General conclusions

High-flow­ accumulators­ are­ especially­ designed­ constructions,­ which­ permit­ volumetric­ flows­ up­ to­ 140­ l/s.­ The­

­distinctiveness­of­these­high­performance­accumulators­is­due­to­the­fact­that­the­fluid­connection­is­enlarged­to­allow­

higher­flow­rates.

One option or version of a high performance accumulator is shown below this paragraph. It can be used for operational pressures up to 330 bar. The adapter contains a pre-charged check valve. A discharge of the bladder due to a sudden pressure­drop­in­the­system­or­a­complete­draining­is­thus­prevented.­Furthermore­the­valve­shaft­is­fitted­with­a­damping­

device,­ which­ ensures­ that­ the­ valve­ is­ not­ damaged­ by­ high­ volumetric­ flows­ during­ the­ opening­ and­ closing­

procedure.

Consequently­the­borehole­in­the­pressure­vessel­for­the­mounting­of­the­fluid­valve­has­a­larger­diameter­than­the­

­borehole­ for­ the­ attachment­ of­ the­ gas­ valve.­ Thus­ the­ fitting­ and­ removal­ of­ the­ bladder­ from­ the­ fluid­ side­ is­

predetermined.

Due­to­the­application­of­different­oil­valve­designs,­the­maximum­possible­volume­flow­can­be­adjusted­to­the­actual­

requirements.

Fig. Low pressure bladder accumulator (High-Flow) Fig. High pressure bladder accumulator (High-Flow)

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3.2 Piston accumulator

Design:

The typical design of a piston accumulator is shown in the following picture. The main components of piston accumulators are­a­cylinder­with­a­finely­finished­internal­surface­and­an­end­cap­on­the­gas­side.They­are­sealed­with­o-rings­and­have­

a­lightweight­metal­piston.­The­cylinder­fullfills­two­functions.­Firstly­it­contains­the­internal­pressure­and­secondly­it­

­guides­the­piston,­which­serves­as­a­separation­element­between­gas­and­fluid­side.

Function:

The­charging­with­nitrogen­up­to­the­pre-charge­pressure­level­causes­the­piston­to­move­to­the­lid­on­the­fluid­side­and­

thus­covers­the­borehole­of­the­fluid­inlet.­Once­the­precharge­pressure­level­has­been­reached­and­the­initial­breakaway­

momentum of the piston has been transgressed, the piston moves into the gas chamber and compresses the gas. The effective volume V can be derived from the compressed gas volume V₁ and V₂. Initial breakaway momentum and losses due to friction during movements depend entirely on the piston design being used.

Fig. Piston accumulator

Fig. Piston accumulator (SK280) (crimped) fluid­port

gas valve body

sealing cover

sealing cover cylinder pipe

piston

Fig. Piston accumulator (screw-type) gas valve body

sealing cover

cylinder pipe

piston

fluid­port

sealing cover

311 V₀ =­effective­gas­volume­ V₂­=­gas­volume­p₂

V₁­=­gas­volume­at­p₁ DV­=­effective­volume

In order to create a preferably balanced pressure level, friction between piston sealing and interior cylinder wall must be as low­as­possible.­Therefore­the­inside­of­the­cylinder­has­to­be­finely­finished.­However­a­pressure­difference­between­gas­

and­fluid­chamber­cannot­be­avoided.

The diagram below shows the course of oil and gas pressure in relation to time for an accumulator cycle with two different sealing systems. As you can see a low-friction sealing system generates smaller differences between both pressures and thus better operational behaviour. However, the friction resistance is not constant but increases with increasing operational pressure. As long as the operational pressures are low, friction resistance dominates in comparison to the movement of the piston. Thus it seems that operations with low pressures are not meaningful. Certain functions of the hydraulic system, like turning­off­the­supply­pump­or­monitoring­the­fluid­level­in­the­accumulator­can­be­directly­influenced­by­modification­of­

the piston accumulator. The problem is solved by attaching the piston rod to the piston and subsequently led out of the accumulator.

This presents the possibility to realize certain control functions by different means, like:

•­mechanically­by­means­of­a­cam­switch­or

•­electrically­by­means­of­permanent­magnets­or

•­inductive­proximity­switches

Another possibility to determine the piston position is to use an ultra-sonic measuring system. By means of a micro processor data, like piston position, together with a simultaneous measurement of gas pressure are being used for various control functions.

Advantages:

Fig. Function principle of piston accumulators

Measurment of oil and gas pressure in piston accumulators 360

Fig. Diagram oil and gas pressure

p₁­=­minimum­operational­pressure p₂­=­maximum­operational­pressure p₀­=­gas­pre-charge­pressure

�_�

312

piston monitoring:

piston accumulator with electric limit switch code letter: A­=­stroke­35­mm

­ B­=­stroke­200­mm

­ C­=­stroke­500­mm

piston diameter (optional) ranges from 100 to 355 mm

extended piston rod code letter: K

piston diameter (optional) from 100 to 355 mm

ultra-sonic position measuring system code letter: U

piston diameter (optional) from 180 to 355 mm measurement­on­fluid­side

cable control code letter: S

piston diameter (optional) from 180 to 490 mm measurement on gas side by means of pressure tight cable duct max. pressure: 350 bar

magnetic flap indicator code letter: M

piston diameter (optional) from 150 to 490 mm for­slow­(<­0,5­m/s)­and­infrequent­

(<­5­/­day)­piston­movements

313 piston positioning switch:

code letter:­ UP­/­UPEX

­ different­designs,­available­for­Øi­=­150­-­490­mm­to­500­mm²/s­viscosity­+­

EX-protection LS 06.14:

standard design, viscosity of­medium­<­100­mm²/s LS 06.14 BF3:

for­highly­viscous­<­500­mm²/s SONOCONTROL 14 BF5:

application: explosion prevention, viscosity of­medium­<­100­mm²/s­

types of pistons design 1:

314