D

Data transmission includes all methods that can be used to transmit information from a sender to a receiver. The process occurs when the sender varies a physical quantity (frequency, voltage) over time and this quantity is measured by the receiver. Transmission can be analog or digital. 

The technical impossibility of shielding the message channel (physical quantity) from outside interference and measuring this physical quantity precisely eventually leads to a loss of data that also cannot be prevented through amplification.

See Fig. 1 Data transmission

Types of data transmission

• Analog: Data is continually related to the physical quantity and values are permitted within a defined interval and are always relevant.
• Digital: Data is related to the physical quantity in binary form (values 0 and 1 are used). Multiple intervals that are not directly consecutive are permitted and are only relevant in specific time intervals that are also not directly consecutive. These intervals must not have any common elements.

Types of digital data transmission

• Serial: Data is transmitted in consecutive bits over a specific line.
• Parallel: Data is transmitted over several physical lines at the same time

DC is the internationally recognised abbreviation for direct current.

Dead time is also referred to as run time or transport time. In control systems, it designates the amount of time that lapses from the point at which the input signal is specified and passes through the controlled system through to the output signal being output. Output signals do not materialise in the absence of dead time.

A system in which dead time is experienced is known as a dead-time element. If substantial dead time occurs, poorer control response can be anticipated since the control parameters are more difficult to set. Dead time is not the same as delay time.

The Declaration of Conformity is a written declaration by the product manufacturer or supplier to confirm that the product meets the basic requirements of the relevant European Directives (also see Declaration of Incorporation). There are no restrictions on issuing a Declaration of Conformity, which means for example that it can be issued in respect of products, processes and sites. Conformity is certified by applying the CE marking.

In accordance with the revised version of the Machinery Directive (2006/42/EC) dated May 2006 a Declaration of Incorporation was introduced as a legally binding replacement for the Manufacturer's Declaration with effect from 29 December 2009. This declaration must include information stating that machinery must not be put into service until all the components have been declared in conformity with the Directive.
Pumps in pull-out design are generally vertically installed tubular casing pumps whose rotating assembly can be easily removed and re-installed.
Sometimes not only the rotating assembly but also the diffuser with the wear ring can be pulled out of the tubular casing without any need to remove the pump casing from the piping and the foundation (also see Back pull-out design).

Deep-well turbine pumps are a type of vertical shaft submersible pump. They are also known as vertical turbine pumps, vertical line shafts pumps and lineshaft turbine pumps. They are borehole pumps, whose pump shaft extends up to ground level, where it is coupled to a dry installed motor (e.g. hollow shaft motor) or gearbox (e.g. hollow shaft gearbox). They are multistage pumps and usually fitted with mixed flow impellers. See Fig. 1 Deep-well turbine pump

Deep-well turbine pumps can be extended in modules (shaft sections). Large installation lengths are produced by means of standard extension sets consisting of an intermediate shaft, an intermediate coupling (e.g. threaded coupling, conical coupling or split muff coupling), a column pipe and a bearing spider.

The suction strainer (also see Valve), foot valve (for installation depths exceeding 10 m), pump and vertical discharge line (riser) are suspended from the motor stool, on which the drive is mounted and which contains the discharge elbow) of the pump. The thrust bearing serves to absorb the axial thrust and support the rotor weight. It is located in the motor stool, hollow shaft motor or hollow shaft gearbox.

The drive shaft transfers the driving torque as well as the axial thrust. It is supported by water-lubricated plain bearings inside the column pipe. If the boreholes or wells are very deep, the drive shaft and intermediate bearings tend to push up the overall cost of the installation (see Economic efficiency), and it becomes more advantageous to install borehole pumps with submersible motors instead.

In the absence of a power grid, deep-well turbine pumps are often driven by combustion engines connected by means of a right-angle gear (e.g. tractors in agricultural applications).

The degree ENGLER is an obsolete unit for quantifying viscosity.

The degree of reaction (r_th), or reaction ratio, is a parameter used for multistage turbomachinery defining the ratio of the static head (see Clearance gap pressure) to the fall head (of turbines) or the pump head (of centrifugal pumps). It is an indication of how the static pressure is distributed between impeller and stage. A degree of reaction of zero means that there is no increase in pressure within the impeller (constant pressure impeller), whereas a degree of reaction of 1 means that the static pressure increase in the stage takes place solely in the impeller.

