The reservoir shown in Figure 15-2 should be separate from the equip-ment base plate and sealed against the entrance of dirt and water. Thebottom should be sloped to the low drainpoint， and the return oil lines should enter the reservoir away from the oil pump suction to avoid disturb-ances of the pump suction. The working capacity should be at least five minutes based on normal flow. Reservoir retention time should be 10minutes， based on normal flow and total volume below minimum operating level. Heating for the oil should also be provided. If thermostatically con-trolled electrical emersion heating is provided， the maximum watt density should be 15 watts per square inch (2.33 w/cm2). When steam heating isused， the heating element should be external to the reservoir.
The rundown level， which is the highest level the oil in the reservoir mayreach during system idleness， is computed by considering the oil contained inall components， bearing and seal housings， control elements， and furnished piping that drains back to the reservoir. The rundown capacity should also include a 10% minimum allowance for the interconnecting piping.
The capacity between the minimum and the maximum operating levels in an oil system that discharges seal oil from the unit should be enough for a minimum operation of three days with no oil being added to the reservoir. The free surface should be a minimum of 0.25 sq ft/gpm (0.023m2/gpm)ofnormalflow.
The reservoir interior should be smooth to avoid pockets and provide an unbroken finish for any interior protection. Reservoir wall-to-top .unctions may be welded from the outside by utilizing full-penetration welds.
.ach reservoir compartment should be provided with two three-quarter-inch minimum size plugged connections above the rundown oil level. Theseconnections may be used for such services as purgegas，makeup， oil supply， and clarifier return. One connection should be strategically located to ensure an effective sweep of purge gas toward the vents.
The oil system should be equipped with a main oil pump， a standbyand，for critical machines， an emergency pump. .ach pump must have its owndriver， and check valves must be installed on each pump discharge to prevent reverse flow through idle pumps. The pump capacity of the main and standby pumps should be 10.15% greater than maximum system usage. The pumps should be provided with different prime movers.
The main pumps are usually steam turbine driven with an electric motor driven backup pump. A small mechanical-drive turbine is highly reliable aslong as it isrunning， but it is undependable for starting automatically after long idle periods. A motor is thus the preferred backup pump driver. A ""ready-to-run"" status light is usually provided for the motor in the control room to give visible evidence that the electrical circuit is viable. Starting of the backup pump is initiated by multiple and redundant sources. The turbine drivers should be maintained for failure by either low-speed or low-steam chest pressure or both.
Low oil pressure switches are provided on the pumps and dischargeheader ahead of the coolers and filters， sometimes after the cooler and filters， and always at the end of the line where the reduced oil pressure feeds the various users. A signal from any of these should start the motor-driven pump and all alarms should be activated in the control room. The emergency oil pump can be driven with an AC motor but from a power source that isdifferent to the standby pump. When dc power is available， DC electric motors can also be used. Process gas or air-driven turbines and quick-start steam turbines are often used to drive the emergency pumps.
The pump capacities for lube and control oil systems should be based upon the particular system"s maximum usage (including transients) plus a minimum of 15%. The pump capacity for a seal oil system should be basedupon the system"s maximum usage plus either 10 gpm or20%， whichever is greater. Maximum system usage should include allowance for normal wear. Check valves should be provided on each pump discharge to prevent reverse flow through the idle pump.
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