# Calculation of HEX U-tubes under external pressure (EN13445+TEMA)

Dear all,

I'm using the Bentley AutoPIPE Vessel for calculation of U-tubes of the bundle under internal pressure and external pressure according EN13445-3 and TEMA.

Tubes are procured with tolerance +/-10%, and customer required to have corrosion allowance of 0,4mm (inside 0,2mm + outside 0,2mm).

I've got the report as shown below, specifically for external pressure (test condition) it seems the result given for Pr/S  is not well calculated, program is assuming the safety factor S=1,5 instead of S=1,1.

Other points are:

1. why the TEMA RCB-2.31 formula is applicable to the U-Tube bend under external pressure?

2. I don't understand from where is coming the value t1 (t1 is the minimum thickness required for straight tube under internal or external pressure)?

Extract below from Microprotol report:

Tubes of the bundle.

Bundle tube under internal pressure.

 Material : X2CrNiMoN22-5-3 Seamless tube temperature in operation : t = 176 °C Weld joint efficiency : z = 1 Alloy Steel Nominal thickness : en = 0,80 mm External Diameter : De = 18,00 mm c = corrosion + tolerance P = internal pressure f = Allowable stress ea = en- c EN 13445:2016 Issue 3 (2016-09) : P (MPa) f (MPa) e+c (mm) s (MPa) c (mm) Pmax (MPa) Horizontal test Operation 7,1 5 428,57 214,67 0,62 0,68 191,7 135 0,48 0,48 15,873 7,9506

 Minimum U-Bends thickness TEMA RCB-2.31 :  to = t1[1+do/(4R)]  = 0,75 mm   (R = 48 mm )

Bundle tube under external pressure.

 Material : X2CrNiMoN22-5-3 Seamless tube P = External Pressure t = Temperature Alloy Steel se = nominal yield strength E = modulus of elasticity S = safety factor Analysis thickness : ea = 0,32 mm External Diameter : De = 18,00 mm c = corrosion + tolerance EN 13445:2016 Issue 3 (2016-09) [8.5.2.2] L = 12.062,00 mm e  : Figure 8.5-3 ncyl : Figure 8.5-4 Pr /Py : Curve 1) Figure 8.5-5 R = (De-ea) / 2 Py = se ea / R Pm = E eae / R Z = p R / L P (MPa) t (°C) se (MPa) E (MPa) S e ncyl c (mm) Pr /S (MPa) 1,2 0,9 20 176 450 322 201.000 190.440 1,1 1,5 0,0004 0,0004 2 2 0,48 0,48 0,8719 0,8267

 Minimum U-Bends thickness TEMA RCB-2.31 :  to = t1[1+do/(4R)]  = 0,91 mm   (R = 48 mm )

INSUFFICIENT THICKNESS