Hanger Sizing

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Hanger Sizing

Spring Selection Procedure in CAESAR II

20 October, 2004CAESAR II Seminar

IntroductionVertical supports for deadweight piping loads are easily located and sized.Differential (thermal) growth between the support structure and the pipe complicates the support selection.A choice must be made between rigid, variable load, and constant effort restraints.

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Deadweight Only

With no thermal growth, a weight analysis with a Y restraint at the support location will produce a load (DW) that can be used to size the rod.

DW


Deadweight Only

DW

The rod diameter is selected to carry the calculated deadweight (DW) at the support location.

33


Add Thermal Effects

As pipe heats up the load on the rod shifts to the pipe, increasing the primary (and secondary) stress and increasing the anchor load.


What If This Is Unacceptable?

The pipe may lift off the support.The support may hold the pipe down.Redistributed pipe stress may be excessive.Support loads, too, are redistributed and they may be excessive elsewhere in the system.

44


A Force Can Replace That Hard Restraint…

DWDW

InstalledPosition

OperatingPosition

……A Perfect SupportA Perfect Support


How Do You Do That?

DW

DW

DW

DW

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But That May Not Be Practical

The structure now carries twice the loadMaintenance may be troublesome

2*DW


A “Constant” Effort Hanger Approximates This Ideal Support

66


So What’s Wrong With That?Constant effort supports are not cheap.They allow position drift if the load is not accurate.Internal friction requires a greater load to start movement


Is There a Compromise?

Between a rigid restraint to carry the deadweight and an applied force to carry the deadweight through a thermal travel? Rigid support has k approaching ∞Constant force has k approaching 0

How about a spring support and its finite k?

77


A Spring Hanger

Min. Load

Max. Load


Only One Balance PointYou can pre-set the ideal design load (assumed “DW” in our example) for only one position.At other positions, the load will change as a function of the spring rate, k, and the position.This imbalance is usually acceptable.

88


So we can tolerate some inaccuracy…

InstalledPosition

OperatingPosition

DW(factory preset)

DW - k∆(close enough?)


Or maybe we can be smarter about it…

InstalledPosition

OperatingPosition

DW + k∆(factory preset)

DW(hits the target!)

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Introducing Hot Load and Cold Load Hot Load (HL) is the target load which the hanger should support in the operating condition.Cold Load (CL) is the intentionally incorrect load at which the spring is pre-set, in order to get to the Hot Load after moving.

CL = HL + k ∆


Load Variation Load Variation = Load Change relative to Hot Load.

| HL – CL | | k ∆ |LV = -------------- = --------

HL HL

Often limited by spec, to 10-25%.

1010


How Can You Select the Correct Spring?

It is a matter of load and deflection.The spring size (using Grinnell terminology) indicates a range of loads that can be carried by a spring.The spring figure number (again Grinnell) relates to support travel.


The Grinnell Spring Table

1111


Hanger Size vs. Load

Increasing CapacityIn

crea

sing

Loa

d


Rec

. Tra

vel

Max

. Tra

vel

Rec. Min. Load

Rec. Max. Load

Min. Load

Max. Load

Recommended & MaximumLoad & Travel

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Spring Travel

Short:Mid:Long::1:2:4

Short Range

Mid Range

Long Range


Spring Rates for the 3 Sizes

MidShortLong

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How Can Differing Travel Limits Provide the Same Load Limits?

By changing the spring rate.

(F=kδ)


Relating k & δ

δ=4:2:1

k=1:

2:4

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How Do You Select the Correct Spring Support?

It’s a matter of load and deflection.Basic input required:

Support load to be carried;Required vertical travel of the support.

Assume balancing load (DW) to be carried in the operating position.Installed load will be DW+kδ.


Determine Data to Pick the Spring

1) Calculate DW by adding a rigid vertical restraint at the hanger location and run a weight analysis.

This will estimate the natural load carried by a support at each selected locationIt can be adjusted to suit designWe usually call this the Hot Load

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Determine Data to Pick the Spring

2) Calculate δ by replacing that Y restraint with a vertical force equal to DW and run an operating analysis.

