V

The term “valve” stems from the Latin “valva”, which means "moving part of a door". In piping technology, a valve is a piping component which controls, directs or mixes the fluid flow by opening, closing, diverting or partially obstructing the passage through itself. Valves are classified according to their basic design type (globe, gate, butterfly, ball or plug and diaphragm valves) and according to the functions they perform, i.e. valves for shut-off and safety (e.g. safety valves, bursting disc safety device), control valves (e.g. control equipment, condensate trap, balancing, distributing and mixing valves) and check or non-return valves. It is also possible to differentiate valves by application, i. e. power station, heating, gas and food-grade valves. Depending on the type of actuation, a differentiation is made between manually operated valves, electrically, pneumatically and hydraulically actuated valves and valves actuated by the fluid handled.

Valve design features

• Globe valve: the obturator or closing element moves in a straight line and, in the seating area, parallel to the flow direction
• Gate valve: the obturator or closing element moves in a straight line and, in the seating area, at right angles to the flow direction
• Ball or plug valve: the obturator or closing element rotates about an axis at right angles to the flow direction and, in the open position, the flow passes through it
• Butterfly valve: the obturator or closing element rotates about an axis at right angles to the flow direction and, in the open position, the flow passes around it
• Diaphragm valve: the flow passage is changed as a result of the deformation of the flexible obturator (diaphragm)
• Strainer

Globe valves

Globe valves are mainly used as shut-off and control valves. They are classified into subgroups according to their body type (straight-way pattern, angle pattern, conventional or Y-valve design), the type of seat/disc interface (soft-seated and metal-seated) and the type of stem passage (e.g. bellows, gland packing, elastomers).

Valve design is governed by various factors. Maintenance-free valves with a low resistance coefficient and low space requirements (compact valves) are suitable for use in building services (e.g. heating, air conditioning, water supply, drinking water, cooling circuits). A commercial design variant is a flanged end soft-seated valve with slanted seat design. See Fig. 1 Valve

Further line connection types include valves with wafer-type bodies and weld end designs. See Fig. 3 Valve

Metal-seated valves are used for higher temperature applications (e.g. in steam boilers, heating and heat transfer systems, pressure vessels). See Fig. 2 Valve

Maintenance-free stem sealing is achieved with metal bellows. See Fig. 2 Valve

In valves with large nominal diameters and high pressures, the forces acting upon the stem are considerable; they can be kept within reasonable limits by using valves with a pilot plug (i.e. for internal pressure equalisation). However, this is only possible if a back pressure can build up in the adjoining piping system (see Obturator).

Lift check valves represent a special type of globe valve. Their function classifies them as check valves and they operate automatically, opening in one direction of flow and closing in the reverse direction of flow. See Fig. 4 Valve

By fitting a lift check valve (foot valve) in a suction line, it is possible to prevent the suction line from emptying after the pump has been stopped. This means that the pump does not have to be re-filled prior to a re-start.

A special lift check valve design is the automatic recirculation valve. It is usually fitted on the discharge side of boiler feed pumps and designed to maintain a minimum flow during low flow operation. During low flow operation, the pump's efficiency is lower and most of the energy consumed
is converted into heat which may result in the partial evaporation of the water volume trapped in the pump. The low water volume and the resultant insufficient water lubrication can lead to damage due to metal-to-metal contact, e.g. at the balance disc (see Axial thrust). The automatic recirculation or minimum flow valve plug moves downwards when the pump's flow rate decreases, opening a bypass in the process through which a defined minimum flow is bypassed back into the inlet tank. The latter helps to dissipate heat, thus preventing evaporation. See Fig. 5 Valve

Other special types of check or non-return valves are ball check valves, safety valves and relief valves. In the case of the last two valve types, a spring- or weight-loaded plug opens when a given overpressure has been reached, and relieves the system. The so-called combined non-return/shut-off valve combines both shut-off and check valve functions.

Gate valves

This type of valve is characterised by low pressure losses because of the straight-line flow path. Gate valves can be built for very large nominal diameters and pressure classes and fluid flow can pass in both directions. The long travel of gate valves make them larger than other valve types.
Gate valves are used as shut-off valves. In order to be used as control valves, gate valves must be specifically designed for this purpose.

Gate valves are classified by their seat design (wedge-type or parallel slide gate valves) and the type of seat/disc interface (metal-seated or soft-seated). Special design variants include block gate, rotary disc and radial gate valves.

