Flexibility Analysis for HighFlexibility Analysis for HighTemperature Piping System -Temperature Piping System -

Case Study for Combined CycleCase Study for Combined CyclePower Plant.Power Plant.

What is Pipe ?What is Pipe ? It is a Tubular item made of metal, plastic, glass etc. meant for conveying Liquid,

Gas or any thing that flows. It is a very important component for any industrial plant. And it’s engineering plays

a major part in overall engineering of a Plant

High Temperature PipingHigh Temperature Piping In Power plant there are some piping which carries steam at high pressure and

temperature.. These pipes carries the main cycle steam and water of the steam power plant.

Pipe material selection - to withstand the high pressure and high temperature.

Steam pipes run at very high temperature and the hot pipes expand.There should be enough flexibility in these pipes so that pipe can itself withstand this thermal loading and high loads should not transferred to the nozzles of Turbine or Pumps.

Pipe Stress AnalysisPipe Stress Analysis The process of checking the stress developed in the piping due to various loading is called Pipe Stress Analysis/Flexibility analysis.

It is a discipline highly interrelated with piping layout and pipe support design and normally associated with analysis of stresses in a piping system, primarily due to thermal expansion or contraction.

The objective of the Pipe flexibility analysis is to ensure safety against failure of the piping material or anchor points from overstress.

Check pipe stresses with governing codes (as Design Base Document) .

Support load & movement for various loading conditions.

Check the terminal point loading (Forces & Moments) generated from pipe to the connected equipment.

Types of LoadsTypes of Loads

Sustained Loads– Dead Weight (Weight Of

Pipe, Fittings, Fluid in Pipe, Piping Components valves, valve Operators, flanges so on.)

Thermal Expansion Loads– Due to the Temperature

Occasional Loads– Seismic– Wind– Snow and etc.,

Also loads on piping can be classified as

Static Loads– Dead Weight– Thermal Expansion and

contraction effect– Effects of Support, anchor and

thermal movements– Internal and external loadings

Dynamic Loads– Impact forces– Wind Load– Seismic Load– Steam & Water Hammer effects– Discharge Loads

Stresses in PipingStresses in Piping

Hoop’s Stress Longitudinal Stress Axial Stress Radial Stress Bending Stress Torsion The failure of structural part occurs when a certain function of the

stress or strain components reaches a critical value. The peculiarity of the piping system is such that, there are possibilities of every possible stresses being generated in it

Stresses in Piping…..Contd.Stresses in Piping…..Contd. Circumferential stresses - Due to internal pressure Bending and torsional stresses - Due to dead load, snow and

ice, wind or earthquake. Primary stresses - Due to external effects are the direct

longitudinal Due to pressure inside the pipe - Three-dimensional stresses in

longitudinal, circumferential and radial direction are generated. Bending and torsional stress - Due to thermal expansion or

contraction because of temperature variations, bending and torsional stress are generated.

There are the direct, bending and torsional stresses - Due to the restrained thermal loadings (the restrained thermal analysis, the external forces being supplied in this case by the line of anchors and other restraints.)

Methods of Flexibility AnalysisMethods of Flexibility Analysis

Code Method Approximate Methods

– Guided Cantilever Method– Chart Solutions– Mitchell Bridge Method

Exact Analytical Methods– Simplified Kellogg's Method– General Kellogg's Method– Using Finite Element Technique

Model Tests

Finite Element MethodFinite Element Method

It is a numerical method of solution of complex problems, which is based on the general principle of "going from part to whole". Finite element method converts a continuous system into a discrete system. (Linear, three dimensional finite analysis program)

Derivation of finite element equations [K] * {u} = {F}

where,

[K] = global stiffness matrix,

{U} = global displacement vector,

{F} = global load vector

Pipe Flexibility AnalysisPipe Flexibility Analysis

Inputs and Various Steps in Flexibility Analysis– Geometric layout of Pipe– Pipe supporting configuration– Pipe Diameter and Thickness– Pressure inside Pipe– Cold and Hot temperatures of

Pipe– Weight of Pipe and insulation– Weight of carrying Fluid– Pipe material Property (Young’s

Modulus, Thermal Expansion Coefficient)

– Thrust on pipe due to blowing wind.

– Thrust on pipe due to earthquake

– Load of Snow on pipe

– Any transient loading like Steam Hammer load

– Any other load on the piping

Pipe Flexibility Analysis…Contd.Pipe Flexibility Analysis…Contd.

Piping Analysis Software

– PIPSYS is a PC-based computer program. This software package is an engineering tool used in the mechanical design and analysis of piping systems.

– There are many other commercial software available are SAP-IV, COSMOS/M, NISA, CAESAR-II & CAE PIPE.

Outputs

– Stress of the pipe at various loading conditions

– Load at various supports and restrains.