The degree of reaction can be calculated from the velocity triangles of an impeller element (see fundamental equation under Fluid mechanics) and is given by:

Typical degrees of reaction of centrifugal pumps vary between 0.5 and 1, and can in exceptional cases also be above 1, for example for propeller pumps.

The delta configuration is a starting method, whereby the three phases of a three-phase system are connected in series (also see Three-phase current).

Density (ρ) is a characteristic property for the material of a body that is independent of its shape and size. It describes the ratio of the mass (m) of the body to its volume (V):

For the density of water as a function of the temperature, see in Appendix under Vapour pressure Fig. 1

The density of liquids changes at constant atmospheric pressure as a function of temperature. See Fig. 1 Density

The density of fluids handled defined here is not identical to the pulp consistency, which is an important concept in the cellulose and paper industry.

In contrast to surface or protective layers, deposits are unwanted effects which impinge upon the values necessary for optimum operation of the centrifugal pump. 

Examples of deposits

• By corrosion products
• By salts (e.g. carbonates, sulphates, phosphates) due to a chemical imbalance (see water hardness,  pH value) 
• By precipitation of dissolved salts from the evaporation of water (e.g. on shaft seals
• By colloids (particles dispersed in another substance) or coarsely dispersed substances (which are not, or only slightly, dissolved in or chemically bonded with each other)
• By sand (see Erosion)
• Fouling (e.g. algae, bacteria) in sedimentation, brackish water and seawater tanks

Depreciation is a term used to describe a loss in the value of a company's assets (specifically non-current assets) over time and is generally calculated from a business administration perspective. This loss in value is caused by factors such as ageing, wear and tear or accidental damage (also see Economic efficiency).

For the hydraulic calculation (selection) of a centrifugal pump, in general, the flow rate (Q), the head (H) and the speed (n) are needed. When used with a HQ curve, these ratings make up the design point of the selected centrifugal pump. This point often does not match the true operating point. Reasons for this include: uncertainties related to the system characteristic curve, properties of the fluid handled differing from those used for initial calculation, manufacturing tolerance, system-dependent control of the centrifugal pump, uncertainties related to the calculations and characteristic curves selection chart of series-produced pumps.

The aim often is to match the design point with the operating point at the best efficiency point or suction characteristics. The actual transient flow conditions significantly differing from those assumed for the design point may result in loss of efficiency in low flow operation and high flow operation and in faults due to vibrations and cavitation.

Dewatering pumps are centrifugal pumps installed in a building that is situated below the groundwater level, to reduce the water level and then maintain it at this level.
One example is in underground mining in which water penetrating into the adits is pumped up to the surface. In open-cast mining, the groundwater level is reduced through drainage well trenches until the pit is dry. The groundwater level in construction pits in civil engineering is reduced in a similar way.

In underground mining, the flow rates (Q) can attain up to 900 m³/h and the heads (H) up to 1050 m; in open-cast mining up to 1800 m³/h and up to 400 m. It is technically feasible to go beyond these limits.

Heads over 1000 m require the use of multistage pumps with single-entry impellers facing the same way or in opposite directions, or with double-entry impellers. See Fig. 17, 18 and 19 impellers

As such water often contains solids and suspended solids (the results of erosion) and is often chemically corrosive (see Material selection) impellers with wide channels, shaft protecting sleeves and wear-resistant or corrosion-resistant materials (see Chemical resistance table) are used.

Dewatering pumps installed as horizontal pumps in dry installation usually have no suction head. They are driven by electric motors suitable for operation in potentially explosive atmospheres. Vertical pumps in the form of submersible borehole pumps offer many benefits for automatically operated dewatering systems. The submersible motor is filled with water and positioned beneath the pumps, flooded and thus adequately cooled. Flameproofing is only required for the cable connections.

In open-cast mining, drainage well trenches equipped with submersible borehole pumps are arranged at the edge of the workings and also in the various levels in a staggered arrangement.
The submersible borehole pumps can also be built for high voltages of 3 or 6 kV (in some cases up to 10 kV) as well as for low voltages.

