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2 OVERVIEW PREHEATER BYPASS SYSTEMS
These valves are designed to bypass high
pressure feed water around the pre-heaters in
the event of high water level in the shell caused
by a defective tube, weld or drain system. To
protect a turbine in modern feed systems, the
bypass valves must be able to operate in less
than 5 seconds for a DN 300 pipe system.
Protection is realized by installing COV
changeover valves and QCV quick closing valves
at in- and outlet around a heater or group of
heaters. The closing function is realized by the
feed water pressure and the system is designed
to 'fail safe' in that the heaters will be by-passed
if either electric or pneumatic supplies fail.
General Description
Technical Data
Sizes : DN 150 - DN 600
Construction : EN / DIN
Pressure Class : PN 160 - PN 500
Changeover valve and quick closing valve
QCVCOV
GS FEED WATER HEATER BYPASS SYSTEM
PREHEATER BYPASS SYSTEMS 3OVERVIEW
In normal operation the COV directs the
water to the preheater en the preheated
feedwater flows to the boiler
In case of damage happening to the preheater pipes, the feedwater flow must be shut off in the
shortest possible time. Quick shut off and deviation of the flow will both prevent the preheater steam
jacket designed for low pressures from being endangered, and also maintain the feedwater supply
to the boiler. Moreover, the turbine is protected against water breaking in by flow backs through the
bleeder pipe.
Due to the automatic shut off of the preheater and the simultaneous opening of the by-pass line, the
feedwater keeps on flowing steadily. Even excessive pipe lengths between the pump and the COV
valve will not create any additional dynamic forces. Due to the deviation of flow, the water column
contained in the by-pass line will have to be accelerated. Short by-pass piping length will keep such
acceleration forces on a low level.
The QCV quick closing valves are operated by the feedwater pressure and will respond within a
second's time.
In normal operation pump pressure will prevail in the inlet of the COV changeover valve. Due to
pressure losses, the pressure in the by-pass line is somewhat lower. This pressure differential as well
as the balanced stem that is passed to the outside, will safely hold the disc of both valves in open
position, providing a shut off towards the by-pass.
Operation of the Preheater Bypass System
4 OVERVIEW PREHEATER BYPASS SYSTEMS
In bypass operation the feedwater flows
uninterrupted from the COV through the
QCV to the boiler and thus bypasses the
preheater
In case of damage in the preheater, the level control emits an electric impulse to the pilot valve, in
addition to possible optical and audible warnings. The valve opens and relieves the bottom sides of
the cylinders for the COV and QCV valve. The Cylinder top side still receives the full pressure of the
feed-water line via a small passage, which causes the discs to move into their closing positions. The
preheater is no longer in operation and the feed-water flow passes through the by-pass line.
When the pilot valve closes again, the COV changeover valve will only remain in its closed position, if
the feed-water pressure in the preheater has lowered by at least 10% as compared with the pump
pressure and does not rise again. Therefore, with power test runs, the preheater bypass valves will
open automatically, and the preheater is put into operation again, as soon as the solenoid control
valve is closed. This automatic opening is caused by the valve stems being balanced and passed
through to the outside. The pistons of the bypass valves have been so dimensioned, that any
opening towards the preheater is prevented, if the latter is pressure less or under low pressure. This
design prevents any sudden and harmful admission of pressure to the preheater piping. For the
start-up or after repairs the preheater is filled via the refill line. Only then the bypass valves can be
opened, or will open automatically after the pressure has been balanced within the system. With
low operating pressures, the optional open, hand-operated, control valve relieves the cylinder top
side sufficiently for allowing the disc to move into its open position.
Closing the solenoid control valve after changeover is necessary, because a permanent relief via the
control pipe may cause erosion. A corresponding closing impulse may be emitted by the limit switch
in "closed" position and a time delay relay, set for approx. 30 seconds.
PREHEATER BYPASS SYSTEMS 5OVERVIEW
Typical feedwater operated preheater
bypass setup
The blocking stem enables the preheater bypass valves to be closed independently from the
solenoid control. The blocking stems must not be used for closing the disc against the full operating
pressure. The QCV non return valve must not be closed before the COV changeover valve has been
blocked. The blocking stem can be operated by hand wheel or by remote drives as per margin. For
the maintenance of the solenoid pilot control valve during operation, the stop valve can be closed
after the preheater protection has been changed over to bypass operation and the blocking stems
have been actuated.
