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

Appreciate your support.

Many thanks

LG