I want to share my experience about sizing of expansion tank especially for thermal oil heater. It is quite simple actually. I will give you an example.
Function of Expansion Tank
The functions of expansion tank are:
- Serves as the safe outlet for the increase in thermal fluid volume due to thermal expansion
- Provides a mechanism for venting water, incondensibles, degradation by products and entrained air during start-up and operation
Sizing Expansion Tank
When selecting an expansion tank, the system volume (including the initial fill) of the expansion tank, the operating temperature and the fluid’s coefficient of thermal expansion should all be considered. Because thermal oils expand at different rates, the expansion tank capacity always should be verified against the oil properties prior to filling the system.
There are some significant aspects that need to be taken care when designing the tank:
- Accommodating thermal expansion of the hot oil heated from minimum to maximum operating temperature
- Maintaining the NPSHr of the hot oil circulating pumps under all operational circumstances
- Venting of possible residual water present in the circuit during start-up
- Allowing filling of equipment and during re-commissioning after shut down for maintenance
The expansion tank is usually elevated so that the normal operating level of the hot oil in the vessel is higher than the highest possible hot oil level in the system.
Volume expansion is considered as difference between specific volume (m3/kg) i.e. inverse of specific gravity of hot oil at maximum and minimum operating temperature of the system which is required to be accommodated between Low liquid level and high liquid level of expansion tank. An additional 20% is added to cater for various operating upsets in the systems, such as vaporization of residual water in the system and a tube burst.
As a general rule, in the liquid phase thermal systems the volumes of expansion tank should be approximately 26%-30% of the total estimated volume of thermal fluid in the thermal system. The properly sized expansion tank should be approximately 1/4 full at start-up and 3/4 full at operating temperatures.
Calculation of Hot oil Expansion Tank
Piping volume = phi (Dp/2)2Lp where Dp = pipe ID and Lp = piping length according to plot plan.
For example, as per P&ID and plot plan, total piping hold up volume and equipment hold up volume = 8.4 m3
Consider the physical properties for commercial grade heat transfer fluid (HTF).
Expansion volume from cold start up to normal operation is considered as design case for the vessel.
If I use Therminol 62 as heat transfer fluid, then the properties is as followed.
Density at temperature 30oC = 948 kg/m3
Density at temperature 310oC = 719 kg/m3
Mass of total hold up (density at minimum operating temperature) = 8.4 x 984 = 7962.8 kg
Volume of oil required based on density at maximum operating temperature = 7962.8/719 = 11.1 m3
Expansion volume = volume at max operating temperature – volume at minimum operating temperature = 11.1 – 8.4 = 2.68 m3
Then, the total volume of expansion tank is = 2.68/0.75 = 3.57 m3
By using checalc.com we can estimate the dimension of the vessel. By using above data we find:
- Orientation: horizontal
- Vessel head: 2:1 ellipsoidal
- Length: 2938 mm
- Diameter: 1175 mm (reccommended L/D for horizontal vessel is 2.5 if the design pressure is between 0-17 barg)
- Level: 815 mm
- Total volume: 3.6105 m3
- Filled volume: 2.6877 m3
- Thermal system expansion tank design. http://www.radcoind.com/TechTips4.htm
- Technical Investigation into Thermal Oil Technology. http://www.investni.com/thermal_oil_technology_technical_investigation_report_sd_march-2010.pdf
- Design consideration of hot oil system. http://www.academia.edu/1514808/design_consideration_of_hot_oil_system
- Guides for vessel sizing. http://www.red-bag.com/jcms/engineering-guides/337-bn-eg-ue109-guide-for-vessel-sizing.html