Low Temperature Steel Pipe Calculations
As stated in a previous post on cold temperature and equipment, low temperatures have an undesirable affect on ductile steels, making them more brittle and prone to failure. This is a problem in process piping, especially in Canada, with a large range of temperatures in the summer and winter seasons. One solution is to use special cold temperature-rated steels in pipes, although this is not always required. Careful reading of ASME B31.3 reveals interesting information that can be applied when using pipe outside of its intended design temperature.
Paragraph 323.2.2 of B31.3, and the associated charts and diagrams, provide a method of determining if a material can be used below its rated minimum temperature without impact testing. For A106-B steel, the following becomes most relevant:
323.2.2 (b): For carbon steels with a letter designation in the Min. Temp. column of Table A-1, the minimum temperature is defined by the applicable curve and Notes in Fig. 323.2.2A.
From chart 323.2.2A, we find that the minimum temperature for A106-B is at -28.9°C (-20°F), depending on the thickness of pipe. Quite often, this is above the required minimum operational temperature. Thankfully, paragraph 323.2.2 (d) of B31.3 provides a method of reducing this temperature even further. For temperatures above -48°C (-55°F), chart 323.2.2B can be used to determine a further reduction in the minimum temperature. However, this temperature reduction comes at the cost of a reduction in the pressure the pipe can handle.
If this method is used to reduce the operating temperature, the piping in question must be hydrostatically tested to 1.5x the design pressure and possibly isolated from any external loads (such as impacts or thermal shocks).
If the required operating temperature is below -48°C (-55°F), but above -104°C (-155°F), the material can operate at a reduced temperature as long as the internal stresses do not exceed 30% of the allowable stress at the minimum temperature (as defined in Table A-1 or 323.2.2A).
For temperatures below what is stated above, or for scenarios that do not meet the above conditions, an impact test is required.
For example, let’s say we have a length of 5” SCHD 80 pipe of A106-B, with an operating pressure of 500psi, and a minimum operating temperature of -40°C. By referencing Table A-1 and Fig. 323.2.2A of B31.3, we find that the minimum allowable temperature is -28.9°C, and an allowable stress of 20kips. Since our operating temperature is above -48°C, we can reference Fig. 323.2.2B. The chart states that we are able to reduce the minimum temperature by the required 11°C, as long as our stresses do not exceed about 80% of the allowable stress of 20kips (= 16000psi).
As our working pressure only results in an internal stress of 3760psi (as determined by paragraph 304.1.2 (a) of B31.3), our pipe section can be used at -40°C, as long as we pressure test it to 750psi, and it is isolated from external loads.
Low Temperature and A106-B Piping
So in conclusion, it is possible to use pipe below the rated temperature as long as you are working sufficiently lower than the rated pressure of the pipe. This methodology can be successfully applied when trying to use material outside of its intended parameters. Obviously the better (simpler) choice would be to choose a material that has a temperature rating that matches or exceeds the operating temperature. However, as equipment moves around the world there isn’t always the chance to make the best material choice for every climate. Occasionally, an engineer gets asked to de-rate a pipe so that it can be used at a lower temperature.
If you want to learn how to determine if a straight section of A106-B pipe can be used in a temperature below the minimum allowable temperature without the need for impact, you can download the Low Temperature Steel Pipe Verification Calculator (simple excel calculator) that uses the formulas discussed above. Finally, we have written extensively on low temperature and materials. Browse our articles below, or send me an email if you have questions.
For further reading, please see: Performance of Steel and Equipment in low temperatures Part 1, Part 2 and Part 3