Page 70 - Fluid mechanics, heat transfer, and mass transfer

VALVES 47

Important types of valves.
FIGURE 3.5
& High flow velocities, particularly at low openings, & In plug cocks, the contact area between plug and
may lead to erosion of the disk. body is large and because of this feature, there is little
& Tight shutoff is not possible when handling fluids likelihood of leakage when closed.
with particulates as particles can get lodged in the & Lubricated plug cocks may use straight or tapered
valve seating preventing full closer, which can give plugs. Lubrication permits easy and quick operation
rise to leakages. For this reason, not suitable for and removal of contaminants from the plug surfaces
flammable and toxic materials. when rotated.
& Cavitate at low pressure drops. & For lubricated plug cocks, the lubricant must have
& Not quick opening type. limited viscosity change over the range of operating
temperature, must have low solubility in the fluid
& A relief valve may be required in cases where pres-
handled, and must be applied regularly. There must
sure build up is likely due to expansion of fluid
be no chemical reaction betweenthe lubricant and the
between two gate valves in closed position.A typical
fluid, which would harden or soften the lubricant or
gate valve is illustrated in Figure 3.6.
contaminate the fluid.
Plug Cocks:
& These are limited to temperatures below 260 C since

& These valves are used in fully open or in fully closed
differential expansion between the plug and the body
position.
results in seizure.
& These are quick opening type, with a quarter turn,
& An elastomer is used in nonlubricated plug cocks as a
that is, 90 rotation of the plug is required to open or

body liner, a plug coating, or port seals on the body or
to close.
on the plug. Provide a low-friction surface for plug
rotation. Eliminate the need for frequent lubrication.
& Temperature limitation is equally relevant for plastic-
coated valves.
& Because there is a large flow change near shutoff at
high velocity, these valves are not normally used for
throttling service.
& In control applications, it can be used for block and
bypass service.
Globe Valves:
& As the valve plug changes, the area for flow between
the plug and the seat (opening) changes (Figure 3.7).
& The name globe refers to the external shape of the
valve, not the internal flow area.
& In cast-iron globe valves, disk and seat rings are
usually made of bronze.
& Most economic for throttling flow. Can be hand
controlled.
& Provides tight shutoff.
& Suitable for frequent operation.
Gate valve.
FIGURE 3.6