Page 210 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

192 Major Process Equipment Maintenance and Repair

support (10) and end of the cylinder also will be noted. Misalignment has
many possible causes. In some installations there is only one support,
i.e., the crosshead guide support (lo), which leaves the cylinder over-
hung. However, there are usually some wedges under the drum (9) and it
is the incorrect usage of the wedges that sometimes causes the trouble.
When the cylinder is aligned and the support (10) grouted, strain is some-
times mistakenly taken on the drum wedges. If the unit is of the bottom
discharge type, the drum will push the cylinder up as it is heated by dis-
charge temperature. Assuming a 48-in. distance and a discharge temper-
ature of 30O0F, it is possible for the expansion of the drum to raise the
cylinder .066 in. The real purpose of the drum wedges, however, is to
keep the drum from vibrating; therefore, the proper method of adjusting
the wedges is to leave them loose until the machine gets up to operating
temperature and only then snug up on them.
In those installations that have the added support (3) at the head end, it
is a common error to adjust it too high and also snug up the drum wedges
before starting.
Some millwrights think they are setting the unit correctly by adjusting
the support (3) until the level (8) reads level. That would be correct if the
level (7) placed in the crosshead guide also indicated level. Sometimes
the level of the crosshead guide is slightly down, and that cannot be
changed because it is determined by the squareness of the guide and
frame at (4). Therefore, the support (3) should be adjusted in height until
the bubbles in both levels (8) and (7) read the same.
Excessive clearances in the crosshead-to-guide , crosshead bushings ,
connecting rods and main bearings will initiate load knocks but they will
be dealt with during discussion of maintenance on the frame or power
end. Other causes of knocks are liquids, loose valve assemblies and pack-
ing glands. Loose assemblies are not too difficult to locate. In regard to
liquids, the knock is spasmodic as the slugs pass through the cylinder.
Now that the importance and method of using each sign or indication
has been explained, it takes very little time to pass through the list of
checks, which are as follows:
1. Checking for excessive rod load is required only when changes in
capacity and pressures are made-the operator need not bother with
non-critical cylinders.
2. Check the packing vents for liquids and discolored oil twice daily.
3. If a packing vent does not clear, conduct a cylinder inspection
through a valve port.
4. Check suction and discharge temperatures and pressures twice a
day.
5. Listen for knocks and noises, and check vibration twice a day.