Your 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). This is why I often refer to hydraulic fluid as the most important component of the system, and certainly not something to be purchased on price alone.

But regardless of whether you use a synthetic, a high viscosity index maximum-efficiency hydraulic fluid (MEHF), ashless, zinc-free, multigrade or monograde, or any other of the myriad of options available today, to do its job well, your hydraulic fluid needs a little help from its friends. The first of these – and possibly the hydraulic fluid’s best friend – is the reservoir or tank.

Figure 1. Ideal Tank Construction for Air Release

Size Matters

Traditionally, recommended tank size for mineral hydraulic oils has been three to five 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 five to eight times pump delivery per minute is recommended.

Clearly, the above formulas were not devised to sell more oil or 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 more of a dream than reality.

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 hog-ties the hydraulic fluid. How? In order to answer this question, the traditional functions of the hydraulic tank – and how these functions can (or can’t) be subrogated to the hydraulic fluid’s other “friends” in the system – must be considered.

Beyond its most rudimentary role of providing fluid storage, the main functions of the hydraulic tank are to dissipate heat and allow contaminants to settle out of the fluid. 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. When it comes to contaminants, the tank’s role in settling out particles and water can be largely subrogated to the 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): the release of entrained air.

Air entrained in hydraulic fluid affects the performance and reliability of the hydraulic system in a number of ways, including:

  • reduced bulk modulus, resulting in spongy operation and poor control system response;
  • increased heat load;
  • reduced thermal conductivity;
  • increased oxidation and thermal degradation (dieseling) of the fluid;
  • reduced fluid viscosity, which leaves critical surfaces vulnerable to wear;
  • cavitation erosion (gaseous cavitation);
  • increased noise levels; and,
  • decreased system efficiency.

I’ve seen much anecdotal evidence over the years which suggests skimping on tank volume compromises hydraulic system reliability. One example that comes to mind is the case of a hydraulic excavator manufacturer who, after increasing tank size and installed cooling capacity, saw typical pump life increase from 12,000 hours to 20,000 hours. This reinforces the point that no matter how good the hydraulic fluid is, it needs help from its friends.

Shape is Important, Too

When it comes to releasing entrained air from the fluid, oil volume and dwell time in the tank is very important, but so too is the way the tank is constructed.

Figure 1 shows ideal tank construction for air release. The tank shown has a longitudinal baffle separating the return from the pump intake. Return fluid is made to travel the full length of the tank twice and pass through a diffuser (designed to collect and float off air bubbles) before re-entering the pump intake.

As an aside, with this tank construction, if the pump was to become noisy, aeration can be ruled out as a possible cause in this design because it is “filtered” out by the diffuser. This leaves vaporous cavitation as the likely cause of pump noise because such cavitation can’t be filtered out.

Also, note that the tank design in Figure 1 features an angled bottom plate to better facilitate drain-off of settled contaminants.

Take Care of Your Friends

From a maintenance perspective, little can be done (economically, at least) about installed tank volume other than specifying minimum required tank volume when ordering new equipment. But the tank, like the hydraulic system’s heat exchanger and filters, must be cared for. This involves regular drain-off of settled contaminants and occasional internal cleaning.