PREHEATER BYPASS SYSTEMS

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Evangeline Peters
5 years ago
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1 PREHEATER BYPASS SYSTEMS PREHEATER BYPASS SYSTEMS OVERVIEW 2014

GS FEED WATER HEATER BYPASS SYSTEM General Description 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

Protection is realized by installing COV changeover valves and QCV quick closing valves at in- and outlet around a heater or group of heaters.

2 GS FEED WATER HEATER BYPASS SYSTEM General Description 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. COV QCV Changeover valve and quick closing valve Technical Data Sizes : DN DN 600 Construction : EN / DIN Pressure Class : PN PN OVERVIEW PREHEATER BYPASS SYSTEMS

3 Operation of the Preheater Bypass System 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. In normal operation the COV directs the water to the preheater en the preheated feedwater flows to the boiler PREHEATER BYPASS SYSTEMS OVERVIEW 3

4 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. 4 OVERVIEW PREHEATER BYPASS SYSTEMS

5 Quick - Closing Check 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. Pilot Control 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. Equipment 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. Typical feedwater operated preheater bypass setup PREHEATER BYPASS SYSTEMS OVERVIEW 5

6 CHANGEOVER VALVE COV Parts list Item QTY Description Material 1 1 Body Bonnet Stem / Plug Base bush Press ring Guiding bush Bypass seat Stuffing box Stuffing box bridge Base bush Press ring Ring 4-part Press ring Yoke Yoke cover Stem nut Blocking stem complete Stud bolt Hexagonal nut Packing set complete RG 98% 23 1 Packing RG 98% 24 1 Packing RG 98% 25 8 Hexagonal bolt Hand wheel St. 6 OVERVIEW PREHEATER BYPASS SYSTEMS

7 QUICK CHANGE VALVE QCV Parts list Item QTY Description Material 1 1 Body Bonnet Stem / Plug Base bush Press ring Guiding bush Bypass seat Stuffing box Stuffing box bridge Base bush Press ring Ring 4-part Press ring Yoke Yoke cover Stem nut Blocking stem complete Stud bolt Hexagonal nut Packing set complete RG 98% 23 1 Packing RG 98% 24 1 Packing RG 98% 25 8 Hexagonal bolt Hand wheel St. PREHEATER BYPASS SYSTEMS OVERVIEW 7

8 PILOT & THROTTLE VALVES Pilot 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. Throttle Valve The throttle valve s function is to adjust the time for the switch-over sequence. 8 OVERVIEW PREHEATER BYPASS SYSTEMS

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 PREHEATER BYPASS SYSTEMS OVERVIEW 9

Typical Feed Water Heater Isolation System.

Type AVS 4/5 Typical Feed Water Heater Isolation System. Features and Benefits These valves are designed to bypass high pressure feedwater around a group of heaters in the event of high water level in