It means a tee of pipe fitting which the branch outlet has a 90 degree direction turning from the main pipe of the tee.Commonly people consider it as the equal tee,it is a mistake.Equal tee means a same sizes of the branch and main pipe of a tee.It means it has a side port at right angle to the run.
On the contrary to the straight tee,it is the lateral pipe tee. It could be a equal tee,also could be a reducing pipe tee (unequal tee).
Officially a straight tee will be indicated by 3 diameters,namely 3 x 3 x 2 (A x B x C).The A & B measure stands for the Nominal Pipe Size of Run-Pipe, C measure stands for Nominal Pipe Size of Outlet.
Standard: ASME/ANSI B16.9, MSS-SP-75, DIN 2615, EN 10253-2 , JIS B2311, GOST 17378
Material: ASTM A234, ASTM 403,DIN 17175, EN10228 …Carbon steel, stainless steel , alloy steel.
Size: 1/2″-96″. DN15-DN2400
Connect method: Butt welding.
Type: Seamless: DN15-DN600,