When we use a lateral tee and a true WYE,they both can divide one flowing pipe to two branches which are at a 45 degree angle in direction.So they have a similar application as pipe fittings.
Then how does the engineer choose lateral tee or WYE when he design a piping system?
We should analyze this from their shapes:the lateral tee divide two branch pipes:one is same direction with the inflow pipe,the other have a 45 degree angle to inflow pipe.The WYE have two branch pipes at a 45 degree angle between them,and the two branch pipes are symmetrical to the centerline of the inflow pipe which means the two outflow direction are both at 22.5 degree to the inflow direction.
Comparing the two lateral pipe fittings,when the inflow pipe direction is horizontal to the ground,we can choose the lateral tee for better support from ground,and when the inflow pipe direction is vertical,then we’d better use the WYE fitting,it will bring a symmetrical interaction force to the piping system.Such designing will make sure the pipe in a stable support when running.(Seamless lateral tee and welded lateral tee)