as ""open"" (de-energized) to alarm and ""close"" (energize) to trip. The pressure switches for alarms should be installed with a ""T""-connection pressure gauge and bleeder valve for testing the alarm.
Thermometers should be mounted in the oil piping to measure the oil at the outlet of each radial and thrust bearing and into and out of the coolers. It is also advisable to measure bearing metal temperatures.
Pressure gauges should be provided at the discharge of thepumps， thebearing header， the control oilline， and the seal oil system. .ach atmospheric oil drain line should be equipped with steel nonrestrictive bull"s-eye-type flow indicators positioned for viewing through the side.
.iewports in oil lines can be very useful in providing a visual check for oil contamination.
In the piping arrangement and layout it is very important to eliminate airpockets and trash collectors. Before starting a new or modified oilsystem， every foot of the entire system.right up to the final connection at themachine.should be methodicallycleaned， flushed，drained， refilled and all instruments thoroughly checked.
Seal Oil System
The compressor seal oil system is designed and furnished with instrumen-tation similar to the lube oil system shown in Figure 15-4. The only essential difference is how the end-supply control is handled. Since much higherpressures (1500.2500 psi) (103.4214.172.36.. Bars) are often involved， the pumps are usually a positive displacement-type. This requires a pressure control valve spilling oil back to the reservoir. This oil supply is available to an elevated head tank that is provided for each shaft seal. The head tank ispressured by its own process-seal pressure connection， so the seal oil supply system pressure must be maintained at a level to supply the highest pressure seal. The oil rate to each seal is maintained by tank level control from the supply system. The tanks are provided with a high/low level alarm to the control room. The low alarm warns of excessive oil consumption by the seal and also calls for backup pump start along with the various pressure switches and primary pump turbine failure in a similar manner to the lube oil system.
A degassing facility is also provided to separate gas contaminants from the seal oil. Figure 15-5 shows a typical degassing drum arrangement. A gas-tight baffle and a liquid seal should divide the degassing drum into two sections to confine the separated gas to one side of the drum. The gas side of the drum should be vented and provided with an inert-gas purge. To assist indegassing theoil， the drum will be heated by electricity or steam.
Figure 15-4. Seal oil system.
Figure 15-5. Typical degassing drum arrangement.
Lubricant Selection
A good turbomachinery oil must have a rustand oxidationinhibitor，good demulsibility and the correct viscosity， and be both nonsludging andform-resistant. Besides lubrication， the oil has to cool bearings andgears，prevent excessive metal-to-metal contact duringstarts， transmit pressure incontrol systems， carry away foreign materials， reduce corrosion， and resin degradation.
For gas turbines， especially the more advanced high-temperature gasturbines， the oil of choice should be syntheticoil， since synthetic oils have a high flash point. .as turbine lubrication systems should be run for about 20 minutes after shutdown since maximum temperatures are reached after 10 minutes of shutdown especially in the bearing area. Most gas turbines are also on turning gear to avoid sagging in the shaft. Mineral oils can be used for the compressor. It is not uncommon to have two types of oil in a petrochemical plant. Mineral oil costs much less than the synthetic oil.
The selection of the correct lubricant must begin with the manufacturer. Refer to the operator"s instruction manual for the oil required and the recommended viscosity range. The local environmental conditions shouldbe seriously considered， including exposure to outside element conditions，acidgas， or steam leaks. As a generalrule， most turbomachines are lubri-cated with premium-quality turbine-grade oil. However， under certain envir-onmental conditions， it may be advantageous to consider another oil. Forexample， if a machine is sub.ect to exposure to low concentrations ofchlorine or anhydrous hydrochloric acidgases， it may be better to select another oil that will outperform the premium turbine oil. .ood results have been recorded using oil containing alkaline additives. Certain automotive or diesel engine oils contain the optimum amount and type of alkaline additives to protect the base oil from reaction with chlorine and HCl. In serviceswhere the attack on the lubricant by the gas is unknown， laboratory tests are suggested.
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