This paper intends to address the proper method for carbon steel pipe-sizing using Barlow's Formula as the initial step in pipe-sizing mechanism; and how this formula will assist us to select the proper grade of line pipes. The paper proposes that the grade of the carbon steel line pipe has to be determined first, and the sizing of the carbon steel line pipe has to be determined second. The proposed method for selecting the proper grade of line pipes comes from personal experience with surface water injection in Saudi Arabia.
Pipelines are major facilities in oil industry; most are metal types of carbon steel. Millions of US Dollars have been spent on funding projects of carbon steel pipelines. In light of changes of world economy where prices of carbon steel line pipes have increased dramatically, either due changes in cost of energy used for manufacturing the line-pipes, or else. As engineers, we have to re-examine requirements and specifications in order to change them to meet the essential requirements and continue using carbon steel line pipes at fair cost. This can be achieved once the proper grades of line pipes are being selected without over sizing in the requirements.
The strength of any metal material is determined by the tension test, which measures the tension force and the deformation of the test specimen. The stress which gives a permanent deformation is the yield strength. The stress which gives rupture is the ultimate strength. Figure-1 shows the common stress-strain curve. Figure-2 shows the yield strength and the ultimate strength of the most common line pipes used in the industry.
The Barlow's formula is not totally new for uses in pipe-sizing. It has been used in every day applications for determining pressure rating of pipes. Also, it has been used with other fluid equations in hydraulic and simulation applications. However, with software available today for hydraulic simulation, we tend to overlook such simple formula and rely on the hydraulic software to perform the sizing calculations, ignoring the selections of the line pipes grades, and the differences between these grades of line pipes. Fortunatly, any grade is often made in all sizes, and that is why we do not pay much attention to the type of grade that would be used in a project as long as we can get the pressure rate and the size we need. Therefore, it is advisable that line pipe grade be selected based on the pressure rating of the system at first; then the hydraulic simulations or other fluid measurements can be utilized for determining: the proper size or any further sizing, fluid velocity, delta pressure, or other fluid parameters that have to be evaluated.