Pipe Schedule Chart Explained: Schedule 10, 20, 40, 80, 160 Comparison

If you’ve ever studied the specification of a pipeline and found the size of the pipe followed by some numbers like “Sch 40” and “Sch 80,” then you’re not alone in having no idea what exactly is changed by this designation. Pipe schedule designations are one of the most confusing elements of pipe specification, but they control the most basic and important characteristic of any pipe: its ability to withstand pressure. At Aashish Metals, we offer pipes and tubes in a range of pipe schedules from Sch 5 to Sch 160 for carbon steel, stainless steel, duplex and nickel alloy pipes, and the question about schedule is asked in virtually all quotations we make. In this article, we will explain in detail what does pipe schedule mean, how it is calculated and how different schedules of 10, 20, 40, 80 and 160 differ from each other.

What Does Schedule Actually Mean in Piping?

Pipe schedule is a common method of indicating the thickness of a pipe as a percentage of the pipe’s outside diameter. It was created for the ease of the engineer, allowing one to use a single numeral to indicate pipe strength, rather than having to write out the decimal wall thickness each time. The schedule numbers in common use today, Sch 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160, are defined in ASME B36.10M for carbon and alloy steel pipe and ASME B36.19M for stainless steel pipe. A schedule number is always higher for a thicker pipe wall in the same nominal pipe size, and hence a higher pressure rating, but there is not always a correlation between the two schedules at the same schedule number when carbon and stainless systems are specified in the same passage.

Nominal Pipe Size, Outside Diameter, and Why Schedule Only Changes the Inside Diameter

As far as a certain Nominal Pipe Size (NPS) is concerned, the outside diameter (OD) will always be constant irrespective of the pipe schedule. However, as the schedule number goes on increasing, there will be an increase in the wall thickness alone while the inside diameter (ID) will decrease. With this in mind, pipes with the same NPS can be joined to the same flanges, fittings, and valves. The other important thing to note is that the nominal size of the pipe is not the same as its actual outside diameter for NPS less than 14.

How Pipe Schedule Is Calculated

The schedule number is derived from a simplified version of the Barlow formula for pressure vessel wall thickness:

Schedule Number ≈ 1000 × P / S

Where P is the internal working pressure in psi and S is the allowable stress of the pipe material in psi. This relationship is the reason that the schedule number approximately correlates with the thickness of the wall needed for a specific combination of pressure and material strength, but is not a firm design constraint. This simplified schedule formula should always be confirmed with ASME B31.3 (process piping) or B31.1 (power piping) calculations, which include corrosion allowance, mill tolerance, and joint efficiency; it should never be used on its own for actual wall thickness selection for a specific project.

Pipe Schedule Chart: Schedule 10, 20, 40, 80, and 160 Comparison

Schedule 10: Thin Wall for Low Pressure and Corrosion-Resistant Systems

Pipe with Schedule 10 has one of the slimmest walls within the standard line and is often used for stainless steel piping where the material is resistant to corrosion by default, hence the need for a thick carbon steel-based corrosion allowance is not necessary. This type of pipe is often chosen for use in sanitary process piping, low-pressure water and air distribution, as well as stainless steel piping within food, beverage, and pharmaceutical industries where the process fluid is not aggressive. Due to its slim wall, Schedule 10 pipes are less heavy and cheaper compared to other schedules of pipe, but they are not recommended for high-pressure hydrocarbon service or buried piping under load.

Schedule 20: A Middle Ground for Moderate-Duty Lines

Schedule 20 lies between Schedules 10 and 30/STD. It is not often used compared to its neighboring schedules and it is found only in large diameter and low-to-medium pressure applications such as utility water, HVAC, and even fire protection systems when there is a need for a little stronger wall than Schedule 10 but not at the level of Schedule 40.

Schedule 40: The Default Choice for General Purpose Piping

Schedule 40 is by far the most frequently referenced schedule for general use industrial and commercial piping. In carbon steel pipe of up to NPS 10, Schedule 40 pipe wall thickness is equal to that of STD (Standard Weight), and hence the interchangeable use of both terms in that range of sizes, even though the terms differ beyond that size.Schedule 40 pipe is a good compromise between pressure rating, weight, and cost, which is the reason why it forms the default schedule in a very large number of process piping systems, plumbing, compressed air, and moderate pressure hydrocarbon applications, unless a specification calls for a heavier schedule. Where a pipe classification specification refers to “carbon steel pipe” in a casual context, Schedule 40 can be safely assumed, although it should never be assumed in an engineering drawing context without verifying the pipe classification.

Schedule 80: Heavy Wall for Higher Pressure and Harsher Service

At most common diameters, Schedule 80 pipe is twice as thick as Schedule 40 and, for carbon steel pipes of NPS less than or equal to 8, Schedule 80 is the same thickness as XS (Extra Strong) pipe, as is the relationship between Schedule 40 and STD. The thicker walls make the Schedule 80 pipe far more resistant to corrosion, erosion and mechanical damage and so much stronger than the Schedule 40 pipe. Schedule 80 pipe is employed in applications that involve high-pressure steam and for chemical processing and hydraulics. In addition, when the service life of the pipe needs to be increased due to either internal or external wear, the Schedule 80 pipe is an appropriate choice. However, there is also a downside to using Schedule 80 pipe, which is its increased weight.

Schedule 160: Maximum Standard Wall Thickness for Severe Service

Schedule 160 is the thickest schedule in the standard list, which is only used where there is a need for the highest pressure and temperatures, for instance in high-pressure steam, boiler feed lines, and severe service oil and gas operations. In small diameters, the Schedule 160 pipe wall can match or even exceed that of Double Extra Strong wall thickness, although they do not necessarily have similar values depending on the particular diameter from the standard schedule chart.Being thicker in wall and correspondingly smaller in bore, Schedule 160 pipe is thus much heavier, costlier, and difficult to weld and fabricate than thinner schedules. This schedule is therefore only specified when the actual design pressure and temperatures require it.

Carbon Steel vs Stainless Steel Schedule Differences

What often surprises buyers is that sometimes, carbon steel pipe (according to ASME B36.10M) and stainless steel pipe (ASME B36.19M) do not have the same wall thickness at the same nominal schedule, especially at schedules such as 5S and 10S which exist in the stainless steel standard but are not numbered in the carbon steel table. If a project has carbon steel connections as well as stainless steel, and/or a change from one material to the other, it is wise to confirm the applicable standard for each material, rather than assuming that the standard numbers apply to each material in the project.

We supply both carbon and stainless steel pipe across the full schedule range through our stainless steel products and alloy steel products lines, along with duplex and super duplex grades such as Duplex Steel S31803/S32205 pipes and tubes and Super Duplex S32750/S32760 pipes and tubes, both of which we stock from Schedule 10 through Schedule 160 to match project specifications without custom mill lead times.

Conclusion:

Pipe schedule is a very minute detail on a drawing that directly impacts system pressures, system fitting and flanging, and project weight and cost. The relationship between the 10, 20, 40, 80, and 160 and how they relate to the older STD, XS, and XXS designations will greatly facilitate correct interpretation of piping specs and help prevent costly ordering errors. With over 50 countries of export experience and full documentation, Aashish Metals provides pipe to cover this entire schedule range, as required by your project. Contact us today to schedule the next piping project.