How much pipe pressure is too much?
In straight pipe, the answer depends on temperature, size, materials, corrosion allowance and/or manufacturer allowance. I’ve noticed over the years that when someone mentions the words, “Maximum Allowable Working Pressure”, that they may have different meanings. It doesn’t help when Regulatory agencies use terms that have relative meanings as well. I’ll explain this more in the video below, but for now, lets back up a bit.
First, what is a pipe? Its that cylindrical shaped “thingy” made of various materials with a hole in the middle. The hole is used to transport fluids, gases, bulk materials or some combination. There are many ways to join these “thingy’s” together to form very simple or complex transport systems. In cases where temperatures change drastically, the materials will expand or contract and create stresses that will cause much hate and discontent if left unaccounted. Many systems do not experience drastic temperature changes, but other influences such as ship movement or equipment movement may cause havoc as well.
Other stresses are created by the internal pipe pressure. Pressure in pipe causes three different varieties;
- Axial Stress – Stress parallel to the length of the cylinder.
- Radial Stress – Stress perpendicular to the length of the cylinder.
- Circumferential or Hoop Stress – Tangential stress in the cylinder wall.
See Wikipedia for more definition.
These stresses are compared against the maximum allowable stress associated with the pipe material intended to be used. So when an engineer wants to calculate the MAWP (Maximum Allowable Working Pressure) of a particular pipe, he/she will turn to governing equations that make use of the maximum allowable stress per the material considered.
In shipbuilding, there are several Classification Societies that offer formulas to calculate the MAWP. I believe most come to the same conclusion with differing variables. The example provided in the spreadsheet download below is calculated with an equation and variables given by The America Bureau of Shipbuilding (ABS). For comparison, I also added a formula from ANSI B31.1. You will be able to see specific differences when varying the corrosion factor. The spreadsheet does include macros for looking up table information, so you’ll have to enable macro’s on your version of Excel. You should get a prompt when you try to open it.
Video & Download
See the video below and download the spreadsheet for free. I hope you get good use from it to confirm your pipe selection.
Download the free spreadsheet here:
*See the disclaimer tab in the spreadsheet concerning liability.
Many thanks for reading this post. I wish you all the best!