Analyze and redesign a steam conduit of a steam turbine by Stress and Fracture Analysis

Presented by:• Shahrior Ahmed • Rezaur Rahman• Rahagir Ridwan Anik• Mohammed Abu Saifullah

Why High Pressure Steam?

• The volume of steam is 1,600 times greater than an equal mass of water. High-pressure steam has a higher density, so pipes can be smaller.

• For steam turbine the pressure of the steam increases up to 15-20 MPa to generate and very good amount of thrust force in turbine blade.

Main Reason of Steam Conduit Failure

• Failure in steam line can be occurred for different reasons. The main reason is internal fracture crack initiation and propagation of a crack. Failure in selecting material and design criteria according to fluid internal pressure is the main reason for crack and line failure.

Problem Description

• A steam turbine power plant, where the conduit to the turbine blade is made of ASTM A47. The material properties of ASTM A47 from the designer of the plant are found that, Yield strength, Sy= 227.5MPa, Outer diameter is 550mm,Wall thickness 40 mm Inner diameter is 510 mm, Fracture toughness: 160 MPa(mm)1/2 , Price: 1200$ /ton.

• The plant layout shows that the maximum steam pressure in the conduit is 15 MPa. The steam quality is highly maintained by checking the drain water regularly. The boiler inlet water is processed and also chemically amicable for the boiler and steam carrying conduits.

• A fracture in the conduit found with the flow of the steam which disturbed the whole electricity generation system by reducing the steam pressure while it is going to the high pressure section of turbine blade.

• We need to analyze the problem with the steam conduit and redesign the conduit more efficiently.

Design Literature• For the thin-walled assumption to be valid the vessel must have a

wall thickness of no more than about one-tenth (often cited as one twentieth) of its radius. The classic equation for hoop stress created by an internal pressure on a thin wall cylindrical pressure vessel is:

• ơhoop = PDm /2t for the Hoop Stress

P = is the internal pressuret = is the wall thicknessr = is the inside radius of the cylinder.Dm = Mean Diameter (Outside diameter – t). Mean diameter of OD and ID… ơhoop = is the hoop stress.

• Most flows encountered in engineering practice are turbulent, and thus it is important to understand how turbulence affects wall shear stress. The shear stress, for a Newtonian fluid, at a surface element parallel to a flat plate, at the point y, is given by:

• This too small compare to the actual hoop stress developed in the wall of the conduit. For this reason the fluid Shear stress in neglected here.

4.0 Assumptions:

In this project there are some assumptions. The assumptions are given below-

• Fluid pressure 15 MPa• The pipe is horizontal and it has a length of 10

meter.• The pipe is not welded, it is seamless.• For existing system safety factor 3.

Mechanical design Existing Conduit

• Here our assumed structure’s material is Cast iron ASTM A47. It has following properties: Yield strength, Sy= 227.5MPa. The nominal pipe size is 550 mm, Outer diameter is 550mm,Wall thickness 40 mm and the inner diameter is 510 mm. Fracture toughness: 160 MPa(mm)1/2 Price: 1200$ /ton.

Choosing New Material

• As the existing system failed to meet the stress criterion of the conventional design method.

• The principal design codes used for piping design are the ANSI/ASME B31.1 (Code for Power Piping) and ANSI/ASME B31.3 (code for process piping), ASTM A53 B, ASTM A106 B and API 5L carbon steel pipes are the ones used for geothermal fields. The Pipe pressure design is assumed to be 15 MPa and the length of the pipe 10m Pipe Steam velocity is 40 ms-1. The materials used in geothermal application are normally A53-B, A106-B and API 5L-B pipe , with mill tolerance. Commercial available pipes normally have a mill tolerance of 12.5% and pipe schedule numbers based in B36.10.

• We have considered the material API 5L-B pipe. Its Yield strength Sy=241.31MPa,The allowable stress is SE=103 MPa, Safety Factor SF 2.34.

• Specification is chosen according ANSI B36.10. The nominal pipe size is 304.8 mm, Outer diameter is 323.85mm,Wall thickness 33.32mmand the inner diameter is 290.53mm. Price :800$/ton. Fracture toughness: Approximately 120 MPa(mm)1/2

Satisfying Design Criterion

Fracture Mechanics Approach:Let us assume that there are cracks on the structure. Here,Crack length = a,Critical crack length = ac

fracture toughness= Klc

Intensity factor= Kl

We know that, Kl=1.12( ơhoop+p)(πa)1/2

& Klc=1.12( ơhoop+p) (πac)1/2

So, ac= (Klc/(1.12* (ơhoop)))2/ 3.1416

Stress Mechanics ApproachIt is important to follow the stress analysis during designing a conduit. The conduit designed should have the allowable stress resisting capability greater than the stress developed during any operation.According to Maximum Shear Stress Theory:ơhoop= PD/(2t) ≤ Sy/SF

Where, Sy = Yield Stress , SF= Safety Factor

Stress Analysis

For Material ASTM A47 considering same diameter and wall thickness and safety factor 3 , from equation (1) we get, 15*0.51/(2*0.04) ≤227.5/3 95.625≤75.83So, this material does not satisfies MSST.

For material API 5L-B, from equation (1) we get, 15*0.29/(2*0.03332) ≤241.31/2.34 65.27≤103.12So it satisfies MSST.

Fracture Mechanics Analysis

For material ASTM A47 ,ac=(160/(1.12*(95.625+15)))2/3.1416 =0.53mm

For material For material API 5L-B,ac=(120/(1.12*(65.27+15)))2/3.1416=0.57mm

Here API 5L-B can be selected over ASTM A47 as, API 5L-B has bigger critical length.

Cost analysis

Assumed Conduit Length, L= 10 Meter

Now, for material ASTM A47:volume= 2*π*(Rout-Rin)*L*t

=2*3.1416*(0.275-0.255)*10*0.04=0.050 m3= 0.017657333 tonCost= 0.017657333*1200=$21.19 (For 10m of length Conduit)

For material API 5L-B :volume= 2*π*(Rout-Rin)*L*t=2*3.1416*(0.161925-0.145265)*10*0.03332=0.0349m3

= 0.012324819 tonTotal cost is= 0.012324819*800=$9.86. (For 10m of length Conduit)

API 5L-B can be selected over ASTM A47 as API 5L-B is thinner, low cost and has higher strength.

Discussion

In the project the existing model is costly and it also has relatively low yield strength. This model has higher wall thickness. The new model has 16.7% less wall thickness than the existing model. The redesigned model’s outer diameter is 41.12% less than the existing model. The redesigned model is also cost effective. It requires 53.47% less cost than the existing pipe. So seeing all the advantages the redesigned model can be effectively used for industrial purposes.

Conclusion

Along with Mechanical design approach fracture mechanics approach is also an essential part of designing any structure. If it is done so then any structure will be economic and safe for the system.