PIPE SYSTEM HYDRAULICS - Environmentally Conscious Materials Handling - page 249

body, the float drops and opens the orifice. As the air is expelled, the float rises
and closes the orifice.
5.6 Combination Valves
The combination valve includes both an air/vacuum valve and an air release
valve. The installation can either consist of an air/vacuum valve and an air
release valve plumbed in parallel, or the two can be housed in a single valve
body. Most air valve installations require combination valves. Guidelines for
sizing air valves are available from valve manufacturers and AWWA (2001).
Large manual air release valves should not be used because they can be can
cause severe transients.
5.7 Pressure Relief Valves
Pressure relief valves (PRVs) are installed to limit maximum system pressures.
They can open automatically when the system pressure exceeds a set pressure,
or they can be programmed to open in anticipation of a transient or pressure
surge. There are two general types of relief valves: nonpilot and pilot actuated.
The type chosen depends on the size of valve required and whether the valve
opening or closing rates need to be controlled.
Nonpilot actuated PRVs have springs that hold the disk closed until the pipe
pressure exceeds the spring setting. The disk opening is proportional to the
overpressurization. They automatically close when the pressure drops below the
spring setting. Nonpilot valves are fast acting with no valve opening or closing
speed control. They are typically limited to smaller valve sizes.
Pilot actuated PRVs are typically globe valves, which are opened and closed
using the upstream and/or downstream line pressure and a restoring spring. Throt-
tling valves in the pilot system controls the PRV opening and closing speeds.
Pilot valves typically actuate more slowly than nonpilot valves. The range of
operating pressures can be adjusted by varying the pilot valve settings, and the
pilot system can be programmed to operate when a pressure surge generated
elsewhere in the system is anticipated. There are no size-specific limits to pilot
activated PRVs.
6 CENTRIFUGAL PUMP SELECTION AND PERFORMANCE
When gravititational forces are insufficient to drive the required flow rate, pumps
must be added to the system. Proper pump selection includes matching the pump
capacity to the system requirements. Present and future flow requirements should
be considered when selecting the design flow rate or range of design flow rates.
The pump selection process requires developing an energy-based head-discharge
relationship or system equation for the piping system. Eliminating the turbine