Fundamentals of Pipe for Oil & Gas Engineer.
Pipe covers very large part of any process plant. If you look at Oil Exploration platform, Refinery and Petrochemical complex one thing that catches the attention is a complex network of piping. Piping is used to transport various process materials from one equipment to another. But why?
Process Plant is a place where a series of activities performed in particular ordered to convert raw material into a useful product and interconnected pipe and pipe components are used to transport raw material, intermediated product and final product to the desired location. Piping components such Pipe, Elbow, Tee, Reducer, caps, flanges, gasket, and Valves are the basic building of Oil & Gas industries.
Let’s learn about first building block of Oil & Gas Industries.
What is Pipe?
The pipe is a straight pressure tight cylindrical hollow, used in the piping system to transport liquid, gas and sometimes solids.
Different Types of Pipe
Different types of pipes used in various design conditions, considering technical and commercial parameters. For small & medium sizes requirement, seamless pipes are more popular whereas, for larger diameter, welded pipes are more economical.
In the chart below you can see the most commonly used types of pipes in Oil & Gas industries.
Pipe Size & Dimensions
Dimensions of the pipes are covered in following Standard
- ASME B36.10 – Welded and Seamless Wrought Steel Pipe (Carbon & Alloy Steel)
- ASME B36.19 – Stainless Steel Pipe
Three different terms are commonly used to define the size of the pipes.
What is NPS, NB & DN ?
NPS – Nominal Pipe Size
All American standard used NPS designation to define the size. This is a modern derivation of earlier IPS – Iron Pipe Size. NPS is not an OD or ID but it is in-between of the outer and the inner diameter of the pipe. For example, NPS 2 size outside diameter is 60 mm or 2.375 inches. In general, NPS 12 and the smaller pipes has outside diameter greater than the size and for NPS 14 and above size pipes outside diameter is the same as the size in inches.
NB – Nominal Bore
This is the European equivalent of NPS. In this standard pipes, sizes are mentioned in millimeter
DN – Diameter Nominal
This is the German equivalent of NPS. In this standard also, pipes sizes are mentioned in millimeter.
In the table, you can see the correlation of size and OD.
Correlation of Pipe size and OD (NPS Vs NB/DN)
|OD in mm||21.3||26.7||33.4||48.3||60.3||88.9||114||168||219||273||324|
Small Bore Pipe Vs Large / Big Bore Pipe
In the project, you will come across the terms such as small Bore & big bore / large bore. Pipes having size range ½” – 1 ½ ” are termed as small bore. Pipes having size range 2” & above are termed as big bore.
Pipe Thickness - Schedule number
The thickness of the pipes is expressed in Schedule number. What is Schedule No?
- A schedule number is an approximate value of the equation = 1000 P/S
- P is the service pressure in (psi)
- S is the allowable stress in (psi)
- Common schedule nos. are 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160.
- Thickness is also expressed as STD, extra strong-XS, double extra strong-XXS.
- Higher the schedule no. higher the thickness and smaller the inside diameter as outside diameter of each pipe size is standardized.
- The thickness of stainless steel pipes is also expressed in Schedule number. Schedule no with S suffix is as per ASME B36.19, and it is used with stainless steel pipes. There is only four schedules are mentioned in ASME code which are 5S, 10S, 40S, 80S. So, please remember schedule no. 10 and 10S do not have the same
- Please note that carbon steel schedules equal stainless schedules for following
- Up to NPS 12, all Sch 10 and Sch 10S wall thicknesses are the same.
- Up to NPS 10, all Sch 40, Std Wt and Sch 40S wall thicknesses are the same.
- Up to NPS 8, all Sch 80, XS and Sch 80S wall thicknesses are the same.
Pipe Length (Single Random Vs Double Random Pipes)
Pipe length is mentioned in either meter or in feet. During production, pipes are not manufactured in same lengths and during construction of process plant, you required various lengths of pipes. To address this issue standard has defined pipe lengths in single random and double random categories.
- Single random pipes comes in 4.8 m to 6.7 m in lengths with 5% of lengths in between 3.7 to 4.8 m
- Double random pipes has a minimum average of 10.7 m and a minimum length of 4.8 m with 5 % of lengths in between 4.8 m to 10.7 m
you can order Fixed length pipes also, but it may cost you more
Pipes comes in following four end types
- Plain End – This kind of end used when socket type weld fittings are used.
- Beveled End – This kind of end used when butt type weld fittings are used.
- Threaded End – This kind of end used with threaded connections in piping system
- Socket & Spigot – This type of end generally used in Ductile iron pipeline and non-metallic piping pipeline such as PVC, GRE/GRP.
Commonly used Pipe Materials
ASME B31.3 provides the list of material used in Process Piping. In below table, you can see the most commonly used material types & their Grades.
Used up to 425°c
|Stainless steel |
Used for corrosive fluid
|Low alloy steel|
Used for temp > (425°c)
|Low temp carbon steel|
Used for temp < (-29°c)
|A53 Gr B||A312 Gr TP304||A335 Gr P2 , P12, P11, P22, P5, P9||A333 Gr 6|
|A106 Gr B||A312 Gr TP316|
|API 5L Gr B||A312 Gr TP321|
Chemical Properties of these piping materials are covered in their respective ASTM standard. ASTM Standard restrict the use of material produced by certain manufacturing process only. For example, ASTM A106 allow only killed steel that produces by the open-hearth, basic-oxygen, or electric furnace, with preferred separate degassing.
Heat Treatment of Pipes (Hot Finished Pipes Vs Cold Finished Pipes)
Heat treatment depends on the manufacturing of the Pipes
- Hot finished pipes, no heat treatment is required. As during the manufacturing, Pipes temp. remain in the range of heat treatment temperature till the final size and thickness are achieved
- Cold finished pipes, heat treatment is required as per applicable ASTM standard. The cold finished pipes are either cold drawn or temperature is not in maintained in the standard specified range during the manufacturing.
Different heat treatment methods are used for different grade of the material of the pipes. These heat treatment methods are
- Solution Annealing
- Stress relieving
Or it is a Combination of above