All good advice, CuzzA. I couldn't agree more.
It might be important to note that even beyond the (hopefully) obvious quality issues, all valves are not created the same. Most, but not all ball valves are full-port valves. That means the passageway through the valve is the same (bore) size as the pipe size it's connected to, which is important if you need to ensure maximum flow. If this is important, be sure to buy a full-port valve of the appropriate size.
With gate valves, there is really no such thing as a full-port valve, although the port sizes can vary considerably, and some valve designs restrict flow at wide open more than others. Because the valve, seat and water passage are more complex, and create more turbulence within the valve body, a gate valve is more prone to become sticky from mineral accumulation, but since they offer more leverage in operating, they are sometimes easier to work free (and also easier to break by using too much force on a stuck valve). In general, think of ball valves as on-off devices, and gate (or other threaded-stem designs) valves as fine throttling devices. Ball valves can of course be used for rough throttling applications, and frequently are.
Here is what I think is the most salient point to make in this whole thread. Several posters in this thread have referred to 'True Union Ball Valves' as if this is some official, accepted plumbing terminology. It is not. There is no such animal, and referring to it as a 'TUBV' still doesn't make it so. Is this some goofy term Savko originated? First of all there is no such thing as a 'true' union. A union is a union. There is no 'false' union. You either have a union or you don't. You can buy valves of almost any design with integral unions at one or both ends. They are almost always much more expensive than the sum of their components. The only reason to ever buy one is if space for the valve and unions is severely limited. If it isn't, you are much better off using separate unions, because on a valve with integral unions, the unions must be replaced with the valve. If you solvent welded the pipe end of the union to your piping and the unions on your new valve are not the same as your original valve, you're going to be cutting and splicing pipe in order to install the new pipe end of the union. Standards are developed for a reason, but they are not always followed, especially with imported components.
Honestly, the most useful purpose for a valve and union is to allow shutoff and disconnection of a device like a pump. In this case, you can probably get by just fine with a ball valve and a single union between the pump and the valve (you'd actually use two such combinations, one in the inflow and one in the outflow sides of the pump). This allows you to shut off the flow, disconnect the union and remove the pump. While the pump is removed, you can work the valve to clear any important debris.
You only need a double union setup with a valve if you expect to need to remove the valve itself... and then, you'll probably need a plan to contain the water in the piping on one or both sides of the valve.
And by all means, as CuzzA pointed out, use threaded fittings for anything that may need to be disassembled. Solvent-welded PVC is great, but when something has to be taken apart, cutting is the only way to do so. Use only Teflon tape for thread sealing. Silicone is not made for this, and you don't want the oils from TFE paste getting into your system water.
As for the Schedule 40 vs Schedule 80 questions, there is no need for Schedule 80 anywhere in any aquarium piping I have ever seen or can imagine. You will never even begin to approach the burst strength of Schedule 40 pipe and fittings (1060 psi for 1-1/2" ips) in this hobby. You won't even begin to approach the maximum operating pressure of 200 psi (1-1/2" ips).
It might be important to note that even beyond the (hopefully) obvious quality issues, all valves are not created the same. Most, but not all ball valves are full-port valves. That means the passageway through the valve is the same (bore) size as the pipe size it's connected to, which is important if you need to ensure maximum flow. If this is important, be sure to buy a full-port valve of the appropriate size.
With gate valves, there is really no such thing as a full-port valve, although the port sizes can vary considerably, and some valve designs restrict flow at wide open more than others. Because the valve, seat and water passage are more complex, and create more turbulence within the valve body, a gate valve is more prone to become sticky from mineral accumulation, but since they offer more leverage in operating, they are sometimes easier to work free (and also easier to break by using too much force on a stuck valve). In general, think of ball valves as on-off devices, and gate (or other threaded-stem designs) valves as fine throttling devices. Ball valves can of course be used for rough throttling applications, and frequently are.
Here is what I think is the most salient point to make in this whole thread. Several posters in this thread have referred to 'True Union Ball Valves' as if this is some official, accepted plumbing terminology. It is not. There is no such animal, and referring to it as a 'TUBV' still doesn't make it so. Is this some goofy term Savko originated? First of all there is no such thing as a 'true' union. A union is a union. There is no 'false' union. You either have a union or you don't. You can buy valves of almost any design with integral unions at one or both ends. They are almost always much more expensive than the sum of their components. The only reason to ever buy one is if space for the valve and unions is severely limited. If it isn't, you are much better off using separate unions, because on a valve with integral unions, the unions must be replaced with the valve. If you solvent welded the pipe end of the union to your piping and the unions on your new valve are not the same as your original valve, you're going to be cutting and splicing pipe in order to install the new pipe end of the union. Standards are developed for a reason, but they are not always followed, especially with imported components.
Honestly, the most useful purpose for a valve and union is to allow shutoff and disconnection of a device like a pump. In this case, you can probably get by just fine with a ball valve and a single union between the pump and the valve (you'd actually use two such combinations, one in the inflow and one in the outflow sides of the pump). This allows you to shut off the flow, disconnect the union and remove the pump. While the pump is removed, you can work the valve to clear any important debris.
You only need a double union setup with a valve if you expect to need to remove the valve itself... and then, you'll probably need a plan to contain the water in the piping on one or both sides of the valve.
And by all means, as CuzzA pointed out, use threaded fittings for anything that may need to be disassembled. Solvent-welded PVC is great, but when something has to be taken apart, cutting is the only way to do so. Use only Teflon tape for thread sealing. Silicone is not made for this, and you don't want the oils from TFE paste getting into your system water.
As for the Schedule 40 vs Schedule 80 questions, there is no need for Schedule 80 anywhere in any aquarium piping I have ever seen or can imagine. You will never even begin to approach the burst strength of Schedule 40 pipe and fittings (1060 psi for 1-1/2" ips) in this hobby. You won't even begin to approach the maximum operating pressure of 200 psi (1-1/2" ips).