Wedge gate valves have either a single solid or a single flexible wedge, or a double disc wedge. They have the following advantages in comparison with parallel slide gate valves: The wedge shape increases sealing forces, resulting in effective sealing even at low pressure differentials. The swift withdrawal of the wedge from its seat helps to avoid a sliding motion and undesirable side effects such as scratches on the seat as a result of foreign particles or seizure. See Figs. 6, 7 Valve

Gate valves of pressure class PN 40 with forged or welded bodies are used in industry, power plants, process engineering and shipbuilding. See Fig. 6 Valve

Slide gate valves include through-conduit-type gate valves and variants with one disc or parallel discs. In order to ensure that parallel slide gate valves provide satisfactory sealing at low operating pressures, it is necessary to include springs (e.g. in the form of elastomers or metal springs) to press the discs against their seats. As a result of the sliding action of the sealing surfaces against one another, a certain self-cleaning effect is achieved.

Soft-seated slide gate valves with one disc are usually called single disc or knife gate valves. They are primarily employed in industry, process engineering and in the waste water sector, with their use being limited by application temperatures. See Fig. 8 Valve

Block-forged gate valves are mainly used in power plants and process engineering because they are engineered to withstand very high pressures (above 600 bar) with their pressure seal bonnets. As their bodies are manufactured from one block, they are called block-forged gate valves. See Fig. 7 Valve

Ball and plug valves

The ball or plug valve is probably the oldest form of shut-off device in piping technology. Bronze or lead ball and plug valves were used by the ancient Romans as early as in the centuries B.C. These valves are mainly used as shut-off valves. Specifically designed variants can be employed as control valves.

As the obturator used in ball and plug valves is rotated within the flow chamber, the overall height is usually low and the maximum actuating distance or travel is small (one quarter turn). If there is no transmission, the position of the actuating lever indicates the position of the valve itself.

Like gate valves, ball and plug valves are characterised by low flow losses. Classification of these valves is performed according to the shape of their obturator or closing element (spherical, conical, cylindrical) and according to their body design (one-piece body, single- or multiple-split body).

Valves with a spherical obturator and cylindrical flow passage are termed ball valves. A frequently used design is the two-piece ball valve which is primarily employed in general industry, in the chemical, petrochemical, pharmaceutical, process engineering, food and paper industries and in power plants.

The obturator (plug, ball) frequently has a floating design. Some ball and plug valve variants are trunnion-mounted and are employed in high differential pressure applications. The seat gaskets are mostly made of a plastic material (e.g. PTFE), but some valves also have metal-to-metal variants. See Fig. 9 Valve

In some applications (e.g. natural gas pipelines), ball and plug valves with very large nominal diameters are used; most applications, however, require small-sized valves. Special types include multiple-way valves. 

Butterfly valves

Butterfly valves are mainly used as shut-off or control valves. They are characterised by their simple design and overall small size. The face-to-face length of butterfly valves with wafer-type design is the shortest of all valve types. As with ball and plug valves, the maximum actuating distance or travel is usually a quarter turn, which makes position indication straight forward.

The flow resistance coefficient of butterfly valves is relatively low, however slightly higher than that of ball and plug valves. They are mainly used in applications where medium-sized to very large nominal diameters are required.

Butterfly valves are differentiated according to the disc's position or the orientation of the sealing surface relative to the axis of rotation. They are classified into centred-disc, offset, double-offset and triple-offset designs. See Fig. 10 Valve

Most centred-disc butterfly valves are soft-seated; their bodies are either lined with or fitted with a liner made from elastomer or plastomer and a metal valve disc. They are suitable for use with many fluids and have a very large range of applications, e.g. in general industry and in energy, heating and air-conditioning, and drinking water systems.

The seat ring of double- and triple-offset butterfly valves is often made of metal or plastomer. This allows their use in higher pressure and temperature ranges and in cryogenic applications, industry (e.g. chemical, sugar and paper industries), geothermal applications, power plants and shipbuilding. The double- and triple-offset design eliminates almost all friction occurring as the disc makes contact with the seat. See Fig. 11 Valve

The offset tilting-disc valve (used as a control mechanism) and swing check valves represent further special types of butterfly valves. Swing check valves are predominantly used when medium-sized and larger nominal diameters are required; lift check valves are employed for smaller nominal diameters. See Figs. 12, 13 Valve

Swing check valves usually can only be dampened by externally fitted devices (which is problematic in the case of pulsating flow). Dual-disc swing check valves are mostly installed in heating and air-conditioning systems and in water supply and treatment installations. See Fig. 12 Valve

Swing check valves employed in power plants, process engineering and industry are designed to handle higher pressure ranges. See Fig. 13 Valve

Diaphragm valves

Diaphragm valves are predominantly used as shut-off valves. Their diaphragms are predominantly made from a (flexible, deformable) elastomer material which is not part of the pressure-retaining body and separates the moving parts from the fluid. They are extremely flexible and allow the valve to be closed before backflow propagates towards the valve. This prevents the valve from slamming shut and helps to mitigate pressure surges. Water supply is the main field of application for diaphragm valves.

