Temperature readings are to be taken 3ft (1m) from any obstructions or the ground. Additionally allow sufficient time for the temperature readings to stabilize. Step 2 : After calculating the TSh (Tank Shell Temperature), the next step is to calculate the difference between the TSh (Tank Shell Temperature) and the TSh REF (Tank Shell Reference Temperature). Δ T = Tank Shell Temperature (TSh) minus Tank Shell Reference Temperature (TSh REF )
Step 3 : Finally, the calculated Δ T value as well as the Linear Coefficient of Thermal Expansion (α) of the tank shell material are used to calculate the CTSh (Correction for Temperature of the Shell). The majority of shore tank are made of mild steel, so in many instances the Linear Coefficient of Thermal Expansion (α) will be either 0.00000620 (U.S.C (in./in. ° F) or 0.0000112 (Metric Units (mm/mm ° C), as illustrated in the below table:
Alternatively, “API MPMS Ch 12.1.1 / EI Hydrocarbon Management HM1 Part 1” includes correction factors for effect of temperature on the tank shell for a) mild steel tanks with a linear coefficient of expansion of 0.0000062/ ° F, and whose capacity tables were calculated at a tank shell reference (TSh REF ) of 60 ° F and b) mild steel tanks with a linear coefficient of expansion of 0.00001116/ ° C, whose capacity tables were calculated at a tank shell reference temperature (TSh REF ) of 15 ° C. To source the CTSh (Correction for Temperature of the Shell) using these tables, merely calculate the TSh (Tank Shell Temperature) as per Step 1 (as above) and locate the applicable shell correction factor which corresponds to the calculated TSh (Tank Shell Temperature). The table is also printed at the back of this book.
Example:
The CTSh (Correction for Temperature of the Shell) is then calculated as follows: CTSh = 1 + 2 αΔ T + α 2 Δ T 2 Although this calculation might look complicated, it really isn’t. Example: α (Linear Coefficient of Thermal Expansion) = 0.00000620 Δ T (Tank Shell Temperature (TSh)) – Tank Shell Reference Temperature (TSh REF ) = 68 CTSh = 1 + ((2 x 0.00000620) x 68) + ((0.00000620 x 0.00000620) x (68 x 68)) CTSh = 1.00084 (rounded to 5 decimal places)
Tank Shell Temperature (TSh) = 128°F
Tank Shell Reference Temperature (TSh REF ) = 60°F
ΔT = 128 °F – 60°F
ΔT = 68
NOTE: Tank Shell Reference Temperature (TSh REF ) is the tank shell temperature for which the capacity tables were calculated to; typically, 60°F, 15°C, and 20°C, although other temperatures are used. NOTE: When calculating ΔT, it is important to maintain the arithmetic sign, as this value can be positive or negative and must be applied as such in the CTSh (Correction for Temperature of the Shell) formula.
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