PIPE SYSTEM HYDRAULICS - Environmentally Conscious Materials Handling - page 244

adjoining pipe. Proper selection of pipe and seal materials will reduce the risk
of leaks and possible contamination of the surrounding environment.
5 VALVES
The minor loss coefficients for a variety of different valve types (in the full-open
position) are included in Table 9.2. Valves differ from other minor loss elements
in that their loss coefficients and corresponding head loss are variable with valve
opening. There are more things to be considered regarding valves than loss
coefficients, however. Valves serve a variety of functions in piping systems, such
as flow isolation, flow rate control, pressure regulation, energy dissipation, reverse
flow prevention, and releasing/admitting air. This section discusses characteristics
of and principles for selecting and operating control valves, check valves, air
valves, and pressure relief valves. For a description of various types of valves
and details regarding their function, see Tullis and Tullis (2005).
5.1 Control Valves
The function of a control valve is to regulate the flow rate through the piping
system. In general, as the valve opening increases, the loss coefficient decreases
and the flow rate increases. The efficiency with which a specific valve controls
the flow rate is system dependent and is related to the ratio of the valve loss
to the rest of the system losses. In a long pipe system where the friction loss
is much greater than the valve minor loss (over a significant portion of the
valve stroke), the valve will only control flow at small valve openings. The
same valve, installed in a short, low-friction system, will have much improved
flow control characteristics. For more information regarding control valves, see
Tullis (1989, 1993).
5.1.1 Cavitation
Cavitation is one possible negative side effect that can result from head loss
across control valves or other minor losses. Cavitation is the process of convert-
ing water to vapor when the local pressure in a section of pipe reaches vapor
pressure. As flow passes through valves or other minor loss elements, the flow
is typically accelerated and concentrated as a result of flow separation from
the boundary. Flow separation typically results from flow path obstructions and
abrupt changes in flow direction. As a result, the flow velocity increases and
the pressure decreases (per the Bernoulli relationship). The turbulent eddies pro-
duced in such flow patterns generate energy dissipation as well as localized low
pressures associated with the large eddy rotational velocities. When the local
pressure inside the eddy reaches vapor pressure, vapor bubbles form. As the
eddies dissipate, the pressure increases (i.e., pressure recovery) and the vapor
bubbles become unstable and collapse. Low levels of cavitation will generate a