Table of contents:
1. Need for
Spring Hangers
2. Types of Spring Hangers
3. Selection Procedure of Variable Effort Springs
4. Selection procedure of Constant effort springs
5. Spring Selection procedure in Caesar II
Generally the use of spring hangers shall be the last resort for a Stress Engineer. However, in some cases depending on the pipe routing and high temperature the use of spring hangers becomes unavoidable.
Some such examples are
piping connected to Pump, Compressors, Reboilers, Tanks, etc.
Whenever some rigid supports are not taking load due to its thermal movement or rigid supports are causing overloading effect to equipment connection Piping engineers suggest the use of a spring hanger to share some of the loads and to keep the piping system safe. Selection of the appropriate type of hanger support for any given application is governed by the individual piping configuration and job requirements.
2. Types of Spring Hangers
There are two types of Spring Hangers:
1. Variable Effort Spring Hangers: Load varies throughout its operating range
2. Constant Effort Spring Hangers: Load remains constant throughout its operating range.
3. Selection Procedure of Variable Effort Springs:
1. Identify the support close to the equipment nozzle that may be lifting up during the operating conditions and hence causing nozzle overload.
2. Remove the rigid support and add a spring hanger in that location.
3. Modify the load cases to include the spring effect and run the analysis. Spring shall be designed at Operating Temperature
4. Determine the hot load or operating load required and the pipe movement (up or down) at that support location.
5. Choose the spring catalogue of any vendor from the project Approved Vendor List e.g. Anvil, Lisega, PTP, Carpenter & Patterson are some of the popular names.
6. Estimate the travel range from the catalogue.
7. Select the smallest spring size which has the hot load within the working travel (midrange).
8. Note the Spring Rate or Spring Stiffness of the selected Spring Hanger.
9. Calculate the cold load as follows:Cold Load = Hot Load + Movement x Spring Rate (For pipe movement up)Cold Load = Hot Load – Movement x Spring Rate (For pipe movement down)
10. This calculated cold load shall be within the working range of the selected spring.
11. In case the calculated Cold load is not within the working range, then select higher spring size or the next travel range.
12. Then calculate the Spring Variability of the selected spring for the system Hot Load and Cold Load.
13. As a general practice, the load variability shall be up to 25% throughout the total travel. However, for critical systems such as piping connected to pumps, compressors, reboilers, etc. lesser load variation is required to meet the allowable load requirements.
Variability (%) = (Movement x Spring Rate) / (Hot Load) x 100
OR
Variability (%) = (Hot Load – Cold Load) / (Hot Load) x 100
14. However, if the load variation exceeds the allowed value, in the same load range selects a spring with lower spring rate. Else, select higher size spring.
15. Repeat the steps no. 8, 9, 10, 11 & 12 until suitable load variation is achieved.
16. Depending on the structural availability the spring can be installed as Hanger Type or “CAN” Type (Bottom Type).
17. In case of Hanger type spring the height of the box above the Pipe is also important for proper functioning of the spring. Note down the height and pipe lateral movement. Calculate the angle of this lateral deflection with respect to the spring box. This angle shall not exceed 4 Degrees. If it is more than try to install the pipe at a lower height from the pipe.
18. In case of “CAN” type spring, Stress Engineer must check the eccentricity of the spring load flange and the spring base plate while providing foundation information to civil.