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Outside the Box
Rethinking the Traditional Reservoir
Presented
by Dan Helgerson CFPS, CFPAI, CFPSD
International Fluid Power Society
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We learned in our hydraulics 101 course that there are four primary functions of a reservoir. First, of course, it is to store the fluid.
Second, it is to allow enough time for the air to escape. Third, is to allow enough time for the particulate matter to settle out. This reduces the amount of contamination in the fluid stream and is thought to be part of the fluid conditioning system. Fourth, it is to allow a certain amount of cooling to take place. Studies have shown that it takes approximately 2 minutes for the air to escape and for the particulate matter to settle out. That is the reason for the rule of thumb that the reservoir capacity should be 2 to 3 times the average pump flow. The idea is that the fluid will be able to reside in the reservoir for 2 to 3 minutes, allowing it time to give up its air, settle out the particles, and give up its heat.
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What actually happens inside the traditional reservoir?
Looking down through the top of the reservoir, we see the fluid enter, but find its own flow path to the outlet port.
The motion of the fluid through the reservoir causes a spinning motion in the corners of the tank.
The fluid in the corners never leaves the tank but simply swirls around and around while the working fluid rushes back out to work.
The same fluid gets used over and over again. It has little chance to give up its heat, its entrained air, or its particulate matter.
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What actually happens inside the traditional reservoir?
Looking through the side of the reservoir, we see the air bubbles being released to the top of the tank.
The particles begin to settle out of the fluid as well.
But the contamination has no place to go and settles onto the bottom of the tank.
This can turn out to be a disaster waiting to happen. Any sudden inrush of fluid, either from a large cylinder retracting or the draining of an accumulator, would stir up the particulate matter and place it back into the fluid stream.
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What actually happens inside the traditional reservoir?
Mineral oil has a relatively low specific heat, meaning it does not like to take on or give up its heat. Carbon steel and plastic each have a low specific heat.
The fluid in the center of the reservoir is insulated from the ambient air by the layers of fluid between it and the tank wall. It has to transfer its heat through the insulating layers of oil and then push it through the wall of the reservoir.
Consequently, there is relatively little heat exchange from the reservoir. In reality, the larger the reservoir, the more difficult it is for the fluid to give up its heat.
So, How does our traditional reservoir compare to its theoretical purpose?
Does it store the fluid?
Yes.
Does it remove the air?
Slowly.
Does it remove particles?
Sort of.
Does it remove heat?
Not so much.
A reservoir with 2 to 3 times the average pump flow, is going to add a lot of weight to a piece of equipment, it will take up a lot of real estate, and it will add cost due to the amount fluid purchased. It also represents a potential environmental hazard in the event of a spill.
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What about the environmental impact?
Oil that is spilled on the ground is covered by State law.
Oil that is spilled on or makes its way to water, is covered by Federal law.
There are some 15,000 pages of federal regulation that govern oil spills.
What is a “harmful quantity” of discharged oil?
A harmful quantity is any quantity of discharged oil that violates state
water quality standards, causes a film or sheen on the water’s surface,
or leaves sludge or emulsion beneath the surface. For this reason, the
Discharge of Oil regulation is commonly known as the “sheen” rule.
Note that a floating sheen alone is not the only quantity that triggers
the reporting requirements (e.g., sludge or emulsion deposited below
the surface of the water may also be reportable).
Under this regulation, reporting oil discharges does not depend on the
specific amount of oil discharged, but instead can be triggered by the
presence of a visible sheen created by the discharged oil or the other
criteria described above.
http://epa.gov/OEM/content/reporting/index.htm#oil
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What about the environmental impact?
So, we can see that any reservoir system that reduces the amount of fluid that must be stored in the reservoir;
Reduces the initial cost of installation,
Reduces the real estate required for the hydraulic system,
Reduces the amount of fluid that must be recycled,
Reduces disposal costs,
Reduces the amount of fluid that can be spilt,
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We are going to explore three types of reservoirs that have been or are being developed to better address the needs of our hydraulic systems.
