Spring hangers are a vital part of a piping system when rigid supports could not be used due to problems with piping “Liftoff” and also to excesive overloads on equipment nozzles.
In the following picture, it can be seen that a rigid support lifts off 25.72mm at an operating temperature of 302°C. Therefore, the support is not able to take any pipe load in the system due to the thermal expansion or movement of the vertical legs.
This thermal displacement can be calculated as a result of the product of the thermal expansion per unit of the material ( ASTM A106 Grade B) and the vertical length of the piping model, which is 6100mm. The ambient temperature is 21°C and the maximum or design temperature is 302°C in this case.
Thermal Expansion=∆T∗Thermal expansion coefficient∗Length
Thermal Expansion=[(Thermal expansion per mm/mm)@(Toper , Tamb)] ∗Length
Thermal Expansion=(0.00373 mm/mm) ∗ 6100mm=22.75mm.
However, this is a simple hand calculation and it doesn’t consider the stiffness of the pipe elements such as, bend, straight pipe, support stiffness, etc. Therefore, the most accurate vertical thermal expansion is not equal to 22.75mm, but instead 25.72mm, which was calculated with the software Caesar II.
Steps to select a variable spring hanger:
1. Determine the hot load or operating load and the pipe movement(working travel of the spring hanger) :
Operating load=30207N . This is the balancing load that should be carried along the thermal expansion.
Due to the stiffness of the spring hanger, the pipe has moved 74.7mm in the operating load case, which is pretty higher than the one calculated without the spring hanger. In this case, this is due to two acting forces: Thermal expansion force + Reaction force of the spring hanger (F=-kx).
2. Calculate the spring rate and the cold load
The spring rate can be selected according to the criteria of the maximum permissible deviation of 25%. Once the max. permissible spring rate is calculated, it is necessary to find the commercial spring rate in the manufacturer’s catalog. In this case, a theoretical spring rate of 101.09N/mm was calculated, while the commercial one is 88.9N/mm and t is found in the Lisega’s catalog.
Once the spring rate (stiffness) and the hot load habe been selected out of the manufacturer’s catalog, the following fomrula can be used to determine the cold load or the hanger preload due to its stiffness(k=-F/x).
Cold Load(Preload)=Hot Load+KX
% Load Variation=|(88.8N/mm.74.7mm)/30207N|
3. Type designation of the spring hanger
Now we are able to choose the type designation of the hanger, which can be either 21.64.18 or 29.64.18. The fourth digit indicates the travel range of the spring can or hanger., which is 300mm in this case.
The use of spring hangers with extra-long travel should be treated carefully, because it could have some issues regarding large spring hysteresis.