Equipment
The feed-water pressure being used as the source of energy for operating the bypass valves
necessitates minimum overpressures in the preheater system. Where valves are to be activated by
solenoid control valves at lower pressures, the bypass valves will have to be modified according to
individual requirements.
Pilot Control Valves
For easy mounting of the preheater bypass valves, they should preferably be installed in vertical
position. Their pressure losses are equivalent to 90° bends and are substantially more
advantageous as compared with T-fittings. In case of preheater groups, the valve bodies being
designed as fittings, may serve for the feed-water line to be distributed to the individual preheaters.
Our technical datasheets will inform you on the possible number of connections, their relative
positions and sizes.
Quick - Closing Check Valves
6 PREHEATER BYPASS SYSTEMSOVERVIEW
Parts list
Item QTY Description Material
1 1 Body 1.6368
2 1 Bonnet 1.6368
3 1 Stem / Plug 1.4057
4 1 Base bush 2.0966
5 1 Press ring 2.0966
6 1 Guiding bush 2.0966
7 1 Bypass seat 1.5415
8 1 Stuffing box 2.0966
9 1 Stuffing box bridge 1.4922
10 1 Base bush 2.0966
11 1 Press ring 1.6368
12 1 Ring 4-part 1.6368
13 1 Press ring 1.6368
14 1 Yoke 1.5415
15 1 Yoke cover 1.5415
16 1 Stem nut 2.0966
17 1 Blocking stem complete 1.4923
20 3 Stud bolt 1.4923
21 3 Hexagonal nut 1.4923
22 1 Packing set complete RG 98%
23 1 Packing RG 98%
24 1 Packing RG 98%
25 8 Hexagonal bolt 8.8
26 1 Hand wheel St.
CHANGEOVER VALVE COV
PREHEATER BYPASS SYSTEMS OVERVIEW 7
Item QTY Description Material
1 1 Body 1.6368
2 1 Bonnet 1.6368
3 1 Stem / Plug 1.4057
4 1 Base bush 2.0966
5 1 Press ring 2.0966
6 1 Guiding bush 2.0966
7 1 Bypass seat 1.5415
8 1 Stuffing box 2.0966
9 1 Stuffing box bridge 1.4922
10 1 Base bush 2.0966
11 1 Press ring 1.6368
12 1 Ring 4-part 1.6368
13 1 Press ring 1.6368
14 1 Yoke 1.5415
15 1 Yoke cover 1.5415
16 1 Stem nut 2.0966
17 1 Blocking stem complete 1.4923
20 3 Stud bolt 1.4923
21 3 Hexagonal nut 1.4923
22 1 Packing set complete RG 98%
23 1 Packing RG 98%
24 1 Packing RG 98%
25 8 Hexagonal bolt 8.8
26 1 Hand wheel St.
Parts list
QUICK CHANGE VALVE QCV
8 PREHEATER BYPASS SYSTEMSOVERVIEW
The throttle valve‘s function is to adjust the
time for the switch-over sequence.
Throttle Valve
To initiate the switching of the bypass
system from normal mode to bypass mode,
the pilot valve has to be operated.
The electric pilot valve is solenoid operated
while basic design can be selected
according to "energized to open" or
"energized to close" causing the failure
position of the bypass system to be in
normal or bypass mode.
As an alternative to the electric pilot valve
the preheater bypass system can also be
operated by a pneumatic piston or
diaphragm actuator in combination with a
small 3-way solenoid valve.
Pilot Valve
PILOT & THROTTLE VALVES
PREHEATER BYPASS SYSTEMS OVERVIEW 9
In bypass operation the feedwater
flows uninterrupted from the COV
through the QCV to the boiler and
thus bypasses the preheater
In normal operation the COV
directs the water to the preheater
en the preheated feedwater flows
to the boiler
The electric preheater bypass system consists of a 3-way valve (COV) at inlet and a quick closing valve
(QCV) or gate valve at preheater outlet.
One disadvantage of electric operation is that the fail-safe position in case of loss of power cannot be
realized. Stroking time for larger valves can also be quite long especially in those cases where a gate
valve is used at preheater outlet.
ELECTRIC OPERATED PREHEATER BYPASS SYSTEM
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