Fluid Control Glossary

Fluid control terminology can be confusing, even for experienced professionals. Moreover, preferred usage sometimes varies according to specific geographic locations, industries and applications. Consult our glossary below for practical explanations of flow and pressure control terms. Feel free to contact us with questions and comments. Disclaimer: While this glossary has been carefully compiled, definitions are based on Equilibar’s interpretation and experience and may at times vary from specific definitions used by others.

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Absolute pressure

Definition of absolute pressure:
Absolute pressure refers to pressure that is measured and expressed in relation to zero pressure rather than atmospheric pressure. It may also be thought of as a pressure measured in relation to absolute vacuum (psia), torr or bar. In contrast, atmospheric pressures are expressed in relation to the pressure of the atmosphere and are referred to as gauge pressures. More information can be found on Science Direct.

Air-loaded valve

What is an air-loaded valve?
An air-loaded valve or regulator is operated by a static air signal applied to the top of a diaphragm or piston to determine the set point. It requires no control spring or pilot.

Back pressure controller

What is a back pressure controller?
Back pressure controller is a term used in some industries and geographic regions for back pressure regulator. See back pressure regulator for a detailed explanation.

Back pressure reducing valve

What is a back pressure reducing valve?
Back pressure reducing valve is a term sometimes used for back pressure regulator. See back pressure regulator for the explanation of this device.

Back pressure regulator (BPR)

What is a back pressure regulator?
A back pressure regulator is a device that maintains an upstream pressure at its own inlet. When fluid pressure exceeds the set point, the regulator opens to relieve excess pressure. Back pressure regulators work similarly to relief valves, but the emphasis is on steady state pressure control instead of on/off actuation. See the Equilibar BPR definition page for more information. Other terms used to describe the same device include back pressure valve, back pressure controller, and back pressure reducing valve.

Back pressure valve

What is a back pressure valve?
Back pressure valve is a term sometimes used for back pressure regulator. For an explanation, see back pressure regulator.

Ball valve

Definition of a ball valve:
A ball valve is a valve used primarily for shut-off service that utilizes a hollow pivoting ball with an orifice so that the valve is open when the ball’s orifice is aligned with flow.

Bernoulli equation

What is the Bernoulli equation?
The Bournoulli equation (or Bournoulli’s principle) describes the various forces that are present and interacting with each other in a moving fluid – in essence, the Bournoulli equation shows that fluid static pressure decreases as flow velocity increases. A detailed explanation can be found on the NASA website.

Butterfly valve

What is a butterfly valve?
A butterfly valve uses a disc or semi-circular plate that pivots to open or close a pipe and thus control flow. As the name suggests, its appearance can resemble a butterfly’s wings. Depending on the exact design, it can be used for shut-off service or throttling service. See this story on the evolution of the Butterfly Valve in Flow Control Magazine.

Bypass valve

Definiton of bypass valve:
Pressure bypass valves serve to control pressure in a system by diverting a portion of the flow. Typically, they bypass fluid from the outlet of a pump back to the reservoir. A pressure bypass valve is essentially a back pressure regulator because both control their inlet or upstream pressure and function by opening just enough to relieve excess pressure on their inlet port. These valves are a class of valve and are referred to by different names based on subtle differences in the application or industry. See the detailed Equilibar bypass valve page for more explanation.

Cavitation

What is cavitation?
Cavitation refers to the formation and collapse of vapor cavities (i.e. bubbles) in a flowing liquid. These occur in areas of low pressure. The instability caused by the creation and collapse of these bubbles can damage surfaces and result in degradation of a pump’s performance.

Centrifugal pump

What is a centrifugal pump?
A centrifugal pump is a device that uses rotating impellers (similar to propellers) to move fluid. Fluid enters the pump and is then expelled via the outlet because of the centrifugal force caused by the impellers’ rotation.

Check valve

Definition of check valve:
A check valve is a valve that prevents backward flow of a fluid. It reacts automatically, closing due to the pressure of the fluid when or if the fluid begins to reverse flow. By closing, the check valve ensures that flow goes in one direction only. There are several varieties of check valve, including swing, lift and angle lift check valves.

