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Obturator

Obturators or closing elements are the moving components inside valves which serve to mechanically obstruct or control the flow of the fluid in pipes. On globe valves, they are mostly tapered plugs. A further differentiation is made between shut-off, on/off, throttling and control valve discs or plugs.
The standard designs of the latter feature linear or equal-percentage characteristics.

Open-loop control

Open-loop control is a process that systematically targets variables in a controllable system which is referred to as a controlled system or controlled object and is typically a device, machine, machine system, or technical plant in which substances or energy are acquired, transferred, converted, stored, or used as intended.

Open-loop control is an effective way of influencing the operation of a device or process in line with a defined plan. Output variables (motor, valve) are set a function of the input variables (switch, time) and status variables (motor running, current temperature).

Unlike closed-loop control open-loop control does not include continuous feedback of the output variable to the input because the result is not checked. It is, therefore, an open loop of action. See Fig. 1 closed-loop control

Operating behaviour

A centrifugal pump's operating behaviour is the collective term for all pump characteristics (e. g. hydraulic, mechanical, acoustic) at a given operating point. The position of the operating point in relation to the design point has a major influence on centrifugal pumps' operating behaviour.
When selecting pumps, the operating point should coincide with or be in the proximity of the design point to ensure very low energy and maintenance costs and minimal hydraulic excitation forces.

In practice, application processes may require that the pump is operated under low flow or overload conditions, i.e. in the off-design range. As the difference between the operating point and the design point increases, unfavourable flow conditions develop at the impeller or diffuser vanes because of an unfavourable approach flow, frequently leading to flow separation, mechanical vibrations, noise and cavitation. See Fig. 1 Operating behaviour

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Under low flow conditions, for example, the meridian component (v1m) of the absolute velocity at the design point decreases to the value v1mT and the relative velocity w1 to w1T.

Depending on the orientation of w1T the result is a highly unfavourable flow to the vane cascade (see also Flow profile). This prevents the relative flow from following the vane contour along the suction side, and flow separation takes place. Similar conditions are encountered in the overload range on the discharge side (see Boundary layer).

Any form of flow separation represents a transient or non-steady flow phenomenon. These transient conditions significantly disturb flow deflection at the vane cascade profile (deflection required to generate the head) and lead to pulsations (noise) in the fluid handled, in hydraulic pump components or in components connected to the pump.

The unfavourable operating behaviour exhibited when centrifugal pumps are continuously operated under low flow conditions is caused not only by flow separation, but also by instability as a result of suction or discharge recirculation. This occurs outside the impeller inlet and inside the impeller outlet if there is a significant discrepancy between flow rate and design point. Suction recirculation (S) and discharge recirculation (D) are transient flow phenomena which may occur independently of each other. If the flow rate is further reduced, they often occur simultaneously.
See Fig. 2 Operating behaviour

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Suction recirculation (S) can be detected over a distance corresponding to several suction pipe diameters in the opposite direction of the incoming flow. To prevent it from extending axially, it is possible to incorporate vanes, elbows or changes in pipe cross-sections.

The higher the specific speed of a centrifugal pump, the more intense the recirculation phenomena relative to the pump's power output. This means that the low flow operating range must be further limited in the case of high specific speed centrifugal pumps as the critical operating limit is reached at an earlier stage (this operating limit is also referred to as the "rumour limit"). See Fig. 3 Operating behaviour

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This limit also exists in the overload range and should not be exceeded by low and high specific speed pumps. It is primarily determined by the pump's suction behaviour and discharge-side flow separations. High specific speed centrifugal pumps such as mixed flow and axial flow pumps are characterised by a more or less pronounced saddle in their H/Q curve due to suction recirculation (S) in the low flow range. This point on the H/Q curve should be passed through as quickly as possible to avoid vibration and possible cavitation. Continuous operation is not permitted in the range from zero flow rate Q=0 to the operating limit. See Fig. 3 Operating behaviour

Depending on the system characteristic curve unstable H/Q curves of low and high specific speed pumps can lead to problems during start-up and shutdown, undefined operating points or pump vibrations.

If a centrifugal pump with a high head and motor power is operated at the lowest limit of the low-flow operating range, or even against a closed

shut-off element the drive's high output power is transferred to the fluid handled leading to a rapid temperature increase. This in turn can lead to evaporation and pump damage (due to seizure in the clearance gaps) or even cause the pump to burst (due to vapour pressure increase in the case of a closed lift check valve). The unfavourable operating behaviour associated with operation in the low flow range can be improved by increasing the flow rates (via a bypass) and by impeller blade pitch adjustment. 

The definition of distinct operating ranges is a necessary measure if the problems encountered due to differences between operating points and design points are to be prevented and damage and trouble-free operation ensured. The four ranges defined are continuous, short-time, minimum flow and impermissible operation.

Definition of operating ranges

  • Continuous operation:
    To prevent damage and unnecessary wear, only the operating points in the region of the design point are permitted.
  • Short-time operation:
    If pump operation at operating points which would lead to damage in the long run is intended, it is necessary to limit the time of operation. Time limits vary considerably depending on multiple parameters.
  • Minimum flow operation:
    A range which is permissible for a very short period of time only and must also be defined on a case-by-case basis.