Both diaphragm and body can be coated or lined with corrosion-resistant materials. Designs include weir-type diaphragm valves, straight-through diaphragm valves and special designs (pinch valve, diaphragm check valve).

Weir-type diaphragm valves are characterised by the fact that the flow passage is closed by pressing the diaphragm onto a raised weir integrated into the body. They can be used in a wide range of applications including general industry, the food and luxury food industry, the chemical industry, process engineering, building services and systems for drinking and service water. See Fig. 14 Valve

Straight-through diaphragm valves have a similar design. Closing is achieved by pressing the diaphragm onto the body wall; the body is shaped such that a largely free flow passage is provided.

In the case of pinch valves, the flow passage's cross-section, which usually consists of a hose-shaped elastomer component enclosed by the valve body, is manipulated via manual, pneumatic or hydraulic "pinching" motion. The closing element of diaphragm check valves resembles a hollow, conically shaped elastomer diaphragm. See Fig. 15 Valve

Strainers

In a wider sense, strainers also belong to the group of valves. The purpose of a strainer is to separate solid particles from the fluid handled. Metal gauze is generally used as filter material; magnetic filters are employed for ferrous fluids.

The suction strainer (inlet strainer) at the inlet of a centrifugal pump's suction line is a special type of strainer. The suction strainer is perforated with slots and round holes, with a total hole area several times larger than the pipe's cross-section area (see Pressure loss).

Valves from KSB – Cutting-edge technology, first-class quality

The vanes of a centrifugal pump are either permanently fixed to or fitted in the impeller or diffuser so as to allow adjustment. They represent the most important structural element of a pump for the conversion of mechanical power (see Power input) into pump power output (also see Internal and hydraulic efficiency) or the conversion of velocity energy into pressure energy.

The vanes are confined in the direction of flow by the leading edge (inlet edge) and the trailing edge (outlet edge), and, at right angles to the direction of flow, by the hub on the inside (in the case of axial and mixed flow Impellers and diffusers) or by the rear (inner or hub) shroud (in the case of radial and mixed flow impellers), and by the pump casing, or by the outer (suction or front) shroud (in the case of closed impellers).

A vane is called adjustable if during pump assembly it can be inserted, have its pitch angle adjusted and then be fixed in position.

If the vane's pitch angle can be altered while the pump is running, then this vane is called a variable pitch vane (see Impeller blade pitch control). The external shape of the vane is usually given as a circular projection in the meridian section (longitudinal section along the pump's rotation axis).

Various types of impeller vane shapes are used in centrifugal pump engineering. They include the conventional axial, mixed flow and radial vane forms; however, a differentiation between different flow directions (i.e. from the inside to the outside or vice versa) is not made. Mixed flow tubular casing pumps (see Mixed flow pumps) for example, often incorporate onion-type diffusers with mixed flow diffuser vanes traversed by the flow from the inside to the outside at the diffuser entry and from the outside to the inside at the diffuser exit. 

As no normal components of the relative velocity can occur perpendicularly to the vane on the impeller (or, in the case of a diffuser, no normal components of the absolute velocity the vane surfaces represent flow areas consisting of stream lines infinitely close to each other.

With regard to the hydrodynamic flow deflection (see Turbomachinery) the effective shape of a vane can only be determined along a stream line. However, this is often complicated because determining the precise stream line pattern in the impeller and diffuser requires a great deal of effort and is only possible if certain assumptions are made (see CFD). 

The velocity triangles on a stream line at the vane inlet and the vane outlet essentially determine the vane's shape, taking into account the vane's thickness and possible vane cascade reactions. The vane centreline section (with half the vane thickness) between vane inlet and vane outlet is referred to as the median line (see Flow profile). It is frequently created by means of a circular arc (e.g. in the case of a circular arc vane), sometimes by means of a parabolic arc, an S-shaped curve or another analytical curve.

As a general rule, the vane inlet is designed to provide shock-free entry and ensure a vortex-free approach flow (see Vortex flow). Ther inducer represents a well-known exception.

