Handbook of Turbomachinery Second Edition Revised - Ventech!

End Seals
End seals or packing glands are used to minimize leakage at the ends of
cylinders. They are intended to prevent air inleakage into the LP and
condenser, and in the case of nuclear units employing boiling water reactors
to prevent leaking of radioactive steam from the cylinders to the
atmosphere. End seals are of the labyrinth design. There are typically
several sections of end seals at the end of each cylinder.
Bypass Systems
Turbine bypass systems offer a variety of benefits, including the following:
1. Ability to match steam and metal temperatures during startups. A
primary function of bypass systems is to permit sufficient firing
rates in the boiler to achieve acceptable throttle temperatures and
pressure while minimizing damage to ancillary systems (such as
overheating of tubes). Bypasses are often considered in conjunction
with moving from base load to two-shift or cycling operation
as a means of reducing the thermal damage induced and to
improve startups [26].
2. Use as relief valves during severe load fluctuations.
3. Operation as overload or bypass during turbine trips or when the
turbine is run back to house load.
4. Operation as a boiler safety valve. Note that the ASME Boiler and
Pressure Vessel Code, Section I, limits such use to once-through
boilers with a minimum of two spring loaded valves.
5. Allows faster unit startups, thereby decreasing startup energy.
6. Provides cooling of all boiler components by providing fluid flow
during all firing conditions.
7. Reduces solid particle erosion on turbine valves and blading by
minimizing the carryover of oxide particles during startups.
Extensive surveys of bypass usage, operation, and design were conducted in
the mid-1980s [11, 12]. Some of the results of those studies indicated:
1. The bypass system size is governed by turbine rotor diameter and
thermal energy distribution in the boiler, as well as startup,
loading, unloading, and shutdown practices.
2. A large bypass system, one capable of diverting at least 40% of the
boiler’s steam, is necessary to avoid boiler shutdown after a
turbine trip at full load.
3. Systems diverting at least 15% of the steam enhance startup
flexibility and load changing capabilities. These systems, sufficient
Copyright © 2003 Marcel Dekker, Inc.