Which would not be an approved irrigation system pipe?

Schedule 20 PVC pipe is not typically approved for use in irrigation systems due to its lower pressure rating and thinner walls compared to Schedule 40 PVC. Schedule 40 PVC is commonly used in irrigation systems because it can withstand higher pressures and is more durable, making it suitable for carrying water over various distances and under varying pressure conditions. PEX tubing is flexible and resistant to freezing, making it appropriate for certain applications, and HDPE (High-Density Polyethylene) pipe is also frequently used in irrigation due to its resistance to corrosion and ability to handle high pressure. Therefore, Schedule 20 PVC's reduced capacity to handle the demands of an irrigation system makes it an unsuitable choice.