6th Chapter Flashcards
Power has to be supplied from an external source → four types of drives are used
Electric
Steam turbine
Gas turbine
Internal combustion engine
Pumps are generally subdivided into
Streaming pumps and Displacement pumps
Types of streaming pumps
Centrifugal pumps
Propeller pumps
Jet pumps
Types of Displacement pumps
Piston pumps Piston diaphragm pumps Screw pumps Eccentric screw pumps Gear pumps Rotary piston pumps Peristaltic pumps
Centrifugal pumps
Most common pump type in the (chemical) industry.
Universally usable pump for low viscous fluids. Large operating range for discharge head and conveying capacity.
Design is inexpensive, simple, robust, has a high
efficiency and low power consumption, and can
be connected to an asynchronous motor.
Suitable for handling pure and impure liquids.
Example wastewater: impeller is constructed especially
to avoid that objects get locked inside the pump
Open design: liquid can pass even if pump is not
running
50 – 90 %
Closed impeller
Series of curved vanes attached to a central hub that extends outward between
two enclosing plates.
Open impeller
Similar design, except that there are no enclosing plates → this type allows visual
inspection of the vanes.
Centrifugal pumps are categorized into
radial flow, mixed flow and axial flow pumps
Radial flow and mixed flow pumps are the most commonly used types
Radial impellers produce relatively large pump heads with comparatively low flow.
Axial impellers lead to a large flow but offer only low pump head.
Mixed flow impellers lie between the two aforementioned.
Standardized chemical pumps
Pumps with standardized (DIN EN ISO 2858) dimensions and hydraulic output.
In-line pumps
Designed with the suction and discharge nozzle on the same axis, with the same nominal diameter.
Advantages: High output, small footprint and flexible installation options → many models of in-line
pumps can be installed in either a vertical or a horizontal position
In-line pumps have become well-established, especially in building services applications, such as
Heating systems Air-conditioning systems Cooling circuits Service water supply systems Swimming pool systems Water supply Industrial recirculation systems
Typical building types of centrifugal pumps
Magnetic-coupling centrifugal pumps (Magnetic-driven pumps)
Have become popular for the transport of aggressive and toxic liquids.
Important: Venting (for the bearings)
Separation by a non-magnetizable can (made from Cr-Ni steel, Ni-alloys or ceramics) which acts
as a hermetically sealed barrier between the atmosphere and the liquid.
Positive displacement pumps
Centrifugal pumps: Head is developed by the speed of the rotor.
Positive displacement pumps: Maximum head achieved by the available power from the driver
that is working on the fluid.
The overall efficiencies of displacement pumps are higher (compared to centrifugal pumps) as
internal losses are minimized.
Oscillating: rapid opening and closing of suction and discharge valves causes pressure pulsation
Flexibility of positive displacement pumps in handling a wide range of capacities is limited.
Piston pumps
Conveyance of the liquid as a result of an oscillating displacing piston connected to a crankshaft.
Working chamber is alternately increased and decreased and is connected to two self-acting valves.
Typically used for smaller applications, requiring minimal space and involving low flow rates.
For low to medium conveying capacities and up to very high pressures. Suitable as dosing pump.
60 – 95 %
Piston diaphragm pump
Perform similar as piston pumps but the displacement element is an elastic membrane made
from rubber, enforced PTFE or steel.
Used for aggressive and solid-containing fluids and
suspensions like acids, brines or toxicants.
Units available that are able to withstand up to
700 bar and deliver capacities > 25 m³/h.
Hermetically sealed → elimination of all packing and seals exposed to the liquid being pumped.
Conveying of aggressive,toxic and flammable liquids
and suspensions. Suitable as dosing pump.
80 %
Screw pump
Screws are rotating contactless (no metal-metal contact) in opposite direction, which allows a good pumping performance even with non-lubricating, corrosive or contaminated fluids Advantages Usually for pressures up to 70 bar (400 bar possible for special applications) Wide range of applications No priming required (self-priming) Pumped capacity proportional to speed Pumping elements are simple to replace For almost all media and fluids High viscosities possible Low pressure pulsation and noise level Dry running possible 80 %
Gear pump
Use of two intermeshing gears rotating against each other.
Characteristics
Quiet-running (gears supported on both sides)
Self priming
Used for high pressure applications up to 200 bar
For low fluid rates
Constant rate of delivery
Medium to high viscosity of the fluid
Sensitive to hard solid particles
For low flow rates and high pressures. Suitable for high
viscous liquids and as dosing pump.
50 – 95 %
Eccentric screw pump
Consists of a rotating displacement element (eccentric screw, rotor) and a stator.
The rotor is in the shape of a rounded single threaded spiral shaft with a large pitch.
Dry running has to be avoided in order not to
damage the stator (e.g. hard rubber, PTFE).
