OT, Hydraulic accumulators

[allscott]

First of all I agree with Peter and Alaric, the energy is what matters not the chicken and egg thing however;

Let's break down the whole pressure vs flow debate into bubba terms. Picture a run of the mill 1750 era piston pump. As the piston retracts gravity (or suction) fills the cylinder cavity with water or oil. As the piston extends the fluid is pushed out. If this piston is open to the world no pressure is developed, just flow.

If you bolt a head on this pump all of a sudden there is a resistance to the flow of the oil out of the cylinder and thus pressure develops. How much pressure the pump can develop is related to the area of the piston and the amount of torque that the rod driving the piston can develop.

If when dead headed the prime mover of the pump can develop more torque and hence stress on the mechanicals of the pump than it can withstand, something breaks. This is why relief valves were invented.

Hydraulics systems aren't developed to introduce a resistance to flow in order to create pressure. It is the actuator of the system that inroduces the resistance that happens to make the pressure, ie: the cylinder that has to move a mass or the hydraulic motor that needs torque to turn something. Everything else in the system is just a parasitic loss.

So yes, pumps create flow not pressure

nuff said.

 

[Peter Nachtwey]

You were doing so well until you got to

So yes, pumps create flow not pressure

Pump convert energy from one form to hydraulic energy. Oil flows from a higher energy state to a lower energy state. In your example the oil just fell out. That doesn't mean the oil didn't have a higher energy at the exit. It could be in the form of kinetic energy or head. Usually the oil is a sump or tank and it is at least lifted a few inches.

In my 'Under Pressure' thread on Ron's forum
http://www.patchn.com/SMF/index.php?topic=612.0
my simple example is a pump because it adds energy to the system by compressing the oil. There is no flow.

It is all about the energy. Sometimes pumps create flow an no pressure. Other times they create pressure with no flow. Other times they create head. These are all different forms of energy. It all eventually gets converted to heat.

I doubt any amount of google searching is going to help allscott with the energy in a cubic inch of oil or air. The answer is can calculated on a page or less. This amount of energy in air is the hardest because the bulk modulus of air changes with the pressure.

 

[allscott]

The amount of energy changes as well with the temperature of the air, significantly. I consider this a challenge and will be back, hopefully with something insightful.

 

[TConnolly]

You all keep thinking the pump creates something. It doesn't. The pump is a power trasmission device, not a creator. Back to my earlier question, an ideal pump with a .3 hp input can can transmit power as .1 gmp at 5,000 psi or 5 gpm at 100 psi. It doesn't make only flow and it doesn't make only pressure, it transmits power as the product of both flow and pressure. Resistance will dictate the proportion of the two at a given power input (like Ohms law) but resistance itself doesn't create the pressure. If that notion was true then I could plug a carbon rod into my house and tell the electric company to eat my shorts.

 

[CroCop]

Resistance will dictate the proportion of the two at a given power input (like Ohms law) but resistance itself doesn't create the pressure. If that notion was true then I could plug a carbon rod into my house and tell the electric company to eat my shorts.

That & retire to a nice beach full of margaritas & senioritas, LOL.

 

[geniusintraining]

Pump convert energy from one form to hydraulic energy..

I disagree... I know this horse is almost dead, but... It does not 'convert' it 'transfers'

Converting is changing (you are not changing it), transferring is moving from one state to another or mass to mass, you take electrical energy to turn the pump then that energy is transferred to the oil by force then your have a charged accumulator

I doubt any amount of google searching is going to help allscott with the energy in a cubic inch of oil or air. The answer is can calculated on a page or less. This amount of energy in air is the hardest because the bulk modulus of air changes with the pressure

So is the reason we use air/nitogen in an accumulator because we can compress the air to a smaller mass, you could do the same with oil but it would just have to be a larger tank

 

[Peter Nachtwey]

I disagree... I know this horse is almost dead, but... It does not 'convert' it 'transfers'

Converting is changing (you are not changing it), transferring is moving from one state to another or mass to mass, you take electrical energy to turn the pump then that energy is transferred to the oil by force then your have a charged accumulator

WTF?

So is the reason we use air/nitogen in an accumulator because we can compress the air to a smaller mass, you could do the same with oil but it would just have to be a larger tank

When you compress the nitrogen the mass doesn't become smaller, only the volume.

