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Hydraulic Design issues to avoid component damages

Bernd Schnabel

Parker Hannifin GmbH Pat-Parker-Platz 1, D-41564 Kaarst, Germany

Standstill of machines can be caused by different issues, mostly happen at an unwanted

and unexpected and unpredictable time, that’s unfortunately bitter reality also in

hydraulic systems.

It is a big problem when the muscles of a machine are out of function.

But it is even a bigger problem, when afterwards the root cause is not recognized and

therefore, not solved.

Especially many service- and repair reports state, “the repair costs of the damaged

components are higher than new components” or another common statement is:

” the defect is caused by oil contamination”.

Therefore it is not seldom that the same component is exchanged for the same reason at

many times.

Beside failure of hydraulic components, many companies who use hydraulic systems

express leakage as a serious trouble.

The mentioned negative impacts of hydraulic systems have motivated me to do find out:

Why do these phenomena happen nowadays so often and have they occurred

much less in the past? What has changed and why?

I am sure, I cannot answer all this questions in this article, but know for sure, that the

general topic “dirty oil” cannot be the only cause of failure, otherwise it could be tackled

by an appropriate filtration.

In many systems, we notice a perfect oil quality, proven with oil analysis but still have

sticking valves, oil leakages, broken O-rings, crying pumps, scratches in cylinders, burst

hoses, cracked pipes and oil which smells as it is burned, although we don’t see any fire.

So the main question is:

Why are we faced with these mentioned kind of problems in hydraulic systems?

In my presentation, I will try to explain the root cause for some different failures. It is

impossible to give detailed explanations but with pictures of the damages, I will try to

make some of the mentioned issues visible.

At the end of the presentation I will focus on the design tips to avoid or at least to

minimize the reason of many component failures in hydraulic systems to improve and

maximize the machine availability.

Air dissolved in the Oil

Naturally air is dissolved in the hydraulic medium. But if we let the air come outside, it

generates noise, leakages, inaccurateness, damages and a lot of other trouble.

At the end that kind of air is responsible for the “bad image of the hydraulic.

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oil molecules

Air bubbles

Undissolved air is omnipresent...

And we can see, hear and smell it:

Noises (cracking, rustling and whistling)

Shocks (pressure peaks)

Offensive smell of burnt oil

Wet and drippy pipes

Leakages are obviously on valves, cylinders and other components

3 different kinds of cavitation

Gas cavitation (Mineral oil based fluids)

Steam cavitation (HFC, HFA based fluids)

Pseudo cavitation (expansion of already existing gas bubbles by lowering a

surrounded pressure) is not part of the presentation.

Air dissolved in the oil

Air is dissolved naturally in the hydraulic medium. The small air molecules are among

bigger molecules of the hydraulic medium, embedded in dissolved form.

Air absorbing capacity

By using the “Bunsen-coefficient” (Dalton’s law)...

.. at ) and (1 bar)

… for mineral oil ca 0,08…0,09

for HFC ca 0,01…0,02

...we can calculate the amount of the absorbed air, depending on temperature and

pressure.

Model: Sack of potatoes

Big molecules of the hydraulic fluid

are represented by the potatoes.

Smaller molecules of the air represented

by small balls under atmospheric pressure

fill out the spaces between the potatoes.

This has no impact on the compressibility

of the medium.

inside

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outside

oil- molecules

Now we create a vacuum

The small balls suddenly grow bigger, escape

from the space between the potatoes and fill

out the total volume of the sack.

Now attributes of the oil are changing.

Which attributes of the medium are different?

When the saturation pressure falls under the limit, the following parameters of the fluid

are extremely impacted:

Specific weight

Compressibility

Viscosity

When the air comes out

A) In the cylinder and piping system

Instabilities

Poor resonance frequency

Hollow spaces (conglomerate of oil and air)

B) Air transported through the oil column into the oil reservoir has to have enough

residence time to release.

The pump must get clean oil – which means free of undissolved air.

Otherwise cavitation damages occur!

Air bubbles

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C) Caused for instance by a pressure drop through

hydro dynamical flow processes, the risk of cavitation is high.

This can also arise in pressurized pipe systems

(flow noise is an evidence).

Cavitation process

The gas or air bubbles are transported by the flow.

By falling under the saturation pressure the bubbles

expand and implode suddenly when they reach the

pressurized side.

This collapse causes the “microjet”.

Implosion process

(Air bubble located on material surface)

Lohe, (7), (8)

1st

step:

Expansion

(air bubble)

2nd

step:

Implosion

(the collapse)

3rd

step:

Strike

(„microjet“)

Material

Pressure at the strike point 14 000 bar

!!!!!!!!!!!!!!

Surface

is rough material

ripped out

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restricted flow redirected flow

attached

flow

separated flow

cavitation zones

separating point

vortex strings

Cavitation zones in geometric forms (7)-(TU DA)

Vacuum in case of redirection flow

p2 = 0

p1 = 40 bar T ≈ 40 °C v ≈ 10 m/s

vortex

- 0,9 bar Vacuum

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Q (v) =50% Q (v) =50%

Q (v) =100%

Cavitation in case of velocity changes

Flow noise = cavitation

Or in another way:

Without cavitation no noise

Differences of flow energy create acoustic sounds

Some cavitation drivers (summary)

Content of air (dissolved and undissolved)

Cavitation rate (“microbes or contamination factor”)

Friction losses in the flow process, depending on:

Viscosity of the medium

Flow velocity

Surface finish

Errors in the pipe and bore crossings (fraying’s)

Back pressure

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Cavitation means as well...

