Designing for maximum conditions can lead to some of the operating and leakage problems with feedwater valves
The feedwater valves are normally sized to accommodate an operating condition that occurs when the safety valves open during a unit trip. It is critical not to allow the HRSG to “dry out”, which can cause severe thermal damage to the boiler tubes and the drums.
To protect against this, the feedwater valves are sized for minimal pressure drop, allowing the maximum amount of water to flow to the drum. At this point, the normal feedwater inlet pressure to the high-pressure (h-p) drum-level control valve is nearly 2400 PSIG. The valve is sized to take a 20-30 psi pressure drop, which increases the required capacity to nearly twice that needed for normal operation. This means that the valve must operate at lower lifts (30%-40% open) than intended during normal operating conditions, thereby exposing seating surfaces to premature erosion during startup.
Since the maximum operating pressure of a combined-cycle plant is around 1850 PSIG with supplemental firing, and the safety valves normally lift at approximately 2000 or 2100 PSIG, the valves are being sized to supply a much higher feedwater pressure to the drum. If the allowable pressure drop across the valve were increased to approximately 100 psi, valve size would decrease and it would operate at 50% to 70% open during normal operating conditions.
Solution
To prevent oversizing your feedwater valves, it is necessary to understand the impact of valve capacity on protecting the HRSG from drying out. As previously stated, slightly increasing the pressure drop across the valve will prevent the valve from being oversized. Retrofit trim packages that alter the performance characteristic of the valve can be supplied. Again, this can be done without the removal of the valves. If a change is made, it is important to ensure that a revised valve characteristic does not interfere with any DCS logic.
(the above is an extract from an Emerson Fisher technical paper)
Additional posts in this series:
- Part 1: Insufficient information at specification time
- Part 3: Failure to specify tight shut-off
- Part 4: Improper operation
- Part 5: Poor control arrangement
- Part 6: Entrained particles
Links for additional steps to take to improve your Critical Control Valve performance or for a copy of the control valve handbook. If you wish to discuss your leaking boiler feedwater valve issues, please contact us using the enquiry form or call Dave Ralph on +44 1633 877505 or Joe Walsh on +353 21 4510900