The reason why air/nitrogen can store more energy is that the
integral of Force(x)*dx as x goes from where x causes the pressure to be 14.7 psi to 1000 psi. It is a calculus problem.
This is an hint to allscott. Now calculating the energy in oil will be easy but the air is tricky because the bulk modulus of oil changes as a function of pressure.

This document has the formula for calculating the bulk modulis of air.
ftp://ftp.deltamotion.com/public/PDF/SpringEffectEffBulkModl.pdf
I did not write this .pdf and I don't remember where I got it was so long ago. There was no copyright notice so I use it.

 

[rsdoran]

I have limtited time at this moment to respond but a quick response is, the end result is you transfer the input energy to output energy BUT in the process you may convert from different forms of energy.

One of the reasons we advance in science and engineering is that those, like Peter, question things. In this case there is no doubt in my mind Peter know and has made great strides in advanced concepts but at the same time loses focus on basic concepts like Fluipower 101.

I am writing an essay or paper to try and explain my perception of the facts and what is taught, I will try to post it later.

 

[fluidpower1]

"Pressure is Resistance to Flow" may not be the correct way to define a principle BUT:

When I am Trouble Shooting a non-working hydraulic circuit and the designer specified a Flow Meter and a Pressure Gauge at the Pump Outlet and the Flow Meter says 20 GPM and the Pressure Gauge reads 17 PSI I know there is a path to tank with 17 PSI resistance.

If I operate a Directional Control Valve to attempt to make an actuator move and it does not move, and the Flow Meter still reads 20 GPM and he Pressure Gauge still reads 17 PSI, I don't replace the pump I look for an open path to tank through some component or any other possible path. If pressure builds some amount, but not enough to overcome the actuators required force and the Flow Meter drops to 0 GPM then I replace the pump.

Still sounds to me like Pumps produce FLOW Resistance to FLOW produces pressure.

 

[rsdoran]

First let’s get some understanding of terms and/or laws of physics that may be involved.


The Law of Conservation Mass and Matter states that mass nor matter may be created nor destroyed except in a nuclear reaction. This concept is widely used in many fields such as chemistry, mechanics, and fluid dynamics.


The Law of Conservation of Energy states that energy cannot be created or destroyed, but can change its form.


DEFINITIONS
Convert: to change (something) into a different form.


Create: this word can be used in different contexts. In the laws it refers to make something come into being without something being there in the first place. Another context is “cause to happen” by intention or design.


Flow: movement in or as if in a stream, to circulate, an act of flowing.


Transfer: to be moved from one place to another


One, I am basically referring to Positive Displacement; there is some differences with centrifugal.


Yes it is about the transferal of energy.


Hydraulics is like any subject, it is taught with certain understandings just like any subject.


Scientists, engineers, and teachers have been teaching that pumps (PD at least) create fluid flow; this has been done since before I was born.


These pumps are usually self priming so when they are operating the inlet does develop a negative pressure to replace fluid at the inlet but they PUSH the fluid out at a predetermined rate for a predetermined rpm for a range of pressures.


Let’s say that an electric motor drives the pump, you are then using electrical energy to create (cause to happen) rotational energy that is input into the pump which uses the rotational energy to create (cause to happen) fluid flow.


At this point you have transferred a small amount of energy, as mentioned 5 gpm @ 100 psi is .3 HP but also converted forms of energy.


To put this simple if you just looped the outlet back to the tank then the pump could pump at its rated value with little pressure involved but at the same time little work could be obtained.


It was stated the actuator, defined as the motor or cylinder, determines the pressure. This is incorrect; it is the system that sets the pressure. It is the designer’s job to determine how much work will need to be done and what components to use to transfer the necessary energy to perform that work.


Once you have created (cause to happen) flow from the pump you then add the components that will inhibit (restrict) the system to create (cause to happen) a predetermined pressure.


This is where the system “attempts” to compress the oil; which in turn develops the pressure; which in turn transfers energy into the fluid.


Are there many more variables involved with developing, maintaining, and using this pressure? YES


Are there more variables involved in the conversion and maintaining of the energies involved? YES.


To me the concept is simple, the pump moves the fluid, and you use components to develop a means to transfer energy with this flowing fluid.


Peter is an engineer that is definitely well versed in motion control using hydraulics and many advanced concepts. I still believe that he tries to apply too much of the advanced concepts to the basics.


This “paper” has probably exhausted all I could ever say on this subject.

 

[plchacker]

This “paper” has probably exhausted all I could ever say on this subject.