For dewatering or reducing groundwater on large building sites, protection is provided by drainage well trenches equipped with submersible motor pumps in excavations such as for underground railways, bridges or in open-cast mining. Portable submersible motor pumps are often used for pumping out heavily contaminated seepage water in such cases. With the latter type of pump, the motor is arranged above the pump. Their impellers are specially designed to handle waste water (see Waste water pump). A mechanical sealing element protects the motor against the ingress of water. The motor cooling is adequately scaled to enable the pump set to also run when not submerged in water.

A diagonal impeller is also referred to as a helical impeller or mixed flow impeller. See Fig. 1 Diagonal impeller

A dielectric is any poorly conducting or non-conductive and non-metallic substance, in which most of the charge carriers are not freely movable. A liquid, gas or solid may be used as a dielectric. It is typically non-magnetic.

Differential pressure is the measured difference between two pressures, such as between the discharge- and suction nozzles of a pump.

Differential pressure flow meters are differential pressure drop devices in which the flow is calculated by measuring the pressure drop over an obstruction inserted in the flow. Installing one in fully flowed straight piping produces a difference in the static pressures between the positive pressure tap in the inlet and the negative pressure tap at the narrowest cross-section. See Fig. 1: Differential pressure flow meter

The flow rate can be determined by this differential pressure from the material characteristics of the fluid from the geometric data (under operating conditions) and the applicable factor (to be found in standards).

Examples of differential pressure flow meters include orifices (standard orifice in accordance with DIN EN ISO 5167-2), nozzles (standard and Venturi nozzles in accordance with DIN EN ISO 5167-3) and Venturi tubes (DIN EN ISO 5167-4).

Fig. 1 Differential pressure flow meter shows the approximate flow and pressure profiles in a measuring section (using an orifice as an example).

The mass rate of flow with respect to the measured differential pressure is determined by the following equation:

Correspondingly, the volume flow rate (q_v) is calculated using the following equation respective of the density (ρ) of the fluids in relation to the temperature and pressure of the relevant volume:

If greater accuracy is required when measuring the flow rate with a differential pressure flow meter, the inlet flow to the differential pressure flow meter must exhibit a high level of uniformity. This is achieved using long, straight inlet sections or various components in the supply section (e.g. flow straighteners or flow converters in accordance with DIN EN ISO 5167-1). See Fig. 1: Differential pressure flow meter

The differential pressure sensor is a measuring instrument (also see Sensor), that converts the difference between two absolute pressures (p_1abs - p_2abs = Δp) into an electric output variable that is proportionate to this difference. It can consist of two measuring chambers that are hermeticallyseparated by a diaphragm. Displacement of this diaphragm quantifies the differential pressure.

The differential transformer is a special type of transformer and is used in sensors. It comprises one primary and two secondary coils, all of which have a common iron core. The secondary coils are connected in series, thus subtracting the voltage at the connections. In symmetrical configuration, the resulting voltage is exactly null. Displacement of the iron core produces an output voltage in the secondary coils.

The diffuser is a component which effects a reduction in flow velocity and corresponding increase in static pressure whilst causing as small a loss as possible (see also Pressure loss). 

A diffuser's distinguishing characteristic is a flow path with increasing cross-sectional area in the direction of flow, in a closed duct or channel. In centrifugal pump technology, diffusers are frequently used on the discharge side of ring-section and volute casing pumps (see Pump casing), they are also used in multistage centrifugal pumps and as a component in piping runs.

As flow separation at boundary layers is a major risk in the region of the diffuser, defined diffuser angles must not be exceeded. The critical diffuser angle (taper angle) of circular-section diffusers predominantly used in centrifugal pump technology is between 8º and 10º. In the event that diffuser elements are fitted to deal with vortex flow or heavy throttling downstream of the diffuser, these angles can be exceeded without the risk of flow separation.

A diffuser can also be used to connect pipe sections of different nominal diameters. In this case, the diffuser selected must be of adequate length so that the critical diffuser angle is not exceeded and additional pipe friction losses and pulsating flow separation are prevented. Under certain circumstances, a sudden transition from a small to a large pipe cross-section (e.g. a Carnot diffuser) may prove more advantageous than installing a diffuser, both for cost and hydraulic reasons.

The opposite of a diffuser is a nozzle (e.g. standard nozzle or bellmouth), also known as a confuser.

