Spring hanger selection and design guidelines for a Piping engineer

Spring hanger selection and design guidelines for a Piping engineer using Caesar II

Introduction:
Spring hangers are an integrated part of Piping Industry. The use of spring hangers for supporting pipe weights are welknown to every piping engineer. Whenever some rigid supports are not taking load due to its thermal movement or rigid supports are creating bad 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. There are two types of Spring hangers. a) Variable Spring Hanger- Loads vary throughout its operating range and b) Constant Spring hanger: Load remains constant throughout its operating range.
The following write up will provide a simple guideline for selection of both Variable and constant Spring hanger while analysing a piping system using Caesar II.

Selection Procedure of Variable Effort Springs:
 
spring hanger
 
1.Determine the hot load required and the pipe movement (up or down).
2.Estimate the travel range from the catalogue.
3.Select the smallest spring size which has the hot load within the working travel (mid range).
4.Ensure that the cold load lies within the working range of the spring i.e. between the two dark black lines shown in the selection chart.
Calculate the cold load as follows:
 Cold Load = Operating Load + Movement x Spring Rate (For pipe movement up)
 Cold Load = Operating Load – Movement x Spring Rate (For pipe movement down)
5. If the Cold load lies beyond the working range in the selection chart, then select higher spring size or the next travel range.
6.    Check the variability in selected spring
Generally for non critical systems, the variability is limited to 25% through out the total travel. For critical systems such as steam connections terminating at turbines and pipes connected to rotating equipment
Like compressor etc. variability is limited to 10%.If the variation exceeds the allowed value, choose higher size spring or smaller spring rate at same load range.
7.Select the type and check the feasibility of the spring depending on space available and  type of structure available.
Selection procedure of Constant effort springs: 
Constant Effort spring shall be selected where the vertical movement exceeds 50 mm, or where it is necessary to restrict transfer of load to adjacent terminal of equipment or where the Spring variability exceeds 25%.
constant spring hanger
1.      Determine the load and the total movement.
         Total movement = design movement + over travel
       Over travel = 20% of the design movement or 25mm whichever is higher.
2.    Select the spring from the load chart keeping in mind that the spring selected must lie within the working range (Between red and black line)
3.    Select the type and check the feasibility of the spring depending on space available and type of structure available.
4.   The Spring box must be able to move freely without any restriction.
5.  Stress Engineer must check the eccentricity (See Fig 1 below) of the spring load flange and the spring base plate while providing foundation information to civil.
Spring Selection procedure in Caesar II:
1.    CAESAR-II Default Setting for Hanger Selection:
Before making input for spring selection it is always better to make a default Caesar setting for hanger design.
                                       Fig 2. Caesar II Default hanger setting
2.    CAESAR-II Auxiliary Spreadsheet setting for Hanger Selection
During spring selection at a particular node the following auxiliary spreadsheet appears. The setting of this spreadsheet is to be done as illustrated in below diagram.
                                     Fig. 3  Caesar II Auxiliary spreadsheet for hanger selection
NOTE-1: Maximum Allowed Travel Limit:
This field is used to specify a limit on the amount of travel a variable support hanger may undergo.  CAESAR will be forced to select a Constant Effort Spring if the movement exceeds the limit in this field, even though a variable effort spring would have fulfilled our purpose.
Constant effort hangers can be designed forcefully by inputting a very small number i.e. 0.001 in this field.
 NOTE-2: Free Code:
Anchor or Restraints from equipment connections which are very near to the hangers are usually freed during the hanger design restrained weight run, so that loads normally going to the equipment nozzle are carried by the hanger.
The hanger can be designed to take almost the full weight of the pipe between the anchor and the hanger
Using this field enter the node number & the direction in which free code is to be used.
Free Codes are:-
1.Free the anchor or restraint in the Y direction only.
2.Free the anchor or restraint in the Y and X directions only.
3.Free the anchor or restraint in the Y and Z directions only.
4.Free all translational degrees of freedom for the anchor or restraint. (X,Y and Z)
5.Free all translational and rotational degrees of freedom for the anchor or restraint. (X, Y, Z, RX, RY, and RZ).Refer Figure below.
The option 5 above usually results in the highest adjacent hanger loads, but should only be used when the horizontal distance between the hanger and the anchor is within about 4 pipe diameters as shown in Fig 4.
                                      Fig. 4  Maximum Spring distance for using Free Code
NOTE-3:  Number of hangers at location:
For better stability, the base type spring support of 24″ and larger is used with 2 spring cans.
Few important points to keep in mind while Spring selection:
  • For can type springs the spring height should be kept minimum from stability point of view. If spring height is less the moment on spring will reduce and tilting of spring (Fig. 5) can be avoided or significantly minimized.
  • The spring which has lower spring rate will have lower load variation.
  • While designing the spring hanger the sustained sagging should be minimized within +/-1 mm so that original piping system is not strained much.
                                          Fig. 5  Effect of Spring Height
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