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Two different HPU systems in different locations with slightly different oils Chevron Ranco HD 68 and HDZ 68 have the same minimum oil temperature 55. Their flash points are are 455 and 414 F respectively. I wanted to know if the oil is a determining factor in the minimum temperature or if the system itself is more critical in this design point.

If it is the HPU, I was curious as to what factors go into determining this value.

Any help is greatly appreciated. Thanks!

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I don't design HPUs, but I'm a mechanical engineer now and have previous experience with them. I would guess that the oil viscosity is the primary driver for its minimum temperature. Below the rated temperature it'll start to solidify or gel and form something more akin to grease or gear oil than a hydraulic fluid.

The viscosity of the oil will have an impact on the system because it'll affect the pressure loss per length of supply hose. The HPU should be rated to supply the specified pressure at some maximum flowrate, plus some margin. That HPU specification should take into account the hose diameters and lengths, etc., to achieve the nominal operating pressure at the hydraulic actuator.

If you run the wrong grade oil, or use it at a lower-than-rated temperature, the oil will essentially congeal, clog the hydraulic hoses, and then you're not going to get the appropriate pressure at the actuator(s) and/or you'll blow a hydraulic hose or lift a relief valve because the HPU is overloaded trying to push that thick oil through the system.

It kind of feels like you're asking if you can use one oil in both systems because they both have the same minimum temperature, and I would caution you against that. You should contact the manufacturer and have them tell you whether or not it's acceptable, because again the system will be designed for some specific viscosity, and the viscosities of the oils may be different at the minimum temperature. What's an acceptable viscosity at 55 for one system may not be acceptable for the other system.

More than that, there may be other characteristics that the oil is specified for, such as corrosion inhibitors, pH, hose/seal compatibility, etc. I looked briefly at the difference between HD and HDZ and saw mention that HDZ is a multi-grade oil, which brings up another good point - viscosity stability. The flash points may also be similar, but the amount of lubrication you'll get as temperature increases may be different. One system may be designed such that it doesn't get very hot and doesn't need to keep its viscosity at higher temperature ranges.

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The pour point of standard ISO32 hydraulic oil is around 40F (4.5C) depending on additives. Below that temperature the oil behaves like a semi solid rather than a linear-viscous fluid. Forcing the oil through pumps / valves / elbows / lines requires far greater pressure differential, as Chuck's answer noted. Typical measurements of viscosity stop around 0C for that reason. Lower ranges become increasingly difficult / impossible to measure.

SAE 30 viscosity and density vs temperature

However that still leaves your original question "why 55F (12.8C)?" Why not 65F or 45F? There are at least 3 factors involved.

  1. Choice of oil. Standard mineral-based hydraulic oil becomes unusable near the freezing point of water. However many synthetic oils exist for use in artic / subsea / aerospace applications. For example most commercial airlines use Skydrol LD-4 with a pour point of -80F (-62C).
  2. System design. It's true that cold oil requires more pressure difference to circulate through downstream valves / elbows / lines. I would refer to this as the "pressure side" of the system. It would include everything after the pump outlet. The pressure side design is rarely the limiting factor for oil temperature (viscosity). The other consideration is the "suction side" of the system. This includes any hoses + components between reservoir outlet to pump inlet. While your pressure side typically has thousands of PSI available to move fluid from pump outlet back to the reservoir, your suction side only has about 5 PSI available to pull fluid into the pump. Without a steady flow of suction oil, the pump will cavitate and ultimately self destruct. System design does play a role here. Safer designs have pumps below oil level in reservoir. Sometimes pumps are even submerged in the reservoir to completely eliminate any hose length required for oil travel to the pump inlet. Another benefit of submerged pumps is noise reduction.
  3. Component design. Most modern hydraulic systems use solenoid valves to control flow directions downstream. All solenoid valves have performance limits. Typically the fluid flow force acts to close the valve, while the solenoid force acts to hold the valve open. Colder oil requires a greater pressure difference to flow through the valve. At some point the solenoid becomes unable to keep the valve open. However this rarely occurs due to oil viscosity alone. At normal temperatures this is usually caused by selecting a valve undersized relative to the pump.
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