4. After Pipe thickness Calculation + Add Corrosion Allowanc

4. After Pipe thickness Calculation + Add Corrosion Allowance…Then…..Why we have deduct 12.5% from Pipe thickness Calculation + Add Corrosion Allowance (12.5%>t). When we get final thickness then were we find final Schedule.

Hi,
The wall thickness of the commercially available pipe you are specifying must be greater than t(m). After you've calculated 't', your pressure design thickness, you have to determine 't(m)', the minimum pipe wall thickness. To get this you must add to 't' the corrosion allowance and 12.5% of the schedule thickness listed in the pipe wall thickness tables of the commercially available pipe you expect to be using. The 12.5% is to allow for mill tolerance (actual pipe wall may be up to 12.5% less than the thickness listed in pipe wall thickness tables).

Let's say that your pipe size is NPS 6 and 't' is calculated as 0.15", the corrosion allowance is 0.0625" (1/16") and you expect to be using Sch. 40 pipe. NPS 6 Sch. 40 pipe is listed as 0.280" WT, and 12.5% of this number is 0.035". t(m) = 0.15 + 0.0625 + 0.035 or 0.2475". Because the calculated t(m) WT (0.2475") is less than the listed Sch. 40 WT (0.280"), Sch. 40 will work.

On the other hand, let's say that 't' is calculated as 0.20". t(m) = 0.20 + 0.0625 + 0.035 or 0.2975". In this case the calculated t(m) is greater than the listed Sch. 40 WT and you'd have to go up to the next schedule of pipe in order to satisfy the code, or in this case Sch. 80.

Another way of looking at this is that the listed WT minus the corrosion allowance and mill tolerance must be greater than 't'. Using the numbers above: 0.280 - 0.0625 - 0.035 = 0.1825. 0.1825 > 0.15 but < 0.20.

Regards

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