First will be a reservoir made by Price Engineering. This reservoir uses the velocity of the fluid to remove entrained air from the fluid.
Second will be a Centrifugal Reservoir that has been written about in Hydraulics and Pneumatics and the Fluid Power Journal. This is a system that was intended for better contamination control. Other features will be presented which add to the usefulness of the reservoir.
Third will be a Variable Volume Reservoir that has been written about in the
Fluid Power Journal. This patented product uses an expanding bellows to provide a closed, yet variable volume, that is sized for only the differential volume of cylinders and potential thermal expansion. Fluid Power Systems Conference 2013
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The Cylindrical Reservoirs Manufactured by Price Engineering
Filter Breather
Mounting Bracket
Sight Glass
Return Line
3.1 gallon (12 liter) Capacity
Suction Line
Sized for a 47 gpm (180 liter) pump
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The Cylindrical Reservoirs Manufactured by Price Engineering
Filter Breather
Float Switch
Mounting Surface
Return Line
1.3 gallon (5 liter) Capacity
Suction Line
Sized for a 20 gpm (75 liter) pump
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The Cylindrical Reservoir Manufactured by Price Engineering
Is made up of:
Top Plate
Return Line
Suction Line
The returning fluid enters the reservoir just under the separation plate.
It makes 1 ½ quick revolutions around the tank before it exits out the suction line to the pump.
Air Vent
Centrifugal force produces Steel or Nylon layers of density, squeezing out the entrained air.
Cylinder filled with fluid
Little bubbles collide with each Separation Plate other, making bigger bubbles which are pushed even further toward the center. Entrained Air
Bottom Plate
Drain or Case Drain
The air enters the upper chamber where the fluid is relatively calm.
The air escapes through the vent.
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The Cylindrical Reservoir Manufactured by Price Engineering
The fluid traveling around the inside wall of the tank sees a pressure of 30 psig.
The fluid near the center of the vortex sees a pressure of about 0 psig.
This makes it possible to use the drain port as the connection for case drains.
Air Vent
Top Plate
Steel or Nylon Cylinder
Return Line
Separation Plate
Suction Line
Entrained Air
Bottom Plate
Drain or Case Drain
The velocity of the fluid produces a positive pressure at the suction line .
This makes it possible to locate the reservoir away from and below the pump without the fear of cavitation.
The reservoir is a guard against both aeration and cavitation.
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High Speed Video
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The Cylindrical Reservoir Manufactured by Price Engineering
The key features of this reservoir are:
The reservoir capacity is only about 2.2% of a traditional reservoir.
This means a much smaller footprint.
There is much less weight.
There is less fluid to purchase.
There is a very small environmental impact as the result of a spill.
The velocity of the fluid in the reservoir produces a positive pressure at the suction line to the pump.
Case drain flow can be directed to the bottom center of the reservoir where the pressure is about 0 psig.
The reservoir is specifically designed for the removal of entrained air.
There is very fast recovery from a large air ingestion.
A centrifugal force of 1 G holds the fluid against the wall and prevents jostling.
The small volume makes for a short warm‐up time.
Heat exchange and filtration are required as separate functions, away from the reservoir.
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Questions?
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The Cylindrical Reservoir
Filter Breather
Fluid returns to the reservoir and begins a gentile, downward spiral toward the suction line to the pump.
Top Plate
Return Line
Suction Line
This motion produces a low pressure area down the center of the cylinder,
Aluminum causing the less dense air and Cylinder
particles under 100 microns to move Kidney Return into the middle of the tank. Small air bubbles combine with other air bubbles until they are large enough to escape the fluid and leave through the breather. Larger, more dense particles are pushed to the inner wall of the cylinder.
Conical Bottom
Kidney Supply
Gravity and the steady flow to the kidney loop, pull the particles down through the conical bottom and into the fluid conditioning system.
