DanielBednar
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Hydraulic reservoir pressurization/ cavitation?

Thu Aug 08, 2019 4:19 pm

Hello,

Why is the hydraulic reservoir pressurized by bleed air? I’ve read it’s to reduce the probability of cavitation and fluid foaming at high altitudes and low pressures. But what is cavitation and why does it happen? Why would pressurization help with that? And what does foam have to do with it? Thanks a lot for answering in advance.
 
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Starlionblue
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Re: Hydraulic reservoir pressurization/ cavitation?

Thu Aug 08, 2019 4:27 pm

Cavitation is the formation of bubbles in a liquid. Foam is a manifestation of cavitation. Air bubbles in hydraulic fluid are bad because air is compressible whilst fluid is not.

Ensuring that hydraulic fluid stays pressurised mitigates cavitation. If you allow a fluid to expand into a space rapidly you can get cavitation. Not sure if the example is completely accurate, but think of shaking a bottle of champagne and then popping the cork. You'll get foam. Conversely, foam will not form if the champagne stays compressed.
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Lpbri
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Re: Hydraulic reservoir pressurization/ cavitation?

Thu Aug 08, 2019 6:58 pm

A 787 hydraulic reservoir does not use pressurized air. An internal piston keeps a positive pressure (75 psi ) a the outlet of the reservoir.
 
stratclub
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Re: Hydraulic reservoir pressurization/ cavitation?

Thu Aug 08, 2019 9:43 pm

Lpbri wrote:
A 787 hydraulic reservoir does not use pressurized air. An internal piston keeps a positive pressure (75 psi ) a the outlet of the reservoir.
Correct. It is called a boot strap reservoir and uses a very small part of system pressure through a tiny piston coupled to a large piston to maintain a positive pressure inside the reservoir. This was done out of necessity do to the fact that bleed air is not tapped off of the engines except for engine nose cowl anti-ice and active clearance control on the engine's compressor.

The DC-10 used bootstrap reservoirs to good effect. So I guess Boeing did get something of intrinsic value for merging with McDougless.

The reason for pressurizing the reservoirs is to maintain a bubble free supply of hydraulic fluid to the hydraulic pumps.
 
DanielBednar
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Re: Hydraulic reservoir pressurization/ cavitation?

Fri Aug 09, 2019 1:17 am

Thanks guys, another follow-up question. I’ve heard the term “hydraulic head pressure” mentioned a lot while trying to get an answer to this question. What does that mean? Thanks again
 
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Starlionblue
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Re: Hydraulic reservoir pressurization/ cavitation?

Fri Aug 09, 2019 1:34 am

AFAIK, head pressure is a measure of the pressure the system can deliver.
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fr8mech
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Re: Hydraulic reservoir pressurization/ cavitation?

Fri Aug 09, 2019 2:15 am

DanielBednar wrote:
Thanks guys, another follow-up question. I’ve heard the term “hydraulic head pressure” mentioned a lot while trying to get an answer to this question. What does that mean? Thanks again


Really, really simple, in hydraulics, head pressure is the pressure exerted on the system by the fluid above the system...or something like that.

In an aircraft hydraulic system, we refer to head pressure as the air pressure in the reservoir.
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DanielBednar
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Re: Hydraulic reservoir pressurization/ cavitation?

Fri Aug 09, 2019 2:49 am

Okay, so basically, just as high altitudes and low pressures are harmful for humans, the same can be said for pumps. At low pressures, cavitation will cause bubbles to form in the fluid at the pump inlet which can be harmful. So they put bleed air inside to pressurize so that cavitation won’t happen and to have a a positive supply of fluid to the pumps. The “head pressure” is just the pressure of the bleed air above the fluid in the reservoir.

So do I understand that right or am I missing something integral????
 
GeneralTiger
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Re: Hydraulic reservoir pressurization/ cavitation?

Fri Aug 09, 2019 3:11 am

Here you can find something like this: https://www.allicdata.com/
 
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Starlionblue
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Re: Hydraulic reservoir pressurization/ cavitation?