Irrespective of the various parameters (e.g. economic efficiency, application, absolute motor power, pump size, pump type, low flow and overload) which determine individual operating ranges, the ranges of operation become narrower for pumps with higher specific speeds.

Operating conditions

The operating conditions of centrifugal pumps represent the requirements laid down by the purchaser/customer with regard to the pump's specific operating characteristics. They have a major influence on the selection of the pumps, e. g. with regard to type, size and drive

The operating conditions primarily encompass the data specified in the supply agreement, i.e. specifications on the fluid handled (e. g. density, temperature, viscosity, solids content, chemical properties), the flow rate, the head, the suction characteristics and possibly the centrifugal pump's rotational speed.

 Further data include size and power supply details of the drive, the operating mode, frequency of starts or the pump set's controllability (see Closed-loop control) and system or environmental factors such as max. permissible noisepiping forces, shaft vibrations and explosion hazards (see Explosion protection). 

Irrespective of the operating points as such, many of these operating conditions apply to multiple  pump applications. For this reason, centrifugal pumps' operating conditions are often laid down in the form of directives which are generally applicable to entire branches of industry (e. g. refineries, power stations, tankers, heating systems) or compliant with environmental protection requirements.

Operating point

The operating point of a centrifugal pump is the intersection of the pump characteristic curve (H/Q curve) and the system characteristic curve Hsys /Q. H/Q is the pump-based variable, Hsys/Q the the system-based variable. See Fig. 1 Operating point

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The operating point's position shifts if the position or the gradient of the pump characteristic curve H/Q and/or the system characteristic curve.

Hsys/Q change:

H/Q changes but Hsys/Q remains unchanged:

  • This takes place in the case of variable speed centrifugal pumps (see Closed-loop control) (See Fig. 2 Operating point) or

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  • When centrifugal pumps of the same size are started up and operated in parallel. See Fig. 3 Operating pointThis is an embedded image

Hsys/Q changes but H/Q remains unchanged: 

  • The system characteristic curve may change during operation as a result of increased head losses (e.g. throttling via check valves, pipe incrustations) or changes in static head (e.g. fluid level fluctuations in tanks).
  • Exact correspondence between the design and duty points (the latter referring to those specified by the customer) and the operating points only exists in rare cases. The operating point is often matched to the required data by throttlingSee Fig. 4 Operating point

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Orifice

In fluid mechanics, an orifice is a plate that is inserted in a line and typically has a round hole in its centre. Orifices are used as fixed throttles that generate head loss. The head loss caused by an orifice can be used to determine the volume or mass rate of flow during flow metering. Orifices used in flow metering are preferably designed as a standard orifice.

Orifices can be fitted to lines branching from piping for throttling purposes so that volumetric flow can be distributed as required.

The hole diameter (dOr) must be dimensioned in accordance with the required flow rate (Q), the difference in static pressure upstream and downstream of the orifice (see standard orifice), and a non-dimensional throttling coefficient (f) respective of the area ratio ((dOr/D)2) of the orifice and design of the orifice hole (applies to sharp-edged orifice openings). See Fig. 1 Orifice

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f     Throttling coefficient
dOr  Inside diameter of orifice in mm
g     Gravitational constant of 9.81 m/s2
ΔH   Pressure head difference to be throttled in m
Q     Flow rate in m3/h

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Since the area ratio is not initially known when the inside diameter of the orifice is determined, the throttling coefficient is estimated and then corrected by iterating the calculation.

If distribution of the volume flow rate must be highly precise, the diameter of the orifice bore should be somewhat larger and the flow rate regulated as required using a control valve.

Outlet cross-section

The outlet cross-section of the system is an agreed cross-section in the discharge-side piping or in another space on the discharge side with known geometric and flow data. A distinction must be made between a system (e. g. pump system) and a pump. The outlet cross-section of a pump is identical with the cross-section of the pump discharge nozzle. If there is no discharge nozzle the pump's outlet cross-section must be defined, e. g. the cross-section at the end of the discharge elbow. See Fig. 2 Head

Outlet width

The outlet width (b2) of an impeller describes the open width at the outlet and is determined by the flow rate level. If the rotational speed, the flow rate and the impeller diameter have been specified, a variation in the outlet width is possible within certain limits in order to influence the head at nominal flow rate and the pump's characteristics under low flow conditions. See Fig. 2 Axial thrust

Over-pressure sensor

The over-pressure sensor is also referred to as the relative pressure sensor and is a measuring instrument (also see Sensor), that converts the physical quantity of pressure as experienced in a closed system relative to the surrounding atmospheric pressure into an electrical output variable proportional to the relative pressure. Over-pressure sensors are a special type of differential pressure sensor from a technical and design perspective, whereby either p1abs or p2abs corresponds to the current atmospheric pressure.

Overvoltage protection

Overvoltage protection protects electrical and electronic devices from excessively high voltage. This voltage can, of course, occur naturally via lightning or other electrical devices/systems (e. g. when fluorescent tubes or motors are switched on).
Surge arresters are used in critical areas in particular to limit dangerous overvoltage.