Does temperature affect pressure? If so, how?

B

berlinmony

#1
Dear Guys,
I am working in a Company as a QA engineer for Hydraulics Based Company.
Last week i am in a inspection for testing Pressure in a Hydraulic Pipe Line..When the Required Pressure was reached, I started checking for leakages in the pipe Line..No leakage was found.But the Pressure in the Pressure Gauge has reduced.
Please note the Pressure in Power Pack is 450 Bar(Input) and the pressure in the Pipe Line is 440 Bar(Output)..This Temperature was around 28 Deg Celcius and the testing was happened @ Night 11.30 pm..
My client did not accepted the smaller pressure drop and he asked me to retest the Complete Pipe Line..
I did it in the Morning Hours around 11.00 Am..The temperature was around 34 Deg Celcius. I did not found any leakage and No Drop in the Pressure also.The Input and Output reading was 450 Bar..I wondered how it happened?
My Query is ;- if temperature decreases do the pressure reduces? I am speaking completely about Hydraulic Pipe Line where I use 60CST Oil as the medium..
Kindly reply me how it happened .If there is any thery also please let me know.
Please Be noted i have raised this query to all Fortune 500 Companies who are the Kings in hydraulics..But Nobody answered..
 
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harry

Super Moderator
#2
Does temperature affects pressure? If so, how?

Read the Wikipedia reference-linkGas laws for the relationship between pressure, volume and temperature.

Combined and ideal gas laws

The combined gas law or general gas equation is formed by the combination of the three laws, and shows the relationship between the pressure, volume and temperature for a fixed mass of gas:

PV = k_5T \,

With the addition of Avogadro's law, the combined gas law develops into the ideal gas law:

PV = nkT \Rightarrow PV = nRT \,

Where R is the gas constant with a value of 8.314472(15) J K-1 mol-1

An equivalent formulation of this law is:

PV = kNT \,

where

k is the Boltzmann constant (1.381×10−23J·K−1 in SI units)
N is the number of molecules.

These equations are exact only for an ideal gas, which neglects various intermolecular effects (see real gas). However, the ideal gas law is a good approximation for most gases under moderate pressure and temperature.

This law has the following important consequences:

1. If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
2. If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present.
3. If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
4. If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.
 
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A

achorste

#3
Re: Does temperature affects pressure? If so, how?

Do the universal gas laws apply to hydraulic systems?
 

Tim Folkerts

Super Moderator
#5
I was putting together an answer about coefficients of thermal expansion, but when I re-read the question, I don't think that applies. (But since I already have it written, I left it at the end).

Theoretically, the pressure in a sealed fluid should be the same everywhere. A change in elevation between the two gauges would make a difference, but it sounds like that did not change between the two tests.

It might be possible that the pressure sensors themselves are affected by temperature.
Was there a valve between the pressure unit and pipe? For example, if the pipe was pressurized and then a valve was sealed so the pump could be shut off, then I could see that the pressure might change due to temperature changes as listed below.

Without knowing more details, it is hard to speculate too much. And I am not a hydraulic engineer - this is all theoretical musings.




The ideal gas law would not apply to a liquid. What you are interested in is the (volume) coefficient of thermal expansion for the oil and the material of the pipe.

There is some info at this wiki page: http://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion

While there is no listing for "60CST Oil", it would appear that the pressure should increase when the temperature increases.

  • The most similar liquids I could find have high coefficients of thermal expansion (gasoline @ 950 x 10^-6 /C or mineral oil @ 640 x 10^-6 /C).
  • Metals used for pipes have a small coefficients of thermal expansion (steel @ 35 x 10^-6 /C or stainless steel @ 52 x 10^-6 /C).
So when the pipe is warmed, the fluid would like to expand more than the steel. Since this can't happen in a seal pipe, the pressure would go up. Of course, the reverse would happen if the pipe cooled.

This assumes that the pipe and fluid were the same temperature all the time. However, if you put warm fluid into a cool pipe and sealed it, the fluid would cool (making it contract) but the pipe would warm (making it expand). As the fluid cooled, the pressure would slowly drop.

This is all theoretical, so talking to a hydraulic engineer might be a good plan.
 

Miner

Forum Moderator
Staff member
Admin
#7
Dear Guys,
I am working in a Company as a QA engineer for Hydraulics Based Company.
Last week i am in a inspection for testing Pressure in a Hydraulic Pipe Line..When the Required Pressure was reached, I started checking for leakages in the pipe Line..No leakage was found.But the Pressure in the Pressure Gauge has reduced.
Please note the Pressure in Power Pack is 450 Bar(Input) and the pressure in the Pipe Line is 440 Bar(Output)..This Temperature was around 28 Deg Celcius and the testing was happened @ Night 11.30 pm..
My client did not accepted the smaller pressure drop and he asked me to retest the Complete Pipe Line..
I did it in the Morning Hours around 11.00 Am..The temperature was around 34 Deg Celcius. I did not found any leakage and No Drop in the Pressure also.The Input and Output reading was 450 Bar..I wondered how it happened?
My Query is ;- if temperature decreases do the pressure reduces? I am speaking completely about Hydraulic Pipe Line where I use 60CST Oil as the medium..
Kindly reply me how it happened .If there is any thery also please let me know.
Please Be noted i have raised this query to all Fortune 500 Companies who are the Kings in hydraulics..But Nobody answered..
Are we talking static pressures or dynamic pressures? You can have dynamic pressure drops caused by restrictions in the flow of the hydraulic fluid. This can be due to friction losses, turbulent flow, reductions in line size, bends in the line, etc.
 

harry

Super Moderator
#8
I think that as the temperature increased the Wikipedia reference-linkvapor pressure of the liquid would also increase. So in a closed container the pressure of the liquid would increase.
That's how I look at it also. We are talking about a closed system where volume change can be assumed to be minimal (but can be worked out) so generally, pressure should be related to temperature change.

Do the universal gas laws apply to hydraulic systems?
It is true that the gas law may not be not applicable to liquids but within the room temperature range, 28-34 C range, the relationship between pressure, volume and temperature should be quite similar and consistent (my argument).
 
B

berlinmony

#9
Guys..Thanks for your post...
I think we don't need to speak about Static & dynamic pressure as we used to give a time duration of 1 hour to the pressure being stabilised across the pipe lines..After that we used to start the Pressure Testing...
So only Static Pressure is applicable for this...
Since we speak about Ideal Gas..is it applicable for Hydraulic Oil...60 CST oil is what I Used..It can be of any Hydraulic Oil with Grade IS22-50..
Guys please solve my issue and give me a clear cut Conclusion...
Whether there will be a pressure drop in a Hydraulic Pipe Line due the effect of Temperature..?
 

harry

Super Moderator
#10
Why don't you extend your suspicion to other things such as trapped air?

I don't have experience with hydraulic oil but I do have experience with testing of water and sewerage pipelines. We use the maintained pressure method and it always take a while before the pressure can stabilized for us to start the maintained pressure cycle. Often, this is due to trapped air and many other minor factors.


Added in edit: You will know what I mean if you ever had air trapped in the brake system of your car.
 
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