SDR-21 vs Schedule 40 Pipe

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SDR-21 and Schedule 40 pipe provide varying degrees of wall thicknesses for piping systems. SDR-21 provides the best choice in most irrigation and drainage applications where flexibility is the top concern. Schedule 40 pipe provides higher strength and is the defacto choice for process piping.

In this article, you will learn about SDR-21 pipe, Schedule 40 pipe, and compare each pipe’s properties.

SDR-21 Pipe

The SDR (standard dimension ratio) is a rating system for pressure pipework. The lower the pressure rating, the greater the SDR. An HDPE (High-Density Poly Ethylene) pipe’s SDR specifies how much pressure in pounds per square inch (PSI) it can withstand. They are highly resilient, with high-tensile and high-impact strength.

SDR-21 pipe wall thickness chart
Courtesy: HARCO Fittings

SDR-21 has a 200 psi rating. The “21” indicates that the outside diameter is 21 times the thickness of the wall. The minimum wall thickness times the SDR is the outer diameter of the pipe. SDR-21 pipe is produced using polyvinyl chloride material to meet or exceed ASTM D2241 standards and is NSF-approved for drinking water use.

SDR-21 is commonly used in:

  • Potable water supply lines.
  • Transmission mains.
  • Sewage force mains.
  • Golf courses.
  • Other irrigation systems.
  • Oilfield saltwater disposal.

Also, stormwater disposal, well casings, industrial process piping, and other pressure applications are suitable for SDR-21 pipe, PR 200, or class 200.

Gravity sewer, force main sewer, and water reclamation projects also see common use. With a long-term 2 to 1 safety factor, the pipe’s pressure rating shows the maximum permitted sustained pressure capacity. 

Schedule 40 Pipe

First, let’s start with what the “schedule” pipe is. Schedule pipe is an earlier method of pipe labeling that uses the standard dimensions of ductile iron and steel pipe, i.e., the wall thickness of the pipe.

Schedule 40 pipe follows the former “standard” piping label and considers the same up to a diameter of 10 inches. The walls of all Schedule 40 pipework, regardless of diameter, are 3/8′′ thick.

Gas and water lines commonly use Schedule 40. This pipe frequently surrounds buildings for drainage, and it’s available at local hardware stores. This pipe serves best in drainage, irrigation, and other cold-water systems.

Schedule 40 pipe is strong, rigid, and can handle pressure applications. In addition, it can withstand temperatures of up to 140 degrees Fahrenheit. Schedule 40 steel pipes made of low-carbon or mild steel. Steels with a carbon content of less than 0.2 percent are classified as low-carbon steels.

SDR-21 vs Schedule 40 Pipe Properties

Material Type

Both pipes are typically made with Type I, Grade I compound, Cell classification 12454-B per ASTM-D1784 (AKA 1120). When considered against each other for service, both SDR-21 and Schedule 40 are typically made of polyvinyl chloride (PVC). Other plastic piping applications may include high-density polyethylene (HDPE), and polypropylene (PP).

Several gray PVC pipe
Courtesy: ThoughtCo

PVC (or Vinyl) is a high-strength thermoplastic material that is widely used in various applications, including pipelines, medical equipment, wire, and cable insulation. In fact, t’s the third-most-produced synthetic plastic polymer on the planet.

Several black PVC pipe
Courtesy: China LESSO

HDPE (High-Density Poly Ethylene) is a petroleum-based thermoplastic polymer. HDPE plastic is one of the most versatile plastic materials available, with uses as diverse as plastic bottles, milk jugs, shampoo bottles, bleach bottles, cutting boards, and pipes.

Polypropylene (PP), is a thermoplastic polymer with many uses. It is made from the monomer propylene through chain-growth polymerization. Polypropylene is a non-polar, partly crystalline polyolefin that belongs to the polyolefin family.

Wall Thickness

A Schedule 40 pipe with a bore of 4.026 inches (100 mm) has an outer diameter of 4.500 inches (114.30 mm), a wall thickness of 0.237 inches (6.02 mm), and an outside diameter of 4.500 inches (114.30 mm) (102.26 mm). On the other hand, a 3.54 (90 mm) SDR-21 pipe has a minimum wall of 4.3 mm.

As a result, the schedule 40 pipe has a thicker wall thickness than the SDR21 pipe. For pipes 2-1/2″ and smaller, it is over 150% thicker.

Usually, SDR has a 4″ interior diameter, the same as schedule 40, but since the wall is thinner, the outer diameter is not the same.

A schedule 40 pipe is universally more common and part of the standard pipe thickness schedule as seen below. A pipe wall thickness calculation determines the pipe thickness of a pipe under pressure.

Pipe wall thickness calculation chart
Courtesy: Tork Systems

Corrosion Resistance

Electrolytic and galvanic corrosion and any other known corrosive soil or water condition do not affect SDR-21 or Schedule 40 specifically. Corrosion resistance depends on the pipe material, process media, and surrounding environment.

Flexibility Strength

SDR stands for standard dimension ratio, and it was created to be more adaptable than Schedule 40. SDR provides flexibility at the expense of structural integrity.

When pipes are underground, they are subject to asymmetrical and hard to predict stresses.

The flexibility of SDR results in fewer breaks from soil conditions. The rigorous Schedule 40 provides little wiggle room if the ground shifts or settles. The pressure will cause it to shatter or break. Schedule 40 gets more brittle in the winter because of its exposure to the elements.