The (impeller) vane outlet angle (β₂) is more or less steep, depending among other factors on the head to be achieved (see Flow profile). On radial impeller vanes, it is usually less than 90° (from 17° to 40° approximately). In this case the vane is called a "backward curved" vane. A "radial end" vane is characterised by a vane angle of 90° and a "forward curved" vane (with extremely high pressure coefficients)by an angle larger than 90°.

The vane thickness is mainly governed by the centrifugal force stresses and the manufacturing method. In the case of profiled propeller vanes, the thickness distribution along the median line in accordance with hydrodynamic factors plays a major role.

The minimum vane thickness is approx. 3 mm for cast iron, 4 mm for cast steel, and in special cases (e.g. inserted or welded-on sheet steel vanes) it is possible to produce even thinner vanes.

The vane angle β_s determines the fluid flow direction in an impeller (also see Vane, Vane cascade). On centrifugal pumps it is defined as the angle between the chord c and the circumferential direction u (see Vane cascade).

A vane cascade consists of regularly arranged vane profiles (see Flow profile) along a straight line (plane vane cascade) or in a circle (cylindrical vane cascade).

The co-axial, cylindrical section through the axial impeller and diffuser of a propeller pump reveals the cylindrical vane cascade of the stage. If this section is projected into a plane, the result is a plane vane cascade of the stage. See Fig. 1 Vane cascade

Different vane cascades can be depicted as one moves from one cylindrical section to the next through the stage of the propeller pump (from radius to radius). See Fig. 2 Vane cascade

The description of the flow across vane cascades is frequently the object of theoretical and experimental investigations on stationary, in most cases plane vane cascades, as well as axial and mixed flow turbomachinery (Fluid mechanics). Such investigations involve two main challenges.

Main tasks of flow description

• Establishing associated flow conditions for a given vane cascade (direct problem).
• Establishing the associated vane cascade for given flow conditions (inverse problem).

While experimental findings are frequently relied on in practice, numerical flow simulation or computational fluid dynamics methods (CFD) have increasingly been applied in recent years.

Vapour pressure (p_D) is a substance- and temperature-dependent gas pressure in a multiphase system. It is necessary to distinguish between vapourisation pressure (pressure where the substance starts to transition to the gaseous state) and saturation pressure (pressure when equilibrium between the phases is established). As the liquid phase ceases to exist, a gas pressure is measured instead of a vapour pressure.

In centrifugal pump technology, vapour pressure refers to the pressure at which the vapour and liquid phases are in equilibrium (see NPSH). The representation of vapour pressure as a function of temperature is known as the boiling point curve. It starts from the triple point (equilibrium condition of the possible phases: vapour, liquid, solid) right up to the so-called critical point (no phase difference between the vapour and liquid phases).

The unit of vapour pressure is the Pascal; the more widely adopted unit in pump technology is the bar. Starting from the triple point condition up to the critical condition, the vapour pressure (p_D) and the density (ρ) of the water vary as a function of the temperature. See Fig. 1 Vapour pressure

Vapour pressure has major significance for various liquids, particularly in chemical pump technology (see Chemical pump).
See Fig. 2 Vapour pressure

A variable pitch vane is a vane which can be turned during operation to change its pitch. It plays an important role in pitch control (see Impeller blade pitch control).

The velocity of a body is defined as the distance it travels in a period of time. In fluid systems, the term flow velocity is also used in this context.

Velocity head is the kinetic portion of the head. In accordance with DIN EN ISO 17769-1, it is the kinetic energy of the fluid handled per unit weight. Alongside pressure head and geodetic height the velocity head is determined in the Bernoulli equation (see Fluid mechanics) as follows:

See Fig. 1 and Fig. 2 Velocity head

The pressure wave propagation velocity is the velocity at which the pressure wave moves in the fluid and is also referred to as the velocity of sound.

In cold, gas-free water in metal pipes it ranges between 1000 and 1400 m/s, depending on the pipe's pressure class. If the fluid contains small proportions of undissolved gases, these values may be lower by a factor of 10 to 20.

Velocity of sound (a), or, to use the correct physical term, pressure wave propagation velocity, is the velocity with which a weak, isentropic (of equal entropy) pressure disturbance is propagated in a fluid or solid object with the following relationship:

Velocity of sound plays an important part in centrifugal pump technology, particularly in the case of surge pressures in piping systems, but also when a mixture of liquid and vapour is pumped (in cavitation conditions). In the latter case, an extremely low velocity of sound of the order of
magnitude of ther absolute velocity (v) can occur (sound limit v/a = 1, compression (shock) wave with sudden and abrupt disappearance of the vapour bubbles).