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The Cylindrical Reservoir
Filter Breather
Top Plate
Return Line
Aluminum Cylinder
The capacity of this reservoir is determined by the volume of fluid needed for differential area cylinders and the capacity of accumulators in the circuit.
It does not depend on the required dwell time of the traditional reservoir.
Kidney Return
The removal of air and contamination is done dynamically.
It does not need a steady pump flow to keep the fluid moving at the right velocity. Suction Line
Conical Bottom
The flow “down the drain” to the kidney loop produces a constant spinning motion of the fluid.
Kidney Supply
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The Cylindrical Reservoir
Filter Breather
Top Plate
Return Line
The heat‐laden return fluid travels around the inner wall of the aluminum cylinder and is more able to release its heat energy.
Depending on the relative efficiency of the hydraulic circuit, it may be possible to reduce the size or even Kidney Return eliminate the heat exchanger.
Aluminum Cylinder
A cylindrical reservoir has a smaller footprint than a rectangular reservoir of the same capacity.
Suction Line
Conical Bottom
Compared to the traditional reservoir, this cylindrical reservoir uses less fluid, takes up less space, is lighter in weight, removes contamination more quickly, and provides better heat transfer.
Kidney Supply
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The Cylindrical Reservoir
The key features of this reservoir are:
The capacity is that which is required to accommodate the differential volume of cylinders, the volume in accumulators and the requirement of a relatively slow rotation of fluid. The reservoir will have a smaller footprint than a rectangular one of equal volume.
The system is part of the fluid conditioning system and assumes a kidney loop for filtration.
Heat exchange and filtration are part of the design intent of this reservoir.
The removal of entrained air is also part of the intent of this reservoir.
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Questions?
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The Variable Volume Reservoir
Patented and Manufactured by Smart Reservoir
Filled with fluid, cylinder retracted.
Filled with fluid, cylinder extended.
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The Variable Volume Reservoir
Case drain or kidney loop port; SAE 6, 8 or 12
Air Bleed Valve
The clear plastic cover acts as a sight glass.
This provides visual evidence of the air purge process at installation.
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The Variable Volume Reservoir
This is a video of the VVR expanding and contracting with the movement of a differential volume cylinder.
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The Variable Volume Reservoir
The key features of this reservoir are:
The capacity is only that which is required to accommodate the differential volume of cylinders and the thermal expansion that may occur. The reservoir does not “breathe” and so there is no ingression of moisture or otherwise contaminated air.
The reservoir size is independent of pump flow or system pressure.
If used for hydraulic motors and/or double rod or opposing cylinders, only the potential change in volume due to thermal expansion needs to be considered.
The reservoir is spring offset in the collapsed position. As the volume in the return line increases above the volume demanded by the pump, the reservoir expands, providing a positive pressure at the suction line of between 0 and .6 bar (1 to 9 psig). Multiple pumps can be used on a single reservoir system using a common manifold.
Multiple reservoirs can be mounted on a manifold to accommodate greater volume requirements.
Heat exchange and filtration are required as separate functions away from the reservoir.
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Reservoir Comparison
Cylindrical, High Cylindrical, Low Velocity Velocity Features
Reservoir
Reservoir
Reduced Weight
Yes
Yes
Reduced size
Yes
Yes
Reduced Volume
Yes
Yes
Multiple Pumps Let’s Talk
Yes
Accumulators
No
Yes
Large Differential Volumes
No
Yes
Large Capacity
No
Yes
Potential Heat Exchanger
No
Yes
Good for Filtration
No
Yes
Good for Air Removal
Yes
Yes
Independent of pump flow
No
Let's Talk
Independent of system pressure
Yes
Yes
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Variable Volume Reservoir
Yes
Yes
Yes
Yes
Let's Talk
Yes
Yes
No
No
Let's Talk
Yes
Yes
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Questions?
Thank you!
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