Thermal expansion for trapped liquid in pipe

[raingal79]

Dear all,

I would like to ask for some advise regarding thermal expansion of liquid in a pipe. There are 2 cases:

1) For liquid trapped in a pipe without external heat source except from the surrounding, I want to know how can I calculate the increase in the pipe pressure if I know the expansion in liquid volume. If I can calculate this, then I will know if the pipe can withstand the delta increase and if a RV is required.

2) Heated liquid is pumped to a header through an insulated pipe. Is it true to say - thermal expansion is not a problem in this case if the liquid is trapped? Because the insulation is to maintain the temperature. Tendency would be for the pipe to cool down rather than to be heated up and expand.

Thanks.

Raingal

 

[Onno]

What temperatures are you considering. I ask you beacuse
above 100 C at 1 bar you know water boils. Or is teh system below 100 C all the time.

 

[raingal79]

Onno,

I'm not referring to water. I'm just taking a general term "liquid" as a whole. Surrounding tempt varies from 28 - 38 degC..

Raingal

 

[raingal79]

Onno,

I'm not referring to water. I'm just using a general term "liquid". Surrounding tempt is around 28 - 38 degC.

Raingal

 

[sailoday28]

Onno---How do you know the expansion in liquid volume?

 

[MikeHalloran]

1)
I assume you've already used the liquid's expansion coefficient to calculate the volume change associated with the temperature change at constant pressure.

Similarly, you use the liquid's bulk modulus to calculate the pressure required to shrink it back down to the original volume at constant temperature. The number may surprise you; in general, you want some trapped gas or other elastance in a closed system.

2)
Maybe true, but beware of buckling if the pipe is relatively thin walled.





Mike Halloran
Pembroke Pines, FL, USA

 

[sailoday28]

OOPS- sorry Onno
raingal79 (Chemical)How did you determine the expansion in liquid volume?

 

[JStephen]

You also need to figure the increase in volume of the pipe due to thermal expansion and pressure- may mostly offset the expansion of the fluid. I suppose it could be more than expansion of the fluid, so that you'd get a vacuum if you raised the temp somewhat.

Steel pipe can probably yield before bursting, which would act to relieve some of the pressure as well.

 

[25362]

These threads may contain useful information:

Thread798-96681, thread391-38740, thread391-15161, thread378-82763, thread378-55232, thread378-55160, thread378-981, thread124-102722, thread124-16098.

 

[Onno]

May I contribute some things. If I have trapped a mixture of gas and liquid in a closed volume the total enthalpy can be determined because the "state" of the mixture is exactly determined in the phase diagram.

H_tot=m_gas*h_gas+m_liq*h_liq

If the closed (static) volume is subject to some heat source or-sink the enthalpy will drop or rise with a known quantity resulting in a new "state" for the mixture.
Use a handbook for enthalpies and phase diagrams.

Bugs are thermal expansion of the liquid/gas mixture and the mechanical deformation of the tube interfering in calculations. You got to decide yourself if approxiamtions can be made giving you more or less exact results.

 

[tesser]

Not quite sure what you are after, but if you can get hold of Chemical Engineering magazine dated May 1989 there is an article dealing with relief valve sizing and predicting potential pressure rises caused by thermal expansion of liquids in piping.