OT, Hydraulic accumulators

[rsdoran]

I agree it is about energy BUT there are rules (laws) to how energy is created and/or transfered.

Like I said, there are some differences in how Peter has to control hydraulics with motion control but it still does not change the laws/physics involved.

Y'all take something that is fairly simple and try to make it sound complicated. There is always pressure on everything, it is called gravity. In a system that pumps water, especially upward, into a tank there will be some added pressure from the weight of the water, it is called head pressure.

If you take that tank of water when filled and no water flowing and you read pressure at the bottom end of the tank it will show a static pressure. P = h x sg/2.31.

IF you pump water into that tank while removing it at the same rate of flow the pressure will drop because some of the pressure is converted to flow (fluid velocity).

As stated it is about energy, whether a hand pump or driven by an electric motor a pump directly transfers the fluid from one place to another with enough force to overcome opposition...whether that be head, gravity, a load, or a system designed to offer opposition to create a higher pressure.

I dislike (or disagree) with the concept of blocking off either inlet or outlet, when that is done there can be no transfer of energy i.e. the pump can not do what it is designed to do; which is to push/pull a fluid from one place to another.

In a basic hydraulic system you normally work within a specific flow rate at a designated pressure. As mentioned above when you have flow you may have a decrease in pressure, that is where accumulators come in. They are pre-charged for a designated pressure, so they fill when the system has developed its operating pressure then discharge when the system may have a pressure drop due to fluid velocity (flow).

In the end it still comes down to a pump provides flow with enough force to overcome gravity and/or head pressure but the system may need to transfer more energy then that therefore the system has to be designed to resist the flow so the fluid can develop pressure so it can store/transfer more of the primary energy it is converting.

 

[TConnolly]

It is all about energy

I'm glad someone finally said it. I wanted to bang my head on the wall as I read this thread.

Energy is proportional to the product of the pressure AND the FLOW. I recently comissioned a system that required low flow rates at high pressure. The energy required worked out to 43 HP. But it also had a portion of the cycle where a large cylinder was moved rapidly at very low pressure, less than 20 PSI. Moving that much hydraulic fluid at low pressure still requires energy because we are moving mass. It worked out to 55 HP including parasitic pump loads. Because the rapid stroke was short, lasting only a few seconds, I calculated the RMS load on the motor and determined that I could safely use a single 50HP motor and have power to spare with a single hydraulic pump with a high/low pressure compensating control on the pump. This saved a lot of money.

The system had a small accumulator to improve servo control under high pressure low flow conditions, but just suppose for the sake of example that the low pressure rapid cycle required more horsepower, lets say 75 HP. Rather than upsizing the motor and the pump I could have then installed an accumulator large enough to supply the required extra fluid and then charged the accumulator over a period of time when the pump output was not being used for other purposes.

Would that save energy? Yes. And No. At the end of a cycle I would have used the same amount of hydraulic energy. So in that sense the accumulator didn't save me any energy, it only changed when I consumed it. But by adding an accumulator, I wouldn't have been turning the mass of a larger motor and larger pump with the corresponding parasitic laods, so I would have saved some energy - you just have to understand why and where the savings occured. That's why a blanket statement that an accumulator saves energy is ipse dixit.

A variable volume pressure compensated pump saves energy because otherwise the unused pump flow would have to be dumped over a relief. Pushing hydraulic fluid over a restriction from a high pressure zone to a low pressure zone creates heat, ie, it consumes energy. We call relief valves "heaters." The pressure compensated axial piston pump removes the need to dump oil over a relief by reducing flow by controlling the angle of the pump swash plate. If the pump is doing constant work, as pressure rises volume will decrease but the horsepower will remain the same. It is all about energy.

What horsepower is required to pump 5gpm at 100 PSI? (ignore parasitic losses)
What horsepower is required to pump .1gpm at 5000 PSI?
What did the pump transmit in both cases?

 

[Peter Nachtwey]

Good post. I just want to expand.

What horsepower is required to pump 5gpm at 100 PSI? (ignore parasitic losses)
What horsepower is required to pump .1gpm at 5000 PSI?
What did the pump transmit in both cases?

I will leave that to others. I don't want to run the fun.

More fun.
Here is a question for all. What stores more energy, a cubic inch of air at 1000 psi or a cubic inch of oil at 1000 psi?

How much energy an accumulator can store is important to the engineers that are using hydraulic pumps/motors to store energy when braking in hybrid vehicles. The energy in an accumulator would not be good for moving down the highway but it sure could absorb energy while braking and cause a wheelie when starting.

From a motion control point of view the main advantage of using accumulators is that the pump only needs to be sized for the average flow plus a little more for safety. It is also possible that one pump can supply multiple actuators. Each electric motor must be sized for the peak load which can be expensive and difficult to mount if the machinery space is tight. Hydraulic accumulators lose their effectiveness as the duty cycle becomes higher because they don't have as much time to store energy.

I have a test system at Delta. It is a 2 inch diameter that is 24 inches long. I have a 5 GPM pump and a 5 gallon accumulator. The load is 600 lbs on a sled. One of the things I like to show students is how the accumulator affects the motion. With the accumulator I can extend the rod at a maximum velocity of 46 inches per second and retract it at maximum velocity of about 33 inches per second. One can see this is much more flow than the pump can produced. Not only that the pump is soooo sloooow that the motion completes before the pump comes on stroke. I can make this fast motion a few times but eventually the oil in the accumulator is used up and the system can no longer go near as fast. When the accumulator is empty or I have the accumulator isolated, the actuator slows down a lot and is just moved by the flow from the pump. It extends at about 7 inches per second and retracts at about 11 if my memory is right.

