Proper Valve Trim Extends Life of Boiler Feedwater Valves
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In competitive power markets, combined cycle facilities are experiencing more operating cycles than owners initially planned for at the design stage. It’s not uncommon for a combined cycle plant to experience more than 250 starts per year these days.
While the ability to cycle a large combined cycle plant is ideal for fleet flexibility, frequent startups and shutdowns strain many critical components and limit their lifetimes. Witness the relatively common boiler tube failures caused by flow-accelerated corrosion (FAC) and the premature failure of steam turbine bypass China valve inspection service. Similar problems with feedwater valves have been experienced at several combined cycle plants with different types of steam turbines and heat recovery steam generators (HRSGs).
Such problems can bring a plant down for extended periods, often without warning. 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 plant can run more than $160,000.
While many problems can be traced to frequent cycling, there are other reasons for feedwater valve damage. The non destructive testing services application is an inherently severe service; a combined-cycle plant using F-class combustion turbines will employ feedwater pressures as high as 3,000 psig, and some repowering projects have pressures climbing above 4,000 psig. Valves operating in such high feedwater pressures encounter a potential for cavitation damage.
Another common feedwater valve issue is excessive leakage, which is typically revealed by an increase in drum water level. With leakage comes damage to the internal throttling and seating surfaces of the valve. Once it is determined which valves leak, they must be opened for inspection to determine the root cause. It is important to understand the type of damage and its cause before the dimension check proper replacement or fix can be applied.
One problem occurs when feedwater valves are not specified for tight shutoff. ANSI (American National Standards Institute) and FCI (Flow Control Institute) have established criteria to denote leakage classes for control valves. Table 1 shows the corresponding leakage of 3-inch and 4-inch feedwater valves. Class V shutoff is recommended for feedwater valves exposed to cavitation conditions.
Looking at the table, it is easy to understand what can happen to the drum-level valves if they are not Class V. Flow that leaks past the seating surface will cavitate, damaging the seating surfaces of the plug and seat ring, exposing the visual inspection to further damage. It is very likely that feedwater valves in existing service may have been installed with a Class IV shutoff rating or less. To upgrade a valve without Class V (CLV) shutoff, a simple trim change (Figure 1) will solve the issue.