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Hydraulic Tank Size

Hydraulic Tank Size

Hydraulic Tank Size

Hi All,

We currently have a emergency steering system in our vehicle that is connected to the driveline to enable power steering should the engine cut out.
The reservoir currently fitted needs to be optimize for space and I was hoping you guys could give me a hand on what the typical size is and how to improve it.

Pump: 27cc
RPM: up to 3000 rpm
Fluid: Mobile 220 ATF
Reservoir Size: 19 l fluid volume
Working Pressure: 110 bar
Maximum flow for working pressure: 30 l/min (the rest will be dumped with a flow control valve)

The system as is has been tested an we did not encounter any heating problems.
Note: The pump will always move fluid when the vehicle is moving but will only generate pressure when needed (this is about 1% of the time)

Thanks for the help!

RE: Hydraulic Tank Size

30l. 1 x flow rate.


RE: Hydraulic Tank Size

27 cc at 3000 rpm = 810 l/min No??

If your 30 is correct then it's a matter of risk and potential vortexing of the inlet pipe so maybe a tall slim tank with 10l may be good, or 5? Any disruption in flow back into the tank though and your pump runs dry - doesn't sound like a good plan for an emergency steering system to me.

We can't see the system or the volume "flex" in the system or the total volume?

19 l might be too small or has some fat in it. V difficult to see from here.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Hydraulic Tank Size

There is always pump pressure when a pump I turnings
30 l/min is 0.5 l/seç
If circulation pressure is 1 bar or 0.1 MPa heat generate is 50 Watt.
So count 0.050 kW per 1 bat pressure. So 10 bar will make 0.5 kW power into heat.
What's the ambient temperature?
What's the outside area of the reservoir?
A spherical reservoir has the smallest outside area and not as great for cooling as a narrow tall rectangular reservoir...

RE: Hydraulic Tank Size

akkamaan, so what size should the reservoir be?


RE: Hydraulic Tank Size

"There is always pump pressure when a pump I turnings"

But if the (typically) positive-displacement pump is pumping fluid into the open-center directional valve that is typical of a conventional hydraulic power steering system, there will be little pressure when there is little or no force/torque demand on the steering system, and a lot of pressure (depending on the force/torque demanded) when there is more load on the steering system.

There isn't enough information on hand to establish, even approximately, the power demand (which in turn defines the heat loading). And it will vary, in the real world, depending on what the driver is doing with the steering wheel.

Same request as in the cross-posted thread on the same topic (not supposed to do that, by the way): Schematic, please.

RE: Hydraulic Tank Size

As Brian Petersen Said, you haven't provided enough information to tell appropriate size of reservoir. It is all a matter of how much heat the pump is constantly generating and that "low" constant pressure number is the key komponent together with ambient operation temperature, air ventilation and possible reservoir design. An old classic estimate is the pump capacity per minute x2-x3 as an "appropriate guesstimate" of reservoir size. I have had heavy mobile equipment with constant flow systems that has had intense duty cycle and still managed with reservoir smaller than pump capacity.

RE: Hydraulic Tank Size

Hi Akkamaan/Brain Petersen,

Thanks for the feedback, let me provide some additional information.

The system is a emergency steering system, I am not concerned that it will overheat under pressure as this will only be for a very short while. My concern is with the circulation of the high volume of oil through the hoses. Steering gear used ZF THP 92, it has a supply for normal steering operation (this system is cooled and not a issue) and a separate supply for emergency steering that is only active should the normal supply be interrupted.

Total hoses loop length: 8m
Maximum ambient temp: 55°C (this will be the air circalating around the tank for cooling)
Surface area: 0.8 m2
Avg hose ID: 16mm

Hope this clears up things
Thanks for the inputs thus far.

RE: Hydraulic Tank Size

I have gone through above posts.
I understand that you are not concerned about heating, but this need to be considered.
As i understood this system does work in emergency only and that too for short period. Pump will start displacing the fluid as soon as prime mover is switched on (Considering pump is directly mounted on engine, with no intermediate gears) and the displaced flow will pass through hose, valves, fitting joints,manifold blocks if any.
This all parts in line will have some pressure drop and cumulative pressure value will decide constant power demand (in non working of steering).
With provided tech. details pressure drop will be around 1 to 1.5 bar and here you can calculate the power.
Apart from this tank construction plays very important role few are listed below.
1. Tank MOC (Material of construction)
2. Thickness ( Important for heat dissipation)
2. Tank position with respect to pump. ( Gravity flow to suction /negative suction)
2. Filter or strainer used in suction line if yes, then assembly type ( Vertical/horizontal, internal/external)

- Suction filter, suction strainer and suction tube position (if not bottom suction) will decide amount of fixed volume inside tank.
- Line mounted components will also have some case volume.
- Actuator volume.

RE: Hydraulic Tank Size

Generally, a tank is of little use for heat rejection. Its greater influence is in the time to reach a temperature.
A small volume warms quickly to operating temperature. A heat exchanger would better serve for heat rejection, temperature control.
A large volume warms slowly, giving more operating time before overheating. It also takes longer to cool.


RE: Hydraulic Tank Size

From what I have seen, when in running condition, the tank needs to be filled at around 40%, and minimum level when stopped should be 60%, according to the SOPs where I have worked (But this is an emergency system, so you can keep it higher). So, that means that the volume of all the pipes, valves, fittings, pump, etc. should equal to 60-40 = 20% of the tank volume. In other words, about 5 times the volume of the oil filled system. But this is only the minimum requirement. The heat dissipation needs to be considered, as stated above.

RE: Hydraulic Tank Size

Now that it's known which steering gear is in use, we can also advise the correct course of action, and it's ... ask ZF.

As far as I can tell, that is a conventional open-center power steering valve that is meant to be connected to a positive-displacement pump. Enabling power steering when the engine is off requires a pump to maintain that flow through the valve. A pressure reservoir will not work correctly.

RE: Hydraulic Tank Size

"My concern is with the circulation of the high volume of oil through the hoses."

What is your concern?


RE: Hydraulic Tank Size

For me, the first step in this puzzle is to better define what is meant by "emergency steering system"
If the purpose of the system is to move the vehicle off the road and out of the way of traffic we may only want to run the system for a matter of minutes?
Understanding this will dictate size and shape etc.

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