Handbook of Turbomachinery Second Edition Revised - Ventech!

to match turbine component and steam temperatures, can reduce
average startup time by 30 minutes.
Small Bypass Systems
The simplest bypass systems consist of a pipe and shutoff valve that can
exhaust steam from the boiler to the condenser, bypassing the turbine. Such
bypasses originate from one or more of the following locations [25]: primary
superheater inlet, primary superheater outlet, secondary superheater outlet,
or turbine inlet.
Small Bypass Systems with Division Values
A more complex superheater bypass systems adds a pressure-reducing valve
(division valve) between the primary and secondary superheater, and
secondary superheater and reheater steam attemperator valves. This permits
bottling up, or shutting off, the boiler drum at high pressure during
overnight outages while providing throttle steam to the turbine at reduced
pressure. This conserves energy and supplies high-temperature steam at the
turbine rotor by avoiding large temperature drops through the turbine inlet
valves and control stage.
Large Turbine Bypass Systems
European power plants use large HP and IP/LP bypasses. The HP bypass
line takes steam from the superheater outlet to the reheater, bypassing the
HP turbine. The IP/LP bypass line takes steam from the reheater to the
condenser, bypassing the IP and LP turbines. These types of systems
accomplish the following objectives:
Steam flows through the reheater as well as the superheater, cooling
the tubes.
Large bypass systems permit firing up the boiler to a minimum stable
load before opening the turbine valves. Thus, the boiler can be
operated at stable conditions and steam outlet temperatures can be
controlled with the need to synchronize the turbine startup with the
boiler startup.
Load rejections can be sustained without tripping the boiler.
This may allow for bypassing of exfoliated oxide during startup
periods and thus reduce or eliminate solid particle erosion damage
to the turbine.
This allows complete feedwater cleanup before admitting steam to the
turbine.
Copyright © 2003 Marcel Dekker, Inc.