Closed-loop control

What is closed-loop control?
A closed loop control system is a set of mechanical or electronic devices that automatically regulate a process variable by using feedback information. By contrast, open loop control systems do not perfectly converge the process variable to the set-point or require human interaction to converge. One of the primary differences between closed-loop control and open-loop control is the use of feedback; therefore, a closed-loop control system is sometimes called a feedback loop. A good explanation can be found in this article in Control Engineering.

Compressible flow

Definition of compressible flow:
Compressibility is a measure of the change in a fluid’s volume due to pressure changes. Liquid processes are generally considered to be incompressible. In addition, gas processes where the gas density changes less than 10% can generally use incompressible equations. When the Mach number is greater than 0.3, density changes require use of compressible equations. Compressible flow is the term used to describe these processes.

Coriolis flow meter

What is a Coriolis flow meter?
A Coriolis flow meter is a type of mass flow meter that introduces a Coriolis force through vibration into the process flow and measures mass flow by interpreting the resulting angular momentum. This type of device offers excellent precision and turndown, and therefore is especially popular for scientific research. An explanation of both the Coriolis force and the history of Coriolis flow meters can be found on the Omega website.

Cv

What does the term Cv refer to?
Cv is used to refer to the flow co-efficient of a valve. See flow co-efficient of a valve.

Density

Definition of density:
In fluid mechanics, the density ( ρ ) of a fluid is defined as the fluid’s mass per unit volume. The density of a fluid in liquid form is usually a given or easily obtained. The density of an ideal gas can be determined from the specific gas constant and the ideal gas law: ρ=P/RT

Derivative control mode

What is derivative control mode?
When using PID (proportional-integral-derivative) control, the derivative mode responds to the rate of change. It offers a type of prediction of error based on the current slope of error. A comprehensive discussion of derivative control can be found on this control blog.

Diaphragm regulator (valve)

What is a diaphragm regulator or diaphragm valve?
A diaphragm regulator or valve is a device that uses a flexible diaphragm to seal against a surface and control the pressure of a fluid. Diaphragm valves are traditionally mechanically actuated against a weir for on/off service. A diaphragm valve is also sometimes called a membrane valve. A more recent technology involves the diaphragm sealing and opening over none, some or all of a field of parallel orifices to allow for instantaneous control and extreme precision. See our How It Works page for more information.

Differential pressure

What is differential pressure?
In essence, differential pressure—DP--is the difference in pressure between two points. There are three primary types of pressure measurements—absolute pressure, which is in reference to zero pressure; gauge pressure, which is in reference to atmospheric pressure; and differential pressure. Engineers often use “psid” to refer to a differential pressure in lb/in2 units.

Dome-loaded regulator (valve)

What is a dome-loaded regulator or dome-loaded valve?
A dome-loaded regulator or valve uses a fluid pressure in its dome (top portion of the valve) to provide a setpoint. In contrast, other types of valves use a spring adjustment to provide setpoint. A sensing element such as a diaphragm or piston separates the process fluid from the dome fluid. Pressure is set by a second regulator referred to as a pilot regulator. The pilot could be either manual or electronic. Dome-loaded regulators are used in applications where traditional spring-loaded designs have poor performance. See this in-depth article in Water Technology Magazine for more information.

Electronic pressure regulator

What is an electronic pressure regulator?
Certain pressure control problems involve factors such as remote control, automation, or data collection that require an electronic device. In the past decade, the fastest growing type of electronic pressure regulator (EPR) has been the servo valve design, which uses two high speed servo or solenoid valves to increase or decrease gas pressure as needed to maintain pressure. These EPRs offer higher pressures and more flexibility and robustness than previous technologies, which were often marketed as electro-pneumatic transducers. Servo EPRs work by using a push valve and a vent valve to maintain the outlet pressure at the desired setpoint. A small internal pressure sensor monitors the output pressure and a digital or analog controller adjusts the timing of the servo valves to maintain the set-point. See our page on How Electronic Pressure Regulators Work for more information.

