PREHEATER

BYPASS SYSTEMS

PREHEATER BYPASS SYSTEMS OVERVIEW 2014

2 OVERVIEW PREHEATER BYPASS SYSTEMS

COV QCV

Changeover valve and quick closing valve

Technical Data

Sizes : DN 150 - DN 600

Construction : EN / DIN

Pressure Class : PN 160 - PN 500

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

GS FEED WATER HEATER BYPASS SYSTEM

PREHEATER BYPASS SYSTEMS 3OVERVIEW

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

In normal operation the COV directs the

water to the preheater en the preheated

feedwater flows to the boiler

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.

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

PREHEATER BYPASS SYSTEMS 5OVERVIEW

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

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

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

Typical feedwater operated preheater

bypass setup

6 PREHEATER BYPASS SYSTEMSOVERVIEW

Parts list

CHANGEOVER VALVE COV

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.

PREHEATER BYPASS SYSTEMS OVERVIEW 7

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.

QUICK CHANGE VALVE QCV

8 PREHEATER BYPASS SYSTEMSOVERVIEW

PILOT & THROTTLE VALVES

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

The throttle valve‘s function is to adjust the

time for the switch-over sequence.

Throttle Valve

PREHEATER BYPASS SYSTEMS OVERVIEW 9

ELECTRIC OPERATED PREHEATER BYPASS SYSTEM

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.

In normal operation the COV

directs the water to the preheater

en the preheated feedwater flows

to the boiler

In bypass operation the feedwater

flows uninterrupted from the COV

through the QCV to the boiler and

thus bypasses the preheater