Accumulators: Energy Savers/Energy Wasters
It looks like the next few articles will be about some specific components and how they can be best utilized in a circuit for maximum energy savings. Previous articles have laid the foundation for understanding the need for efficient systems both in terms of fluid power and electricity. We will build on that knowledge as we move forward.
I started out my career in fluid power in the mobile and marine markets. We had a limited product line and an even more limited knowledge of fluid power. In some of my earliest training, when the question of accumulators came up, I was told that it would be best to stay away from them. They were really complicated and would only be confusing. I was led to believe that accumulators were only used in special industrial applications. I did not need to be bothered with them. Seven years later, I found myself working for a company that was the U.S. distributor for a major line of accumulators. It seemed the cure for every problem was to throw an accumulator into the circuit.
Somewhere between never and always is the right time and place to use an accumulator. Now, accumulators have a number of uses such as pulsation dampening, surge suppression, emergency energy storage, and cushioning, but we are only going to be looking at the applications for energy savings. We will give some guidelines to help you choose, not only whether or not to use an accumulator but also how to properly make the application for maximum energy efficiency.
As a general statement, whenever there is a substantial dwell time, i.e. when the regular source of fluid power is not being used, it is a good idea to consider the use of an accumulator in the system. Remember, from the point of view of the Power Company, the electric motor driving the system is most efficient when it is running near its nameplate power rating. If there is a dwell time when our hydraulic pump is unloaded or in low flow compensation, we tend to be pretty content because we are not generating much heat and our hydraulic system is not working very hard. However, the electric motor driving the unloaded pump becomes very inefficient. Typically, the motor cannot drop below one-third of its rated amperage load, and at that point its power factor becomes very low. The ideal situation is to find the average system flow and then use a pump that will provide only that much flow.
For example, let’s say we have a system that requires 20 gpm at 1,714 psi for 5 minutes and then rests for 5 minutes. To supply the system without an accumulator, we would have to provide a 20-gpm pump driven by a 20-hp electric motor and with some type of unloading circuit. However, during the resting time, our electric motor would be drawing about 7 hp with a low-power factor that would send a ripple back through the entire facility.
By taking the average flow requirement of 10 gpm, we have an opportunity to dramatically reduce the size of the electric motor. We could use a 10-gpm pump with a 10-hp motor and an accumulator. Half the time, we would be supplying 10 gpm more than was needed and would store the excess in the accumulator. The other half of the time we would be providing 10 gpm less than was needed, but we would borrow from what was stored in the accumulator. The electric motor would run continually at its rated power, and everybody is happy!
Okay, I see a couple of you in the back row waving frantically. What’s the matter? You say it won’t work? Why not? You want me to do the math? But the rest of these guys are sick of doing math. They want some easy fixes. Okay, Okay! I’ll do the math. Let’s see…
Oh, oh! We have a problem. If I need 10 gpm from my accumulator for 5 minutes, that means I need to have 50 gallons stored in the accumulator. When I use my handy-dandy accumulator-sizing formula and assume a pre-charge of 50% of minimum pressure (rule of thumb), it turns out we will need at least 170 gallons of accumulator gas volume with a maximum pressure of 5,000 psi. We will have to use a 30-hp electric motor to drive the pump, and we will have about a 3,300-psi pressure drop adding heat to our system. The electric motor will sometimes be operating at only 30% of its rated hp. This system will not provide any significant benefit to the facility. In fact, it will actually use more energy than if we hadn’t messed with it. The system will be more complicated and harder to maintain. It looks like it was a good thing we did the math.
Let’s change the scenario. We still have a requirement of 20 gpm at 1,714 psi, but now we will be on for 30 seconds and off for 30 seconds. We will still have the same average pump flow of 10 gpm, but we will only need 5 gallons stored in the accumulator. This makes a dramatic change in the accumulator gas volume, bringing it down to only 20 gallons. But we are still driving our pump at 5,000 psi and so will still require a 30-hp motor to operate the system. We still have a 3,300-psi pressure drop adding to the heat load. We will have done no favors to anyone.
Take a look at our “rule of thumb” pre-charge of half the minimum system pressure. Where did that come from, anyway? This is an example of a “rule of thumb” with a built-in “fudge factor.” Bladder-type accumulators have a check valve that closes to prevent the bladder from extruding into the plumbing if the gas pressure is higher than the system pressure. If the gas pressure is set right at minimum system pressure, the check valve may have a tendency to pound away on the valve seat and cause excessive wear, resulting in a premature failure. To prevent this, the pre-charge pressure needs to be below the minimum system pressure so that the fluid pressure is always holding the check valve off its seat. A pre-charge of anything below system pressure would be adequate to accomplish this, but as a safety measure and as a way to avoid doing a lot of math, using half the minimum pressure will always ensure the longevity of the valve seat. Oh, and as a side benefit, it requires a substantially larger accumulator.
So let’s pretend we really like doing our math homework and say we are quite confident that our minimum pressure is really 1,714 psi. If we pre-charge the accumulator to about 96% of minimum pressure (about 1,650 psi) and change the maximum pressure to 2,000 psi, take a look at what happens. We will now need 50 gallons of gas volume, and our power requirement will become about 12 hp. We will have less than a 300-psi pressure drop, so there will be a very manageable heat load with which to deal. If this system runs 24/7, the resultant savings will be about 55,000 kWh/yr with a cost benefit of about $6,000/yr.
When deciding to use an accumulator as an energy saving device, remember to take into consideration not only the average flow and system pressure but also the relative dwell time, the pre-charge gas pressure, and the maximum accumulator pressure. Remember also that the larger the accumulator, the lower the maximum pressure will be and the greater the energy savings. Reject the rule of thumb, forget the fudge factor, and shun the shortcut. It is tempting to be content with simply providing a smaller pump, but if it requires running at a much higher pressure, we may not be providing any substantial savings. The goal is not the smallest flow or the smallest accumulator, but the optimum system for low-energy consumption.
The previous discussion has all been related to using accumulators with a fixed gas volume. There is another way. For maximum energy efficiency, the best accumulator is one that is weighted instead of charged with a gas. In our example, we needed 5 gallons of stored fluid at 1,714 psi. If we took a 4-in. bore cylinder with a 92-in. stroke mounted vertically and placed a weight of 22,000 pounds on it, we could store our fluid in the cylinder at 1,750 psi. We could then drive our 10-gpm pump with the 10-hp motor. The resultant savings would be about 64,000 kWh/yr with a cost benefit of about $7,000/yr. This may be unconventional, but we need to allow ourselves to think creatively if we are serious about being Fluid Power Professionals.
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