13.1.1 Calculation of CTSh
Example (non-insulated tank)
13.1 Correction for Temperature of the Tank Shell Any tank, when subjected to a change in temperature, will change its volume accordingly. Assuming that a tank has been calibrated in accordance with API MPMS Chapter 2, upright cylinder tanks have capacity tables based upon a specific shell temperature (TShREF). If the observed tanks shell temperature (TSh) differs from the capacity table tank shell reference temperature (TShREF), the volume extracted from that table can be corrected accordingly using the correction for temperature of the shell (CTSh). Correction for Temperature of the Shell (CTSh) (or more commonly referred to as the Tank Shell Correction Factor within the Industry) is the correction factor for the effect of the temperature, both ambient and liquid, on the shell of the tank. The application and calculation of the CTSh is included in “API MPMS Chapter 12.1.1 / EI Hydrocarbon Management HM1 Part 1: Calculation of Static Petroleum Quantities – Upright Cylindrical Tanks & Marine Vessels”. As shore tanks cannot be readily sheltered from the elements, the ambient temperature, as well as product temperature, must be considered when calculating an appropriate correction for the effect of temperature on the shell of the tank.
Tank Ambient Temperature (T a ) = 80 ° F Tank Liquid Temperature (T L ) = 135 ° F
The calculation of the CTSh (Correction for Temperature of the Shell) considers the following:
TSh REF = Tank Shell Reference Temperature
TSh (Tank Shell Temperature) = ((7 x 135) + 80) ÷ 8
TSh = Tank Shell Temperature
TSh (Tank Shell Temperature) = 128 ° F (rounded to the nearest degree)
T L = Tank Liquid Temperature T a = Tank Ambient Temperature
Ambient air temperature surrounding a storage tank can vary widely. For practical operational purposes, “API MPMS Ch 12.1.1 / EI HM1 Part 1”. recommended methods of taking the ambient temperature are: • a temperature device carried by the measurement technician into the tank area when gauging tanks. Take at least one temperature reading in a shaded area. If more than one temperature is taken, then average the readings;
α = Linear coefficient of expansion of the tank shell material
Step 1 : Firstly, the T L (Tank liquid Temperature) & T a (Tank Ambient Temperature) are used in a calculation to determine the TSh (Tank Shell Temperature): TSh for Non-Insulated Tanks = ((7 x T L ) + T a ) ÷ 8 TSh for Insulated Tanks = T L Basically, when the tank is not insulated, the TSh (Tank Shell Temperature) is considered to be 7/8 of the T L (Tank Liquid Temperature) and 1/8 of the T a (Tank Ambient Temperature), and when the tank is insulated, the TSh (Tank Shell Temperature) is deemed to be the same temperature as the T L (Tank Liquid Temperature); and therefore, the ambient temperature is not considered.
• shaded external thermometers permanently mounted in the tank farm area;
• local on-site weather stations
All on-site temperature devices used to record ambient air temperature for the calculation of the tank shell correction factors during custody transfer shall have their accuracy of ± 2 ° F ( ± 1 ° C) verified every month.
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