Subject: Combined-cycle power plants can suffer premature failure of boiler feedwater valves. Proper valve trim can solve the problem.
Shelby Richardson, a sales engineer supporting flow controls products at Emerson in Marshalltown, Iowa, says combined cycle facilities are experiencing more operating cycles than owners initially planned for at the design stage, leading to premature failures of boiler feedwater valves. Her article, Proper Valve Trim Extends Life of Boiler Feedwater Valves in the April 2019 issue of Power magazine, says installing the proper valve trim can solve most problems.
It’s not uncommon for a combined-cycle plant to experience over 250 starts per year. Frequent startups and shutdowns strain many critical components. Problems with feedwater valves have been experienced at several combined-cycle plants with different types of steam turbines and heat recovery steam generators (HRSGs).
Globe valves, like the Fisher HP shown here, are commonly used in boiler feedwater applications.
Shelby says 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.
One problem occurs when feedwater valves are not specified for tight shutoff. Flow that leaks past the seating surface will cavitate, damaging the seating surfaces of the plug and seat ring, exposing the valve 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. Upgrading a valve to Class V shutoff just requires a simple trim change.
Upgrading the valve trim can stop leaks.
Sometimes problems are introduced by the way the valves are operated, Shelby notes.
All control valves have a minimum operating point. If the valve is opened only a minute amount off the seating surface, the plug and seating surfaces can experience erosion.
To protect against this, valves should not be throttled below the manufacturer’s minimum travel recommendation. A rule of thumb is to not operate valves below 10% open.
Standard seal ring designs may work sufficiently to maintain valve shutoff. However, as increases in plant cycling continue and process pressure and temperatures rise, older sealing types may not be able to withstand the process conditions.
Sealing technologies have developed substantially over the past years. For moderate process temperatures below 600°F, common seal materials are sufficient. But for higher temperature operations, a newer sealing technology can often be retrofitted into the existing valve trim design, she says.
Metal seals are available for process temperatures above 600°F. Upgrading the seal material is also a simple change of a few trim components.
Contaminants cause particulates to build up in the system. These entrained particulates create accelerated wear to the plug and seat ring. Fine particulate matter essentially “grit blasts” critical surfaces.
A valve plug with “gear-tooth” erosion at the bottom of the plug and on the seat ring has decreased shutoff capability, and the additional leakage through the valve will only exacerbate the problem. This is commonly found in boiler feedwater valves that have been exposed to particulates.
Particulates caused “gear tooth” erosion to this valve plug.
One solution is to install trim with a protected inside seat design. This design moves the seating surfaces out of the direct flow path and to the center of the plug, prolonging trim life. Protected inside seat designs can be installed by a simple change of plug/stem and seat ring.
Valves equipped with anti-cavitation trim that become plugged may no longer stage the pressure drop effectively. This leads to cavitation damage on many trim components, and in severe cases within the valve bodies. Anti-cavitation trims that have been plugged can be retrofitted with trim that has larger flow paths and a protected seat design to provide cavitation protection.
Fisher Dirty Service Trim(DST) provides cavitation mitigation in dirty environments.
The protected seat design removes the shutoff areas away from locations experiencing pressure drop. By ensuring that a pressure drop does not occur across the seating surface, the high-velocity “grit blasting” effect from the entrained particulate cannot occur, and the trim is protected against premature erosion.
Much of the entrained particulate in boiler feedwater can be linked to inadequate pipe blowdowns during plant startup and commissioning. Inadequate pipe blowdowns have caused the failure of many valves, not just feedwater valves. But feedwater valves are particularly susceptible to particulate damage because debris can become lodged in the flow passages and reduce capacity. Shelby says it’s important to ensure a proper piping blowdown, or blow.
Prior to a pipe blow, it is necessary to remove the normal operating trim from the valve and install sacrificial trim. Emerson offers such trim along with support from Lifecycle Services to handle operational trim removal and replacement.
Boiler feedwater valves operate under severe service conditions and are subject to premature failures from seat leakage, cavitation, particulates and frequent cycling. Once the type and root cause of damage is properly understood, installing the proper trim can solve most problems.
This is the official online community site of the Emerson Global Users Exchange, a forum for the free exchange of non-proprietary information among the global user community of all Emerson Automation Solution's products and services. Our goal is to improve the efficiency and use of automation systems and solutions employed at members’ facilities by sharing our knowledge, experiences, and application information.
User Groups |
World Areas |
Community Guidelines |
Legal Information |
Contact Community Manager
Website translation provided by
© 2015-2020 Emerson Global Users Exchange. All rights reserved.