When designing the piping in a small system or machine, the pipe
fittings are often what drive the pressure drop due to the fittings.
Understanding how to handle the pressure drop through these fittings is
an important step in your journey to design maturity.
Flow through straight pipes has a pressure drop associated with it.
Flow through an abrupt opening or abrupt restriction has more pressure
drop than flow through a gradual opening or restriction. Turning a corner
though a close radius 90 has more pressure drop than turning a corner through a
long radius 90. Turning a corner through the branch port of a tee has
more than both of them.
A way of characterizing these pressures drop differences is something
called equivalent length. There are standard tables of equivalent length
for all standard fittings. All that you need to do to estimate pressure
drop in a piping design is tally all the fittings sorted by type. Each
type of fitting has an equivalent length. Multiply the number of that
type of fitting times that fittings equivalent length and add all of those
numbers to the length of straight pipe in your system. It really is that simple. Here is an example:
Assume that you have a system with 34 feet of 1″ pipe. There are 6
90′s in the system as well. Each 90 contributes 2 equivalent feet of
pipe, so you can consider the total length of pipe in the system to be 34 + 6*2
or 46 feet of 1″ pipe.
