Page 106 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

2.56 CHAPTER TWO

3. Shutoff. The degree of allowable shutoff must be known. For utility piping, some minor
leakage should be allowed and would prove extremely costly to eliminate. Bubbletight
valves are those that exhibit no visible leakage through the elastomeric seat of the valve
for the duration of a test, as defined by MSS SP 82.
4. Valve operation. It must be determined whether the valve be used only for ON/OFF use
or for throttling.
5. Pressure drop. Allowable pressure drop must be established and the size (equal to or less
than that figure) selected.
6. Corrosion resistance. This is affected by the nature, concentration, and temperature of
the fluid.
7. Velocity. The velocity of the fluid through the valve must be considered.
8. Firesafe. It must be known if this is a requirement. (Refer to previous discussion.)
9. Hazardous material. When the fluid being transported is considered hazardous or lethal,
valves must be specifically designed to handle these materials. Redundant stem packing
and leak detection ports are typical design features. ANSI B 31.3, category M, defines
this category of fluids.

GATE VALVES

Gate valves, illustrated in Fig. 2.24, use a wedge-shaped disk or gate as the closure member
operating perpendicular to the flow; it is raised to open and lowered to close the valve. As
the disk closes, it fits tightly against the seat surfaces in the valve body. A gate valve is used
fully opened or closed only. It should not be used for throttling service (partly open), as the
gate will vibrate and quickly become damaged and subject to wire drawing caused by the
velocity of the liquid flowing past the disk.

Primary Gate Valve Components
There are four main features that dictate gate valve design: the disk, stem, bonnet / body
connection, and body. The body materials and end connections have been previously
discussed.
Often, iron body valves use a combination of materials to provide corrosion-resistant
bearings for stems and other wear points such as seating surfaces. These valves are called
iron body bronze mounted (IBBM).
Disk Design. There are three types of disk constructions: solid wedge, split wedge, and
flexible wedge (illustrated in Fig. 2.25).
Solid Wedge. Solid wedge disks are most prevalent because of their simple and usually
less expensive design.
Split Wedge. Split wedge disks, also called double disks, have somewhat better seal-
ing characteristics than solid disks because the two disk halves are forced outward against
the body seats by a spreader after the disk has been fully lowered into its seating position.
When the valve is opened, pressure on the disk is relieved before it is raised, eliminating
the friction and scoring of body seats and disk.
Flexible Wedge. Flexible wedge disks are solid only at the center and are flexible at the
outer edge and seating surface. This design enables the disk face to overcome the tendency
to stick in high-temperature service where wide swings in temperature occur. This type of
disk is generally found only in steel valves.

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