I highly doubt that Ron. A man with your experience and understanding certainly could if prodded add to the paper. Sill, you sir are correct in every aspect. I'd bet you know how to feel an operating accumulator to see if it is over or under charged.



I look for and read a lot of Peter's work. Motion control is fascinating and math is what defines a science. Clearly Mr. Natchwey understands the science of fluid dynamics better than most, at least when it concerns hydraulic control systems.



My only real addition is this: Accumulators store energy. If you are working on a hydraulic system please make sure that you have isolated/bled the accumulators before opening the system. That energy will be released in the form of pressure and flow as soon as the system is compromised. If you don't know what an accumulator looks like, find someone who does.

 

[TConnolly]

This document has the formula for calculating the bulk modulis of air.
ftp://ftp.deltamotion.com/public/PDF/SpringEffectEffBulkModl.pdf

Interesting document Peter. It motivated me into risky behavior: thinking.

Many years ago not long after my introduction to servo hydraulics I observed two things that would dramatically degrade the performance of a servo system. One is entrained air in the oil and the other is very hot oil. They are of course obvious, but taking it to the next level...

Before seeing the article I had never actually bothered to mathematically consider entrained air's impact on the bulk modulus of the hydraulic fluid. The hotter the oil the lower its viscosity and that obviously has an effects, but I got to wondering about the effects of temperature on the bulk modulus of hydraulic oil. Its not an area that I have ever explored. So I googled it and found a very recent article (last week) from Hydraulics & Pneumatics Magazine.

From the article

Raising the temperature of commercial hydraulic fluid by 100° F alone reduces its bulk modulus to 61% of its roomtemperature value. Introducing 1% air by volume reduces the bulk modulus to 55% of its room temperature value. If these two conditions occur simultaneously, the net effect is to reduce the bulk modulus by 67%.

Really interesting stuff. Thanks for the link and thoughts.

 

[allscott]

My only real addition is this: Accumulators store energy. If you are working on a hydraulic system please make sure that you have isolated/bled the accumulators before opening the system. That energy will be released in the form of pressure and flow as soon as the system is compromised. If you don't know what an accumulator looks like, find someone who does.

Amen. Hydraulic accumulators are very common where I work, however I'm surprised at how many new maintenance people to our facility have never seen one. When anyone asks me what "that bottle thing does" I tell them it is a bomb and treat it as such.

To anyone who is involved in specifying, designing, or installing a hydraulic system with accumulators please do two things.

1. Install a safety relief that dumps all of the system including the accumulators in the event of the pump loosing power (lockout E-stop etc...) whenever practical. I know there are some systems where this isn't practical as has been mentioned but if you can, do it.

2. Install pressure gauges at each accumulator just because it makes me feel better when I have to crack a line.

Not only is pressurised hydraulic fluid dangerous, it stinks too. I've been covered in it one too many times......

As far as Peter's challenge goes. I spent some time with Mr. Google along with some text books I haven't opened in years and have decided that I am beat.

It soon became apparent that some calculus would be invlolved which brought back a nervous tick that I haven't had since taking my final calculus course in college.

So to some one with some more math skills than me, take up the challenge. Like I tell everyone, I'm just the guy that hooks the wires up.

 

[Peter Nachtwey]

At the risk of ruining allscott's challenge

Attached is the solution to one of my extra credit questions about how much energy is in a cubic in of oil compressed to 1000 psi. It isn't much.
I had this done so I can post it. However, if you work through the calculations you will see that the lower the bulk modulus the higher higher the amount of energy store in the oil. I should recalculate using a bulk modulus of 100000psi

When doing motion control you want to have the controlled object have the exact amount of kinetic energy since kinetic energy is related to the speed. Some of the energy that gets added to the system is wasted due to friction. Some of it is absorbed in the oil. ( potential energy ) Oil can absorb absorb a lot of energy. This is why one want to reduce the trapped volume of oil between the piston and the valve as much as possible. Hoses absorb energy too. The problem the control system has it that it must compenstate for energy absorbed in oil when accelerating and the energy release by oil when decelerating. This is why there is acceleration feed forwards. Without the feed forwards the actuator would always lag while accelerating and lead or overshoot when decelerating.

 

[allscott]

Yep, glad I didn't go any further. I really like the "simplified manually" comments.

We all have a purpose in this world. Some people are simply on different planes than others. I have met some truely brilliant people in my career (Peter being one of them), that think in different leagues than most of us. It is because of these people that technology advances. It is up to the rest of us to try and keep up.