Flow chemistry

Definition of flow chemistry:
The term flow chemistry, also called continuous flow chemistry, refers to the process of running chemical reactions in a continuously flowing stream through tubular reactors, as opposed to running one batch at a time in a vat or other container. While flow chemistry is not a new technique, it is becoming more and more popular for laboratory and pharmaceutical applications because of its many advantages, which include improved heat transfer, ease of automation, and safety. For more information, see this discussion on the Equilibar Flow Chemistry application page.

Flow coefficient of a valve (Cv)

What is the flow coefficient of a valve (also known as Cv))
Represented by the symbol Cv, the flow Coefficient of a valve is a number used to represent how easily fluid flows through the valve. A low flow coefficient represents high resistance to the passage of flow, while a high flow coefficient means there is little resistance and therefore flow happens easily. In the United States, a Cv value of one is the volume of water at 60 °F that will flow per minute through a valve with a pressure drop of 1 psi across the valve. Kimray has a good video explaining Cv in depth.

Flow control valve

Definition of flow control valve:
In fluid control systems, flow and pressure are related but distinct. Flow is movement of fluids while pressure is force. A flow control valve controls the volume of fluid flowing through it while a pressure control valve controls the amount of force. In the simplest of scenarios, flow rate is set by changing the size of the orifice of a traditional flow control valve. For more challenging applications, traditional flow control valves do not always work and alternatives are sometimes required. See the Equilibar flow control page for more information about innovative flow control valves.

Gate valve

Definition of a gate valve:
As its name implies, a gate valve uses a flat barrier (like a gate or door) that can be moved to either open or block the path of a fluid in a pipeline. It is often either lowered into a seat to seal or raised into an external recess to open. A gate valve is sometimes referred to as a sluice valve.

Gauge pressure

What is gauge pressure?
Gauge pressure refers to pressure that is measured and expressed in relation to atmospheric pressure rather than zero pressure. PSIG refers to pounds per square inch gauges or bar(g), as a gauge would read in the atmosphere. In contrast, absolute pressures are expressed in relation to zero pressure and differential pressure refers to the difference in pressure between two points. Gauge pressure is the most common type of pressure measurement and is sometimes written as gage pressure, reflecting an archaic spelling.

Globe valve

Definition of a globe valve:
A globe valve uses a plug to control or stop the flow of fluid through a pipe. It is suitable for throttling service. Despite similarities in name, globe valves and ball valves are completely different devices. See this in-depth description on corrosionpedia.

Ideal gas law

Definition of the ideal gas law:
The ideal gas law is a formula that describes the relationship of pressure, volume and temperature for a hypothetical “ideal gas.” It is expressed as PV=nRT, where P is gas pressure, V is volume taken up by the gas, T is the temperature of the gas, R is the gas constant and n is the number of moles of gas. While there is no such thing as an ideal gas, many gases are close enough to make the concept a useful approximation. Khan Academy has an easy to follow explanation of ideal gases and the ideal gas law.

Inches water column (in WC)

What does the term inches water column or inWC refer to?
Inches of water column is a unit of pressure mainly used in North America to measure small differences in pressure. 1 inch of water is the pressure exerted by a 1 inch high column of water at a given temperature. It can also be represented as in WC, inAq, inches water gauge, or in H2O. Air and other industrial gasses are often measured in inches of water when at low pressure rather than inches of mercury or pounds per square inch (psi).

Incompressible flow

Definition of incompressible flow:
Incompressible flow refers to a flow in which the density remains constant. It is also referred to as isochoric flow. It is distinct from an incompressible fluid, which describes the material rather than the flow. The densities of liquids, as opposed to gases, are basically constant at a certain temperature; therefore, the flow of liquids is considered incompressible. See this discussion on ResearchGate.

Integral control

What is integral control?
When using PID (proportional-integral-derivative) control, the integral mode accounts for past values of the setpoint to process variable error and integrates them over time. A comprehensive discussion can be found on this control blog.