...Mr. Diesel is not far away.

The Diesel effect: How is it generated and what are the consequences?

The ignition

Diesel Effect in general is defined as an adiabatic compression of an air and oil

conglomerate. This mixture is produced during the cavitation process.

With a pressure jump we create a “Diesel ignition2.

The ignition formula

Assuming that the factors TA and for the medium are constant, the adiabatic

compression of the medium from p1 up to p2 level is only a question of the pressure

jump ratio (P1 / P2).

And this is typically around 1:20 (Diesel engines work with a ratio between 1:18 and

1:22)

That means at an adiabatic compression...

...we get a Diesel ignition from a pressure jump 1 to 20 bar and nevertheless also from

0,1 to 2 bar.

Temperature level at adiabatic compression...

2

1

AZ

p

pTT

TZ…Ignition temperature

TA…start temperature medium

p1…pressure point 1

p2…pressure point 2

k…. polytrophic coefficient

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Acrylic tube

vacuum by pulling the rod

through a handle

The Diesel Ignition...

...created by hand force?

Cylinder manually operated

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Creating Diesel ignition

by pushing back the rod

abruptly

Creating Diesel ignition by

injecting 2,5 cm³ air volume into

the oil and switch the valve in

8ms

Diesel Ignition crated by hand force

Test Setup

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Ash and other combustion residue

High speed image of a Diesel lightning (8)

Pressure measurements w & w / o air (8)

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Oil column stops with a pressure peak of:

pressure spec. weight (oil)

flow velocity

speed of sound (oil)

p… (N/m²) ~ 900kg/m³ v ~ 10 m/sec c ~ 1250 m/sec

p = 900 x 1250 x 10 x 10

-5

p = 112,5 bar

The pressure shock (Joukowski shock)

Closing a pipe abruptly a pressure increase will occur on a level of:

(l)

3 (without oxygen)

Diesel p ca. 40 bar

ca. 270bar

dynamic of the oil column (Joukowski

shock)

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Formula (II) in Formula (I)

By stopping an oil column a pressure pulse is generated:

Than it hits into the vacuum (oil/air hole)

It creates a pressure shock and a Diesel ignition

Pressure rise generated by a ramp whether an oil column is accelerated or decelerated, it will create a pressure rise.

The critical closing time tcrit tells the time, the pressure impulse is running two times

through the pipe (5):

Relation:

(II) c

l

critt

2

Speed of sound (oil)... c (m/s)

Critical closing time... tcrit (s)

No shock when:

realcrittt

Real closing time... treal (s)

Length of the pipe... l (m)

Stop an oil column slowly By closing a pipe with a deceleration ramp (~ 0,02 sec) a pressure rise is created on a

level of:

Length of pipe I ≈ 5 m

Closing time tcrit ≈ 0,02 sec

The oil column is reflected (depending on the oil compressibility) and still on the backwards

direction while the much faster pressure pulse (by a sonic speed of ca. 1250m/s) already is back-

p = 45 bar

Oil column stops with a pressure rise of:

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Pictures of damages

Caused by cavitation (microjet) and “Diesel effect” (ignition, temperature)

Cylinder head – cavitation damage:

Picture 1

Cavitation and Diesel damage

Pictures 2 /3

Cartridge spring Y

drain was connected

to tank line

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Diesel damage

Pictures 4 /5 / Cartridge sleeve located in the tank line of a manifold

Cavitation, Diesel and pressure shock damage

Picture 6

D3FP sleeve broken over T (Y) drain connected to T line.

Oil column in T line hits back into a vacuum.

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System design recommendations

separated chambers in the oil tank (have time to release undissolved air)

limit the temperature of the oil max. 50°C (local temperature in

components can be higher)

don’t extend the recommended oil velocities in pipes and

manifolds

suction ………..< 0,8 m/s (top mounting)

< 1,5 m/s) (supply mounting) return line <tank> (< 4 m/s)

return line……..< 4 m/s (tank)

pressure line …2-5 m/s (up to 100 bar)

pressure line …3-10 m/s (up to 350 bar)

take care to use the right sizes of components (NG 6 valve

have a max. bore diameter of 7.2 mm that means:

a velocity of more than 24 m/s @ 60 l/min)

Cavitation damages

PV pump tank top mounting

Up-stroke time too fast

The oil column in the suction side could

not follow

Pictures 7/8/9

PV pump tank top mounting

Up-stroke time too fast

The oil column in the suction

side could not follow.

Erosions due to cavitation

Erosions due to cavitation

Erosions due to cavitation

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use an appropriate filter cooling system (with internal magnet or magnet in

the tank and a high contamination means the cavitation starts earlier)

medium should have a good air release capability (< 4 min @ 50°C)

avoid abrupt flow changes (cavitation can be created depending on pressure level)

install tube bending instead of elbow gland

don’t stop the oil column in an abrupt way (creates a pressure pick of

100bar per 10m/s) install a precharge valve in the tank line (avoids vacuum in the system and

the oil column can’t flash back)

never connect pilot drain to tank line (bring it separated to the oil tank)

X

Oil tank design

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References: