Pipe Schedule describes the pipe wall thickness. With an increase in pipe schedule number pipe thickness increases. The main function of the pipes is to carry fluid (liquid, gas, fluidized solids, slurry, mixed-phase products, etc) under pressure (internal, external or both), therefore to sustain the fluid pressure the pipe has to be strong enough to perform its intended duty without failure. Obviously, for pipes containing pressurized fluids the wall thickness, and by implication the pipe’s strength, is the most important parameter. Wall thickness of Pipe is expressed by “schedules or Schedule numbers“, referred to as pipe schedules or Piping Schedule.
What is a Pipe Schedule?
Pipe Schedule or Schedule number of a Pipe is a dimensionless number that is related to Pipe Wall Thickness. Piping Schedule Number for a specific pipe size is a pipe thickness designator for that pipe size.
How to Calculate a Pipe Schedule?
Schedule Numbers for pipe size/wall thickness combinations are calculated (approximated) to get a uniform relationship equal to 1000 times the P/S (P=Design Pressure and S=Allowable Stress) expression contained in the modified Barlow formula for pipe wall thickness. The pipe schedule is abbreviated as SCH. So,
Features of Pipe Schedule
For a given size and schedule the thickness of the pipe is fixed and defined in the applicable ASME standard (B 36.10M/B 36.19M). Even though Pipe thickness can also be specified in mm or inches to the value corresponding to that specified in the ASME standard, The Schedule Numbers are strictly used as a convenient designation system while ordering piping items.
For any given pipe size and varying schedule numbers or thicknesses, its Outside Diameter (OD) remains constant and Internal Diameter (ID) varies. With an increase in thickness the strength increases but its ID decreases. OD is kept constant to help the support hardware design so that the same support hardware can be used for the same pipe size (varying thicknesses).
Pipe Schedule Governing Standards
In the oil and gas and related downstream industries the most common standards are
- ASME B 36.10 Welded and Seamless Wrought Steel Pipe, and
- ASME B36.19 Stainless Steel Pipe
What is Nominal Pipe Size
The size of all pipe is identified by the nominal pipe size. The manufacture of pipe NPS 1⁄8 (DN 6) to NPS 12 (DN 300), inclusive, is based on a standardized outside diameter (OD). This OD was originally selected so that pipe with a standard OD and having a wall thickness that was typical of the period would have an inside diameter (ID) approximately equal to the nominal size. Although there is no such relation between the existing standard thickness — OD and nominal size — these nominal sizes and standard ODs continue in use as ‘‘standard.’’
The manufacture of pipe NPS 14 (DN 350) and larger proceeds on the basis of an OD corresponding to the nominal size. So the OD in mm of a pipe NPS 14 or higher can easily be calculated by simply multiplying the NPS (here 14) by 25.4. But for lower sizes, the OD calculation is not so easy.
Pipe Schedule vs Pipe Size
For all pipe sizes the outside diameter remains constant. Therefore any variation in schedule i.e. wall thickness affects only the inside diameter. As the schedule number increases, the wall thickness increases, and the actual bore is reduced.
- Standard (STD) is identical to SCH 40 up to NPS 10. All larger sizes of Standard (STD Schedule) have 9.53 mm wall thicknesses.
- Extra-Strong (XS) is identical to SCH 80 up to NPS 8. All larger sizes of Extra-Strong have 12.70 mm wall thicknesses.
- Double Extra Strong (XXS) wall is thicker than SCH 160 from NPS 1/8 to NPS 6, and SCH 160 is thicker than XXS wall for NPS 8 and larger.
- With an increase in pipe thickness internal diameter of the pipe reduces as pipe outer diameter remains constant.
Pipe of sizes and wall thicknesses other than those of Standard, Extra-Strong, and Double Extra-Strong, and Schedule Number were adopted from API Specification 5L.
Pipe Schedule / Pipe wall Thickness Calculation
The calculation of wall thickness varies depending on the usage of code. All codes (B 31.3, B 31.1, IBR, B 31.4, B 31.8, nuclear code, etc.) provide equations for calculating the minimum wall thickness based on pressure which the pipe has to withstand. The major parameters involved for thickness calculation are Design pressure, Pipe OD, and Allowable Stress at design temperature. After calculation of minimum wall thickness, corrosion and mechanical allowances need to be added with that. After that, the actual pipe thickness is selected (immediate higher size thickness of the calculated value) from ASME code tables depending on pipe material (CS or SS).
You may be interested in the following two articles:
Pipe Schedule Chart/Pipe Dimension Charts
The wall thickness associated with a particular schedule depends on the pipe size. Dimensions (OD, ID, Thickness, and Schedule Number) and Weights of CS and SS pipes are given in ASME standards mentioned above. The tables in the respective codes are dimensionally complete for all sizes and wall thicknesses within its scope, but some of the larger, heavier wall sections are beyond the capability of seamless mill production and must be obtained from forged and bored billets or other sources.
Stainless steel pipe is more often available in standard weight sizes (noted by the “S” designation, for example, “NPS SCH 10S”). However stainless steel pipe can also be available in other schedules.
- DN-Nominal Diameter,
- NPS- Nominal Pipe Size,
- NB – Nominal bore,
- STD – Standard,
- XS – Extra Strong,
- XXS – Double Extra Strong,
- ASME-American Society of Mechanical Engineers,
- CS-Carbon Steel,
- SS-Stainless Steel.
The following table developed by Stressman engineering provides a Nominal Pipe Size vs Pipe Schedule Chart.
Pipe Schedule 40 Pipe Dimensions
The following table will provide an example for Schedule 40 Steel Pipe Dimensions.
|Pipe NPS||Schedule No||Outside Diameter, Inches||Wall Thickness, inches||Plain End Weight, lb/ft||Outside Diameter, mm||Wall Thickness, mm||Plain End Mass, Kg/m|