Design calculation is one of important steps to build new steam boiler. Cylindrical parts such as water wall tubes, header, downcomer pipe, riser, and drum are designed under stress both external pressure and internal pressure. One of them is longitudinal stress which is caused by load from internal pressure that performs along the lengthwise or long axis of tube, pipe or other cylindrical parts.

Material should be designed to resist longitudinal stress unless the material will impact to break material and split along the long axis (please see Figure 1 and Figure 2). The following below is an approach to get the formula of longitudinal stress.

From figure 1 above, longitudinal force is resisted by resistance force. Cylindrical part should have enough strength to resist longitudinal force, so minimum requirement of resistance force value should has minimum equal, not less than longitudinal force unless the material will be crack along longitudinal axis (see Figure 2).

Figure 1: Working Principal of Cylindrical Part |

Longitudinal Force = Internal Pressure x Projection Area

= Internal Pressure x Inside Diameter x Length

= P x D x L

Resistance Force = Longitudinal Stress x Projection Area

= Longitudinal Stress x Thickness x Length x 2

= SL x t x L x 2

**RĂ©sistance Force**

**= Longitudinal Force**

SL x t x L x 2 = P x D x L

SL = PD / 2t

So the formula of longitudinal stress in the cylindrical part is PD / 2t

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