Pipe Schedule (SCH) is the term used to define a pipe’s wall thickness. It is a reference value derived from a wall thickness formula rather than an actual measurement. Pipe schedule numbers rise along with pipe thickness. The primary purpose of pipes is to transport fluid under pressure, hence to maintain the fluid pressure, the pipe must be robust enough to complete the task at hand without fail. The wall thickness, and consequently the pipe’s strength, are the most crucial factors for pipelines carrying pressurized fluids. Schedules or Schedule numbers, often known as pipe schedules or piping schedules, are used to express the wall thickness of pipes.

Differing schedules can result in different wall thicknesses in two pipes of the same diameter. Therefore, if someone is choosing a pipe for a high-pressure application, they will choose a higher number, which indicates a more expansive schedule (wall thickness).

Additionally, the letter “s” is added as a suffix after the number to define stainless steel piping schedules. NPS 14 pipe with a 40s schedule.

Schedules of stainless steel are handled in this way because of their higher strength. In comparison to other steels, less wall thickness is needed to resist the same pressure. In use today are the following pipe schedules: 5, 5S, 10, 10S, 20, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160, STD, XS, AND XXS.

The process of pipe sizing has been impacted by the diverse ways that different pipe diameters are made.

This indicates that pipe sizes (NPS) 1/8th ′′ to 12′′ have an outside diameter larger than their nominal pipe size, whilst pipe sizes 14′′ and above have an outside diameter identical to their nominal pipe size.

Three pipe wall thicknesses were the only ones used in the past, but these measurements are no longer used because they are too ambiguous.

- Standard (STD)
- Extra Powerful (XS)
- Extra Strong by Two (XXS)

SCH 40 is the most popular pipe schedule. SCH 40 is the same wall thickness as the previously applied value for NPS 12 or smaller pipes or pipes having an outside diameter of approximately 12 inches. On the other hand, for pipe sizes up to NPS 10, the SCH 80 wall thickness is equivalent to the earlier XS wall thickness.

**Characteristics of Pipe Schedule**

The thickness of the pipe is fixed and specified in the relevant ASME standard (B 36.10M/B 36.19M) for certain pipe sizes and schedules. The Schedule Numbers are only used as a practical designation method for ordering pipeline products, even though pipe thickness can alternatively be specified in mm or inches to the value equivalent to that stated in the ASME standard.

The pipe’s internal diameter (ID) fluctuates and its external diameter (OD) is constant for any given pipe size and any given schedule number or thickness. Although its ID drops as thickness grows, an object’s strength increases. To aid in the design of the support hardware, OD is kept constant to enable usage of the same support hardware for pipes of the same size (varying thicknesses).

Even if the pipe schedule numbers for various pipe diameters may be the same, the pipe thickness may not be. Concerning pipe sizes, they might be the same or different. For instance, while both schedule 40 pipes are 8 inches in length, the thickness of the 8-inch pipe is 8.18 mm as opposed to the thickness of the 6-inch pipe, which is 7.11 mm. Again, the 9.53 mm pipe thickness is the same for both the 15-inch Schedule standard and the 16-inch Schedule standard.

**What is Nominal Pipe Size?**

The North American standard for measuring pipe diameters used for various pressures and temperatures is called NPS, which stands for “Nominal Pipe Size.” When there is a reference to a pipe as being 6″, it means that it is the pipe’s nominal size. The NPS standard is extensively used in several places but some requirements must be met to determine the proper nominal pipe size.

Moreover, NPS 14 and above Outside Diameter is the same as NPS for pipe diameters. NPS only indicates the pipe’s outer wall diameter for diameters greater than NPS 14. Based on a fixed outer diameter, NPS 1/8 (DN 6) to NPS 12 (DN 300) pipe is manufactured (OD). Therefore, the pipe’s interior diameter (ID) decreases as wall thickness does. An NPS 12 pipe will have a 12.75-inch outer diameter, while an NPS 14 pipe will have a 14-inch outer diameter.

Therefore, the NPS will be between the pipe’s OD and ID. The nominal size of a pipe is corresponding to the manufacturing of NPS 14 (DN350) and above pipe OD.

The pipe schedule and the metric DN standard, where the outer wall diameter is measured in millimeters, are typically included in tables with the NPS number. Because they are two somewhat different standards, there is a difference between smaller and larger pipe sizes. Since there were three wall thicknesses available STD, XS, and XXS the NPS values always corresponded to the same pipe inner diameter. However, because there are so many different pipe thicknesses available, the NPS number is now only loosely tied to the inner and outer diameter of the pipe.

