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Turbine Bypass Valve Challenges

Turbine bypass valves are one of the tough functions in an influence plant. Keeping these valves working efficiently and avoiding sudden failures is important for plant operation.
When engineers are asked to listing the most difficult management valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive stress drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many crops are probably to ignore these valves until they fail, creating unplanned outages, lost production, and high costs for reactive maintenance. This article offers instructed strategies of inspection to anticipate and mitigate points beforehand, and it presents upgrade alternatives ought to a valve have to be repaired or replaced.
Strictly Severe Service
Severe service control valves are used in probably the most troublesome installations inside process crops. These installations generally embody cavitating, erosive, corrosive, noisy, high strain, excessive temperature, high stress drop, or high velocity media. Turbine bypass valves are exposed to many of these course of circumstances; yet, they have to respond flawlessly and remain leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, as properly as when a turbine journeys off-line (Figure 1). In normal operation the valves are fully closed, forcing all of the steam via a turbine. During startup, bypass valves divert steam away from the turbine until the properties and situations of the steam are acceptable for sending it to the turbine. This course of happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting potentially moist steam, and by making certain only acceptable steam circumstances and flows make their way to the turbine itself.
1. Depending on the power plant design, a quantity of turbine bypass valves may be employed to instantly shunt steam round a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to keep away from gear injury due to overpressure and excessive temperature, so the turbine bypass valve immediately opens to maintain circulate through the system.
As it operates, a turbine makes use of steam to perform work, reducing outlet steam temperature and pressure. When a turbine bypass valve opens, it will drop the strain, however the exit steam will remain fairly superheated, probably destroying downstream equipment. To keep away from that state of affairs, turbine bypass valves both incorporate a water injection system in the valve body, or employ a separate water injection desuperheater simply downstream, in both case to lower the exit steam temperature.
As a result, turbine bypass valves face an ideal storm of extreme service situations. While the plant is in operation, these valves should stay tightly closed to keep away from wasting energy. When a turbine journey happens, the bypass valves should reply instantly, exposing them to rapid temperature modifications and requiring them to pass very high flows at high strain drops, creating excessive noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that virtually each turbine bypass valve will ultimately fail in some way. Unfortunately, many of those valves are installed in difficult-to-access locations, are sometimes welded in place, and are usually closely insulated. As a result, they’re usually ignored until problems begin to floor. Steam leakage through the valve is usually the first symptom seen, but far more significant and probably harmful damage can occur.
The excessive stress drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metallic in the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can result in catastrophic failure underneath strain.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in both the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this downside, every turbine bypass valve and its associated piping ought to be routinely inspected. Some type of non-destructive examination must be periodically employed to detect metal fatigue issues that could be growing but usually are not but visually obvious. If a plant lacks the data or equipment to carry out these inspections, the valve vendor or authorized consultant may be utilized to perform turbine bypass well being check providers (Figure 3).
3. เกจ์วัดแรงดันน้ำ bypass valve health examine and full inspection is strongly advised. Metal fatigue can develop in numerous welds located on the valve itself, and within the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, buyer connection welds, diffuser welds, and surrounding piping can be inspected to identify any developing issues earlier than the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from problems associated to quenching and cracking.
There are additionally threats to trim parts, similar to seat floor erosion. One widespread expression of such erosion, particularly on the plug, is often referred to as “gear toothing.” This happens extra commonly in flow down valves, where the steam accelerates by way of the cage holes after which strikes the trim/seat space immediately, inflicting extreme wear and decreasing service life. Over time, this high-velocity steam, which may comprise water throughout startup situations and magnetite during any situation, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve body designs are prone to trim damage because the moist, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like soft goods and spray nozzles can often be replaced comparatively simply and at minimal cost. Trim components that need to be replaced due to heavy put on, corresponding to gear toothing, can be fairly costly, particularly if their delivery must be expedited. The worst-case situation for a turbine bypass valve is metal fatigue growing in the valve physique or diffuser, with excessive restore procedures, or even a complete valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been installed through the heyday of mixed cycle plant development from 1998 to 2004. Most of these valves and piping techniques are exhibiting their age and sometimes have important indicators of metal fatigue. Others have loads of life left in them but may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the most recent sealing expertise could inject new life into the valve.
Another example of a know-how upgrade is when there have been repeated failures with a welded diffuser. In this occasion, a removable two-in-one seat diffuser will scale back required upkeep.
If a turbine bypass valve should be changed, plant personnel are strongly inspired to look past a direct substitute and evaluate potential improvements. A significantly necessary merchandise to consider when looking at alternative and improvement opportunities is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a flow down trim design paired with a horizontal actuator (Figure 5, left). This arrangement is ideal for places with low overhead clearance, and it locations the actuator closer to the deck or floor for easy access, however it creates numerous long-term operational issues. Gravity tends to result in increased and uneven put on on trim parts, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the primary possibility a couple of many years in the past for turbine bypass valves. New circulate up valve designs (right) could additionally be installed without piping modifications in most cases, providing prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for circulate down is vertical mounting of the actuator. This arrangement ends in decreased and extra even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that alternative alternatives are usually for horizontally mounted actuators, and the two designs aren’t readily interchangeable without important piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead house is out there with current horizontal actuator circulate down designs, a flow up design can simply meet the existing face-to-face necessities of the old valve, whereas decreasing a number of the wear frequent to circulate down designs with a horizontal actuator, corresponding to gear toothing. Because the trim just switches from flow all the means down to move up, this change could be made with out modifications to existing piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life as a end result of the steam is moving comparatively slowly because it passes over the plug and seat (Figure 6). As the steam moves via the small holes of the cage, it accelerates considerably, however that power dissipates into the massive body cavity of the valve as an alternative of putting the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease because they cross over the seat and accelerate later because the steam moves through the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely exhibiting erosion, but when compared to the flow down trim proven above (Figure 4 right), the seating floor on the seat ring and plug stay relatively undamaged, permitting the valve to close off utterly, the required mode in regular operation.
7. These photos of move up trim clearly present the erosive influence of high-velocity steam, but not like the circulate down trim damage proven previously, this valve has no injury to the seating surface and can still shut off tightly. Courtesy: Emerson
The move up valve style design nonetheless supplies the same management responsiveness and move capability, however the brand new circulate association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized items of engineered gear that should be rigorously specified, installed, and maintained. It is due to this fact clever to be proactive by reaching out to trusted specialists prior to purchase as they will help with valve sizing and specifications based mostly on specific operating situations.
For existing installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while since the plant turbine bypass valves have been fully inspected, it will be clever to consult your turbine bypass valve vendor for help and help.
Many of those distributors supply full inspection and well being check services to identify areas the place metallic fatigue and stress cracking are developing. Ideally the valve may be repaired, but when a valve substitute is warranted, the seller can also provide steering in evaluating bypass valve design types and selecting the most suitable choice for the specific utility.
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