OEM's "Tank Design Key To Hydraulic Performance"

engine tank design

Let's start at the beginning - "Hydraulic fluid" Optimal hydraulic fluid performance is critical. It’s a power transmission device, a lubricant, provides heat dissipation and even a sealant (in most hydraulic components). This is why hydraulic fluid is often referred to as the most important component of the system.

To achieve maximum performance your hydraulic fluid needs to be operating in the most favourable conditions.

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.

The above equations were devised with hydraulic system performance and reliability in mind. With increasing demand for lighter, more compact hydraulic equipment (we see particularly in mobile markets), tank oil volumes in this equated size are becoming less of a 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 hinders the hydraulic fluid.

Beyond its primary 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
  • decreased system efficiency

There is much evidence which suggests reducing tank volume compromises hydraulic system reliability. An example we have come across is the case of a OEM customer of ours who after increasing tank size and installed cooling capacity, saw typical pump life increase from 13,000 hours to 21,000 hours. This reinforces the point that no matter how good the hydraulic fluid is the tank is critical.

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.

The below illustration 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 the below features an angled bottom plate to better facilitate drain-off of settled contaminants.

Interesting reading? OEM’s, don’t hesitate to contact me regarding your current tank requirements on 00353864681333 or at slawlor@keltechengineering.com.

“Keltech – leaders in tank manufacturing for over 20 years”


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