Pipe wall thickness is based on the internal pressure of a pipe and, if necessary external pressure or any additional loads. The pipe wall thickness is calculated by using the equation for internal pressure thickness in the applicable ASME B31 Code, modifying the thickness for any external pressure or additional loads, selecting pipe schedule based on manufacturer's tolerance. Also calculating the Maximum Allowable Operating Pressure (MAOP) for the pipe after determining the pipe wall thickness will be covered.
Factors Affecting Wall Thickness Calculation
In the process of calculating the wall thickness of a pipe several parameters must be considered and accurately specified. These parameters are listed below.
Pipe Material-
The pipe material will be specified as has been discussed in the previous chapter. The material type identifies basic design parameter, which is the allowable stress to be explained in the coming sections
Internal pressure -
This is the crucial parameter in piping design because the main purpose pressure piping codes and standards is to contain the pressurized fluid from escaping out or rupturing the pipe.
Fluid temperature -
The fluid temperature affects the allowable stresses of the material and the designated code because each code has temperature limitations.
External Loads-
These loads could be caused by external pressure due to vacuum or water static head for submarine lines. Also, dead weight of soil, snow or sand over burden load on the pipe.
Construction -
Construction of pipelines whether on land or off shore could add additional construction requirement that may increase the wall thickness of the pipe. In fact for offshore pipeline, stresses on the pipe exerted on the pipe during lay-off from the barges are the detrimental factor of the pipe wall thickness. Another example is the elastic bend requirement to follow the pipeline terrain for on land pipes.
Objective of calculating pipe wall thickness
Piping systems are designed and constructed to convey pressurized fluid, mostly flammable and could be toxic, therefore codes tend to be very conservative in respect to pressure containment of the pressurized fluids. This explains why there are few failures reported due to over pressure of the piping systems.
The basic theory for designing of wall thickness of a piping system that contains the internal pressure is based on limiting the hope stress developed by this pressure to an acceptable value by the code. The calculated hope stress in pipe shell is based on Lame equation as follows.
Sh=P[0.5* (D/t)^2 –(D/t) +1] / [(D/t)-1]
Boardman's empirical equation has further simplified Lame equation based on an intermediate diameter and introduced a Y factor to correct the difference. Boardman's equation has become the basis for calculating wall thickness B31 Codes for pressure containing piping. Boardman's equation is as follows.
Sh=P(D-Yt)/2t
For thin pipe where D/t > 6, and outside diameter D, Y is considered 0.4.
Each code provides an equation that is used to calculate internal pressure thickness. The equations may look different and/ or the approach varies, but the basic concept is the same. The main concept is to limit the hoop stress, Sh , to an acceptable and safe level.
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