Diffuser elements are hydraulically relevant components built into centrifugal pumps which alter the vortex flow up- or downstream of the impeller. When positioned upstream of the impeller, they are used as pre-swirl control equipment (see Closed-loop control) to increase swirl. Fitted downstream of impellers, they work like diffusers and convert the kinetic swirl energy into pressure. Depending on the pump type, the diffuser elements can be radial, mixed flow or axial diffusers, but also volute or annular casings.

Diffuser vanes are stationary vanes, fitted in the pump casing or diffuser. They convert velocity into pressure energy and guide the fluid handled to the impeller vanes of the subsequent stage.

A digital signal has several information parameters (e.g. 8, 16, 32, or 64) that can be provided consecutively (series signal) or at the same time (parallel signal). The benefits of digital signals include high representation accuracy (dependant only on the number of digits used), easy long-term storage, and the possibility of linking many parameters.

A DIN rail is part of the mounting rail group and consists of a standardised metal profile rail. It has a hat-like U-shaped profile and is used to attach components such as relays, contactors or time switches inside control cabinets, distribution boxes and terminal boxes.

It is also known as a top hat rail.

Direct current is abbreviated internationally as DC and refers to the unidirectional flow of electric charge in theoretical electrical engineering. If fluctuations in current do not have a negative effect on consumers connected to the power source, current with minimal pulsation is also regarded as direct current.

See Fig. 1 Direct current

The direct current face plate starter is an electrical switchgear that starts direct current motors, by changing the ohmic resistance in the armature and excitation circuit. The contact pieces of the step switches have a planar layout. DC face plate starters are being increasingly replaced with electronic power converters such as thyristors.

The direct current motor is an electric motor that uses direct current and comprises a stator and a rotor. Most direct current motors are internal rotor motors.

The armature winding and, thus, the number of windings and poles, as well as the field strength of the excitation winding, characterise the general motor behaviour of conventional direct current motors.

Direct starting is also known as DOL starting and is a starting method for three-phase asynchronous motors.

The direction of rotation is also referred to as the sense of rotation and indicates the direction (clockwise or anti-clockwise) in which bodies rotate around an axis. The direction indicated depends on the reference (e.g. viewing) direction, which must also be specified (typically towards the drive or direction of inflow).

Disc friction (P_Ldiscfric) in centrifugal pump engineering is the friction loss caused by the fluid between the impeller shrouds and the pump casing.

Among all losses caused by friction (losses in bearings and seals, disc friction loss), the power loss resulting from disc friction usually dominates (see Internal efficiency). 

As disc friction is the result of viscous friction between the fluid rotating in the side gap (at circumferential speeds differing from that of the impeller) and the outside impeller surfaces, disc friction is greater for a closed impeller than for an open impeller with one shroud.

The equation for disc friction loss is as follows:

c_M        Moment coefficient
ρ         Density of the fluid handled
u₁, u₂   Circumferential speed of impeller at D₁ and D₂
D₁        Internal diameter of impeller shroud
D₂       Outside diameter of impeller
n         Impeller speed

The moment coefficient (c_M) takes into account the influences of the Reynolds number, the surface roughness and the side gap geometry. As the equation shows, disc friction changes at constant circumferential speed with the square of the outside diameter of the impeller and at constant rotational speed with the fifth power of the outside diameter of the impeller. If the outside diameter of the impeller and the rotational speed are constant, the ratio of disc friction (P_Ldiscfric) to power input (P) depends on the flow rate (Q), i.e. this ratio is a function of the specific speed (n_s). 

Experiments have shown that the difference in surface roughness between a rough (cast) impeller shroud and a polished one reduces the disc friction loss by approx. 30 %. The influence of the side gap geometry can also make a difference of up to 10 %. Both changes should always be considered in conjunction with the specific speed. See Fig. 1 Disc friction

In the specific speed (n_s) range from 40 to 60 rpm the losses caused by disc friction only amount to between 1 and 2 % of the power input (P). From a specific speed of roughly 30 rpm downward, disc friction loss increases rapidly with decreasing specific speed, amounting to approx. 5 % at a speed of 20 rpm and as much as 10 % at a speed of 10 rpm. %.

This means that the surface roughness and side gap influences can hardly be measured for specific speeds higher than 40 rpm. Marked improvements can however be achieved at low specific speeds by using smooth impeller shroud and casing surfaces. As disc friction increases with the fifth power of the outside diameter of the impeller, it will suffice to machine only the outside portions (outside of approx. 0.7 of the impeller diameter).