Fri Aug 09, 2019 4:23 am

DanielBednar wrote:
Okay, so basically, just as high altitudes and low pressures are harmful for humans, the same can be said for pumps. At low pressures, cavitation will cause bubbles to form in the fluid at the pump inlet which can be harmful. So they put bleed air inside to pressurize so that cavitation won’t happen and to have a a positive supply of fluid to the pumps. The “head pressure” is just the pressure of the bleed air above the fluid in the reservoir.

So do I understand that right or am I missing something integral????


Seems about right.
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vikkyvik
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Re: Hydraulic reservoir pressurization/ cavitation?

Fri Aug 09, 2019 2:07 pm

fr8mech wrote:
In an aircraft hydraulic system, we refer to head pressure as the air pressure in the reservoir.


Air pressure? Or hydraulic fluid pressure?

Starlionblue wrote:
Cavitation is the formation of bubbles in a liquid. Foam is a manifestation of cavitation. Air bubbles in hydraulic fluid are bad because air is compressible whilst fluid is not.


Presumably, cavitation would cause hydraulic-fluid-bubbles, rather than air bubbles, correct?
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acmx
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Re: Hydraulic reservoir pressurization/ cavitation?

Fri Aug 09, 2019 3:02 pm

DanielBednar wrote:
Okay, so basically, just as high altitudes and low pressures are harmful for humans, the same can be said for pumps. At low pressures, cavitation will cause bubbles to form in the fluid at the pump inlet which can be harmful. So they put bleed air inside to pressurize so that cavitation won’t happen and to have a a positive supply of fluid to the pumps. The “head pressure” is just the pressure of the bleed air above the fluid in the reservoir.

So do I understand that right or am I missing something integral????


That’s basically it. Head pressure is the pressure on the fluid in the reservoir to ensure positive flow to the inlet of the pump. Its often bleed air but can be done other ways like a bootrap reservoir like was mentioned before. But it doesn’t matter if you’re at altitude or on the ground. It’s a sealed system and could cause cavitation at the pump without head pressure either way. You can hear it when you run a pump without head pressure, sometimes it sounds pretty angry.

Googling pump cavitation:
“The product of excessive vacuum conditions created at the hydraulic pump’s inlet (supply side), cavitation is the formation and collapse of vapors within a hydraulic pump. High vacuum creates vapor bubbles within the oil, which are carried to the discharge (pressure) side. These bubbles then collapse – i.e. cavitation.”

Head pressure on the supply fluid prevents this condition.
 
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fr8mech
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Re: Hydraulic reservoir pressurization/ cavitation?

Fri Aug 09, 2019 8:13 pm

vikkyvik wrote:

Air pressure? Or hydraulic fluid pressure?


Air pressure exerted on the fluid.

vikkyvik wrote:

Presumably, cavitation would cause hydraulic-fluid-bubbles, rather than air bubbles, correct?


Air bubbles & foaming. Not sure what a hydraulic-fluid-bubble within the hydraulic fluid is.
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Weatherwatcher1
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Re: Hydraulic reservoir pressurization/ cavitation?

Sat Aug 10, 2019 2:41 am

fr8mech wrote:
vikkyvik wrote:

Air pressure? Or hydraulic fluid pressure?


Air pressure exerted on the fluid.

vikkyvik wrote:

Presumably, cavitation would cause hydraulic-fluid-bubbles, rather than air bubbles, correct?


Air bubbles & foaming. Not sure what a hydraulic-fluid-bubble within the hydraulic fluid is.


Cavitation is where a fluid boils due to low pressure. If there isn’t enough head pressure to force the fluid into the pump, the suction of the pump can result in sufficiently low inlet fluid pressure so that hydraulic fluid bubbles form. The hydraulic fluid boils (not due to heat, but due to low pressure). This will quickly destroy the pump.
 
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Starlionblue
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Re: Hydraulic reservoir pressurization/ cavitation?

