The minimum pressure for niobium and niobium alloy tubes is listed in the above tables. The pressure increases with wall thickness and the addition of zirconium and decreases with the outside diameter. Given a tube with a certain OD, the heat exchanger customer shall order tubes with thicker wall (or with 1% addition of zirconium) if the customer would like tubes to hold higher pressure. Normally in the heat exchanger industry unalloyed niobium tubes are seldom ordered, to our knowledge.

The numbers in Tables 1 & 2 are minimum values. Our actual numbers are typically at least 25% higher. The above data are calculated as follows:
P=2 S t / D
P: Minimum Hydrostatic Pressure, psi (or MPa)
S: Allowable Fiber Stress of 75% of the Minimum Yield Strength, psi (or MPa)
t: Average Wall Thickness of the Tube, inch (or mm), and
D: Outside Diameter of the Tube, inch (or mm)