Accumulators can be used to help stabilize the lube system against pressure transients such as that from the turbine power operator during a large correction. For a sizing rule of thumb, the system pressure should not vary by more than 10%, while the turbine servo travels full stroke in a one second interval. The role of accumulators for pump switching was covered earlier in the section on Startup Control.

The preferred accumulator is the bladder type as shown in Figure 8-9. Body material should be 300 series stainless steel, in accordance with ASTM A 240. Either a manual precharge valve or automatic charging system can be used, based primarily on the user's preference. The alternative choice is a direct contact accumulator as seen in Figure 8-10. This accumulator has several problems. The gas used to pressure the top of the oil will eventually be absorbed by the oil, which can lead to drain line foam-

ing. The other problem is that if the oil level isn't carefully controlled, it may dump gas into the oil system. The direct contact accumulator is used primarily when the requirement exceeds the available capacity of the commercial bladder type. With either type of accumulator, additional check valves may be required in the oil system to keep the accumulator from delaying the standby pump start or other similar functions.

Seal Oil Overhead Tank

For liquid film seals, an overhead tank is normally used. The tank functions as part of the differential pressure control. The process gas is referenced to the top of the tank, and the tank's physical height becomes a manometric leg. The oil level is controlled in the tank. Figure 8-11 shows an alternate arrangement for the overhead seal tank, including the recommended operating levels and volumes. The material of the seal oil tank should be stainless, the same as that recommended for the accumulator

Figure 8-11. Direct contact type overhead tank module for a seai oil system including an optional transfer barrier (bladder type) vessel [1].
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