, each of which has specific advantages in certain applications. For process control, the protocols of the Profibus and Foundation systems will hold their own on the market in the long run, as they both support a power supply on the network.


The use of flanges is a method of connecting pipe sections leak-tight with one another. The pressing force of two flange discs on the gasket in-between is decisive. The pressing force is usually generated with bolts that are guided through the flange discs. These flanges are usually welded to the pipe. They belong to the pipeline parts (fittings). Such flanges are often cast on valves.

Flow direction

In the case of globe (angle) valves this is predominantly against the closing direction of the throttling plug. In the closing direction the sealing is assisted, but protects from the risk of unstable (pneumatic) actuators and, in particular in the case of fluids, of pressure shocks.

Foundation Fieldbus

Worldwide fieldbus concept, allows the distribution of control to the field, has timestamps for alarms in the field devices, extensive interoperability functions and a unique user layer for advanced functions. www.fieldbus.org


Highway Addressable Remote Transducer Fast addressable direct transmission signal converter. Proven digital field communication variant. Already includes fieldbus technology functions, but the analogue measurement signal is used to transport the digital information. The HART communication protocol has been introduced to the market and is standard with the present-day generation of 4-20 mA devices. www.hart.com


Trade name for nickel alloy with additions of, for example, molybdenum, chromium, manganese, copper, silicon and iron in varying quantities depending on use. Hastelloy alloys provide resistance to high temperatures, acids, chlorine and chlorine compounds, molten fluorides in nuclear reactors, seawater, etc. (Hastelloy = Haynes Steel Alloy, registered trademark of Haynes Int. Inc., Kokoma, USA).

Heating jacket

Pressure-tight enclosure of the valve body and the bonnet, through which steam or heat transfer oil flows. Heating jacket valves are often used in smelting and for polymerising media.

Hygienic valve

A valve for applications in the food and pharmaceutical industries with a special form of housing that counteracts the formation of deposits and enables simple cleaning.


In a control valve the difference between the stroke positions that ensue with the same required stroke signal but with opposing directions of movement.

Instrument air

Dry and oil-free air is indispensable for operation of positioners and other pneumatic components without malfunctions (IEC 770).

Insulating column

Extension of the valve stem used with cryogenic temperature media in order to protect the packing, actuator and positioner from icing up caused by the cold medium, as well as to enable effective insulation.


International Organization for Standardization. Worldwide federation of national standardization institutions from more than 130 countries. The term ISO corresponds to the first three letters of the Greek word isos, which means “equal” in the sense of equality.


Order-driven concept for the timely delivery of goods before further processing.

Kv value

Coefficient for the through-flow capacity of a valve. 1 Kv corresponds to the flow of 1 m³/h water at a differential pressure of 1 bar.

Laminar flow

Characterised by the uniform flow of flow particles on neighbouring paths without turbulence. Laminar flow only occurs in control valves at extremely low flow rates or with high viscosity fluids.


Life Cycle Costs Life cycle costs refer to the entire life cycle of an object and include the costs of investment, installation, energy, operation, maintenance, downtime, environment and disposal.


A leakage is a hole in a technical system through which fluids, gases or solids can unintentionally enter and exit.

Leakage class

In a closed valve there is always some level of leakage flow between seat and plug. The requirement for the sealing function and its testing is established in international standards (EN 60534-4).


The pressure-bearing metallic form of the valve is clad on the side exposed to the medium with a chemically-resistant plastic.

Mounting position

A mounting position with a vertical stem is preferred, because valve actuators cannot then exert any shear forces or bending moments on the axis-symmetrically aligned valve trim and their seals.


Normenarbeitsgemeinschaft für Mess- und Regeltechnik in der chemischen Industrie [Standards Working Group for Measurement and Control Technology in the Chemical Industry], founded in 1949 under this name. According to the current status, the subtitle today is “Interessengemeinschaft Automatisierungstechnik der Prozessindustrie” [Community of interests for Automation Technology in the Process Industry]. NAMUR is an association of users of automation technology in the process industry. It has about 100 member companies from Germany and other European countries such as Spain, Austria, Hungary, Switzerland, Belgium and The Netherlands. It deals with planning and construction, with solutions and systems for the process control and operational control level, with measurement and control device technology as well as with the operation and technical support of process control facilities up to decommissioning. www.namur.de

Nozzle-flapper system

In 1917, the original patent for the nozzle-flapper system was applied for in Sweden by RagnAR CArlstedt, after whom ARCA is named. The nozzle-flapper system forms the basis of the continuous pneumatic control of control valves, which enables the optimal control and metering of gases and fluids in industrial processes. This innovation accelerated the industrial development of the entire industry and paved the way for completely new and significantly faster production processes in large-scale industrial plants.


Dynamic sealing of the valve stem against the external environment.

Parabolic plug

Simplest shape of a control plug that in conjunction with a circular opening (valve seat) forms an annular throttle point.

Perforated plug

Control plug, configured as a perforated cylinder, that slides in a seat ring and according to position opens up more or less holes and thus alters the throttling cross-section.

Piston actuator

Pneumatic actuator that possesses a dynamic piston seal instead of a diaphragm. Is often double-acting and used for long strokes.

