why is venturi blowing water instead of sucking air ?
i put a weld T joint into the return feed from my skimmer and used 20mm pipe down from the top it blew water so i reduced the size of the pipe and it still blew water what am i doing wrong ?
I'm not sure what your configuration is, pictures would help, but my guess is there's too much back pressure in the pipe leading from the venturi. For example, if you put a venturi close to where water dumps out of a pipe the water is happy to dump out the end of the pipe as you would expect and also draw air into the pipe.
However say you added a venturi right after the pump and after that there were lots of pipes, filters, maybe the pipe goes up...that all creates back pressure...the water flowing down the pipe just can't move as fast. That increased pressure pushes water up into your riser and if there is enough pressure the water comes out the end.
If you increase the height of the riser you will eventually create enough pressure so water doesn't blow out the top, but you also won't draw any air. You would still have a venturi, but venturi doesn't mean air is drawn into the pipe. Venturi is just a pressure thing. To draw air into the pipe take a bit more care...or luck.
If you decrease the size of the pump, or amount of water going to the venturi, it would probably start working as you expect (but I don't know what you have so don't know for sure).
If you increase the diameter of all the pipes after the venturi that can fix it, but it still depends on how much pipe and where it goes.
If you look at Koiguy's venturi you'll see it's located close to where the water dumps out. That helps. But also that venturi, for thiose size pipes
, will only work for certain size pumps. If the pump were changed to say a 100 GPH pump there probably wouldn't be enough pressure on either side of the venturi to create a venturi effect. If the pump were changed to say a 4000 GPH pump you'd probably see the same thing you're seeing, too much pressure in the tail pipe and water blowing out of the air intake.