Hydraulic reservoir design is critical in power unit performance, so selecting the right style elements will ensure efficient circuit design.
If you have a hydraulic system, you have a reservoir. Full stop. Even “reservoir-less” systems still require an expansion tank to hold excess fluid. Aside from such unconventional designs, hydraulic reservoirs offer benefits above and beyond their prominent fluid storage role. Your hydraulic machine simply wouldn’t perform to its peak potential without the advantages a reservoir provides.
When designing a hydraulic reservoir, consider how to best take advantage of the reservoir’s resources, not the least of which is its size. The size chosen for a reservoir dictates the many parameters of its performance. Rather than simply consider hydraulic reservoir size, consider time instead. Indeed, a larger reservoir buys you more time. But time for what?
Manage heat generation
In many ways, a hydraulic reservoir is a buffer. Fluid returning from the hydraulic systems doesn’t meet the standard criteria for healthy hydraulics. The returning fluid is hot, contaminated and possibly aerated.
Heat generation in hydraulic machines is all but guaranteed. Any energy you input without creating practical work converts to pure heat alongside perhaps an iota of sound. If we provide heat a hall pass to amble around your system unchecked, damage to pumps, valves, actuators, seals, and even the fluid itself will eventually penalize you in some regard.
To reduce the damaging effects heat wracks on your valuable hydraulic components, a large reservoir volume helps in two obvious and one less-obvious way. The more fluid inside your reservoir, the longer it takes before those same heated molecules are back on the city bus travelling throughout and collecting more heat along the way. We define dwell-time as the volume of the reservoir divided by the flow rate of your pump. For example, 10 gpm of pump flow guzzled through a fifty-gallon reservoir will see its volumed exchanged every five minutes.
The above five-to-one example is typical for hydraulic systems, and in fact, many consider this to be a minimum value (See related article with sizing formula for reducing heat generation). Some feel three-to-one or less is fine, but I’m talking about optimization. When you offer the hydraulic oil more time to rest before re-entering the circuit, you provide it with more cooling time.
Simplify system design and configuration
So long as you have the real estate to accept a large reservoir, the second obvious advantage to size is surface area. Because most reservoirs are made from steel, they make excellent radiators. A large reservoir has the obvious two-fold advantage of more cooling surface and more time for the radiation to dissipate away from the system.
You expert designers know where I’m going next with my less-obvious advantage to a large reservoir: real estate. A sizeable hydraulic tank has plenty of surface area for cooling. That surface area makes a perfect canvas for laying out and mounting the various hydraulic components required to operate and condition your entire system. One of those critical components to help remove heat are coolers, and providing a large surface area for the cooler to mount is critical.
Beyond heat removal, a large tank provides an area for more than just coolers. The pump-motor group most often finds itself mounted atop the reservoir, drawing fluid up into its suction port. You’ll have to search far and wide to locate a power unit without a filter, level/sight gauge, and filler/breather mounted to it. However, nearly everything else may be mounted here, outside the actuators; the valve stack/manifold, the accumulator(s), heat exchangers and conditioning accessories make happy homes on the reservoir.
Reduce aeration and contamination
Let’s return to your consideration of time rather than just size; other factors better suit a more extensive reservoir. Just as heat has more time to escape the confines of the fluid, that extra time allows air bubbles to float up and solid particles to settle down. Should the reservoir find itself undersized, these bubbles (known as aeration) and particles may “river” right back into the pump suction where they can damage the pump or other downstream components. The aeration effect is similar to cavitation, where the bubbles implode on the pressure side of the pump, creating tiny, heated jets that damage metal.
Particle contamination may be ingested or internally generated. Should you not provide the time for particles to settle down to the bottom of the reservoir, they will happily recirculate through the rest of the downstream components (hint, it’s all of them), causing wear or even damage. Of course, every hydraulic reservoir should have a filter to trap as many particles as possible returning from the circuit. However, even the most efficient filters still allow 0.1% of all particles to pass right through. Those 0.1% remaining particles best make a bed on the reservoir floor rather than being sucked through the pump.
