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# Valve pressure drop at high viscosities / low temperature

## Valve pressure drop at high viscosities / low temperature

(OP)
I am designing a hydraulic system that needs to operate as low as -10 C with ISO22 oil. I am trying to estimate pressure drops through valves at this lower temperature.

The datasheets typically show pressure drop vs flow through the valve using an oil with a viscosity of 22-46 cSt. I am wondering how the pressure drop would change if the viscosity increased to roughly 200-300 cSt?

Thanks for the help

### RE: Valve pressure drop at high viscosities / low temperature

I am confused
Are going to cool the oil to minus 10 C during cycling?
Usually system warm quick especially when there is a pressure drop...
So do mobile systems working at minus 30 C...

### RE: Valve pressure drop at high viscosities / low temperature

Well pressure drop isn't just the valves, but also the tubing.

It's not easy to extrapoloate viscosity to pressure drop as it's not in proportion, but my guess would be about 3 to 5 times as much pressure drop to go from say 30cP to 300cP.

300cP really is pretty "thick". With smaller diameter tubing this could be a lot more.

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### RE: Valve pressure drop at high viscosities / low temperature

(OP)
Thanks for the replies.

The system needs to be functional with the thicker fluid as it can only be operated for a limited amount of time. We cannot wait for the fluid to warm up and thin out, and it needs to be functional right away at temperatures just below 0 C.

I have already accounted for the pressure drop through tubing, I am just unsure of how to scale it for the valves. For ex, if I have a needle valve that drops 100 psi at 32 cSt, how would the pressure drop change if the viscosity increases to 200-300 cSt. I want to say that it the pressure drop will be pretty similar, as the valve total distance is very small and viscous affects are less prominent on short runs.

### RE: Valve pressure drop at high viscosities / low temperature

But the valve pressure drop is proportional to flow so if the flow is reduced because the frictional resistance in increased in the tubing then it will impact unless the pressure drop int he valves is so much more than the pressure drop in the tubing. It will balance out and my guess is that flow will slowly increase as the temperature increase assuming the pump is putting out the same pressure.

For valves try this https://www.emerson.com/documents/automation/manua... and go to page 3-14

This also might help https://www.womackmachine.com/engineering-toolbox/...
or search "valve pressure drop viscosity correction"

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

### RE: Valve pressure drop at high viscosities / low temperature

Why not swap it for hydraulic oil of a lower viscosity which is more capable of flowing at the lower temperatures seen?

### RE: Valve pressure drop at high viscosities / low temperature

Hello,
I have an excel table to do these calculations. But I don't know how to pass it on to you; If you can read:
You must use cell D1, G1, I1, K1 and after column C,D,E. If you have a valve: the flow is turbulent. In this case put a length of pipe of 1mm in column C and enter column D the diameter which gives the pressure drop corresponding to the values ​​given by the manufacturer at the corresponding viscosity.

### RE: Valve pressure drop at high viscosities / low temperature

And if you have a filter or a refrigerant, you must consult the manufacturer's instructions, which often indicate the variations in pressure drop depending on the viscosity.
Good work

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