The low sound velocities in a mixture of liquid and vapour are explained by the fact that considerable changes in density (dρ) of the mixture occur in relation to the change in pressure (dp), because of evaporation or condensation (particularly in the region of low pressure and low vapour contents).

Unexpectedly low sound velocities also occur in mixtures of liquid and gas, such as inside a waste water pump or pulp pump. 

In fluid-filled pipelines the velocity of sound is also influenced by the material of the pipe and the ratio between the pipeline diameter and the wall thickness. The velocity of sound for water in pipelines made of steel, cast iron or concrete is approx. 1000 m/s.

The velocity triangle is the vectorial representation of kinematic movement. A vector is a directional magnitude. This relationship is illustrated in the velocity parallelogram of a liquid particle. See Fig. 1 Velocity triangle

The illustration of the relationships in the form of the velocity triangle, which is often also called a velocity diagram (not to be confused with forces diagram or vane profile) also shows the flow angles. See Fig. 2 Velocity triangle

Flow angles in the velocity triangle

• Angle α between v and positive u direction ("absolute angle")
• Angle β between w and negative u direction ("relative angle")

The position of one velocity triangle in relation to another velocity triangle does not indicate whether the flow velocities plotted on these triangles are in fact situated in the same positions in relation to each other in the actual flow. See Figs. 3 and 4 Velocity triangle

The absolute velocity of a liquid particle may be divided into axial, radial and peripheral components. See Fig. 5 Velocity triangle

The vent valve is a device installed to prevent negative pressure (e.g. as a result of surge pressures) in a piping system. Vent valves are also installed at the highest points in a piping, system downstream of shut-off elements or pumps, and at the highest points in domestic installation systems.

Venting is a necessary process which serves to release accumulations of air from pressurised water pipes and allows a centrifugal pump to be primed with the fluid handled prior to its start-up. To this end, centrifugal pumps in suction head operation are equipped with a vent valve or a manually operated air vent at the pump casing's apex.

Venting systems use water ring pumps or jet pumps (see Eductor-jet pump). They are employed as automatic systems which keep the pump ready for operation and as start-up systems that prime the suction line and the pump just before it is started up.

Waste water pumps, for instance, are continuously vented via an open vent line with a diameter of at least 25 mm. The leakage water flow is allowed for in this arrangement and is led back to the intake chamber or similar.

If centrifugal pumps are installed above the suction-side water level and their suction line is equipped with a foot valve, they are filled either manually by means of a filling funnel at the pump suction nozzle or via a venting system; this is necessary unless the pumps are self-priming (see Priming). 

When calculating the vent time of suction and siphoning lines, the air volume to be drawn off should be established separately for the continuously rising section and the horizontal section of the piping, and the two added together.

The following equations apply:

• For vertical pipes

• For horizontal pipes

The expressions in brackets are combined in a factor f which is plotted in the graph as a function of the geodetic suction lift (H_s.geo) See Fig. 1 Venting

T   Vent time in s

V   Gas-filled volume of vertical or horizontal pipe in m³; the volume within the pump is added to that of the horizontal pipe.

f    Factor See Fig. 1 Venting

Q_s  Flow rate of a vent pump in m³/s

p₁  Pressure in bar at which the venting process of the
     pipe's volume (V) starts

p₂  Pressure in bar at which the respective pipe's volume (V)
     is filled with water

A Venturi nozzle that meets the relevant standards is also referred to as a standard Venturi nozzle.

The Venturi tube is a differential pressure flow meter, which, in terms of its geometric dimensions, surface quality, and installation conditions, meets the requirements of the DIN EN ISO 5167-4 standard.

It consists of a narrowing inlet section, followed by a cylindrical section and a conical divergent section called a diffuser. See Fig. 1 Venturi tube

With the same nominal diameter and diameter ratio, the overall pressure losses of the Venturi tube are smaller than with a standard Venturi nozzle.

For a measurement accuracy that is comparable to other differential pressure flow meters, the lengths of the inlet sections are considerably shorter than with a standard Venturi nozzle.

The flow rate is calculated from the measured differential pressure level and the flow coefficient (C) using the formula for differential pressure measuring instruments.

A vertical pump is a centrifugal pump with a vertical shaft, for example, a condensate pump or a vertical shaft submersible pump. 

See Fig. 1 Vertical pump

A vertical shaft submersible pump is a single-stage submersible pump. The vertical, radially split, seal-less pump set is fitted with a radial impeller or channel impeller.