Laminar flow (laminar regime)

What is laminar flow or laminar regime?
Flow occurs in three basic states—turbulent, transitional and laminar. In simplistic terms, laminar flow, which is also called laminar regime, can be thought of as smooth flow in which all of the fluid flows in layers (laminae) as opposed to turbulent flow, which is chaotic with whirls and other disruptions. Laminar flow tends to occur at lower flow rates and is related to pressure variables. The flow can appear so smooth that it visually looks as if the fluid is a solid or frozen, as demonstrated in popular videos such as this one. The degree to which a flow is laminar or turbulent is described by its Reynolds number. For a more in-depth discussion, see Alicat’s excellent description.

Lobe pump

What is a lobe pump?
A lobe pump is a positive displacement pump that uses rotating lobes that almost meet. The action moves material through the pump. This type of pump is used in a wide range of industries because it has good sanitary qualities, efficiency, corrosion resistance and clean-in-place capability.

Magnetic flow meter

What is a magnetic flow meter?
A magnetic flow meter is a device that uses a sensor and transmitter to measure flow. The sensor is placed in the line of flow and measures the voltage generated by fluid flowing through a pipe. The transmitter converts the voltage into a flow measurement. These types of devices are also called electromagnetic flow meters or magmeters. Emerson explains in depth on their flow measurement page.

Mass flow meter (MFM)

What is a mass flow meter?
A mass flow meter measures the mass flow rate of a fluid moving through a tube. It does this by measuring the force that results from acceleration of mass. In other words, the force is measured from mass moving per unit time, not volume per unit time. There are two primary types—Coriolis mass flow meters and thermal dispersion meters. Mass flow meters are commonly used in chemical processes, gas and oil, pharmaceutical, and wastewater.

Mixed-phase flow

Definition of mixed-phase flow:
See multi-phase flow.

Multi-phase flow

What is multi-phase flow or mixed-phase flow?
Multi-phase or mixed-phase flow is the flow of material that exists in more than one state of matter. Most often it is the flow of liquids and gases together and so is called two-phase flow. Multi-phase flow is a challenging process condition because it introduces complexity and makes it difficult to control pressure. Common applications for multi-phase flow include laboratory reactors for petrochemical catalyst research and fuel cell testing systems involving condensation. See our two-phase flow page for more information.

Needle valve

Definition of needle valve:
A needle valve allows precise flow control by using a port and a narrow pointed rod resembling a needle that can be moved to open and close an orifice. It is generally used for low flow scenarios.

Non-Newtonian fluid

What is a non-Newtonian fluid and why is it important?
When selecting pumps and other equipment that apply shear to fluids, it is important to understand non-Newtonian fluids. These are fluids that do not follow Newton’s law of viscosity. In other words, the viscosity of non-Newtonian fluids can change when under force to become either more liquid or more solid (as demonstrated in the famous Oobleck science experiment). Usually the viscosity of non-Newtonian fluids depends on shear rate. Ketchup is shear thinning. Plaster of Paris is shear thickening. Water is Newtonian (no large effect of viscosity with shear). This engineering blog describes different types of non-Newtonian fluids and what to consider when working with them.

Open-loop control

What is open-loop control?
An open-loop control system relies on a person to make changes if desired results are not being achieved rather than using feedback from the system to automatically make changes as is done in closed-loop control. Open-loop control is sometimes chosen because an experienced operator can respond more quickly than waiting for the system to interpret feedback. See more on this engineering blog

Peristaltic pump

What is a perastalic pump?
A peristaltic pump contains fluid in a flexible tube inside a circular pump casing. A rotor compresses the tube to force fluid through it. As the tube reopens to its natural state, fluid again flows into it. This suction and discharge process is called peristalsis and is used in natural biological systems such as intestines. A peristaltic pump is a type of positive displacement pump, meaning a pump that moves a fluid cyclically with energy added intermittently. It is also called a roller pump.

PID controller

What is a PID controller?
A PID controller refers to a closed control loop that uses proportional, integral, and derivative modes of control to automatically adjust process variables using feedback so that they always equal desired setpoints. By using proportional plus integral plus derivative modes, the control loop is able to respond to three different parameters—the amount of offset, the duration of offset and the rate of change.

