If the flow-detection element is compact rather than distributed (as is certainly the case with the
turbine flowmeter shown above), care must be taken to ensure correct positioning within the pipe.
Since flow profiles are never completely flat, any insertion meter element will register a greater flow
rate at the center of the pipe than near the walls. Wherever the insertion element is placed in the
pipe diameter, that placement must remain consistent through repeated extractions and re-insertions
or else the effective calibration of the insertion flowmeter will change every time it is removed and
re-inserted into the pipe. Care must also be taken to insert the flowmeter so the flow element points
directly upstream, and not at an angle.
A unique advantage of insertion instruments is that they may be installed in an operating pipe
by using specialized hot-tapping equipment. A “hot tap” is a procedure whereby a safe penetration
is made into a pipe while the pipe is carrying fluid under pressure. The first step in a hot-tapping
operation is to weld a “saddle tee” fitting on the side of the pipe