Hydraulic fluid has a BIG job to do. It’s a power transmission device, a lubricant, a heat transfer medium – and even a sealant, in some hydraulic components at least. And this is why I consider the fluid to be THE most important component of a hydraulic system. And certainly not something to be purchased on price alone.
But regardless of whether the hydraulic fluid you use is a synthetic, high VI, zinc-free, multigrade or monograde — or any other of the many options available today, to do its job well, the hydraulic fluid needs help from its friends. The first of these, and possibly the hydraulic fluid’s BEST friend, is the reservoir or tank.
Size Matters
Traditionally, the recommended size for a conventional tank design filled with mineral hydraulic oil has been 3 to 5 times Q plus a 10 percent air cushion. Where Q is pump flow per minute – or mean pump flow per minute where a variable pump is used.
For some special fluids, recommended tank size is even larger. For example, for hydraulic systems using HFC and HFD fluids, a tank volume of 5 to 8 times Q is recommended.
The thing is, the above formulas were not devised to sell more oil or to increase the size of the spill risk. They were devised with hydraulic system performance and reliability in mind.
But these days, with increasing demand for lighter, more compact hydraulic equipment – particularly in mobile markets, tank oil-volumes of this order are becoming a thing of the past.
If tank oil-volume or more precisely, the lack of it, affects hydraulic system performance and reliability, then it follows that less than ideal tank volume compromises the hydraulic fluid.
How? Well, in order to answer this question, the traditional functions of the hydraulic tank – and how these functions can or can not be subrogated to the hydraulic fluid’s other ‘friends’ in the system – must be considered.
Beyond its most basic role of providing a store of fluid, the main functions of the hydraulic tank are to:
- dissipate heat; and
- allow contaminants to settle out of the oil.
In practice, the amount of heat dissipated from even a large tank is relatively small, so this function is easily and more efficiently subrogated to a heat exchanger. And when it comes to contaminants, the tank’s role in settling out particles and water can be largely subrogated to the hydraulic system’s filters.
This leaves one important function of the tank for which there is no clear substitute – other than adequate oil volume and therefore dwell time. And that is the release of entrained air.
I’ve seen a lot of anecdotal evidence which suggests skimping on the volume of a conventional tank compromises hydraulic system reliability. One example that comes to mind is a hydraulic excavator manufacturer who, after increasing tank size and installed cooling capacity, saw typical pump life increase from 12,000 to 20,000 hours!
So if you design or build hydraulic equipment and you care about its reliability (and you should) don’t skimp on tank oil volume. Doing so can be a costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.
HI,
If the oil is not kept clean of micronic particles and if moisture is not lept at less than 200 ppm (atleast) then, no matter what volume, these contaminants create a chance for thermal degradation at the place where there is a restricted oil flow because of the tight tolerances required for the system. Thats how the hydraulic oil gets affected. Keep the oil clean and dry- always. It helps.
Thanks for the enlightenment
We have had success downsizing hydraulic tank sizes but it is not just a matter of making the tank smaller. As mentioned in the article, getting the air out is critical and the goal of all of these downsizing efforts is to remove air.
For high volume builds, the reservoir geometry can be designed to force the fluid motion to cause air to propagate out of the quicker. This can be through optimally placed weirs or setting up cyclonic motion etc. This usually involves analysis using CFD (Computational Fluid Dynamics). That expense can only be borne when you are building many, many systems.
We are starting to see components that aid in air removal such as filters that flow from inside to outside. The traditional OUT->IN filters cause the air bubbles in the fluid to become smaller. Smaller bubbles take longer to rise out of the oil to the surface as they are less buoyant. When the oil flows from in to out, the bubbles become larger. Larger bubbles are mote buoyant and rise out of the oil faster.
For some critical systems, there are vacuum sealed reservoirs which preclude any air from being entrained in the oil. Some create the vacuum at startup and seal the system. Others have active vacuum generators onboard that continuously remove air and water vapor from the oil. In this case the reservoir size needs only address the volume changes and temperature expansion/contraction of the system (plus a bit of buffer volume of course). But again, not for everyone as initial cost is higher and maintenance is more complicated.
For one off basic systems, the best path is to still follow the 3-5Q rule for standard fluids.
Hi, PMC Hydraulics has a ready solution of Cyclones which can be used in Tanks to remove the entrained air at a faster rate and thus help downsizing.
Ofcourse the heat exchanger needs to be sized adequately to compensate the heat increase due to smaller tank volume