Flexibility Analysis for High Temperature Piping System Case Study for Combined Cycle Power Plant.
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 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.
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Pipe material selection - to withstand the high pressure and high temperature.
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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 Analysis ❜
The process of checking the stress developed in the piping due to various loading is called Pipe Stress Analysis/Flexibility analysis.
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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.
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The objective of the Pipe flexibility analysis is to ensure safety against failure of the piping material or anchor points from overstress.
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Check pipe stresses with governing codes (as Design Base Document) .
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Support load & movement for various loading conditions.
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Check the terminal point loading (Forces & Moments) generated from pipe to the connected equipment.
Types of Loads
Sustained Loads – Dead Weight (Weight Of Pipe, Fittings, Fluid in Pipe, Piping Components valves, valve Operators, flanges so on.)
– Dead Weight – Thermal Expansion and contraction effect – Effects of Support, anchor and thermal movements – Internal and external loadings
Thermal Expansion Loads – Due to the Temperature
Occasional Loads – Seismic – Wind – Snow and etc.,
Also loads on piping can be classified as Static Loads
Dynamic Loads – – – –
Impact forces Wind Load Seismic Load Steam & Water Hammer effects – Discharge Loads
Stresses 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.
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 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 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] {U} {F}
= global stiffness matrix, = global displacement vector, = global load vector
Pipe 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. 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 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 Loops
Consideration 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 Ushaped 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 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 - Example
Flexibility of Piping - Example
Flexibility of Piping - Example Expansion Loop
Types of Pipe Supports
Constant Load Spring
There are three general types Rigid type (no flexibility in the direction of restrain) Spring type (Allows pipe movement in direction of loading)
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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.
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Constant load support, the load remains constant within some range of movement.
Rigid Support
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Variable Spring
Rigid Hanger
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Dynamic Support, Snubber Rigid Support
Case Study for Combined Cycle Power 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 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 Steam Piping above 750 F less than 955 F Steam piping above 955 F to 1050 F Steam piping above 1051 F to 1200 F Flashing heater drain service Mild corrosive service Severe corrosive service Low pressure and temperature Concentrated acid handling systems Fire protection
ASTM A 106 Grade B or A53 Gr. B ASTM A 335 Grade P11 ASTM A 335 Grade P 22 ASTM A 335 Grade P 91 ASTM A 335 Grade P5 ASTM A 312 or A 367, Grade TP304 ASTM A 312 or A 367, Grade TP316 ASTM A-53 Grade B Alloy 20 or HDPE / PVC / Rubber lined Carbon steel
Design data Pipe Size Pipe Thickness Insulation Thickness Pipe size Pipe Thickness Insulation Thickness Design Temperature Design Pressure Pipe Material Insulation Material Pipe Construction Flange type Fittings Greater than 2 inch ASTM Spec. ASME STD. Type Type Fittings Less than 2 inch ASTM Spec. ASME STD. Type Rating Type Attemperator weight
= = = = = = = = = = = =
8 inches for Main Steam Pipe 160 Sch 7.5 inches 24 inches for Bypass connection STD 2.5 inches 955.4 ° F 1450 psi ASTM A335 P22 Calcium silicate per ASTM C533 for heat retention Seamless Not Allowed
= = =
A234 WP22 B16.9, B16.28 Butt Weld
= = = = =
A182 F22 B16.11 9000 Class Socket Weld 1322.5 lbs per 7.87ft
Stress/Node Isometric
Analysis 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 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
Pipe Behaviour In Thermal Condition - Iteration -I
Nodes Failing
Nodes Failing
Maximum 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 reaction Hrsg LOAD CASE HOT & WEIGHT COLD & WEIGHT
FORCES (LBS) FR = 4082. FR = 3516.
MOMENTS (FT-LBS) MR = 60244. MR = 72069
Turbine. HOT & WEIGHT COLD & WEIGHT
FR = 6068. FR = 6679.
MR = 36673. MR = 44044.
FR = 1102. FR = 1734.
MR = 6646. MR = 9026.
Condenser HOT & WEIGHT COLD & WEIGHT
Pipe Behaviour In Thermal Condition - Iteration -II Expansion Loops
Guide Support
Spring Hanger
Maximum stressed Node - Iteration II NODE NODE TYPE
STRESS (PSI)
325 305 5 330 55 95
24500. 21700. 19800. 18600. 16000. 14400.
11 8 7 7 8 8
ALLOWABLE RATIO STRESS(PSI)
29443. 29332. 28606. 29494. 28651. 29015.
Equipment Nozzle reaction Hrsg LOAD CASE HOT & WEIGHT COLD & WEIGHT Turbine. HOT & WEIGHT COLD & WEIGHT Condenser HOT & WEIGHT COLD & WEIGHT
FORCES (LBS) FR = 3924. FR = 3260.
MOMENTS (FT-LBS) MR = 56488. MR = 67373
FR = 5983. FR = 6503.
MR = 33526. MR = 40128
FR = 1109. FR = 1674.
MR = 9508. MR = 12032.
0.832 0.740 0.692 0.631 0.558 0.496
Final 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.
Conclusion 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 You