Vertical shaft submersible pumps are used for a great diversity of applications, ranging from automatic drainage of water from pits (e.g. waste water) to tank-installed versions transporting condensate, heat transfer fluids or chemically aggressive fluids. See Figs. 1, 2 Vertical shaft submersible pump

Vibration of a physical quantity is said to occur when its time profile shows regular or irregular reversal points.

Mechanical vibrations are almost unavoidable with rotating or oscillating machinery, and with flowing fluids. As with all turbomachinery vibrations are also evident in centrifugal pumps and machinery influenced by the fluid handled (see Smooth running). 

Measurable acceleration, displacement and velocities of a mechanical vibration are governed by national and international codes such as DIN ISO 10816; DIN ISO 7919; ISO 5799; DIN 45662; VDI 3839 and API 610. These contain the general values for overall assessment of vibrations in machinery.

There are special codes relating to the effects of mechanical vibrations on humans, for example VDI 2057 and ISO 2631.

Vibrations are important indicators when monitoring the operating behaviour of machines. When selecting and designing machinery and systems, it is important to eliminate foreseeable vibration-induced problems such as resonances (see Critical speed or Unbalance).

The acronym VIK stands for "Verband Industrieller Kraftwerksbetreiber" (German Energy and Power Stations Association), which introduced the VIK recommendation for the technical requirements to be met by three-phase asynchronous motors in Germany in 1975. Its key objectives were to adapt standard three-phase asynchronous motors to the special application conditions in the primary industry, power plants and refineries, and to enable streamlined order processing and parts storage by standardising special equipment.

The VIK requirements for three-phase asynchronous motors apply to:

• Stator winding
• Connection
• Construction and materials
• Bearings
• Bearing lubrication
• Shafts and balancing
• Run-up behaviour
• Motor protection 
• Explosion protection

Viscosity is the property of a fluid, to resist the relative displacement of adjoining layers (internal friction). It is necessary to differentiate between dynamic and kinematic viscosity. The physical definition of viscosity originates in the NEWTONian formulation of shear stress (NEWTONian liquid).

The viscosity curve in the shear stress diagram (τ = f(∂v_x/∂y)) is, therefore, a line through the origin, see Fig. 1 Pulp pumping. All other curves denote non-NEWTONian liquids handled in Pulp pumping applications, whose effects on the operation of centrifugal pumps cannot easily be calculated by the usual methods.

However, in practice it is normal to specify the viscosity-density ratio (kinematic viscosity).

The SI unit of dynamic viscosity is Ns/m² = Pas, the SI unit of kinematic viscosity is m²/s.

The term "dynamic viscosity" is derived from the Greek "dynamis", which means force, since the SI unit contains a unit of force (N). The SI unit for kinematic viscosity, on the other hand, contains only kinematic dimensions for length (m) and time (s).

The dependence of kinematic viscosity ν on temperature t is shown both for water,

see Fig. 1 under Vapour pressure

and for mineral oil distillates; with the axes being partitioned in such a way as to provide straight lines. See Fig. 1 Viscosity

As the temperature increases, almost all liquids become thinner, i.e. their viscosity decreases. See Fig. 2 Viscosity

Dynamic viscosity η can be measured for all liquids using a rotational viscometer, in order to record the viscosity curve, as follows: a cylinder with freely selectable rotational speed rotates in a cylindrical pot filled with the liquid to be tested. Drive torque, circumferential speed, the size of the wetted cylinder surface and the wall clearance in the pot are measured at several rotational speeds. Viscometry also includes other processes, e.g. falling sphere and flow measuring methods.

Effect on the characteristic curves of pumps

The characteristic curves of centrifugal pumps show noticeable effects only at a kinematic viscosity of

ν > 20 ·10 ^–6 m²/s and must be converted using empirically calculated conversion factors only after this limit is reached. The two most well-known techniques are those according to the Standards of the Hydraulic Institute (HI) and according to KSB. Both techniques use diagrams to illustrate the conversion factors, which - though they are used in a similar way - differ in that the KSB technique includes the influencing variables Q , H and ν as well as being clearly influenced by the specific rotational speed ns in addition. The HI technique, see Fig. 3 Viscosity, was measured only at n_s = 15 to 20 rpm and, in this narrow application range, leads to similar results to the KSB technique see Fig. 4 Viscosity, which was measured in the n_s range from 6.5 to 45 rpm and at viscosities of up to ν_z = 4000 · 10^–6 m²/s. The use of the two diagrams is explained by means of the depicted examples.