This vertical growth, δ, must be less the travel range of the supportThis δ is used with the Hot Load and proposed spring rate to calculate a proposed Cold Load


Test the First Possible SpringEnter the hanger table with the balance load – DW and the vertical growth at the support point – δ.Find a smallest spring size that can carry the operating load (DW).Use the k of the short range spring (highest k) of this size and see if it can carry the installed load (DW+kδ).

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Search for the First One That Works

If both operating load and installed load are within the recommended range for the spring, a workable spring is now identified.If not, try the midrange spring of the same size (divide k by 2).If not that, try the long range spring (once again dividing the k by 2).


Moving to Other FiguresAnd if that doesn’t work, move up to the next larger figure and repeat until a spring is found.If this fails, divide the support load by 2 (DW/2) and restart the selection process. This time selecting two springs to support the pipe.

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Variations

The pipe grows down.δ is negativeCold = DW+kδHot Load > Cold Load

Cold load design.Cold Load = DWHot Load = DW-kδ

Installed

Operating

Inst.=DW

Op.= DW+kδ


An ExampleUsing the Grinnell table, select a spring that will carry the balancing load (DW) in the operating position.

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Compute Data (DW)Run a weight analysis with a rigid vertical restraint at this location.The load on this restraint will be the balancing load for the support in the operating condition.For this example, let the load (DW) be 900 lbf.


Compute Data (δ)Now remove the rigid vertical restraint and replace it with an ideal support in the form of an upward force equal to DW.Run an operating analysis with this force and compute the vertical growth at this location.For this example, let the vertical thermal growth (δ) be +1.2 inches at this location.

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Go to the Table


Find the First Spring to Carry DW Size 9

works

2020


What’s the Spring Rate?

The short range spring rate (k) is 400 lbf./in.


Calculate the Installed LoadOperating load is 900 lbf.Op. = DWThe installed load for a short range spring is 1380 lbf.

Inst. = DW+kδ = 900 + 400(1.2)

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Check If This Spring Can Carry the Load

(The table shows only 3/4 inch travel available.)

Maximum recommended load for a Size 9 spring is 1200 lbf.


Continue the SearchClearly a short range spring does not work.Move to a midrange spring.kmid = kshort/2Try k = 200 lbf./in.Inst. = DW+kδ = 900 + 200(1.2)Inst. = 1140 lbf.This works; max. load is 1200 lbf.

2222


Check Load VariationIt is important to minimize the load shift at supports in moving from the installed position to the operating position.This is measured by Load Variation (L.V.)Load Variation = (Inst.-Op.)/Op.

Inst. = Installed (usually Cold) LoadOp. = Operating (usually Hot) Load


Using Load VariationWith DW and δ given, L.V. can only change as k changes.Remember that k changes by 4:2:1 in going from short to long range springs.Moving to the next longer spring will halve the Load Variation and the load need not be checked.

2323


Check Load VariationL.V. = load change / balance loadL.V. = (Inst.-Op.)/Op.or = (Cold-Hot)/Hot

L.V. = kδ/DWL.V. = 240/900 = 27%This load variation is excessive.Move from midrange to long range spring to cut L.V. in halfL.V.long = L.V.mid/2


Review the SelectionWe have a long range (Fig. 98), size 9 spring.It will carry a balancing load of 900 lbf. in the operating position,and carry 1020 lbf. [900+(100)1.2] in the installed position.The load variation for this spring is an acceptable 13.5%.

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CAESAR II Listing


Check the Spring CapacityMaximum recommended load = 1200Minimum recommended load = 700At 900-1020, we are in the middle; OK

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Actual Installed LoadSpring support balances the design load (DW) in the operating position. This is the Hot LoadTypically, the Theoretical Installed Load is DW+kδ. This is out of balance.The Actual Installed Load is a separate calculation to check for hanger deflection due to this imbalance.


Actual Installed LoadMost spring hangers have little difference between the Theoretical and Actual Installed Load.A flexible system or a large load variation will cause the Actual Installed Load to differ.Look at the restraint loads in the installed position to check or run the extra load case in hanger design.

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CAESAR II Hanger DataNode Info.

Design Data

Defined HangerData


You can modify the support load

Set your own Hot Load

Shift load from existing supports

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Hanger Sizing

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