Chapter 10J

10J. Temperature Drop in Buried Lines Using Equation 10.55 or 10.56 Back to Main

This page is used to determine two different things. Method "A" is used to calculate the required distance for the temperature to be equal to the hydrate formation temperature. Method "B" is used to calculate the outlet temperature for a specific pipe length. The equation used in Method B’s calculation is

(Equation 10.57)

Where: = temperature at

= temperature at

J = Joule-Thomson coefficient

= distance from initial point

= distance from initial point

= ground or water temperature

L =

a = dU / (mCp)

U = overall heat transfer coefficient

# B = constant Metric 408, English 24.4

= gas sp gr (air = 1.00)

= heat capacity

To calculate the temperature drop, the method of calculation must be decided. Choose Required pipe length to reach hydrate formation temperature or Pipe outlet temperature calculation by given length by clicking and shading the circle provided for that selection.

Once the method of calculation has been chosen, by clicking and shading the circle provided for that selection, the Inlet gas temperature, Water or ground temperature, Hydrate formation temperature, Pipe outside diameter, Gas flow rate, Overall heat transfer coefficient, Gas relative density, Pipe length, and Gas heat capacity of the gas must be entered into their entry spaces. To do so use the mouse to click the curser in the entry spaces and input the data. Once this has been done, select Run to execute the solution.

Water or ground temperature is not a constant quantity but depends on the air temperature to some degree. At a burial depth below the "frost line" the soil temperature generally will vary from 2-16°C [35-60°F] seasonally. About the same temperature range will be encountered with burial in water of 30 m [100ft] deep.

Pipe outside diameter is the length from one outer edge of the pipe to another outer edge of the pipe, on the exact opposite side.

Gas flow rate is the flow rate of the gas that flows inside the pipe.

Overall heat transfer coefficient (U) is the single number that represents all of the resistances in series. In a buried pipe, all of the following resistances to heat flow can occur:

1. Film coefficient between fluid and pipe wall

2. Inner pipe wall conductivity (tubing)

3. Annular space between inside and outside pipe

4. Pipe wall

5. Pipe coating

6. Insulation

7. Concrete layer or bond

8. Sand backfill

9. Native soil near pipe at temperature above normal due to heat gain from pipe

As with heat exchanger, the most reliable values of "U" are found by test.

Gas relative density is the density of the gas divided by the density of air or the molecular weight of the gas divided by the molecular weight of air at atmospheric pressure.

(1.2)

Gas heat capacity can be found from the slope of h Vs T plots at a given pressure or for pure substances are readily available from many handbooks and similar reference materials.

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