Counter balance valve and pressure due to thermal expansion of oil in cylinder

[MGZmechanical]

Hi, I've a question regarding counter balance valves. In a typical circuit as attached but in my case the cylinder pulls downwards. Sometimes the load is stopped at intermmediate positions for long times and sometimes the pressure is needed to keep the load firmly attached to the ground.
The cylinder is under the sun and will not be operated for long times. So, If I think right, the oil in the cylinder will heat and expand. The counter balance valve will be closed because there's no pilot pressure and the stem will develop huge foce.
To avoid this I'd have to add an overpressure valve between the cylinder and the counterbalance but this valve is not that tight so pressure in the system will be lost in those cases I need pressure against the ground.
Is there a way, purely mechanical (without using pressure switches and PLC) to keep the cylinder in intermmediate positions for long times and avoid the thermal pressure?

Best regards,

 

[3DDave]

Even without all of that, cylinders are subject to internal leakage across the piston and are therefore not suitable for long term position and load holding.

It looks like the valve has a separate over-pressure relief in it already on the piston end.

 

[kiwitom235]

the only pressure you will develop between the cylinder and counterbalance is that of the load applied to the cylinder.

If thermal expansion occurs, it will simply push the cylinder out as the rod side is open to tank.

can you provide a drawing and hydraulic schematic of what you are doing?

do not add an RV between the counterbalance and the cylinder, the counterbalance valve should be mounted as near as possible to the cylinder in case of hose failure. The counterbalance valve acts as an RV anyway.

 

[hydtools]

You might consider an accumulator to absorb the thermal expansion if the piston and load are not free to move during the period of load holding mid-stroke.

Ted

 

[Compositepro]

An accumulator would help to keep pressure (and thus cylinder force) constant but you would completely lose control of cylinder position. So I do not think that is a viable solution.

 

[hydtools]

Perhaps another solution is a cylinder with higher pressure rating to tolerate pressure increase due to thermal expansion and load locking pilot operated check valves to hold load position.

Ted

 

[hydtools]

What is the anticipated temperature change from system operating temperature to sun-baked temperature?

Ted

 

[MGZmechanical]

Hi, the temperature wouod go up around 15 ºC so that's an incriment pressure in the oil of around 100 bar. The problem here is that the cylinder has some overstroke then ensure "positive" sealing so all the force is transmited to the stem and the structure.
The only way I've found so far in small cases is dimensioning everything against 160 bar of max pressure + 100 bar due to temperature. But with big cylinders is becoming a problem to size everything that big.

 

[kiwitom235]

manologalarraga, you descriptions are hard to understand. please post a drawing of your set-up and also a hydraulic schematic if you have it.

 

[gruntguru]

Sure the oil will expand but so will the cylinder. If both materials had the same coefficient of thermal expansion, there will be no pressure increase. The actual volume delta will depend on the difference between the two coefficients of (volumetric) thermal expansion.

je suis charlie

 

[3DDave]

gruntguru,

It's good to consider, but matching the expansion of oil to the expansion of metal is unlikley to happen.

https://en.wikipedia.org/wiki/Thermal_expansion#Apparent_and_absolute_expansion_of_a_liquid

From the Wikipedia article on the topic, steel (in SI units) is 32.4 vs gasoline at 950 and glycerin at 485, though turpentine is a low of 90. Still, between 3 and 31 times the rate.

<rant>
I'd be more interested in hydraulic oil actual values, but the lack of standardized units really irritates me. Can everyone just get along with PPM-linear and PPM-volumetric? How hard is that? So tired of 0.000005 and 0.005% and 5e-5 and 50E-6 and if those aren't the same number, then that's what I'm getting to.
</rant>