4. Make sure that a foamy surface does not exist immediately below the sole plate. A foamy surface is caused by an insufficient quantity of aggregate in preparing the epoxy mortar. The epoxy adhesive has a density of about nine pounds per gallon. The aggregate hasa bulk density of about 14 pounds per gallon， which assumes about25-30% voids. In preparing an epoxy mortar， the resin and hardener components are always mixed together before the addition ofaggregate. When the aggregate is added to themix， it obviously falls to the bottom and introduces air into the mix. If a soupy mortarisprepared， the air will simply rise to create aweak， foamy surface.
Increasing Mass and .igidity
When excessive vibration is detected in the gear box of a compressor train(Figure 21-14) and is transferred to the platform below， a dampening effect can be created by increasing the rigidity of the support below. This effect can be accomplished by first filling the platform cavity and then the gear-box support with epoxy mortar.
In the case where the turbine and supports have a minimal crosssection， then the ability to increase the stiffness of these pedestals is minimized.Consequently， the objective is to concentrate on increasing the mass of the pedestals. This increase is accomplished by filling the cavities with a special mortar prepared with epoxy and steel shot. The density of this special mortar can be in excess of 300 pounds per cubic foot. To inject this specialmortar， a pipe has been installed in the access hole that was drilled in the side of the pedestal near the top. These same techniques can be employed to stabilize the foundations under much smaller equipment.
Large Machinery Startup Procedure
Many of these problems can be quickly resolved by deductive reasoning if sufficient care has been taken in obtaining startup data. The following aregood guidelines， but they are not intended to be complete:
1. Beforestartup， become generally familiar with the train. Find out:
a. Critical speeds of the train"s rotors
b.Operatingspeeds， temperatures， and pressures
c. .nusual operational characteristics
d. If all vibration monitoring systems are functioning and what are the alert and danger limits
2. For
turbines， find out:
a. Slow roll (from one-half to threehours， depending on operational experience) to relieve rotor bow and allow for system warmup
3. At
a slow-roll speed of less than 800rpm， take the applicable slow-roll vibration data:
a. Gap voltages
b. Probe identification
c. Total electrical and mechanical run-out
d. .eyphazor relationships
e.Vibration values inmils， displacement，velocity， and acceleration.
4. Observe allmeters， guages， sight glasses， oil temperatures， dischargetemperatures， balance-line pressures， surface condenser temperature， etc.
5. Bring the machine through the first critical and observe the machine"s performance for 15 minutes
6. Bring the machine midway between the first critical and minimum governor. Observe performance for 15 minutes.
.. Go quickly to minimum governor to ensure that the rotors go through any other criticals as easily as possible.
8. At minimum governor， get another set of vibrational readings.
9. Document all readings and observations and file for future reference with a copy sent to the unit supervisor and unit engineer.
During the startup， all vibration signals should be tape recorded. The recordings can be analyzed and plotted to provide baseline vibration data for future evaluation of machine performance.
Typical Problems .ncountered in .as Turbines
There are many types of failures associated with a gas turbine， since these units are very complex in their overall makeup. The failures in the hot section far outnumber the problems in the compressor due to the high temperatures associated with the hot section. Hot-section failures are usually connected to problems associated with fuels. Turbine failures can be verycostly， the average cost runs about.500，000 for units between 10 and50 MW and about..00，000 for units above 50 MW. These average failures result in downtime of between 12 and 16 weeks. The type of operation the unit experiences is a major factor in the problem. The unit has a more trouble-free operation if it is a baseload unit.
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