A double union ball valve is the most basic type of valve, with a union either side of it that enables the valve to be removed from the system and repaired or replaced quickly and efficiently, saving you time and effort. The double union ball valve operates on a full bore flow, which means that there is nothing to catch any debris within the valve. If you want fine control of your flow, this valve is not the best choice, because there's only a quarter of a turn between the valve being fully open and fully closed, which doesn't leave you much room to adjust in small amounts.
There are two types of gate valves: one that operates with a push-down and pull-up mechanism, and one which operates using a gear mechanism. The first gate valve operates on a full bore flow, but there is a little ridge inside the valve that can catch grit and sand, for example. Too much debris can compromise the valve as it will build up at this ridge. The valve is better suited to low pressure systems and is fitted with a bolt and nut rather than a union on the side of it. However, you can still attach a union to either side, and this is something which we recommend for that quick and easy check.
The gear mechanism gate valve takes about eight turns to go between fully open and fully shut, which allows for the fine control you might want on your flow. It operates on a full bore flow, but also has a potentially compromising feature inside - a small dip which can also cause debris build up. If you have a high flow rate the risk lessens, however, because the increased flow can push the debris out. Again, we would recommend fitting a union to either side of this valve.
A diaphragm valve is, in our opinion, the best valve for fine control of your flow. Our diaphragm valves come with unions fitted to either side. When you turn the gear mechanism on the top of the valve, you cannot see inside it to tell when it is fully open or fully closed - to rectify this, the valve has a riser in the centre of the gear mechanism that rises up and down when you turn the valve from fully open to fully closed. There is a weir inside the valve which the water flows over and a rubber seal that compresses down or lifts off of the weir, which is what the gear mechanism operates. This valve can be easily compromised by debris coming down the system - small pieces can be cleared by opening the valve fully, but larger pieces will require the valve being taken out of the system to clear.
A butterfly valve gets its name from the shape of the wafer inside, which looks like the wings of a butterfly. This wafer rotates as you turn the handle, physically blocking the water when you close the valve and letting the water through when you open it. This valve is not ideal for fine control of flow because, like the double union ball valve, there is only a quarter turn between being fully open and fully closed. There is a risk that debris can get stuck across the wafer, blocking the valve from being lifted out and cleared if the debris is large enough that you cannot close the wafer. The butterfly valve comes with flanges on either side of it, rather than unions, because of the size of it, but flanges work in much the same way as unions - all you need to do is unbolt the flanges and you can lift the valve out. Butterfly valves are used in systems which have gone above four inches/110mm.