– Movement of pipe at support locations

– Pipe terminal point (anchor, equipment ) loading.

Piping FlexibilityPiping Flexibility

The major requirements in high temperature piping design is to provide adequate flexibility for in the piping system to allow the thermal expansion of the pipe without causing excessive stresses and without exceeding the terminal equipment allowable loadings.

Flexibility can be provided using Expansion loops, offsets, bends, etc., In piping designing, elbows, Bends, and Pipe Expansion Loops normally provide adequate flexibility for thermal expansion.– The stress can be reduced by introducing an expansion loop.– Expansion loops provided in the pipe length perpendicular to

the direction of straight pipe.

– The expansion of straight pipe will be accommodated between the anchors by flexing the loop legs, thus reducing the stress in

the pipe and loading on anchor.

Expansion LoopsExpansion Loops

Consideration for Piping FlexibilityConsideration for Piping Flexibility

Avoid the use of a straight pipe run of pipe between two-equipment connection or between two anchor points.

A piping system between two anchor points in a single plane shall have as a minimum configuration L-Shaped consisting of two runs of pipe and a single elbow.

A piping system between two anchor points with the piping in two planes may consist of Two L-Shaped runs of pipe. For e.g. One L-shaped run in the horizontal plane and another in vertical plane.

A three-plane configuration may consist of a series of L-shaped runs or U-shaped expansion loops designed into the normal routing of the system.

For high temperature piping following minimum consideration are required to ensure adequate flexibility :

– Adequate developed length of piping system between anchors/ equipment connection with in the physical design constraints as functional design requirements.

Consideration for Piping Consideration for Piping Flexibility….Contd..Flexibility….Contd..

– Provision of flexible supports, when up or down movement of pipe at support location will be made possible

– Provide single or multi direction restrain at strategic location to guide the pipe thermal expansion in a predictable manner and also to constraint where necessary.

– Further guides and restrain help to the control the excessive pipe rotation and resulting the stress in the pipe on moments on the equipment nozzle.

– Provide flexible supports in vertical raiser.

For systems consisting of large diameter main and numerous smaller branch lines, the designer must ascertain that the branches are flexible enough to with stand the expansion in the main header.

Systems that are purged by steam or hot gas must be reviewed to assure that they will be flexible during the purging operation.

Closed relief valve and hot blow down systems should be given special attentions.

Flexibility of Piping - ExampleFlexibility of Piping - Example

Flexibility of Piping - ExampleFlexibility of Piping - Example

Expansion Loop

Constant Load Spring

Variable Spring

Rig

id H

an

ger

Rig

id S

up

po

rt

Dynamic Support, Snubber

Rigid Support

Types of Pipe SupportsTypes of Pipe Supports

There are three general types

Rigid type (no flexibility in the direction of restrain)

Spring type (Allows pipe movement in direction of loading)

Dynamic Support (Degree of restrain depends on acceleration of load)

There are two types of spring support

Variable load type, here support load changes as the pipe moves.

Constant load support, the load remains constant within some range of movement.

Case Study for Combined Cycle for Combined Cycle Power PlantPower Plant

Main Steam Piping System The High Pressure (HP) steam system is designed per

ASME-B31.1(Power Piping Code) to convey HP superheated steam, from the HP superheater outlet to the high pressure section of the steam turbine. HP steam line is provided with a bypass line, with a combined pressure reducing and steam desuperheating valve and is connected to the Condenser.

Normal Operation Start-Up/Shutdown Operation

Piping Material SelectionPiping Material Selection

Piping material selection is based on established industry practices for the temperature, pressures, services and fluid type

General water and steam services less than 750 F ASTM A 106 Grade B or A53 Gr. B Steam Piping above 750 F less than 955 F ASTM A 335 Grade P11 Steam piping above 955 F to 1050 F ASTM A 335 Grade P 22 Steam piping above 1051 F to 1200 F ASTM A 335 Grade P 91 Flashing heater drain service ASTM A 335 Grade P5 Mild corrosive service ASTM A 312 or A 367, Grade TP304 Severe corrosive service ASTM A 312 or A 367, Grade TP316 Low pressure and temperature ASTM A-53 Grade B Concentrated acid handling systems Alloy 20 or HDPE / PVC / Rubber

lined Fire protection Carbon steel

Design dataDesign data Pipe Size = 8 inches for Main Steam Pipe Pipe Thickness = 160 Sch Insulation Thickness = 7.5 inches Pipe size = 24 inches for Bypass connection Pipe Thickness = STD Insulation Thickness = 2.5 inches Design Temperature = 955.4 ° F Design Pressure = 1450 psi Pipe Material = ASTM A335 P22 Insulation Material = Calcium silicate per ASTM C533 for heat retention Pipe Construction = Seamless Flange type = Not Allowed

Fittings Greater than 2 inch ASTM Spec. = A234 WP22 ASME STD. Type = B16.9, B16.28 Type = Butt Weld

Fittings Less than 2 inch ASTM Spec. = A182 F22 ASME STD. Type = B16.11 Rating = 9000 Class Type = Socket Weld Attemperator weight = 1322.5 lbs per 7.87ft

Stress/Node IsometricStress/Node Isometric

Analysis MethodologyAnalysis Methodology The Piping System is considered as an assembly of many pipe segments connected by analytical node

points.

