Page 363 - Steam Turbines Design, Applications, and Rerating

Rerates, Upgrades, and Modifications 337

should not exceed 0.002 in (0.05 mm). Problems often develop when the
turbine is brought up to operating conditions too rapidly. When this
happens, the shaft expands faster than the carbon and seizure can
occur. Seizure generally causes severe vibration and carbon ring break-
age. Hence, closely following a proven run-in procedure is recom-
mended to reduce the risk of carbon ring seizure and to ascertain
steam leakage in the more reasonable range.
Because steam leakage is not normally considered an environmental
problem, it has not undergone the same scrutiny as other process leak-
ages. Controlling steam leakage, though, would be beneficial for a num-
ber of reasons, including environmental reasons. Steam released to the
atmosphere represents a loss in revenue, since the user has invested
money in water, its preparation and chemical treatment, and energy to
create the steam. Since 1985 the significant potential cost savings
achievable using gas seal technology in lieu of carbon rings was
reported by reliability engineers who had successfully retrofitted high-
temperature metal bellow seals to medium-size steam turbines.
Figure 16.8 shows a cloud of steam around the turbine and adjacent
areas that was generated as a result of steam leakage. Clouds of steam
and condensed water promote corrosion and in turn make working
conditions for operators unpleasant and potentially unsafe. Unless
advanced bearing housing protector seals are used, the escaping steam
will find its way into the bearing lubrication and associated systems.

Figure 16.8 Steam clouds due to an ineffective sealing device. (Saudi Aramco)