ARTÍCULO EN PROCESO DE REVISIÓN (ESTUDIO 1)

Design:

Instead­of­a­bladder­/­diaphragm,­special­types­of­construction,­e.g.­metal­bellows­accumulators,­use­a­metal­bellows­to­

separate­gas­and­oil­side.­Therefore­they­are­available­as­virtually­gas-tight­dampers.­A­baffle­plate­/­diffuser­block­is­

integrated to improve damping characteristics. A metal bellows serves as separator between gas and oil side.

Function:

Without friction and wear the metal bellows moves inside the accumulator. Adjusted only once it can operate over a long period of time (years). Monitoring and maintenance are minimalized.

Advantages:

•­up­to­160°C­resistant­to­all­conventional­fuels

•­virtually­gas-tight,­no­friction­between­parts­(no­wear),­thus­maintenance­and­monitoring­is­minimalized

•­maintenance­free

•­no­recharging­necessary

•­can­be­used­with­all­hydraulic­fluids

•­low­operational­costs Application:

•­pulsation­damping­in­ship­diesel­engines­and­power­plant­motors,­as­well­as­in­proportioning­pumps­/­metering­pumps

•­volume­compensation­and­energy­storage­in­aircraft­/­aviation Fig. Metal bellows accumulators

317

3.5 Weight reduced hydraulic accumulators

Bladder, piston and diaphragm accumulators are also supplied in a weight-reduced hydraulic accumulator version. Weight reduction can be achieved by a reduced accumulator wall thickness or by using aluminum or compounds. Application of these materials lead to a weight reduction of up to 80% in comparison to standard accumulators made of C-steel.

Weight reduced accumulators save energy in various ways:

On the one hand the weight reduced accumulators contribute to saving energy, i. e. fuel, on the other hand these accumulators are being used to store energy. For instance during break application the energy is stored and used again when the engine or machine accelerates.

Aided by weight reduced hydraulic accumulators conventional transport vehicles can save energy particularly in „stop and go“ situations. Some examples are buses (shuttles, public transport in inner cities), public service vehicles, delivery trucks and rail-based vehicles (driving units of locomotives).

Application:

•­mobile­technology

•­automotive­industry

•­aviation­and­aerospace­industry­

Fig. Hydraulic accumulator

318

3.6 Hydraulic dampers

In­hydraulic­systems­pressure­fluctuations­can­occur.­

already during the planning stages of a hydraulic unit. Appropriate countermeasures have to be taken. Possibilities for damping are numerous. However, hydraulic dampers applied in hydraulic systems have turned out to be particularly suitable­and­efficient.

Pressure pulsations occur at both ends of the pump (pressure side and suction side).

The requirements made on such dampers can be categorized into physical, constructive and operational or managerial aspects.­The­physical­parameters­relate­to­a­very­distinctive­damping­process­covering­a­large­frequency­range­and­–­at­

the­same­time­–­a­low­drop­in­pressure.­The­constructive­design­essentially­comprises­a­simple­design­combined­with­

excellent­fitting­possibilities­and­sufficient­temperature,­fluid­and­pressure­reliability.­The­managerial­aspect­deals­with­

maintenance work (input) as low as possible in such a way, that the operational reliability is not compromised.

General aspects:

Depending on the mode of operation hydraulic dampers are based on the physical principle of hydropneumatic bladder or­diaphragm­accumulators­or­fluid­based­sound­absorbers.­In­hydropneumatic­dampers­the­compressability­of­a­gas­

(nitrogen) is applied. For instance in bladder accumulators depending on the amount of pressure applied, the bladder is compressed or extended.

Something similar can be said about membrane (diaphragm) accumulators. During the application of standardized bladder or diaphragm accumulators the dampening process can be jeopardized due to the unfavourable connection

­between­fluid­and­gas­volume.­Therefore­special­hydropneumatic­dampers­(pulse-tone­dampers)­have­been­developed,­

which­by­means­of­an­inline-connection-block­couple­or­link­the­fluctuations­of­volume­or­pressure­to­the­volume­of­gas­

perfectly well. Thus excellent damping characteristics can be achieved up to a frequency level of approximately 500 Hz.

We distinguish between the following types of damping processes: Fig. Hydraulic damper

319 3.6.1 Pulsation dampers

Design:

Pulsation dampers consist of a welded or forged pressure container made of C-steel resistant to chemically aggressive fluids.­A­specially­designed­fluid­valve­with­inline­connection­guides­the­volume­flow­into­the­container.­

Function:

A­pulsation­damper­has­two­fluid­ports.­Therefore­it­can­be­fitted­directly­into­a­pipe.­Due­to­the­diversion­of­the­volume­

flow­inside­the­valve,­the­flow­is­immediately­directed­on­the­bladder­or­diaphragm,­in­other­words,­there­is­an­immediate­

contact­between­bladder,­diaphragm­and­fluid,­which­compensates­the­fluctuations­of­the­volume­flow­via­the­gas­volume.­

This­ also­ includes­ the­ pressure­ fluctuations­ with­ higher­ frequencies.­The­ charging­ pressure­ has­ to­ be­ attuned­ to­ the­

existing operational conditions.

Application:

Pulsation dampers are predominantly used in hydraulic units, displacement pumps, sensitive measuring and control units and­widely­ramified­pipe­systems,­for­instance­in­process­circuits­of­the­chemical­industry.

when pressure relief valves are activated, especially when the operational pressure is close to the opening pressure.