The capacity Q, the head H and the efficiency η of a single-stage centrifugal pump, which are known for operation with water (subscript W), can now be converted for operation with a viscous liquid (subscript Z) as follows:

The f factors are called k in the HI technique; both are illustrated graphically in Figures 3 and 4; in Fig. 4 Viscosity, in addition, the pump rotational speed n must be read in and the specific rotational speed n_s of the pump impeller must be known.

With these factors, the operating data known for water mode can then be converted for liquids of a higher viscosity; the conversion applies in the load range:

0.8 Q_opt < Q < 1.2 Q_opt

Thus simplified at three flow rates 0.8 and 1.0 and 1.2 Q_opt, with the only exception:

Q = 0.8 Q_opt For H_z = 1.03 • f_H • H_W

At flow rate Q = 0, put simply:

H_z = H_w and ŋ_z =ŋ_w = 0

A calculation scheme simplifies the conversion. See Fig. 5 Viscosity

After the power at the three flow rates (see load range equation) has also been calculated according to

all characteristic curves can then each be plotted from either 4 or 3 calculated points over Q_z. See Fig. 6 Viscosity

If the terms of reference are reversed so that it is not the water values that are given, but the data for operation with a higher-viscosity liquid (e. g. to determine a suitable pump for a required duty point), the water values are first estimated and the solution is then approximated using the conversion factors f_Q , f_H and f_η iteratively in two (or, if necessary, three) steps.
Above a specific rotational speed of n_s ≈ 20 rpm, the better adapted KSB calculation technique leads to lower drive ratings; below this limit the drive ratings calculated according to HI are too small!

Effect on the characteristic curves of the system

Since all hydrodynamic laws remain valid without restriction for NEWTONian liquids all the calculation formulae and diagrams for the pipe friction coefficients and for the loss coefficients in valves also continue to apply. It is only when calculating the REYNOLDS number Re = v · d/ν that, instead of the kinematic viscosity ν_w of water, ν_z, that of the respective higher-viscosity liquid must now be used, resulting in a lower Re number and consequently a greater pipe friction coefficient λ_z (see also head losses).

The term "visualisation" refers to the graphical representation of data or relationships provided by a database. It is used for controlling and monitoring automated machines and systems (also see communications system). 

Electronic displays (such as monitors or graphic displays) are used to represent the information. This means, for example, that machine operator stations can take the form of text/graphical displays or PC-based solutions.

Within the context of flow process experiments, the flow can be visualised by adding dyestuffs or tracer particles to the fluid.

A voltage converter is a device that converts a specific voltage into a different one, whereby the following distinctions are made depending on the type of input and output voltage (direct current or alternating current):

• Inverter (DC/AC converter, e.g. for generating mains voltage in vehicles or supplying solar power to the grid)
• DC/DC converter (e.g. switching controller for DC/DC conversion)
• Transformers (AC/AC conversion, identical frequency, e.g. for adapting devices to different mains voltages)
• Switched-mode power supplies (AC/DC conversion)

The term voltage drop as used in electrical engineering describes a difference in potential between two points of an energised electrical resistor. This corresponds to the loss of voltage, across the length of an electrical line, between the terminals of the power supply and the electrical load. The extent of the voltage drop depends on the electrical resistance encountered and, thus, the diameter of the line.

See Fig. 1 Voltage drop

The volume flow coefficient is a characteristic coefficient and is also referred to as the flow coefficient ϕ .

The term volume flow rate describes the volume of a fluid flowing through a cross-section over a specific period of time (also see Flow rate).

The volute casing is designed to guide the flow out of the impeller, in order to convert the fluid flow's kinetic energy into static pressure it serves to collect the fluid discharged from the impeller and route it to the discharge nozzle (also see Pump casing).

The volute casing pump is the most common type of centrifugal pump. Its characteristic feature is the volute-shaped pump casing which is typical for single-stage centrifugal pumps.

There are designs whose main dimensions are standardised in accordance with EN 733, ISO 2858 and ISO 5199, but this standardisation still leaves ample room for many design variants. See Figs. 1 to 6, Volute casing pump 

The volute casing pump is usually a single-stage design, but sometimes incorporates two stages (see Multistage pump).
See Fig. 2 Pipeline pump

Both single-suction and double-suction variants are common (see Double-suction pump). Impellers can be of either radial or mixed flow type, as defined by the specific speeds of approx. 12 to 70 rpm (in individual cases up to 100 rpm and higher).

