6 Reasons Boiler Feedwater Control Valves Leak: Suggested Solutions

Emerson Fisher Control Valves have extensive experience in control Valves for the Power Industry. Drawing on this experience they have published the following paper

First though, what are the 6 most common reasons for Boiler Feedwater  Control Valve leakage and what to do when you isolate the problem?

1. Insufficient information at specification time (part 1)
2. Oversizing for maximum conditions (part 2)
3. Failure to specify tight shut-off (part 3)
4. Improper operation (part 4)
5. Poor control arrangement (part 5)
6. Entrained particles (part 6)

Combined-cycle powerplants offer a great deal of operating flexibility, typically having the ability to respond to changing load demands much faster than large fossil-fired steam stations. So it is not surprising that in competitive power markets combined-cycle facilities are experiencing more operating cycles than owners planned for at the design stage

While the ability to cycle a large combined-cycle plant is ideal for fleet flexibility, frequent start-ups and shutdowns strain many critical components and limit their effective lifetimes. Witness the relatively common boiler tube failures caused by flow-accelerated corrosion (FAC) and the premature failure of steam-turbine bypass valves. Such problems can bring a plant down for extended periods, often without warning

Another maintenance issue, one that has not received much publicity to date, has to do with valves in the boiler feedwater system. The design of boiler feedwater systems varies somewhat from plant to plant, but the valves used are fairly repeatable

The number of valves depends on the design of the steam turbine and the heat-recovery steam generator (HRSG). If the turbine and HRSG are of the multi-pressure type, the number of valves increases. To illustrate: For multi-pressure HRSGs, drum level may be controlled by a one-valve or two-valve arrangement. Depending on plant design, there may be between two and six drum-level control valves. In addition, for each boiler feed pump (most plants have two per HRSG), there is a recirculation valve that recycles a portion of feedwater flow back to the low-pressure (lp) drum or deaerator to prevent the boiler feed pump from overheating and potentially cavitating

Similar problems with feedwater valves have been experienced at several combined-cycle plants with different types of steam turbines and HRSGs. The most common issue noted is excessive leakage. With leakage comes damage to the internal throttling and seating surfaces of the valve. This damage often is incorrectly attributed to faulty design or misapplication

In many cases, the resultant leakage damage can be traced to the frequent cycling of the unit. It is not uncommon for a combined-cycle plant to experience over 250 starts per year. This is more than the number of starts that most large coal-fired plants experience in a lifetime

Given that there are up to eight critical boiler feedwater valves per HRSG, maintenance or replacement of these valves can be very expensive. The cost of feedwater valves for a two-on-one combined cycle can run more than $100,000. While many problems can be traced to frequent cycling, there are other reasons for valve damage as detailed above. It is important to understand the type of damage and its cause before the proper replacement or fix can be applied

What causes feedwater valves to leak? The first indication of leaking feedwater valves normally is an increase in drum water level. After you determine which valves leak, they must be opened for inspection to determine the root cause

Click here to read how ESI Technologies can assist with optimising the performance of the critical control valves in your plant, or you can contact ESI using the enquiry form, or call Joe Walsh on +353 21 4510900 or Dave Ralph on +44 1633 877505 if you have leaking boiler feedwater valves and want to discuss options.