VHB Feedwater heater bypass z Protects the feedwater heater jacket in z Allows bypassing of the feedwater heater dur- case of tube rupture or leakage. ing plant start-up at low turbine load. z Protects the turbine extraction from excessive z Allows bypassing of the feedwater heater at backpressure. peak loads to increase power output from the turbine. z Allows the feedwater heater to be isolated from the system for inspection or repair without dis- rupting the circulation through the plant.

Water

Feedwater Steam Control Valve Turbine

Steam bypass Water valve valve

Condenser HP heater and bypass

Feedwater Pump

Applications which provides a tight isolation of the feedwater heater. The valve body is manufactured of forged steel and is specially The BTG feedwater preheater bypass valves VHB-3, VHBS- designed to handle the high pressure levels and the thermal 2 and VHBS-3 are designed for use in power plants to allow loads typical of feedwater heater bypass applications. bypassing of the feedwater heater. The Kv/Cv-value of the valve is dependent on the pressure The VHB-3 is installed upstream of the feedwater heater and ratio p1/p2 and must ñ for each valve ñ be calculated by the the VHBS-2 or VHBS-3 is installed downstream. CCI/BTG computer program, where all throttling points in The VHBS-2 and VHBS-3 are on-off valves with one plug the valve are taken into consideration. A certified dimen- and one seat, and VHB-3 is an on-off valve with two seats, sional drawing will be supplied by CCI.

©CCI 2003 1 PS570.06/1en 621en Function Design The bypass system is always in stand-by position, ready to The VHB valves are specially designed for the feedwater act if the feedwater heater malfunctions, which will cause heater bypass application. Reliable function and long ser- the water level in the heater jacket to increase. By monitor- vice life with a minimum of maintenance are the basic ing the water level, the feedwater heater bypass system can design criteria for the VHB valve series. be automatically controlled. When the water level becomes too high, the feedwater heater bypass valve will open and Body bypass the feedwater around the heater, which is isolated. The valve is manufactured from fully machined forgings Another common malfunction may occur when the pressure with a body shape of smooth design and all body penetra- in the steam heating line increases. This can be monitored tions circular to prevent asymmetrical stress patterns. and used as a signal to isolate the heater. The switch-over to bypass mode is automatic and very fast. Resetting the sys- Connections are adapted to match the connecting piping. tem to normal operating mode can be done manually or Material is typically carbon steel or CrMo low alloy steel. automatically depending on specifications. Bonnet At the inlet side of the feedwater heater, VHB-3, a three- way block valve passes the feedwater from the feedwater Pressure seal bonnet is standard which provides a safe and pump side, either to the feedwater heater (normal operation) tight connection that is easily assembled and disassembled. or to the bypass line (bypass). A two-way block valve All internal parts can be removed through the bonnet cavity. VHBS-2 is installed at the feedwater heater outlet side. This Material is typically CrMo low alloy steel. valve either allows the feedwater flow to pass (normal oper- ation) or to isolate (bypass). The VHBS-2 is also available Plug in a three-way version - VHBS-3 - with integrated T-piece. The VHB valves provide an on-off control function. The A separate T-piece is thus not required. plugs are accordingly contoured and designed to minimize the pressure drop over the trim. Normal operation The plug for the VHB-3 is provided with two tight shut-off The VHB-3 passes the feedwater from the pump to the feed- areas as this valve has two seats. water heater (FWH). During normal operation, the plug is so positioned, that the flow is allowed to pass through the feedwater heater. At the same time, it keeps the passage Seat through the by-pass line closed. The internal pressure acting The seats are stellited and integrated with the valve body on the plug results in a force, which tends to maintain the and the bonnet (only for VHB-3). plug closed. The actuator is also acting in this direction to secure a tight shut-off towards the bypass line. Pressure drop The feedwater from the FWH to the passes through The pressure drop over each valve can be as low as 0.5 bar the VHBS-2 or the VHBS-3. The internal pressure in the (8 psi). valve, acting on the plug, tends to keep the valve open. The actuator on the VHB is also acting in the same direction to secure the free passage from the FWH to the boiler.

Bypass operation When the water level in the heater jacket actuates the level switch, the feedwater heater bypass system automatically switches the VHB-3 to close the passage to the FWH and opens the bypass line. At the same time, VHBS-2 or VHBS- 3 will close the passage to the FWH, and the FWH becomes isolated from the feedwater flow. Other signals, such as high pressure in the steam line, will similarly switch the valves.

Manual opening of the bypass system is performed exactly the same way, except that the signal to open is generated in the control room from a push-button. Normally the VHB valves cannot be stroked while being in bypass mode due to the large pressure differential over the plugs owing to the force available due to the size of the actuators. Fig 1 VHB3 - Sectional view

PS570.06/1en 2 ©CCI 2003 621en Actuating system Pneumatic actuators Three actuating systems are available: Standard piston pneumatic actuators are used. Air is required at a minimum pressure level of 4 bar (60 psi). The 1 Hydraulic, using the feedwater as fluid system control can be included in the actuator assemblies. The actuator controls can be configured to provide safe 2 Pneumatic, using standard piston actuators operation (normally bypass mode).

3 Electric, using standard electric actuator. Electric actuators The actuating time for the valves can be varied, but typical Standard electric actuators with all on the market available is approx. 3 sec for hydraulic and pneumatic actuators and voltages. System control to be included in DCS. approx. 15-25 sec for electric actuators. Three different versions are available Hydraulic actuators using the feedwater 1 Automatic bypassing and manual resetting. The hydraulic actuators are double-acting or single-acting and use the feedwater as power fluid. No external power is 2 Automatic bypassing and spring-opening upon signal, required for this system. The control system can be supplied except electric actuators. such that the valves will fail in the safe mode (normally bypass mode). 3 Automatic bypassing and resetting upon signal with hydraulics in both directions. The actuator control manifold contains both the valves and the electronics for the control of the system. Configurations

VHB-3

VHBS-2

VHBS-3

Fig 2 VHB valves in shown bypass position

©CCI 2003 3 PS570.06/1en 621en Bypass signal from DCS De-energize for bypass

Air supply

Vent

Solenoid valve

Limit switch, normal operation Limit switch, bypass mode

Switching valve (4-way ball valve) Valve for pressurizing of heater (not CCI scope)

Feedwater heater

Flow control valve for adjustment of speed

Limit switch, bypass mode Limit switch, normal operation

Fig 3 Boiler feedwater heater bypass self acting hydraulic control system shown in bypass mode

PS570.06/1en 4 © CCI 2003 621en CCI reserves the right to make technical improvements. For sales contacts, please refer to www.ccivalve.com