Plant characteristic

The characteristic that ensues from the characteristic of the control valve, taking into account the pump characteristic as well as all pipework components. The valve authority is calculated from the plant characteristic.


Profibus user organisation, www.profibus.com

Pressure relief

Elimination of the hydrostatic valve plug forces by means of pressure compensation between the upper and lower sides of the valve plug. Requires complex shaping of the control plug into a piston-cylinder system with a sliding radial seal.

Pressure shock (Joukowski shock)

Sudden pressure rise evoked by the rapid deceleration of a flowing fluid in pipework caused as a result of shutting a valve. The pulse of the flowing mass has a powerful potential for destruction.

Process industry

Term for industrial sectors in whose plants process engineering processes take place, i.e. in which the flows of materials, mass or energy relating to the main process are treated or processed continuously or discontinuously, e.g. in large-scale chemical plants, in plants in the pharmaceutical industry, in steel and cement production, in the food and beverage industry, and in waste incineration plants, foundries, etc.

Process technology

engineering discipline that deals with the technical-industrial implementation of all processes in which substances are changed in terms of structure, property or composition.


Process Field Bus Today, serial fieldbuses are mainly used as communication systems for the exchange of information between automation systems and with the connected decentralised field devices. The PROFIBUS is the leading, universally applicable, open fieldbus system in Europe. Transmission speeds: up to 12 Mbit/s; usually: 0.5 to 4 Mbit/s. www.profibus.com

Quality assurance

A company is only successful in the long run if it is better able than its competitors to offer products and services that sufficiently meet the quality-relevant interests of potential customers. Such interests are, for example: Usability of a product, adherence to delivery deadlines, technical service, value for money, safety, environmental compatibility and resource conservation. The effective satisfaction of these interests requires efficient quality management. In connection with the establishment of a corresponding quality assurance system, there are a number of company-neutral aspects as well as a large number of company-specific ones. The former are laid down in international specifications (ISO 9000 family of standards) and national standards (EN ISO 9000 series), the latter usually in company-specific quality manuals according to ISO 9000.


Generally the ratio of the largest to the smallest controllable mass flow rate. The inherent rangeability corresponds to the ratio of the largest to the smallest through-flow coefficient.


For pneumatic actuators the return of the safety position (spring force) in the event of air supply failure.

Safety position

Valve position prescribed in the event of failure of the actuator energy source: closed, open or paused.


Abbreviation for Safety Integrity Level. Serves to protect the health and well-being of those employed, the environment and of goods. Control valves are essential components in the assessment of process technology plant in this respect.


Downstream throttling stages for noise reduction purposes (see downstream pressure increase).

Split range operation

Distribution of the flow between a large main control valve and a small fine control valve if the rangeability of the large valve is insufficient.

Start-up valve

Additionally installed valve whose prime function is to bring the process operating condition up to speed. It is characterised by extreme operating conditions, but is not subject to constant loading.

Steam-conditioning station

A valve for the combined pressure and temperature reduction of superheated steam by means of simultaneous throttling and injection of cooling water.

Stick-slip effect

The difference in frictional forces during sliding and releasing the radial seals can in conjunction with weakly (i.e. purely statically) designed pneumatic actuators lead to undesirable jerky movements that prevent an exact positioning of the valve stem.

Stuffing box

Module for sealing of the valve stem with the packing as an essential component.

Subcritical pressure ratio

The pressure difference across the throttle point is relatively small so no choked flow or cavitation occurs.

Supercritical pressure ratio

The pressure difference across the throttle point is relatively high and causes choked flow or cavitation.

TA Luft

Abbreviation for “Technische Anleitung zur Reinhaltung der Luft” (Technical Instructions for Maintaining Clean Air). German standard that places increased requirements on the stem sealing function in particular.

Turbine bypass station

Steam-conditioning station which in the event of failure of the turbine assumes its throttling and cooling functions.

Turbulent flow

Unordered non-parallel flow. Flow-conditioned pressure differences are generated within the fluid and thus turbulence is generated.

Two-phase flow

A mixed gaseous and liquid (e.g. wet steam/condensate) flow or a fluid flow with solid components (e.g. cellulose).


The valve controls the flow of materials in pipelines (gas, fluids, solid state flows). Shut-off device for fluids, consisting of a housing with a partition wall with circular passage. This is closed by a plug sitting on a stem, which controls the flow. In addition to valves that can only be completely open or closed (open/close valves), there are control valves for controlled influencing of the flow. Valves are operated by a valve actuator.

Valve authority

A key figure that describes the influence of the valve on the process parameter to be controlled.


Partial conversion of a fluid medium into the gaseous state during the throttling process to a pressure that lies below the vaporisation pressure of the fluid.


Verein Deutscher Ingenieure [Association of German Engineers]. www.vdi.de


Verband Deutscher Maschinen- und Anlagenbau e.V. [German Machine and Plant Manufacturers’ Association]. www.vdma.org

Washing trim

Placeholder (mainly only for welded-in valves) which is introduced in exchange against the internal parts to protect the valve during washing and pickling processes.