You have to calculate two times for internal pressure and external pressure.
The design pressure should be maximum differential pressure under the worst case. The vacuum conditions should be considered if there are.

Here is a sample calculation.

Component: Tubes

ASME Section VIII-1 2004 A05 UG-27 Thickness of Shells under Int. Pressure

--- Calculations --- Cylinder Internal Pressure
Material: SA-214 K01807 Wld. tube
Design pressure P = 310 psi Design temperature T = 650 F
Radiography = - Joint eff.circ str. E = 1
Design stress S = 10900 psi Joint eff.long str. E = -
Design stress, long S = - (circum. butt welds)
Inside corr.allow. CAI = 0 in Outside corr. all. CAO = 0 in
Material tolerance Tol = 0 in TEMA min. thickness tm = 0 in
Outside diameter OD = 0.75 in Corroded radius OR = 0.375 in

Required wall thickness of the cylinder , greater of:
Circumferential stress
t = (P*OR / (S*E+0.4*P))+cai+cao+tol = 0.0105 in APP.1-1(A)
Longitudinal stress
t = (P*IR / (2*S*E+0.4*P))+cai+cao+tol = - UG-27(c)(2)
Actual wall thickness of cylinder: tnom = 0.083 in
(Required wall tks. for nozzle attachments, E=- , tri = - )

ASME Section VIII-1 2004 A05 UG-28 Thickness of Shells under Ext. Pressure

--- Calculations --- Cylinder External Pressure
Material: SA-214 K01807 Wld. tube
Design pressure PE = 215 psi Design temperature T = 650 F
Inside corr. allow. CAI = 0 in Corrosion allow. CAO = 0 in
Radiography = Full Material tol. Tol = 0 in
Cyl. outside dia. Do = 0.75 in Cylinder length EP L = 192 in
Nominal thickness tnom = 0.083 in (tnom-CAI-CAO-Tol) t = 0.083 in
L/Do ratio Ldo = 256.0 Do/t Dot = 9.0361
(2*S) or (0.9*yield) SE = 20745 psi Mod. of elasticity ME = 25849998 psi
A factor SII-D-FigG A = 0.014136 B factor CS-1 B = 10880
Max allowed external pressure: Pa = B*((2.167/Dot)-0.0833) = 1702.81 psi
Actual external design pressure: PE = 215 psi
(Required cyl. tks. for nozzle attachments at PE, tre = 0.019 in )