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Spring hanger selection and design guidelines for aPiping engineer using Caesar II

want2learn August 25, 2015 4 Comments

Introduction:

Spring hangers are an integrated part of Piping Industry. The use of spring hangers for supportingpipe weights are welknown to every piping engineer. Whenever some rigid supports are not takingload due to its thermal movement or rigid supports are creating bad effect to equipment connectionPiping engineers suggest the use of a spring hanger to share some of the loads and to keep the pipingsystem safe. Selection of the appropriate type of hanger support for any given application is governedby 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 constantSpring hanger while analysing a piping system using Caesar II.

Selection Procedure of Variable Effort Springs:

[1]

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 blacklines 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

[2]

Generally for non critical systems, the variability is limited to 25% through out the total travel. Forcritical systems such as steam connections terminating at turbines and pipes connected to rotatingequipment

Like compressor etc. variability is limited to 10%.If the variation exceeds the allowed value, choosehigher 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 andtype ofstructure available.

Selection procedure of Constant effort springs:

Constant Effort spring shall be selected where the vertical movement exceeds 50 mm, or where it isnecessary to restrict transfer of load to adjacent terminal of equipment or where the Spring variabilityexceeds 25%.



[3]

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 theworking range (Between red and black line)

3. Select the type and check the feasibility of the spring depending on space available and type ofstructure 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 thespring base plate while providing foundation information to civil.



[4]

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 hangerdesign.

[5]

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 settingof this spreadsheet is to be done as illustrated in below diagram.



[6]

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 thisfield, 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 thisfield.

NOTE-2: Free Code:

Anchor or Restraints from equipment connections which are very near to the hangers are usuallyfreed during the hanger design restrained weight run, so that loads normally going to the equipmentnozzle are carried by the hanger.

The hanger can be designed to take almost the full weight of the pipe between the anchor and thehanger

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 whenthe horizontal distance between the hanger and the anchor is within about 4 pipe diameters as shownin Fig 4.

[7]

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. Ifspring 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 sothat original piping system is not strained much.

[8]

Fig. 5 Effect of Spring Height

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