Fig. Pulsation damper

320

3.6.2 Suction stabilizers

Another way to dampen pulsations is to apply suction stabilizers, especially when they have to compensate pressure fluctuations­generated­at­the­suction­side.

Design:

These stabilizers essentially consist of a fairly large housing (compared to the size of the bladder), which serves as a reservoir,­a­gas­valve­fitting­and­a­cage­to­accommodate­the­bladder.

Function:

The­gas­volume­is­surrounded­by­a­fluid­volume­several­times­larger­than­the­gas­volume.­Due­to­this­fact­and­its­special­

design­in­the­vicinity­of­the­ports,­the­effects­of­acceleration­of­the­volume­flow­are­reduced.

This leads to the following effects:

•­improvement­of­NPSH-values­(unit­holding­pressure)

•­cavitation­of­pump­can­be­avoided

•­prevention­of­pipe­vibrations Application:

•­in­displacement­pumps­of­all­designs

•­in­sensitive­measuring­and­control­instruments

•­in­process­cycles­of­chemical­industries

•­in­the­low­pressure­range­on­the­suction­side­of­displacement­pumps

In general auxiliary pumps (rotatory pump) are connected in line before said pumps to generate a pilot pressure.

Fig. Suction stabilizer

gas valve insert

accumulator bladder

bubble sieve

housing Fig. Suction stabilizer

321 3.6.3 Silencers

Design:

A silencer consists of a welded or forged outer housing, an internal pipe and two pipe ports at opposite ends. A silencer has­no­moving­parts­and­no­gas­filling.­Therefore­it­does­not­require­any­maintenance­work­whatsoever.­It­can­be­used­

for­mineral­oils,­phosphoric­acid-ester­and­glycol.­Other­fluids­might­require­a­stainless-steel­version.

Fig. Silencer Function:

A­silencer­is­based­on­the­principle­of­an­extension­tank­with­interference­duct.­The­oscillations­are­reflected­within­the­

tank. A major part of the oscillations are damped for a wide range of frequencies.

Application:

•­displacement­pumps­(all­types)

•­vehicles,­machine­tools,­injection­moulding­machines,­aircraft

•­hydraulic­drive­units­and­other­systems­with­large­„active­area“

They are used to:

•­reduce­pressure­fluctuations­in­dB

•­reduce­noise­level­(dB)­by­means­of­a­silencer

•­compensate­pulsations­with­different­pressure­changes

•­no­maintenance­required­(no­initial­pressure) Application:

All displacement pumps, like axial and radial piston pumps, vane pumps, gear pumps or screw spindle pumps generate fluctuations­of­volume­and­pressure.­This­causes­vibrations­and­noise.­Considerable­noise­levels­are­not­generated­by­

the­pump­alone,­but­also­by­the­fluids­and­their­mechanical­pulsations.­If­they­are­transferred­to­larger­surfaces,­they­are­

even­amplified.­Insulation­and­the­application­of­flexible­hoses­or­sound­insulation­caps­resolve­only­part­of­the­problem,­

since they cannot prevent the effects of the pulsations being transferred to other areas. A silencer can improve the situation considerably.

fluid­chamber

inlet outlet

Fig. Membrane accumulator housing (body) SD330

322

3.6.4. Shock absorption

For pressure shock damping purposes all hydraulic accumulator designs can be used:

•­bladder­accumulator

•­diaphragm­accumulator

•­piston­accumulator

Design:

A­welded­pressure­container­made­of­C-steel­or­stainless­steel,­the­fluid­port­with­a­punched­disk,­which­prevents­the­

bladder to be pushed out, plus a gas valve are the main components in a pressure shock damper.

Function:

Sudden­changes­in­the­stationary­status­of­pipes,­normally­exposed­to­fluid­flow,­as­may­occur­during­pump­failure­or­when­

a valve is opened or closed, can lead to pressure levels several times higher than permitted. A pressure shock damper prevents this incident by translating potential into kinetic energy or respectively kinetic into potential energy. Thus pressure surges are avoided and pipes, control units and other components are protected against destruction.

gas valve insert

accumulator bladder

mounting­flange

pressure vessel

Fig. Low pressure bladder accumulator (shock absorber) Fig. Pressure shock damper

323 Where are pressure surges generated?

Pressure surges can be generated by control and adjustment procedures:

•­influx­into­pipe­systems

•­opening­and­closing­of­components

•­rapid­pressure­unloading­of­containers­(accumulators)

•­quick­flow­control­procedures

Surges can also be generated by operational failures:

•­pump­or­compressor­failures

•­pipe­bursts Application:

•­pipe­systems­with­quickly­closing­valves­or­flaps

•­switching­on­and­off­of­pumps

•­rapidly­changing­operational­pressures General remarks:

A pressure shock damper or shock absorber reduces pressure surges and prevents both pipe systems and components from­being­destructed.­Pressure­surges­are­defined­as­pressure­waves­in­fluid­systems,­which­occur­discretely­as­an­

individual damped oscillation. These pressure surges cannot be repeated. Depending on the circumstances, they cannot be predicted either (cavitation at peak level).

325

4 Accumulator station

In document Influencia de la ansiedad en el recuerdo de información emocional : meta-análisis sobre los sesgos de memoria y su estudio en personas mayores (página 154-200)