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Pressure thrust/end cap loads at are self equilibrating loads. This means that for any given piping component, the distribution of pressure load is in equilibrium with the pressure load at the inlet/outlet. The pressure load at the inlets/outlets is commonly referred to as "End-Cap forces" calculated as shown in Figure 1 and follows a sign convention that a positive end-cap force induces tension on the pipe. Note that for the calculation of analysis loads, no material allowances are considered.
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For pipes with variable cross section (reducers or reducing elbows) there is a pressure thrust in addition to the end-cap force. For symmetric cross sections, the radial pressure load cancels and doesn't contribute to pressure thrust, only Only the component of pressure acting parallel to the longitudinal axis of the pipe contributes to the pressure thrust. The end-cap forces are calculated as previously using the inside and outside diameters at each end of the reducer. The pressure thrust acts as a uniformly distributed axial load and is equivalent to the pressure acting on projected area of a segment of length 'dx'. For a reducer that varies linearly, and is equivalent to the difference in end-cap force divided by the reducer length (F2-F1)/L. The small side end-cap force combined with the total pressure thrust load is equal and opposite to the large side end-cap force. Figure 2 shows the End-Cap/Pressure Thrust forces on a reducer.
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