Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most difficult functions in a power plant. Keeping these valves working effectively and avoiding sudden failures is crucial for plant operation.
When engineers are asked to list essentially the most troublesome control valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive stress drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many crops tend to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and high costs for reactive maintenance. This article offers advised strategies of inspection to anticipate and mitigate points beforehand, and it offers improve options should a valve must be repaired or changed.
Strictly Severe Service
Severe service control valves are used in the most tough installations within process vegetation. These installations commonly embody cavitating, erosive, corrosive, noisy, high pressure, high temperature, excessive stress drop, or excessive velocity media. Turbine bypass valves are exposed to many of these process situations; but, they have to reply 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 nicely as when a turbine trips off-line (Figure 1). In regular operation the valves are completely closed, forcing all the steam through 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 guard the turbine by diverting probably wet steam, and by ensuring solely acceptable steam conditions and flows make their way to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves may be employed to immediately shunt steam around a turbine should it trip offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to keep away from tools harm as a outcome of overpressure and excessive temperature, so the turbine bypass valve instantly opens to take care of circulate by way of the system.
As it operates, a turbine makes use of steam to perform work, reducing outlet steam temperature and stress. When a turbine bypass valve opens, it will drop the stress, however the exit steam will remain quite superheated, probably destroying downstream gear. To avoid that state of affairs, turbine bypass valves both incorporate a water injection system within the valve body, or employ a separate water injection desuperheater just downstream, in both case to decrease the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of extreme service situations. While the plant is in operation, these valves should stay tightly closed to avoid wasting vitality. When a turbine trip occurs, the bypass valves should respond immediately, exposing them to fast temperature modifications and requiring them to pass very excessive flows at excessive stress drops, creating excessive noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that just about each turbine bypass valve will ultimately fail indirectly. Unfortunately, many of those valves are put in in difficult-to-access areas, are typically welded in place, and tend to be closely insulated. As a end result, they’re typically ignored till issues start to surface. Steam leakage via the valve is usually the first symptom noticed, but much more important and probably harmful damage can happen.
The excessive pressure drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue may find yourself in catastrophic failure under pressure.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metal and create cracks in each the valve itself and the surrounding piping. Courtesy: Emerson
To keep away from this drawback, every turbine bypass valve and its associated piping should be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect metallic fatigue issues that might be developing but usually are not but visually apparent. If a plant lacks the information or gear to perform these inspections, the valve vendor or approved representative may be utilized to carry out turbine bypass well being examine services (Figure 3).
three. A routine turbine bypass valve health examine and full inspection is strongly advised. Metal fatigue can develop in varied welds situated on the valve itself, and in the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, customer connection welds, diffuser welds, and surrounding piping could be inspected to identify any growing problems before the equipment is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are additionally threats to trim components, such as seat floor erosion. One widespread expression of such erosion, particularly on the plug, is also known as “gear toothing.” This happens extra generally in circulate down valves, where the steam accelerates through the cage holes after which strikes the trim/seat space immediately, causing extreme put on and reducing service life. Over time, this high-velocity steam, which might contain water during startup situations and magnetite during any situation, will erode and injury the valve seat and trim (Figure 4).
4. Flow down valve body designs are susceptible to trim injury as the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like soft goods and spray nozzles can normally get replaced relatively simply and at minimal price. Trim elements that must be changed as a result of heavy put on, such as gear toothing, could be quite costly, especially if their supply needs to be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue growing in the valve physique or diffuser, with extreme repair procedures, or even a full valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service have been installed during the heyday of mixed cycle plant building from 1998 to 2004. Most of those valves and piping techniques are displaying their age and infrequently have important signs of steel fatigue. Others have plenty of life left in them however may benefit from upgrades in technology. Technology and practices have improved, and upgrading to the most recent sealing expertise could inject new life into the valve.
Another instance of a expertise upgrade is when there have been repeated failures with a welded diffuser. In this instance, a removable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve should be replaced, plant personnel are strongly encouraged to look past a direct replacement and evaluate potential improvements. A significantly important item to think about when looking at replacement 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 association is ideal for places with low overhead clearance, and it places the actuator closer to the deck or ground for easy entry, however it creates a selection of long-term operational problems. Gravity tends to end in elevated and uneven put on on trim parts, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the primary choice a quantity of decades in the past for turbine bypass valves. New move up valve designs (right) may be put in with out piping modifications in most instances, offering extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for flow down is vertical mounting of the actuator. This association results in decreased and more even put on, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that substitute opportunities are sometimes for horizontally mounted actuators, and the 2 designs are not readily interchangeable without important piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead space is available with existing horizontal actuator circulate down designs, a flow up design can easily meet the existing face-to-face necessities of the outdated valve, while lowering a variety of the wear frequent to flow down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from move down to circulate up, this modification can be made with out modifications to current piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life as a result of the steam is moving relatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates significantly, but that power dissipates into the big body cavity of the valve as a substitute 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 speed up later because the steam strikes through the small trim holes, tremendously extending trim service life. เพรสเชอร์เกจ : Emerson
Notice how the flow up trim (Figure 7) is certainly showing erosion, however when in comparison with the flow down trim proven above (Figure 4 right), the seating surface on the seat ring and plug remain comparatively undamaged, allowing the valve to close off fully, the required mode in normal operation.
7. These photos of circulate up trim clearly show the erosive impression of high-velocity steam, but not like the circulate down trim damage proven previously, this valve has no harm to the seating floor and can still shut off tightly. Courtesy: Emerson
The flow up valve type design still provides the identical management responsiveness and circulate capability, however the new move association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised pieces of engineered gear that should be rigorously specified, installed, and maintained. It is due to this fact wise to be proactive by reaching out to trusted consultants prior to purchase as they can help with valve sizing and specs based on particular operating circumstances.
For current installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time since the plant turbine bypass valves have been totally inspected, it would be clever to consult your turbine bypass valve vendor for help and support.
Many of those distributors supply full inspection and well being examine providers to establish areas where metallic fatigue and stress cracking are growing. Ideally the valve could be repaired, but if a valve alternative is warranted, the vendor can also present steerage in evaluating bypass valve design styles and choosing the finest choice for the particular software.
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