Pilot operation

What is pilot operation?
Pilot operation refers to the method of setting the pressure for a dome-loaded regulator or valve by using a second standard valve or regulator called a pilot regulator. (The term dome loaded means that a fluid is fed into the top (dome) area of the regulator to provide the pressure setpoint for the process.) The pilot regulator can be manual or electronic, depending on the application’s requirements. The advantage of pilot operation is that it allows a small and easily operated feed to control a larger or otherwise much more complicated feed. Pilot pressure, setpoint pressure and reference pressure are all synonyms.

Pipe friction

Definition of pipe friction:
When a fluid flows through a pipe, friction between the pipe wall and the fluid causes a loss in pressure or head (height of a fluid column). This translates into a loss of the fluid’s potential energy; therefore, determining this loss is important. A common calculation in fluid controls is the estimate of pipe friction using flow rate, viscosity, density, and pipe roughness. Most of the complexity of this formula comes from estimating the effective friction factor using either the Darcy or Fanning formula. See the Neutrium website for more.

Positive displacement pump

Definition of positive displacement pump:
Pumps convert mechanical energy into fluid energy and are classified according to the way they do this. A positive displacement pump is a machine in which a chamber fills and empties during each cycle—ie each stroke or revolution of a piston, plunger, vane, etc. See our controlling pump slip for positive displacement pumps page for more information.

Pressure reducing regulator (PRR)

What is a pressure reducing regulator (PRR) or valve?
Pressure reducing regulators are the most common type of regulator. They reduce an unregulated inlet pressure to the desired regulated outlet pressure. They open to increase pressure downstream and close to decrease pressure downstream. Back-pressure regulators work the opposite way. Read more about pressure reducing regulators VS back pressure regulators on Equilibar’s Definition of a Back Pressure Regulator page. Note that a pressure reducing regulator is also also referred to as a pressure reducing valve. (PRV)

Pressure reducing valve (PRV)

What is a pressure reducing valve?
See pressure reducing regulator, a term that refers to the same device as a pressure reducing valve.

Pressure sustaining valve

Definition of pressure sustaining valve:
Pressure sustaining valves maintain pressure in a fluid circuit by opening only as much as necessary to maintain the required pressure on their inlet port. The term pressure sustaining valve has essentially the same meaning as back pressure valve or back pressure regulator. See the definition of back pressure regulator for a detailed description.

Proportional control

What is proportional control?
Proportional control is a type of feedback control system that applies an adjustment to the controlled variable that is in proportion to the difference between the desired value (or setpoint) and the measured value or process variable. A toilet bowl float valve is a classic example. In PID (proportional-integral-derivative) control, the proportional mode of control responds to the magnitude of the offset between the desired value and the measured value.

Proportional valve

Definition of proportional valve:
Proportional control valves produce pressure or flow that is in proportion to an electrical signal. For more information see this explanation on machine design.

Pump slip

Definition of pump slip:
Also known as pump runaway, pump slip refers to a loss of pumping capacity when fluid leaks back through a pump. It can affect the efficiency of all types of pumps in different ways. Factors leading to pump slip are differential pressure, viscosity of the fluid involved, and sometimes clearance dimensions. See this useful blog from a UK engineering firm: https://www.michael-smith-engineers.co.uk/resources/useful-info/pump-slip

Reference

What does the term reference mean in fluid control?
When discussing dome-loaded pressure regulators and other fluid control devices, the term reference is used to describe the setpoint pressure for the process. In other words, both reference and setpoint pressure refer to the amount of pressure or vacuum that is desired for a given process.

Relief valve

Definition of relief valve:
A relief valve is a valve used to enable the escape of steam or other fluid in cases of excessive pressure. By allowing some of the steam or fluid out, the build-up of pressure is reduced and explosions or other catastrophes are prevented.

Rotameter

What is a rotameter?
A rotameter is an instrument that consists of a tapered tube (usually glass) that contains a small float for measuring the flow of a fluid by forcing the float upward until its weight is equal to the fluid force. It is in a class of meters called variable area meters.