A discharge casing must be capable of withstanding a given internal and external pressure. The wall thickness is selected accordingly (see also Pump casing).

The discharge-side elbow (see Elbow) of a tubular casing pump is termed a discharge elbow.

A discharge line is a section of piping where the pressure is higher than atmospheric pressure (e.g. Pump system).

Discharge nozzle is the short name for the pump discharge nozzle.

DN (i.a.w. ISO 6708) is an alphanumeric identifier of size for components in a piping system and is used for reference purposes. It comprises the letters DN, followed by a dimensionless value (e.g. DN 100) that is indirectly related to the physical size of the hole or outside diameter of the connections (in millimetres), on piping, valves and nozzles of pump casings (also see Nominal diameter). Further identifiers apply to products with threads or cutting ring connections as well as soldered or welded connections and products and include NPS (Nominal Piping Size), OD (Outside Diameter) and ID (Inside Diameter) (EN 1759 or ASME B 16.5).

Fig. 1 DN provides an overview of preferred DN values in accordance with ISO 6708.

Dock pumps are centrifugal pumps which are used for draining dock facilities. The dock pumps used depend on the type of docking system, namely graving docks and floating drydocks.

Pumps for graving docks

See Fig. 1 Dock pump

The pump installation of a graving dock is commonly located on one of the longitudinal sides, near the dock gate. The water enters the intake chamber via the dock floor and is then evenly distributed to the bilge pumps (usually between two and four pumps).

When pumping commences, the geodetic head equals zero. Only the pipe friction losses and the discharge loss have to be overcome. Due to the large suction head of 10 to 12 m the NPSH value of the pump is generally unproblematic.

As the water level in the dock sinks progressively, the head of the system rises and the flow rate of the pump decreases (see Characteristic curve). 

Towards the end of the draining process the bilge pumps are stopped one after the other, and smaller tubular casing pumps with a diffuser (after-bilge pumps, see Mixed flow pump) are started up.

When the dock is completely empty, drainage pumps are employed for draining any leakage water. As coarse contamination of the leakage water caused by repair work in the docks has to be expected, such drainage pumps are often fitted with channel impellers (see Impeller). 

Pumps for floating drydocks

See Fig. 2 Dock pump

In a floating drydock the bilge pumps (usually four to six pumps, or more if required) are installed in one of the legs of the U-shaped floating structure. These pumps are vertically installed. They are usually double-suction pumps of radially split design with annular or volute casing suitable for flow rates from approximately 500 to 3500 m³/h.

The rotating assembly can be pulled out from the top. The pump shaft and the exposed drive shaft, which extends vertically upwards, are supported by grease-lubricated plain bearings. The thrust bearing and the drive are arranged in a water-tight chamber some metres above the pump.

The complete floating drydock is divided into chambers, which can be flooded individually. When the dock is submerged (flooded) the pumps and drive shafts are also submerged (see Wet well installation). Each chamber is equipped with its own suction line, which is connected to the corresponding bilge pump via a shut-off valve (see Valve). 

The pumps and shut-off valves can be operated from a control station, from where the water levels of each chamber as well as the longitudinal and transverse inclination of the dock can be monitored. To lift the dock and the docked vessel the amounts of water which are pumped off from the individual chambers may vary to accommodate the weight distribution. In contrast to graving docks, the head of this installation fluctuates by no more than a few metres.

DOL stands for “direct on line“ and refers to DOL starting.

Direct-on-line (DOL) starting is also known as direct starting and is a starting method for three-phase asynchronous motors.

Domestic water supply systems serve to supply individual residential houses or farms with water sourced from a well of adequate yield on the premises.

They can also be used for fire protection, cooling circuits, general and spray irrigation, drawdown of ground water levels, pressure boosting, fountains and air-conditioning systems.

Domestic water supply systems are equipped with either horizontal pumps or vertical pumps. The pumps must be fitted with a foot valve in the suction line. Exceptions to this rule are submersible borehole pumps, which are suspended beneath the lowest water level.

See Fig. 1 Domestic water supply system

To balance radial thrust on volute casing pumps a double volute is installed instead of a single volute. This volute type consists of two partial volutes which have the same effect and are arranged at an offset of 180° ending in the same discharge nozzle. See Fig. 1 Double volute

A double-entry impeller is an Impeller with a suction entry on both sides. A double-entry impeller is employed to reduce the axial thrust and the NPSHr value. See Figs. 6, 18 Impeller

In double-suction pumps the total flow rate is distributed over several impellers arranged on a pump shaft. The head remains constant.

