By Josh Cosford, Contributing Editor
When fluid power designers think about an optimized reservoir, the goal is rarely to optimize the reservoir itself, but rather to consider the needs of the hydraulic system as a whole. We like to provide the pump with superior inlet conditions that avoid cavitation, enough volume to aid with cooling and contamination control, while providing real estate for the neighborhood of filters, valve banks and other hydraulic accoutrements.
But what if we wanted only to optimize the reservoir itself to most effectively control and manage the oil it’s intended to house and circulate? What if we focused solely on maintaining fluid levels and ensuring the best possible conditions for the pump to operate?
First, let’s make this reservoir tall. The height increases the head pressure available to the pump, and to be honest, I’m not sure why more designers don’t favor tall skinny reservoirs over flat wide versions. For every inch of height above the suction port, you increase head pressure by 0.032 psi (assuming standard hydraulic oil). A six-foot tall reservoir could offer over 2 psi of head pressure to the pump, which is not exactly charge-pump level, but allows higher inlet velocity compared to the typical wide, flat JIC style reservoirs. While we’re at it, let’s seal the reservoir entirely and add a few psi of nitrogen to allow for thermal expansion.
To further improve inlet conditions above and beyond our already-cavitation-resistant tallboy, let’s also employ horn-shaped diffusers. Such devices are rarely used in fluid power, but they even out the velocity profile into the pump, reducing gradients, and lowering Net Positive Suction Head (NPSH). Although such horns need to be correctly calibrated and executed for the given inlet flow rate, they calm the inlet flow and reduce localized (incipient) cavitation, such as that created from suction strainers.
Even with an appropriately sized suction strainer, they’re complex shape leads to vortical cavitation (tiny swirling regions), tip/edge cavitation (flow over sharp edges) and unpredictable micro-cavitation. As you’d expect, our optimized reservoir laughs at suction strainers while providing the most pristine laminar flow directly into the pump suction. Such a pump should be mounted either inside the reservoir or directly adjacent, with the shortest possible flow path. Hey, we’re not claiming this is the easiest reservoir to maintain.
All filtration should take place in the settling tank, which is a small tank designed to house the return line filter assembly and provide a location for air bubbles to escape. This settling tank is also pressurized, which inherently reduces air bubbles because of the increased saturation pressure. It is connected to the primary reservoir with a pair of horn diffusers, allowing fluid to pass downstream with no drama.
The fluid entering through the horn diffuser into the primary tank is also located at the bottom, where fluid must rise over five feet (in this example) to another pair of horn diffusers that join each side of the reservoir. The height provides settling opportunities for contamination and any leftover air bubbles. Such a system encourages flow passing so smoothly that if observed through a glass side panel, it would appear as a tank of stationary, amber oil, despite a high flow rate.
Our optimized reservoir would be expensive and possibly impractical to install, but would provide the absolute best possible conditions for any pump to inhale hydraulic fluid. Despite its downsides, adopting a few of these concepts could potentially lead to higher-performing pumps with increased reliability.