Sat Aug 10, 2019 3:13 am

Weatherwatcher1 wrote:
fr8mech wrote:
vikkyvik wrote:

Air pressure? Or hydraulic fluid pressure?


Air pressure exerted on the fluid.

vikkyvik wrote:

Presumably, cavitation would cause hydraulic-fluid-bubbles, rather than air bubbles, correct?


Air bubbles & foaming. Not sure what a hydraulic-fluid-bubble within the hydraulic fluid is.


Cavitation is where a fluid boils due to low pressure. If there isn’t enough head pressure to force the fluid into the pump, the suction of the pump can result in sufficiently low inlet fluid pressure so that hydraulic fluid bubbles form. The hydraulic fluid boils (not due to heat, but due to low pressure). This will quickly destroy the pump.


Lower pressure means lower boiling temperature, so I think temperature may still the cause of the boiling.
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strfyr51
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Re: Hydraulic reservoir pressurization/ cavitation?

Sat Aug 10, 2019 4:50 am

DanielBednar wrote:
Thanks guys, another follow-up question. I’ve heard the term “hydraulic head pressure” mentioned a lot while trying to get an answer to this question. What does that mean? Thanks again

Many manufacturers of hydraulic reservoirs use pressure above the upper diaphragm assy. to keep pressure on the fluid in the reservoir to prevent foaming of the fluid.
It's usually around 10 PSI.
 
masi1157
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Re: Hydraulic reservoir pressurization/ cavitation?

Mon Aug 12, 2019 7:42 am

fr8mech wrote:
In an aircraft hydraulic system, we refer to head pressure as the air pressure in the reservoir.

Which is of course the same as the pressure of the hydraulic fluid in the reservoir.

Starlionblue wrote:
Lower pressure means lower boiling temperature, so I think temperature may still the cause of the boiling.

Don't think in "boiling temperature", think in "boiling pressure". There is always some gas dissolved in the hydraulic fluid which will stay fluid itself as long the static pressure of the fluid is higher than the vapor pressure of that gas. That is what you try to maintain by applying the "head" pressure (never heard that term before) to the low pressure side of the hydraulic system. Now when you accelerate the hydraulic fluid you can locally get rather high velocity meaning high dynamic pressure and thus reduced static pressure in the fluid. If the static pressure locally drops below the vapor pressure gas bubbles will form and collapse again. That process is like a series of explosions in the fluid, that produce an awful lot of noise and can easily destroy pumps, tubes, fittings etc. Sharp edges in the low pressure flow path are particularly dangerous, because the fluid is locally accelerated around them.


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fr8mech
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Re: Hydraulic reservoir pressurization/ cavitation?

Mon Aug 12, 2019 4:36 pm

masi1157 wrote:
Which is of course the same as the pressure of the hydraulic fluid in the reservoir.


Which, is of course, controlled by the air pressure in the reservoir.

On aircraft where air is used to pressurize the reservoir, we measure and regulate air pressure, not hydraulic pressure, in terms of head pressure.
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masi1157
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Re: Hydraulic reservoir pressurization/ cavitation?

Mon Aug 12, 2019 7:09 pm

fr8mech wrote:
On aircraft where air is used to pressurize the reservoir, we measure and regulate air pressure, not hydraulic pressure, in terms of head pressure.

In a way you control both simultaneously. What you want to achieve is a certain pressure in the fluid on the low pressure side of the system. What you to achieve that is to apply the exact same pressure in the air volume in the reservoir. And what I wanted to say is, there is no need to distinguish between air and fluid pressure in the reservoir, they are the same.


Gruß, masi1157
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JayinKitsap
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Re: Hydraulic reservoir pressurization/ cavitation?

Tue Aug 13, 2019 4:02 am

masi1157 wrote:
fr8mech wrote:
In an aircraft hydraulic system, we refer to head pressure as the air pressure in the reservoir.

Which is of course the same as the pressure of the hydraulic fluid in the reservoir.

