What oil reservoir size should I specify?
Is it just about “three times flow rate”?
The widespread use of the “three times flow” rule of thumb serves well but current pressures on space, economics and environmental issues warrant a closer examination of this rule.
So what are the factors to be considered?
• Hold enough oil for system function
• Sufficient surface area to dissipate heat to the surroundings
• Large enough volume so turbulence is minimized allowing entrained air to escape and contaminations to settle
• Separating the suction from the return areas.
• Access for maintenance and cleaning
• Air space conditions, pressure, dryness and cleanliness
• “Real-estate” for fitting of main system components
Basic features of a traditional oil reservoir:-
Hydraulic reservoir design recommendations and standards NFPA/T3.16.2 and ISO 4413:2010 are essential documents when designing a new hydraulic power pack.
Hydraulic reservoir size
Firstly basic hydraulic oil storage must be determined, eg cylinder displacements or accumulators volumes, these will govern the minimum quantity of oil required.
Second main consideration must be heat. Hydraulic oil is the means of power transmission around a hydraulically operated machine. Work done by the hydraulically operated machinery can be a fraction of the power of the hydraulic power pack so these in-efficiencies will therefore result in heated oil in the hydraulic reservoir.
Dissipating this heat can only be effective if there a temperature difference exists between the highest allowable oil temperature and the ambient temperature (T). Combining this with a reservoir of a sufficiently large surface area the heat can be dissipated to the environment.
In order to reduce hydraulic oil reservoir volumes several problems must be overcome.
1) Limited free fluid surface area for de-aeration. The use of baffle plates between the pump suction and return oil lines will allow time for air bubbles to rise to the surface. A return line diffuser will split the flow into numerous lower velocity paths, thus allowing more time for de-aeration.
2) Entrained air takes longer to reach the surface in thicker oil. The use of a high viscosity index oils(HVLP), these have additives that minimize these prevent the oil viscosity changes.
3) Contaminant density will be increased in a smaller oil volume, so filtration must be improved.
4) The energy transmitted by every litre of oil will be increased. This extra stress will reduce the oil life. The use of a high quality oil with additives to reduce ageing and wear and increase oil lifetime.
Power pack configurations
1) Horizontal tank top mounted – Where the pump motor assembly is mounted horizontally on top of the reservoir lid. Design of a rigid hydraulic reservoir structure and lid is important to support the rotating mass of the motor/pump and minimize vibration transmission. This configuration allows easy maintenance and access, but ideal pump suction conditions can be difficult to achieve and vibration isolation is essential.
2) Horizontal below tank – Mounting the hydraulic reservoirs above the motor pump set give a natural “head” of oil to the pump inlet which helps reduce cavitation. Access to the reservoir is simplified but the physical structure required makes this configuration bulky and more costly. A configuration commonly used for water-glycol systems.
3) Vertical submerged pumps – Pumps submerged in the hydraulic oil reservoir through the lid has several distinct advantages. Size is minimized, often utilizing a deeper tank shape. Pump noise is dampened by the surrounding oil. Suction lines can be minimized thus reducing the likelihood of cavitation.
All shapes and sizes
Hydraulic oil storage containers come in all shapes, sizes and orientations. Industrial hydraulic power packs are normally rectangular where many mini hydraulic power packs have cylindrical reservoirs. Thermodynamic considerations should be given to the shape, as hot oil rises, so the upper surfaces are most important to radiate heat. Keeping tank surfaces free from contamination and oxidation is critical to optimal heat transfer. Aluminum has a thermal conductivity three times that of steel so has become more commonly used on smaller power units.
Dependent upon the power pack configuration lifting and handling must be considered. Storage bunds are a common requirement and will normally hold 120% of reservoir volumes in the event of reservoir failure.
So the three times flow rule can be useful but is by no means a guarantee for a successful and economic hydraulic power pack design. With a thorough understanding of the complete hydraulic machine and a thermodynamic appreciation of what is happening to the hydraulic oil a more informed design can often be considered.