The axial thrust can be balanced by 

• Thrust bearings
• Balancing holes in the impeller shroud, often combined with a second sealing clearance on the impeller side
  See Figs. 1 and 2 Volute casing pump
  See Figs 7, 8 and 10 Axial thrust
• Back vanes See Fig. 9 Axial thrust
• Impellers in back-to-back arrangement on double-suction or two-stage volute casing pumps
  See Figs. 3 and 4 Axial thrust
  See Figs. 18 and 19 Impeller

The pump casing can be

• Radially split relative to the shaft (see Radially split design)
  See Fig. 1, 2, 4, 5 Volute casing pump
• Axially split at shaft centreline (see Axially split design)
  See Fig. 3 Volute casing pump

A volute casing pump can be a vertical or horizontal pump and the pump shaft can be supported by rolling element bearings or plain bearings on one side only or on either side of the impeller.

On a horizontal volute casing with an overhung impeller, the bearing unit is integrated:

• In the drive
  see Fig. 2 Volute casing pump (also see Closed-coupled pump) 
• In the bearing bracket
  see Figs. 1, 4 Volute casing pump
• In the support bracket
  see Fig. 5 Volute casing pump 

The bearing bracket is designed for a pump in back pull-out design. The support bracket has the advantage of being able to directly transmit the radial and axial forces from the impeller via the shaft to the pump foundation and enables smaller baseplates to be used.

The pump discharge nozzle can be arranged either tangentially on the volute casing or radially in the shaft plane through the provision of a "gooseneck". It can be arranged at the top, bottom or sides of the volute casing, providing it does not interfere with the pump feet on the casing.
The pump suction nozzle  of volute casing pumps with an overhung impeller (cantilever pumps) is frequently arranged axially, and either radially or tangentially in the case of in-line pumps and volute casing pumps in between-bearings design.

Instead of radial or mixed flow impellers, special impellers such as single-channel or multi-channel impellers can be fitted if required. A diffuser is sometimes fitted between the impeller and volute of large volute casing pumps to improve the pump efficiency and balance the radial thrust.

A double volute, i.e. two axially symmetrical volutes that are offset by 180° but normally have a common discharge nozzle, also serves to balance the radial thrust.
See Fig. 6 Volute casing pump

Depending on the fluid handled or the degree of maintenance required, a variety of different shaft seals can be fitted. It is also possible to heat or cool the volute casing pump, e.g. when used for chemical processes. See Fig. 12 Pump casing

Volute casing pumps are designated according to their

• Drive, e.g. canned motor pump or closed-coupled pump
• Application, e.g. water supply pump, marine pump, chemical pump or fire-fighting pump
• Fluid handled, e.g. waste water and sewage pump, pulp pump or heat transfer pump
• Volute casing material, e.g. plastic or concrete volute casing pump

The term volute casing pump can therefore only express one of several aspects.

In fluid mechanics, vortices are the circular flows of a fluid and form as a result of different velocities. They develop if one part of the fluid flows notably faster than the other, a situation which occurs in areas of flow separation or when two fluids with different flow velocities meet.

The vortex flow exhibits rotational symmetry (see Fluid mechanics) with tangential components. For vortex flow in pipes, flow velocity often comprises both tangential and axial components. Particles carried in the flow thus move along helical paths, and compared with vortex-free flow, they have to negotiate a longer path along the pipe wall for a given length of piping at the same flow velocity resulting in a higher pressure loss.

The tangential component (v_u) at radius (r) in a vortex flow usually follows the law applicable to potential flow,

and the flow is then known as a potential vortex flow. The above relationship is however not valid for very small radii, as in this case the tangential component of the vortex flow would have to increase to infinity as the radius decreases. In fact, a vortex core is present at the centre of a vortex flow below a given limit diameter (Rankine vortex), in which the axial component is notably smaller in the area of the potential vortex, while the tangential component (vu) of flow velocity follows the law for solid body vortices:

The static pressure in a vortex flow increases with the radius. A vortex flow is always present at the outlet of a rotating impeller and the tangential components of this vortex flow can be partially converted into static pressure by suitable elements arranged downstream (e.g. diffuser).

In a vortex flow encompassed within solid walls in a pipe or annular chamber, the intensity of the swirl decreases slowly along its travel path (as the distance increases) due to wall friction.

Undesirable vortex flow in pipes, e.g. upstream of a measuring point or in the approach flow to a pump (see Inlet conditions) can be suppressed by fitting flow straighteners, for example of the cruciform flat plate type.

Vortex flow pump is another term for torque flow pump. The latter must not be confused with the peripheral pump, which is sometimes erroneously referred to as vortex flow pump.