Starlionblue wrote:
Lower pressure means lower boiling temperature, so I think temperature may still the cause of the boiling.

Don't think in "boiling temperature", think in "boiling pressure". There is always some gas dissolved in the hydraulic fluid which will stay fluid itself as long the static pressure of the fluid is higher than the vapor pressure of that gas. That is what you try to maintain by applying the "head" pressure (never heard that term before) to the low pressure side of the hydraulic system. Now when you accelerate the hydraulic fluid you can locally get rather high velocity meaning high dynamic pressure and thus reduced static pressure in the fluid. If the static pressure locally drops below the vapor pressure gas bubbles will form and collapse again. That process is like a series of explosions in the fluid, that produce an awful lot of noise and can easily destroy pumps, tubes, fittings etc. Sharp edges in the low pressure flow path are particularly dangerous, because the fluid is locally accelerated around them.

Gruß, masi1157


Masi - you are spot on but it may be over the head of someone not familiar with piping systems. I'll throw out a different way to look at it.

Many of us understand air conditioning cycles which consist of a low pressure reservoir, a pump, a radiator to expel heat, an expansion valve, and a radiator to absorb heat. A refrigerant fluid is used in the system selected for its properties, while hydraulics selects for far different properties. The reservoir is basically at the pressure and temperature of its boiling point with the pump inlet below the liquid level. The pump pressurizes the fluid creating heat which is removed at the 1st radiator, then it goes thru an expansion valve, basically a pin hole. This warm liquid is suddenly in a near vacuum so it boils to a gas, the energy to boil the gas takes heat from the 2nd radiator. What happens at the expansion valve is cavitation, the total energy = kinetic energy + pressure + heat. To jump the KE, usually both pressure and temperature drop, each fluid has a boiling point for a given set of temp and pressure. Tiny drops of gas are now in the stream, but once the pressure increases at all and the bubbles collapse, it can eat thru a super hard steel valve seat in days if the conditions are bad enough. Similar in propellers and pumps, but if the pressure of the incoming fluid is high enough to avoid the boiling point in the pump, cavitation is avoided and things are smooth.

hydraulic fluids are picked for other properties but many systems basically use vegetable cooking oil as it is more safe. On an aircraft the fluid would be picked to have a low viscosity still at -70F so it isn't thick. But it must also resist degradation at high temperatures and heating. A 3,000 psi pump can be imparting 5KW per gallon flow thru the pump. It takes about 0.5KW to boil a gallon of water. At the 3,000 PSI supply pressure the hydraulic fluid does not boil, but at the pump inlet on the back side of the vanes (like lift on wings) it is enough of a vacuum that the fluid boils.

So on the closed loop system enough pressure is supplied by bleed air or a bootstrap on the low pressure side of the pump to prevent said boiling. The 15 psi head pressure on the low side is sufficient to do this. The low pressure side of the system is probably designed for 150 PSI working pressure, and the high pressure side is the 3,000 psi.
 
masi1157
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Re: Hydraulic reservoir pressurization/ cavitation?

Tue Aug 13, 2019 7:37 am

JayinKitsap wrote:
What happens at the expansion valve is cavitation

I would not call it cavitation but rather vaporization: The fluid, all the fluid, turns into a gas and stays a gas for a while. But it does not have the local and very explosive and destructive formation and collapse of gas bubbles inside the fluid. In a cavitating system it is normally not the hydraulic fluid itself but rather contamination with other gases dissolved in the fluid causing the cavitation. It is helpful here to understand the difference and relation between static and dynamic pressure of the fluid: Increasing the velocity of the fluid means higher dynamic pressure and lower (by the same amount) static pressure. That is what usually causes the local static pressure drop below the vapor pressure of the dissolved gases. Often you can avoid cavitation by simply rounding the sharp edges that caused the fluid to accelerate locally. If you really had cavitation at your expansion valve it wouldn't survive very long. [url="https://en.wikipedia.org/wiki/Cavitation"]Wikipedia[/url] has some impressive photos of damages by cavitation.


