flow, or regulate pressure? Look for the information
stamped on the valve body by the manufacturer: type
of system (oil, water, gas); operating pressure;
direction of flow; and other information.
You should also know the operating
characteristics of the valve, the type of metal it is made
of, and the type of end connection it has. Operating
characteristics and material affect the length and type
of service a valve can provide. End connections
indicate whether or not a particular valve is suited for
installation in the system.
Valves should be installed in accessible places and
with enough headroom to allow for full operation.
Install valves with stems pointing upward whenever
possible. A stem position between straight up and
horizontal is acceptable, but avoid the inverted
position (stem pointing downward). When the valve is
installed in the latter position, sediment collects in the
bonnet and scores the stem. When a line is subject to
freezing temperatures, liquid trapped in the valve
bonnet may freeze and rupture it.
Globe valves may be installed with pressure either
above or below the disk. It depends upon what method
is best for the operation, protection, maintenance, and
repair of the machinery. You should ask what would
happen if the disk became detached from the stem?
This is a major consideration in determining whether
pressure should be above the disk or below it. Check
the blueprints for the system to see which way the
valve should be installed. Pressure on the wrong side
of the disk can also cause serious damage.
Valves that have been in constant service over a
long period of time eventually require gland
tightening, replacing, or a complete overhaul. When a
valve is not doing the job, it should be dismantled and
all parts inspected. For proper operation, parts must be
repaired or replaced.
Spotting-in is the method used to determine
visually whether or not the seat and the disk make good
contact with each other. To spot-in a valve seat, first
apply a thin coating of prussian blue evenly over the
entire machined face surface of the disk. Then insert
the disk into the valve and rotate it a quarter turn, using
light downward pressure. The prussian blue adheres to
the valve seat at those points where the disk makes
contact. Figure 4-11 shows what correct and imperfect
seals look like when they are spotted-in.
Figure 4-11.Examples of spotted-in valve seats.
After you have examined the seat surface, wipe all
the prussian blue off the disk face surface. Apply a
thin, even coat of blue to the contact face of the seat.
Again, place the disk on the seat and rotate the disk a
quarter of a turn. Examine the blue ring that appears on
the disk. It should be unbroken and of uniform width.
If the blue ring is broken in any way, the disk does not
Grinding-in is a manual process used to remove
small irregularities by grinding together the contact
surfaces of the seat and disk. Grinding-in should not be
confused with refacing processes in which lathes,
valve reseating machines, or power grinders are used
to recondition the seating surfaces.
To grind-in a valve, first apply a small amount of
grinding compound to the face of the disk. Then insert
the disk into the valve and rotate the disk back and forth
about a quarter of a turn. Shift the disk-seat
relationship from time to time, so the disk is moved
gradually, in increments, through several rotations.
During the grinding-in process, the grinding
compound is gradually displaced from between the
seat and disk surfaces; therefore, it is necessary to stop
every minute or so to replenish the compound. When
you do this, wipe both the seat and the disk clean before
applying the new compound to the disk face.
When it appears that the irregularities have been
removed, check your work by spotting-in the disk to
the seat in the manner described previously.
Grinding-in is also used to follow up all machine
work on valve seats or disks. When the seat and disk
are first spotted-in after they have been machined, the