The most common type of multi-suction design in centrifugal pump engineering is a double-suction pump whose impeller pair is arranged back to back on the pump shaft (see Back-to-back impeller pump). This arrangement serves to balance the axial thrust. Due to this advantage and also due to their common pump suction nozzle and common  volute casing double-suction pumps with back-to-back impeller arrangement can be compact in design, e.g. for use as water supply pumps. See Fig. 1 Double-suction pump

A double-suction pump with impellers arranged in parallel can be used to increase the flow rate at a constant head. Double-suction pumps are employed when the flow rate required of a centrifugal pump becomes too large for the inlet cross-sections of one impeller or when the flow velocity in the inlet cross-section of the first impeller has to be reduced to prevent cavitation.

In principle, fourfold or sixfold suction designs are possible. However, as the branching and connecting elements required for such designs are very costly, they are used far less frequently than double-suction designs. See Fig. 18c Impeller

Double-suction or multi-suction pumps can also be designed with multiple stages, for example for use as pipeline pumps or large water supply pumps. See Fig. 2 Double-suction pump

The drainage pump is used for draining pits and deep roadway underpasses, as well as for dewatering basements and courtyards subject to a flooding risk or groundwater. It is usually started automatically via float switches. The water to be handled may contain dirt and solids whose permissible particle size depends on the pump size and impeller. 

The conventional pump design, see Fig. 1 Drainage pump, where the pump is rigidly connected to an air-cooled flanged motor via a long support column, is being replaced more and more by submersible motor pumps. See Fig. 2 Drainage pump

In these latter machines, usually installed vertically, the motor situated above the pump (oil-filled for better cooling) is sealed by mechanical seals and shaft sealing rings. The chamber located between the sealing elements installed in pairs is filled with grease or oil.

The compact design makes the submersible motor pump a portable, transportable pump suitable for draining.

Centrifugal pumps are usually driven by electric motors. Piston engines (e.g. diesel engines) and gas and steam turbines also provide drive power, however. Hydraulic engines are rarely used. Electric motors and turbines generate uniform torque, whereas piston engines produce non-uniform torque. This irregularity is largely compensated by implementing appropriate design measures (e.g. flywheels, changing the number of cylinders and their arrangement).

In the low power range (up to 1 kW), single-phase AC motors (see Alternating current) with squirrel-cage rotors (see Asynchronous motor) are the preferred choice of drive.

The medium to high power range (up to 8000 kW) is dominated by three-phase motors (e.g. asynchronous motors) with squirrel-cage rotors.

Another option in the top power range are synchronous motors due to their relatively high level of efficiency and ability to compensate reactive power. In storage power stations, they operate as generators in turbine mode.

The drive rating is the drivepower available at the shaft coupling and is measured (see Unit) in watts (W). For high drive ratings more convenient units are used, such as kW or MW.

Series-produced pumps are typically assigned a fixed choice of drive. Their drive rating is selected based on the maximum power input within the permissible operating range and, in the case of standardised motors (in accordance with IEC), is rounded up to the next highest motor rating level. The rated power specified on the motor rating plate of standardised motors must not be exceeded by more than 3 % during continuous operation. Additional safety margins to account for potential long-term wear or sedimentation are not common for series-produced pumps, since their power input as a function of flow rate typically changes but slightly at the upper limit of the operating range (see Characteristic curves).

Engineered centrifugal pumps are designed to match the targeted operating point. Their drive rating must always exceed the power input (P) required by the pump to accommodate increased power input due to, for example, operation under off-design conditions (see Design point), variable operating points, changes in speed (see Pump affinity laws), changes in the density of the fluid handled, manufacturing tolerances, long-term pump wear or sedimentation.

A margin of 10 to 20 % will be sufficient to account for these factors in engineered clean water pumps.

The safety margins can be specified by the customer ordering the pump or in codes and standards. See Fig. 1 Drive rating.

Safety margins may also be necessary for dimensioning a waste water pump drive to accommodate potential problems at start-up, varying density of the fluid handled, high wear, single-pump operation of parallel pumps (see Parallel operation), environment-related factors (e.g. operation in a tropical environment), or closed-loop control involving super-synchronous speeds (see Synchronous speed).