Consider style and mounting as well as size
On the other hand, many of the vital reservoir design elements have little to do with size but rather function within the scope of their limitations. For instance, hydraulic power units operating the few functions on machine tool applications are offered very little in floor space or surplus of electrical power and make do with tiny reservoirs. It’s rare to see a CNC mill with anything larger than a five-gallon reservoir tucked under a panel.
Small doesn’t always mean insufficient. A surprising number of components may be packed atop or within a compact power unit. Small, vertical-style tanks employ removable lids. The pump and motor essentially sandwich the lid using special bell housings, leaving the motor atop and the pump submerged. Valves, filters, coolers and electronics may still find a home, albeit with close company. However, nobody said valves or components couldn’t function below the tank lid; plumbing especially may be partially hidden away down low.
A vertical style power unit with the pump mounted below oil level provides the pump with ideal suction characteristics, preventing suction-related cavitation. The horizontal reservoir (such as JIC or DIN styles) offers a large tank top area for mounting various components, not the least of which is the pump motor group. Unfortunately, the pump must now pull fluid up from below, which creates a vacuum. Excessive vacuum results in the spontaneous formation of bubbles known as cavitation. Cavitation is bad.
The top-mounted pump doesn’t guarantee cavitation, of course. Limiting how high the pump sits above fluid level and then choosing correctly sized (i.e., large diameter) suction plumbing should satisfy the pump. You must still be cognizant of the cavitation potential, such as with cold oil, reduced pump displacement, elevation and other factors.
To prevent any potential for suction-related cavitation, design your reservoir to provide a flooded inlet. Elevating the reservoir using a steel frame and then mounting the pump below guarantees positive pressure at your pump’s suction port. The elevated reservoir takes up no more floor space but does require more height. This configuration provides you with the side benefit of an additional mounting surface not enjoyed by floor models — the bottom.
My personal favorite reservoir design is the L-shape. This tank style uses a horizontal reservoir with a large drip tray protruding out the bottom of one side of the tank (although inverted T-shaped reservoirs are also available). The L-shape design provides you excellent serviceability, where all components are easy to access. The pump mounted on the tray pulls its suction directly from the side of the tank, where a small amount of positive pressure help improves suction conditions.
Design challenges and maintenance needs
Either L-shaped or elevated designs do have some challenges you should address. Because both designs offer flooded suction, a failure in the suction plumbing spells disaster to the power unit, as an unnoticed leak may drain the entire reservoir quickly. A tank level switch should be mandatory for these designs, which will automatically E-stop the power unit should the oil level drop below a critical level.
For maintenance purposes, the L-shaped and elevated reservoirs must also use ball valves in the suction line. Closing the suction line ball valve before pump maintenance avoids the need for draining the tank before removing the pump. It’s also wise to add a locking mechanism to said ball valve to prevent accidental closing during operation or accidental opening during maintenance. Some designers go so far as to electrically monitor this critical ball valve to signal the PLC to shut the unit down should the ball valve close during operation.
Taking your reservoir design to the next level means offering more features than just physical size and pump location. The ideal design provides you with a baffle plate separating the suction and return sides of the reservoir. This baffle may have tiny cutouts at the bottom of either end, providing only a small area for the fluid to travel from the return to the suction side. This configuration prevents the rivering I mentioned earlier and nearly guarantees that aeration must settle to the top of the return side instead of flowing into the pump suction. The baffle adds buffer time to the theory expressed earlier.
Reservoirs often need maintenance, such as regular cleaning and inspection. Removable cleanout panels on either side of the reservoir allow maintenance personnel easy access to the tank’s interior when needed. And, of course, the original assembly technician installs the below-tank items via these large cutouts. If you’ve seen inside one of these reservoirs, you’d also have noticed the bottom plates are situated in a vee shape, ensuring that oil drains more efficiently when you empty the tank.
As you can see, reservoir design is more thoughtful and complex than most know. The design of your reservoir dictates your power unit’s performance and, therefore, your entire hydraulic circuit. Be sure to consider size, pump orientation and supplementary features the next time you select a reservoir for your power unit.
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Filed Under: Fluid Power Basics, Fluid Power World Magazine Articles, Reservoirs