By multiplying an inch dimension by 25.4 and rounding, one can simply convert it to millimeters, more than 16-inch outside diameter, rounded to the closest 1 millimeter, round to the closest 0.1 mm for outside diameters of 16 inches or less and the nearest 0.01 mm is used to round the pipe wall thickness.

The NPS standard makes no mention of elements like the inner diameter or wall thickness. The parameters of the pipe schedule contain those specifics. Please be aware that the name “NPS” is also used in the American Standard for funnel assignments, although there it refers to “National Pipe Thread Straight.”

**What is Pipe Nominal Bore?**

NPS is usually used to describe an NB (Nominal Bore). There is therefore no distinction between NB and NPS. Another American abbreviation for pipe dimensions is NB. I’ve also seen that individuals often refer to pipe sizes in NB when pipe measurements are given in millimeters (DN). Therefore, whether someone refers to 25nb pipe or 50nb pipe, they are essentially referring to DN.

**What is Diameter Nominal Pipe Size?**

Diameter Nominal, sometimes known as DN or Diameter Nominal, is a designation used internationally (together with the SI or Matric Designator) to indicate pipe sizes. Here, it’s important to understand that DN and NPS display pipe sizes in different ways.

DN 50 is used to refer to a 2″ pipe. By multiplying any NPS by 25, you can convert it to DN. Pipe sizes increase incrementally by 1/4, 1/2, and finally 1. It rises from 6″ to 42″ in two-inch steps, then in four.

**Difference between Pipe Schedule and Pipe Size**

The exterior diameter stays the same for all pipe diameters. Therefore, any change in the pipe schedule, such as a change in wall thickness, solely impacts the inside diameter. The pipe schedule number increases, the wall thickness increases, and the actual bore lowers.

- Standard (STD) and SCH 40 are equivalent up to NPS 10. Standard (STD Schedule) bigger sizes all have walls that are 9.53 mm thick.
- Extra-Strong (XS) and SCH 80 are equivalent up to NPS 8. Wall thicknesses for Extra-largest Strong’s sizes are 12.70 mm.
- From NPS 1/8 to NPS 6, the double Extra Strong (XXS) wall is thicker than SCH 160, and for NPS 8 and larger, SCH 160 is thicker than the XXS wall.
- The internal diameter of the pipe decreases as pipe thickness increases while the pipe’s exterior diameter stays the same.

The API Specification 5L was adopted for pipes with wall thicknesses and diameters other than Standard, Extra-Strong, Double Extra-Strong, and Schedule Number.

**How to calculate Pipe Schedule?**

Schedule numbers for pipe size/wall thickness combinations are computed to obtain a uniform relationship equal to 1000 times the P/S (P=Design Pressure and S=Allowable Stress) expression found in the modified Barlow formula for pipe wall thickness (approximatively). The pipe schedule is denoted by the initials SCH. SCH = 1000 * (P/S). Here, P stands for the pipe’s internal working pressure, while S is the maximum amount of stress that the material can withstand.

An illustration of a pipe with an S value of 12,000 and an internal operating pressure of 450 psi is:

1,000 * 450/12,000. This is 37.5, or around an SCH 40. The modification in wall thickness in relation to NPS is another crucial point to consider (Nominal Pipe Size). Even for the same SCH number, the wall of the pipe becomes thicker as the nominal diameter increases. Take the same SCH 40 wall thickness that we previously used. The SCH 40 wall thickness for an NPS 1 pipe is 0.133 inches, whereas the SCH 40 wall thickness for an NPS 2 pipe is 0.154 inches.

**Pipe Schedule for Stainless Steel**

Compared to carbon steel pipe, stainless steel pipe is significantly more expensive. Less thick pipe may function well without worrying about early failure because of the stainless steel’s inherent resistance to corrosion, the development of high alloy stainless steel, and fusion welding.

For stainless steel pipes and fittings, ASME has established various schedule numbers to lower material costs. For SS pipe, ASME B36.19 introduces Schedule No. with the “S” suffix. Instance: 10S

**What is a Pipe Schedule Chart?**

A table listing pipe NPS and their thicknesses concerning different schedule numbers is known as a pipe size chart or pipe schedule chart. Depending on the pipe size, a certain schedule has a specific wall thickness. In the aforementioned ASME standards, dimensions (OD, ID, Thickness, and Schedule Number) and weights of CS and SS pipes are provided. The tables in the relevant rules are dimensionally comprehensive for all sizes and wall thicknesses covered, although some of the larger, heavier wall sections cannot be produced in a seamless mill and must be purchased from billets that have been forged and bored or from other sources.

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