Wall Thickness Calculation of Pipes

Nominal Wall Thickness Calculation Of metallic pipes needs the following information as a Pre-requisite.

a) Design Pressure, b) Design Temperature, c) Material Specification, d) Pipe Size e) Applicable Code

As Per B 31.3, the required thickness of a Straight Section of Pipe is given by the relation

t_m=t+c

Where,

t_m =Minimum thickness required including, mechanical corrosion and erosion allowance

t=Thickness Required to withstand internal pressure

c=Sum of Mechanical Allowance (thread, groove depth) added with corrosion and erosion allowance.

Let's Go through each term in the above equation

i)Thickness Required to withstand internal pressure [t]

Nominal wall thickness should be Greater than or equal to the sum of the minimum required thickness and Manufacturer’s Negative tolerance.

As per the Code Minimum Thickness t =PD/2(SE+PY)

Here,

P=Design Pressure (Gauge)

D=Outside Diameter of Pipe

S=Allowable Stress Value of the Particular Pipe material

E=Longitudinal Weld Joint Efficiency

Y=Co-efficient for Pipes, in cases where “t” < D/6. This value is available in Table form for different Materials, for a specific material coefficient will be different for different Temperature.

This is for situations where thickness calculated based on Outside diameter.

When we have to calculate thickness based on inside diameter, we

can use the following formula

t=P(d+2c)/2[SE-P(1-Y)]

Among the above two equations for thickness calculation, Equation for t<=D/6 might be the usual case .If not, additional considerations such as thermal stress, the theory of failure and fatigue effects etc should be considered.

For calculating the thickness, Allowable Stress value for different materials are listed Under Table-A1 ANSI B31.3.

Similarly, Weld Joint Efficiency (Quality Factors) for different ASTM Pipe Material Specification are listed under Table A-1B.

A few are listed below

A53   ERW=0.85

A53   Seamless=1

A312 Seamless Pipe=1

ii) Sum of mechanical Corrosion and Erosion allowance [c]

a)Threads:

applicable if the pipes are threaded for making joints as is the case with Galvanized piping. The nominal thread depth has to be calculated under such situation.

b) Corrosion/ Erosion Allowance:

Its Depend upon the type of fluid going to flow through the pipe and can be provided by the process licensor. It may vary from 1m.m- 6 m.m and even more in some cases.

As a good Engi Practice, it is advisable to consider minimum 1 mm as corrosion allowance for all other services where Process Licensor didn't specify the thickness.This will also take care of the external corrosion if any.

c) Bending Allowance:

If Pipes are to be used for making bends, then it is necessary to increase the thickness "t_m" by a factor called bend thinning allowance. During bending the outer fibers get stretched and in order to maintain minimum wall thickness t_m at all point in a complete bend one has to add an allowance for thinning.

As an alternative, one can use eccentric bore pipes exclusively for making bends where the requirement is a large and material cost is significant.

d) Manufacturers negative tolerance:

As we require tm at all points in pipes after the pipes are manufactured, while ordering its necessary to account for manufacturers negative tolerance

This tolerance depends upon the method of manufacturing and these are given on respective ASTM pipe material Specs.

For Seamless Pipes, if minimum thickness required is "t_m" then Nominal thickness should be equal or greater than t_m/0.875

Similarly, for ERW Welded Steel Pipes, this tolerance value is 0.3mm.These are for Piping conforming to ASTM A672.

Schedule number selection

After Calculating "T" the nominal wall thickness which is required for the design conditions, one can order the pipes provided the quantity is large enough for special Rolling.

Otherwise, as per  ANSI B36.10 -for Carbon steel and ANSI B36.19 -for Stainless Steel pipes, are readily available in various thickness specified by their schedule numbers

It is recommended to make use of these Standard Pipe thickness which is available. Schedule number selected should be equal or Greater than the Nominal thickness calculated for the Given conditions.

But sometimes when we calculate the required thickness, we get very low thickness values. This values may be enough to withstand the Pressure-Temperature effects. But may not have the structural rigidity and chances are high that may get deformed even during handling, care should be given for such lines.

Unless the material is very costly we usually decide the minimum thickness for various Pipe sizes at the initial stage of the project depending on structural rigidity criteria.

For Carbon Steel Piping in Refinery and Petrochemical Plants, Specially Process Lines, following minimum schedules are usually used.

1/2" to 4"  Sch.80

2 1/2"to  10" Sch40

12" 24" STD wall

For utility line, can go for lower schedule than mentioned above.