We can summarize some of the things that support the fact that in many applications where metal valves are specified, plastic one would do a better , more cost effective job.

First and foremost is the corrosion resistance of plastics over metal. This results in some obvious and not so obvious benefits. More than just being a "low maintenance" valve a plastic one will never jam, stick, or fail because of rust or corrosion. Another often overlooked benefit of corrosion resistance is that plastic valves never have to be painted to stand up in corrosive environments or harsh climatic conditions. They can be installed and used right out of the box in places a metal valve would have to be epoxy coated just to survive.

Another benefit is cost. Not only the cost of the individual valve but the total cost of the installed system. When all cost are considered, including freight, installation and service life a metal system will in most cases be more expensive. The differences can be significant. Exotic metal alloy systems can cost up to 13 times what a plastic system would. Even carbon steel can be almost twice the cost of PVC depending on the size and complexity of the piping system involved.

Flow rates are another area where plastic pipe has and advantage. The interior of plastic pipe is smooth and clean and it will stay that way year after year. Metal pipe can rust , corrode and scale resulting in reduced flow rates and higher pressure drops over time.

Some people are not aware of the engineering advance that have been made with regard to plastic piping materials over the last several years. Users of metal pipe, valves, and fittings are often concerned with what they believe to be the mechanical strength limitations of plastics. While it's true that there is no commonly available plastic system that can match metals temperature/pressure service levels, advances have been made. Plastic piping systems are now commonly available that provide adequate tensile strength or operations up to 200 F. Plastic pipe is also available that maintains its pressure bearing capabilities for over 50 years.