Eccentric screw pumps are self-priming and can
deal with fibrous media and fluids with high
viscosities.
Conveying of muddy and paste-like fluids. Direction of flow invertible.
50 – 70 %
Rotary piston pump
Rotors made of steel touch neither each other nor the casing (mechanical clearance) and offer
comparably low pressures.
Fluids with high turbidity (particles) and a
wide viscosity range can be conveyed.
Two rotors on parallel axes run by a synchronized gear and rotate in the opposite direction.
Peristaltic pump
The conveyed fluid is contained within a flexible tube (e.g. silicone hose).
A rotor with a number of rollers compresses the flexible tube.
As the rotor turns, the part of the tube under compression is pinched closed thus forcing the
fluid to move through the tube.
The fluid is not in direct contact with moveable pump parts.
Newtonian Fluids
The viscosity of Newtonian fluids ① is not affected by the magnitude and the motion that it is
exposed to. In other words, the viscosity is load-independent. Examples are water or mineral oil
Non-Newtonian liquids
The viscosity of Non-Newtonian fluids is dependent on the shear rate or the shear rate history. In
other words, the viscosity does change when the liquid is agitated.
Shear thinning fluids
Behavior is generally not seen in pure
liquids with low molecular masses or ideal
solutions but is often seen in polymer
solutions, molten polymers and complex
fluids and suspensions such ketchup, paint
or blood.
Cavitation
If the NPSHA is lower than the NPSHR gas bubbles will form in the fluid and cavitation will occur
Changes that affect the NPSH
Source liquid level Height of source Pressure on fluid surface Vapor pressure Piping design Fluid velocity Dirty or restricted pipes Fluid properties
Resistances connected in parallel
Components installed in parallel reduce the total resistance in the system and consequently the
head loss.
This results in a more flat system curve
Qtotal = Q1 + Q2
Resistances connected in series
The total head loss in a system consisting of several components connected in series is the sum of
head losses each individual element represents.
Open systems
Open systems are used to transport liquid from one point to another like in water supply systems,
irrigation systems or many industrial processes.
The pump has to overcome both the geodetic
head of the liquid and the friction losses in the
pipes and the installed equipment.
Open systems with positive geodetic head
The pump has to provide a head that is higher
than the geodetic head (h)
The pump has to provide the required head to
overcome also the total friction losses (Hf
) due
to the pipeline, fittings, valves, etc.
This pressure loss is dependent on the flow Q.
The flow Q1 and the pump size have to match
the need of the specific system.
Open systems with negative geodetic head
An example of an open system with negative geodetic head is a pressure booster system.
The geodetic head brings water to the consumer although the pump is not running.
The height difference h results in the flow Q0 but it is too low to ensure the required flow Q1 to the
consumer.
The pump has to booster the head to the level H1
Closed systems
Closed systems are commonly used to transport heat energy in heating systems, air-conditioning
systems, process cooling systems etc.
The circulated liquid is often the carrier of heat energy.
The pump only has to overcome the sum of friction losses which are caused by all installed
equipment and pipelines.
Speed-controlled pumps connected in parallel (1)
Useful method to realize an efficient pump performance when the flow demand is varying.
Commonly used in water supply and pressure boosting systems.
Pumping systems consisting of two speed-controlled pumps with similar size can cover a wide
performance range.
Pumps connected in series
Generally used in systems with a high pressure demand.
Multi-stage pumps are also based on the series principle with one stage equals one pump.
Htotal = H1 + H2
Adjusting pump performance
It is important to select a pump where the duty point is in the high-efficiency area of the pump.
Failing this, the power consumption of the pump is unnecessarily high.
It is not always possible to select a pump that hits the optimum duty point due to the requirement
of system changes or the system curve changes from time to time.
Most common methods Adjusting pump performance
(1) Throttle control
(2) Bypass control
(3) Modifying impeller diameter
(4) Speed control
Observar Diapositivas
Throttle control
The power consumption is reduced to about 94 %* Head is increased from 70 to 76 m The overall efficiency of the pump system is considerably reduced Continuous adjustment is possible
Bypass control
To reduce the flow in the system to 50 m3 h-1 the
head of the pump has to be reduced to 55 m
This is achieved by an increase of Q to 81 m3 h-1
Power consumption is increased up to 10 %*
whereby the degree of increase depends on the
pump type and the duty point
Continuous adjustment is possible
Modification of impeller diameter
Drop of flow and pump head if the pump impeller diameter is reduced Power consumption is reduced to around 67 %* The overall efficiency of the pump system is reduced only slightly No continuous adjustment possible
Speed control
Flow and head are decreasing when the speed of
the pump is reduced
Power consumption is reduced to about 65 %*
The overall efficiency of the pump system is
reduced only slightly
Continuous adjustment is possible