The stress is computed based on internal forces and moments in each segment at all node points.

The reactions at each pipe support location are calculated; force equilibrium check is made at all node on support points.

The stress value as calculated in the analysis for sustained load and thermal expansion load at each node will be verified as per ASME B31.1 code equations for code compliance.

Dead weight AnalysisDead weight Analysis The PIPSYS checks the node formation and end connection of fittings, if it is properly sequenced it will

further proceed by forming a matrix for further analysis else error will be indicated for the specified Node and the same should be corrected.

it is checking the dead weight supporting is within the permissible limit. If the pipe is not properly supported in dead weight the support location should be changed to minimize the sag.

NODE TYPE FACTOR STRESS IN PSI DISPLACEMENTS IN INCHES (GLOBAL COORDINATE)

I (I*M)/Z X Y Z

--------------------------------------------------------------------

5 7 1.46 1389. 0.000 0.000 0.000

10 1 1.00 373. 0.000 -0.010 -0.001

Pipe Behaviour In Thermal Condition - Iteration -IPipe Behaviour In Thermal Condition - Iteration -I

Nodes Failing

Nodes Failing

Maximum stressed Node - Iteration IMaximum stressed Node - Iteration I

NODE NODE STRESS ALLOWABLE RATIO

TYPE (PSI) STRESS(PSI)

95 8 144000. 29180. 4.935

320 1 60800. 29028. 2.095

50 1 42200. 28083. 1.503

5 7 33000. 28619. 1.153

55 8 31400. 28639. 1.096 Ratio are more than 1, means that the stresses are exceeding the allowable stress limits and thus the nodes get

fails.

Equipment Nozzle reactionEquipment Nozzle reactionHrsgHrsg LOAD CASE FORCES (LBS) MOMENTS (FT-LBS)

HOT & WEIGHT FR = 4082. MR = 60244.

COLD & WEIGHT FR = 3516. MR = 72069

Turbine.Turbine. HOT & WEIGHT FR = 6068. MR = 36673.

COLD & WEIGHT FR = 6679. MR = 44044.

CondenserCondenser HOT & WEIGHT FR = 1102. MR = 6646.

COLD & WEIGHT FR = 1734. MR = 9026.

Pipe Behaviour In Thermal Condition - Iteration -IIPipe Behaviour In Thermal Condition - Iteration -II

Expansion Loops

Guide Support

Spring Hanger

Maximum stressed Node - Iteration IIMaximum stressed Node - Iteration II

NODE NODE STRESS ALLOWABLE RATIO

TYPE (PSI) STRESS(PSI)

325 11 24500. 29443. 0.832

305 8 21700. 29332. 0.740

5 7 19800. 28606. 0.692

330 7 18600. 29494. 0.631

55 8 16000. 28651. 0.558

95 8 14400. 29015. 0.496

Equipment Nozzle reactionEquipment Nozzle reactionHrsgHrsg LOAD CASE FORCES (LBS) MOMENTS (FT-LBS)

HOT & WEIGHT FR = 3924. MR = 56488.

COLD & WEIGHT FR = 3260. MR = 67373

Turbine.

HOT & WEIGHT FR = 5983. MR = 33526.

COLD & WEIGHT FR = 6503. MR = 40128

Condenser

HOT & WEIGHT FR = 1109. MR = 9508.

COLD & WEIGHT FR = 1674. MR = 12032.

Final IterationFinal Iteration

As the same Lot of trail and error iteration has been done to keep the pipe within permissible limit in dead weight, minimum stresses at all nodes and all the three equipment nozzles within the allowable limits as specified by the manufacturer of the same.

Finally by doing lot of iteration the best solution has arrived which gives

Minimum stresses in Piping Meets the code limits and Meets the Equipment forces and moments.

ConclusionConclusion

As seen in the case study in detail, the piping stress analysis checks with

The Acceptance of piping system per applicable design code, Requirement related to equipment limitation and

which ensures

The Safety of piping and piping components against failure Maintain system operability to comply with legislation / Indian boiler regulation the piping is well supported and does not sag or deflect in an unsightly way under its

own weight the deflections are well controlled when thermal and other loads are applied the loads and moments imposed on machinery and vessels by the thermal growth of

the attached piping are not excessive

Gives the input for

Input to civil for Structure design And loads and displacement for support design and for hanger design.

Thank YouThank You