attach the handle to the lap and begin the lapping. The
wedge gate can be lapped to a true surface, using the
same lap that is used on the seat rings. (CAUTION: DO
NOT use the gate as a lap.)
Do not remove any more material than is necessary.
You can resurface a gate valve only a limited number of
times. By removing too much material, you will reduce
the number of times the surface can be refinished and
the life of the valve.
Leakage around the stem of a gate valve is caused
by troubles similar to those in leaking globe valves. The
same procedure is used to stop these leaks in both
Leaks are the principal trouble found in check
valves. Leakage is caused by a pitted disk or valve seat.
Pitting is usually caused by abrasives caught between
Figure 16-44.--Piston-type flushing valve.
the disk and the seat.
When a check valve requires maintenance because
If the relief valve fails to seat properly, there may
of pitting, the work will depend upon the type of disk
be enough leakage to prevent the upper chamber of the
in the valve. With a ball-type disk, you will have to
valve from filling. The piston will therefore remain in
replace the ball and grind the seat. A flat or conical disk
the open position. Inspect the relief valve seat for dirt
can be repaired by grinding in the disk to its seat with a
or other foreign substances that may be causing the
fine grinding compound.
relief valve to tilt. If these substances are present,
Remember that fluid will flow through them in only
disassemble the piston, wash the parts thoroughly, and
one direction. Be sure they are installed correctly.
reassemble. Replace wornout washers, making sure that
the surface upon which the washer sits is perfectly
clean. If any old rubber sticks to the metal surface,
scrape it off.
Any flush valve will give years of adequate and
Corrosion of the bypass valve in the center of the
trouble-free operation if it is properly maintained.
top plate will also cause continuous flow. The water is
However, there are two major problems that may occur
unable to pass into the upper chamber of the valve.
in flush valves: (1) the valve may run continuously
Therefore, no force is exerted on the piston to move it
instead of shutting off at the right time, or (2) the valve
downward to its seat. Very dirty water passing through
may fail to deliver the desired amount of water (short
the system can clog the bypass and deprive the upper
flushing). Both of these defects can waste a lot of water.
chamber of water. Pipelines in a new installation should
One of the reasons for a flush valve is to reduce water
be thoroughly flushed out before they are placed in
waste; therefore, proper maintenance is important.
operation. If not, the pipe dope or dirt accumulated in
Once you understand the principle and the operation of
them can also stop up the bypass valve.
a valve, you will know what to look for when anything
goes wrong. The internal working parts of a typical
The same condition can arise in a diaphragm valve.
piston-type flushing valve are shown in figure 16-44.
If chips or dirt carried by the water lodge between the
relief valve and disk, the relief valve will not seat
Continuous flow through a piston-type flush valve
securely. The leakage will then prevent the upper
is almost always caused by one of two things. Either the
chamber of the valve from filling with water. The valve
relief valve fails to seat properly, or the bypass valve is
corroded. In both cases, there is not enough force on the
will then remain in the open position, since there will
piston to seat it.
be no pressure to force the diaphragm to seat.