In the context of drive systems, starting torque refers to the torque that is applied to accelerate the machine to the nominal speed when it is switched on (see also starting method). 

Drive systems convert electrical into mechanical (kinetic) energy using electrical machines and play a key role in automation systems, which require many movements to be carried out using electric drives. Electronics are vital and used to control the drives as well as supply them with electrical energy. Classic electrical machines are synchronous, asynchronous and direct current motors. 

Decentralised drive systems

In a decentralised drive system, only the power supply and, if required, components of a central open-loop control system of the multiple-motor configuration are installed in a control cabinet. All other functional units such as frequency inverters and closed-loop control systems are fitted near the motors or directly on them.

This concept is of particular advantage for large machines or plants, or machines or plants that are spread out over an extensive area. See Fig. 1 Drive systems

Centralised drive systems

In centralised drive systems, the power supply, frequency inverters, motor controllers and various control units of the multiple-motor configuration are housed in a central control cabinet. The motors and other process-related control elements are supplied or controlled from this cabinet, and required sensor signals are returned there. See Fig. 1 Drive systems

Dry cooling is a cooling process in which the coolant (water) is not in direct contact with the air that absorbs the heat. A differentiation is made between direct and indirect air cooling.

Alternative cooling processes are hybrid cooling and wet cooling. 

Direct air cooling

• The process of direct air cooling uses only air for cooling and condensing vapour in a closed circuit. Cooling water pumps are not needed in direct air cooling.

Indirect air cooling

• In indirect air cooling, the cooling water is pumped through a dry cooling tower and then to a mixing condenser using a circulating pump. 

The term dry installation is mainly used in conjunction with large vertical pumps. A dry installed centrifugal pump is installed in a dry area and its pump casing is never submerged, wholly or partially, in contrast to wet well installation.

Dry running is usually undesirable in a centrifugal pump it occurs in the total absence of the liquid component of the fluid handled (e.g. following the ingress of air in the suction line) or if under normal operating conditions gas bubbles (see Formation of air pockets) attach themselves to normally wetted rotating components (partial dry running).

During normal operation of a well-designed centrifugal pump, the fluid handled completely fills the flow space inside the pump, including narrow controlled gap seals on impellers, gland packings and mechanical seals. The liquid helps to cool and lubricate the components in contact with each other and exercises a centring action in the clearance gaps of the impellers and shaft passages (see Multistage pump) so that long and slender ring-section pumps for example are able to run without the rotor touching the casing.

In the absence of liquid, dry running can occur in certain areas because of insufficient cooling and centring action. The consequences are overheating, abrasion, seizure of the materials, vibrations and other phenomena which may in due course lead to the complete disintegration of the pump.

If the pump operator cannot avoid such instances of absolute or partial dry running, it may be necessary to invest in the optimisation of the centrifugal pump's design. An improved pump design should include reinforced shafts which prevent radial contact between the rotor and the casing, specially designed clearance gaps (controlled gap seal) and mechanical seals, gland packings and bearings which are supplied with external lubricating or barrier fluids rather than using the fluid handled.

Pumps fitted with a hydraulic axial thrust balancing device must be equipped with an additional thrust bearing to prevent rubbing contact or seizure. In the event of incipient dry running, the pump can also be stopped by a dry running protection device.

Self-priming centrifugal pumps must always be filled to a certain liquid fill level prior to start-up in order to be able to self-prime. During the self-priming phase they operate under partial dry running conditions.

Dry running protection is intended to prevent a pump from operating without fluid. For this purpose, the pump is monitored by means of various measuring methods. If a specified limit is undershot, the protection function will ensure that the system is shut down completely and the respective message is displayed. Depending on the safety requirements, the system can then be re-started either automatically or manually.

Ductility means the ability of a material to withstand the propagation of a crack or rupture. This is associated with an energy input in the case of plastic deformation. The opposite is brittleness.

Dynamic pressure is an abbreviated designation for hydrodynamic pressure and results from the kinetic energy of a body with mass which moves with a velocity (fluid velocity).

Dynamic seals perform contact sealing between moving parts. Depending on the nature of the relative movement, a differentiation is made between translational (back and forth) and rotational sealing types (See also Shaft seal).