Measuring the diameter of a plastic pipe might seem quite straightforward at first – you just measure the diameter of the pipe. However, it's important that you measure the right diameter with the right unit, so you can tell exactly what size it is, and make sure that when you're reordering fittings or more pipe, that you choose the right size.
A union is a three-part fitting. The first part is a section which has a large thread on it and an O-ring inside which makes the seal. On the other side of this first part is a plain socket. The other half of the union consists of two parts: a union nut and a socketed section. The nut slides over the socketed section and, together, they pair up with the threaded section of the union. The O-ring sits against the flat surface of the socketed section to give you a good seal.
Double union ball valves have both advantages and disadvantages – figuring out where you need the valve in your pipe system and what kind of control you want to have over the flow will help you decide whether a double union ball valve is the right choice for you.
A PVC gate valve is quite a complex valve compared to more simple valves, like a ball valve, but is fantastic for fine control of flow. The wheel at the top of the valve has about eight/eight-and-a-half turns from fully shut to fully open, which allows for very small movements in the internal workings of the valve. This gives you fine control of your flow.