maximum and minimum; delivery pressure, maximum and minimum; system
temperature, maximum and minimum; flow-control requirements. The preliminary
fluid flow calculations are based on estimates of piping size and component
location. The use of simplified fluid flow calculations is described in this
section. (Detailed flow calculations, made after design information is more
complete, are described in Section 5, Paragraph 4.)
The designer should note that Section 5, Paragraph 4,
provides a reference for making detailed flow calculations.
The designer should review that reference so
he will not be misled by the brevity of the section
into thinking the procedure in itself is short or simple.
(1) Fluid Flow Approximations. The piping configuration must
initially be approximated for each system to provide the required flow rate.
This requires an estimate of the piping arrangements and of the locations and
types of valves and fittings. The basic problem becomes one of selecting the
smallest pipe size for each system that does not cause an excessive pressure
drop. Excessive pressure drop in piping containing a liquid can result in
cavitation as well as require excessive pumping pressure (requiring an
expensive pump or high-strength pipe), while excessive pressure drop in
piping containing a gas can result in excessive noise or insufficient
capacity if sonic flow is encountered. (Thus, "excessive" will be defined by
the designer within the context of the system.) Using a larger pipe size
will reduce the noise of the fluid flow. The Piping Handbook by Crocker and
King (Reference 14) and the Crane Company Technical Paper No. 410, Flow of
Fluids Through Valves, Fittings, and Pipe (Reference 15), as well as other
sources of the designer's choice, provide guidance for making fluid flow
calculations. The initial sizing of a fluid system can be accomplished
through the use of Darcy's formula (Reference 15) and the use of selected
tables and simplified calculations.
(2) Darcy's Formula. Darcy's formula expresses the pressure drop
resulting from fluid friction in a pipe. In use of this formula, pressure
drop through valves and fittings is expressed in terms of pipe length. One
form of this formula is the following equation:
[DELTA]P = --
pressure drop, psi
Moody friction factor
fluid weight density, pounds/feet3
mean flow velocity, feet/second
pipe diameter inches.
(a) Friction factor. The friction factor, f, in Equation
(1) is an experimentally determined value which varies in accordance with
several fluid properties. For approximation purposes it is convenient and
conservative to assume that complete turbulence exists in the fluid system.
For this condition, f can be obtained from Figure 5-1 using only the
appropriate inside pipe diameter. Values for f will normally range from