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kalvado
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Re: Hydraulic reservoir pressurization/ cavitation?

Thu Aug 15, 2019 1:27 pm

masi1157 wrote:
JayinKitsap wrote:
What happens at the expansion valve is cavitation

I would not call it cavitation but rather vaporization: The fluid, all the fluid, turns into a gas and stays a gas for a while. But it does not have the local and very explosive and destructive formation and collapse of gas bubbles inside the fluid. In a cavitating system it is normally not the hydraulic fluid itself but rather contamination with other gases dissolved in the fluid causing the cavitation. It is helpful here to understand the difference and relation between static and dynamic pressure of the fluid: Increasing the velocity of the fluid means higher dynamic pressure and lower (by the same amount) static pressure. That is what usually causes the local static pressure drop below the vapor pressure of the dissolved gases. Often you can avoid cavitation by simply rounding the sharp edges that caused the fluid to accelerate locally. If you really had cavitation at your expansion valve it wouldn't survive very long. [url="https://en.wikipedia.org/wiki/Cavitation"]Wikipedia[/url] has some impressive photos of damages by cavitation.


Gruß, masi1157

One thing to remember: pressurizing the fluid with gas increases the amount of gas dissolved in the fluid, so pressurization can make the problem you describe actually worse.
It depends a lot on the nature of fluid and gas (for example see divers using helium mixtures instead of nitrogen-oxygen mixture, aka air).
Maintaining large enough positive pressure differential between head reservoir and pump inlet to assure enough flow to pump inlet seems more plausible. Since pump inlet cannot go below zero pressure (unlike our green friends with lots of negative pressure!), that requires pressurizing things further upstream.
 
JayinKitsap
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Re: Hydraulic reservoir pressurization/ cavitation?

Thu Aug 15, 2019 6:16 pm

kalvado wrote:
masi1157 wrote:
JayinKitsap wrote:
What happens at the expansion valve is cavitation

I would not call it cavitation but rather vaporization: The fluid, all the fluid, turns into a gas and stays a gas for a while. But it does not have the local and very explosive and destructive formation and collapse of gas bubbles inside the fluid. In a cavitating system it is normally not the hydraulic fluid itself but rather contamination with other gases dissolved in the fluid causing the cavitation. It is helpful here to understand the difference and relation between static and dynamic pressure of the fluid: Increasing the velocity of the fluid means higher dynamic pressure and lower (by the same amount) static pressure. That is what usually causes the local static pressure drop below the vapor pressure of the dissolved gases. Often you can avoid cavitation by simply rounding the sharp edges that caused the fluid to accelerate locally. If you really had cavitation at your expansion valve it wouldn't survive very long. [url="https://en.wikipedia.org/wiki/Cavitation"]Wikipedia[/url] has some impressive photos of damages by cavitation.


Gruß, masi1157

One thing to remember: pressurizing the fluid with gas increases the amount of gas dissolved in the fluid, so pressurization can make the problem you describe actually worse.
It depends a lot on the nature of fluid and gas (for example see divers using helium mixtures instead of nitrogen-oxygen mixture, aka air).
Maintaining large enough positive pressure differential between head reservoir and pump inlet to assure enough flow to pump inlet seems more plausible. Since pump inlet cannot go below zero pressure (unlike our green friends with lots of negative pressure!), that requires pressurizing things further upstream.


Good practice is to always have either a diaphragm or piston between the pressurization and the fluid. Besides for gas bubbles entering, bleed air is not 'pure', there is water vapor and traces of oil and other contaminants, as well as particles. Just a smidgen of sand would tear up all of the seals quite quickly. Must be a closed system to avoid problems.

With water even the best pumps are not operated with a negative suction head more than about 5 PSI (1 atmosphere is about 14.5 psia) so the pump cannot be more than about 10 feet above the fluid level. With pumps handling more viscous fluids a positive suction head is usually required. Jet fuel and hydraulic fluids at -60F would be a bitch to pump.

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