Tangential flow filtration (TFF)

What is tangential flow filtration, also known as TFF?
Tangential flow filtration (TFF) is used in the biopharmaceutical industry as an efficient method for purifying biomolecules. During TFF, fluid passes across the membrane rather than through it. This decreases clogging problems and makes it well suited for continuous flow processes. In TFF, the differential pressure across a membrane must be maintained within a narrow range. The process is also referred to as cross flow filtration. See the Equilibar tangential flow filtration page for more.

Thixotropic shear thinning

Definition of thixotropic shear thinning
Shear thinning is the Non-Newtonian behavior of fluids in which viscosity lessens due to the force of shearing. Thixotropy is time-dependent shear thinning. In other words, some fluids that are normally thick will become thinner and flow more easily over time when shaken, agitated or shear-stressed. KThey then take a set amount of time to return to a more viscous state. Thixotropic shear thinning is often observed in polymer solutions, complex fluids and suspensions.

Turbulent flow

What is turbulent flow?
Flow occurs in three basic states—turbulent, transitional and laminar. While laminar flow occurs when the fluid flows in smooth layers, turbulent flow occurs when the fluid swirls and mixes chaotically with wakes and vortexes. Constant changes make it difficult to measure flow. It usually occurs at high flow rates or in larger pipes. While challenging, turbulent flow offers the benefit of helping solids remain suspended in a fluid to prevent settling. Turbulent flow is explained in more detail in this informative blog post from Bronkhorst.

Two-phase flow

What is two phase flow?
See the definition of multi-phase flow for more information about two-phase flow or mixed-phase flow.

Vacuum breaker

Definition of vacuum breaker:
There are two major types of vacuum control devices—a vacuum regulator and a vacuum breaker. A vacuum breaker lets ambient air into the vacuum system to control the vacuum pressure. It acts similarly to a pressure reducing regulator because the pressure is controlled at the outlet port. It opens to increase absolute process pressure or reduce vacuum level. It is also known as a vacuum relief regulator.

Vacuum regulator (valve)

What is a vacuum regulator or vacuum valve?
There are two major types of vacuum control devices—a vacuum regulator and a vacuum breaker. A vacuum regulator is a device that maintains a desired vacuum pressure in a system. It works by throttling flow between the vacuum pump and the process in order to control the process vacuum. A vacuum regulator is also known as a vacuum valve by most segments of industry.

Valve Authority

Valve Authority is a concept used to determine the optimal size of a control valve. It describes the correlation between control valve resistance and resistance across the frictional components of an entire system and is expressed as the ratio of differential pressure (dP) across the control valve (dPv) compared to the total dP across all frictional components in a system (dPsys). Mathematically, Valve Authority = dPv / dPsys. Valve Authority is important when designing a system because control problems are likely to occur if a valve is either oversized or undersized. Read more here: New Video, Web Page Focus on Valve Authority & Equilibar Valves -

Viscosity

Definition of viscosity:
Viscosity is a measure of a fluid’s resistance to flow due to an external force such as gravity. Jelly is an example of a fluid that is highly viscous while water is an example of a fluid that is not viscous. Temperature, stress and pressure are all factors that affect the behavior of a fluid in different ways depending on the characteristics of a particular fluid. In general, most viscous fluids flow more easily when temperature is high. There are a variety of equations used to determine viscosity in different circumstances. See this discussion on engineeringtoolbox.

Volumetric flow rate

What is volumetric flow rate?
Also known as volume flow rate or volume velocity, volumetric flow rate is the measure of the volume of fluid that passes through a point in unit time. In equations, it is represented by Q. A straightforward discussion can be found on Khan Academy.

Water hammer

Definition of water hammer:
A pressure surge or wave caused when a fluid is forced to stop or change direction abruptly is called water hammer. Often it is caused by a valve closing. The pressure wave can cause major problems, including pipe rupture. Using devices such as pulsation dampeners can reduce the effects. If you are having issues with water hammer in your Equilibar fluid control valve, please visit our chatter support page. For a detailed explanation of how to analyze water hammer issues, see this video from Mike Crowley with Fluid Mechanics in the UK.