 

[TConnolly]

A pants wetting examle of accumulators in action is Kingda Ka, the roller coaster at Six Flags in New Jersey. Eight 500HP motors charge a bank of accumulators for several minutes. Then all the stored energy stored is released through 32 hydraulic motors, accelerating the roller coaster train to 128mph in 3.5 seconds and throwing it up a track 57 stories high.

http://themeparks.about.com/od/rollercoasterarticles/a/KingdaHydraulic.htm

While I haven't experienced it yet, if I'm ever anywhere near Six Flags in NJ, I plan on it.

 

[fluidpower1]

Peter wrote:
What if I pump oil up hill into a tank? The oil can flow freely but the pressure still rises at the pump. In this case one is increasing head.

In my way of thinking that is still "Resistance to Flow"????? and would be added to Conduit Resistance.

I don't think it matters what the resistance is, restriction, work, conduit length, fittings, elevation, flow direction change or anything else that makes it harder to move the fluid

Put the pump at the tank and pump the water down the hill and the heighth would appear to reduce resistance because the Head Pressure would be assisting flow.

 

[rsdoran]

What horsepower is required to pump 5gpm at 100 PSI? (ignore parasitic losses)
What horsepower is required to pump .1gpm at 5000 PSI?
What did the pump transmit in both cases?

1. 100% efficiency? Approximately .3 HP
2. Same answer as 1, you just inverted the numbers.
3. Had to rething and restate this one, if the pump is not used to control something like a cylinder, motor etc. it has not transmitted anything, it has just converted from one source of energy to another source with a potential.

What stores more energy, a cubic inch of air at 1000 psi or a cubic inch of oil at 1000 psi?

Pressure is the force applied therefore its equal.

 

[allscott]

More fun.
Here is a question for all. What stores more energy, a cubic inch of air at 1000 psi or a cubic inch of oil at 1000 psi?

I'll say a cubic inch of air. A cubic inch of oil at 1000 PSI is storing almost no energy because it isn't compressed. A cubic inch of air at 1000 PSI is compressed a lot.

 

[TConnolly]

1. 100% efficiency? Approximately .3 HP
2. Same answer as 1, you just inverted the numbers.

Intentionally, to illustrate the point.

3. Had to rething and restate this one, if the pump is not used to control something like a cylinder, motor etc. it has not transmitted anything, it has just converted from one source of energy to another source with a potential.

I could have stated the question better, sorry about that. Assuming that the pump is continously delivering 5 gpm at 100 PSI or .1gpm at 5000 psi, it is transmitting power, specifically .3HP. Flow and pressure are different in each case, but the power is the same.

 

[rsdoran]

I got you, was not sure exactly where you were going. There can be a multiplication of the forces but Work in = Work out.

 

[geniusintraining]

I'll say a cubic inch of air. A cubic inch of oil at 1000 PSI is storing almost no energy because it isn't compressed. A cubic inch of air at 1000 PSI is compressed a lot.

I say equal, 1000psi is 1000psi, oil may be more efficient, but air and oil both will retain the same energy.

If you are going to move a cylinder you can do more work with 1000psi of stored oil, but energy is equal, just the displacement is different.

But then again... this is not my strong point

 

[Peter Nachtwey]

allscott is correct

Extra credit. How much energy is there in that cubic inch of oil and cubic inch of air.

It is the nitrogen in the accumulator that stores energy. That is why one should pre-charge the accumulator as high as possible but not so high that the accumulator runs out of oil. This way the accumulator stores the most energy.

Bud, the problem I have with the pressure is resistance to flow is that is backwards thinking. I can live with a pressure drop is caused by a resistance to flow. Pressure is created by adding energy to the oil by compressing it. It isn't the resistance to flow that adds the energy to the oil.

 

[allscott]

Busy typing with one hand on the keyboard and one patting my back.


Anyway, the reason I know this is that I spent some time working on hydrostatic testing machines for testing steel pipe. The goal is to pump the pipe full of water and then pressure it up. The trick is purging the air out before pressuring up. If a piece of pipe fails at 5000psi that is 98% full of water it is fairly uneventful (ie, not much energy released). If a pipe fails that is 50% full of water and 50% air it can only be described as an explosion (ie, large amount of energy released both in the pipe and any bystandards pants!)

I don't know the answer to the extra credit question off hand but with google by my side I'm sure I can figure it out.

 

[geniusintraining]

Extra credit. How much energy is there in that cubic inch of oil and cubic inch of air.

Ok I suck (failed) on the first question, so I stayed after class for this one...

But I still can't get this either... if (energy = force x distance) and (pressure = force / area) how can you calculate 1000 psi of air?

I don't think you can truly, its not a conversion... its a transfer of energy, 1000psi is still 1000psi no mater what

To Peter's and allscott point, you can compress 1000psi or air into a smaller mass, thus this would be the reasoning for the accumulators using nitrogen and not oil

So I conclude that its determined by mass not pressure

But then again... my conclusions sometimes have (or lack of) well you know

 

[rsdoran]

The fact is a pump develops flow, since there are natural forces that oppose any action then there will be always be a resistance to the flow. To obtain "operational" pressure the circuit must be designed with added resistances. Why do you need "operational" pressure; so you can tranfer the necessary energy.

The 5GPM pump at 100 PSI needed a .3HP motor but 5GPM at 1000 PSI would be 3HP.

I can live with a pressure drop is caused by a resistance to flow.

I think that is backwards, a pressure drop would be caused by a lowering of resistance.

Pressure is created by adding energy to the oil by compressing it.

Exactly, but how do you compress it? By resisting the flow.

Been through all this afore, not going thar again, y'all play.

 

[fluidpower1]

Peter wrote;

Pressure is created by adding energy to the oil by compressing it. It isn't the resistance to flow that adds the energy to the oil.

I'll go along with that explanation if you will explain how pressure builds without resisting the added energy that produced the flow.