Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most tough purposes in a power plant. Keeping these valves working efficiently and avoiding unexpected failures is critical for plant operation.
When engineers are asked to list probably the most troublesome management valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, high pressure drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many crops are inclined to ignore these valves till they fail, creating unplanned outages, misplaced production, and excessive costs for reactive upkeep. This article provides suggested strategies of inspection to anticipate and mitigate points beforehand, and it presents improve options should a valve have to be repaired or replaced.
Strictly Severe Service
Severe service management valves are utilized in probably the most troublesome installations inside process vegetation. These installations commonly embrace cavitating, erosive, corrosive, noisy, high pressure, high temperature, excessive pressure drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these course of circumstances; yet, they must reply flawlessly and stay leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam turbines during plant startup and shutdown, as nicely as when a turbine trips off-line (Figure 1). In normal operation the valves are completely closed, forcing all of the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine until the properties and circumstances of the steam are acceptable for sending it to the turbine. This process occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting doubtlessly moist steam, and by making certain only acceptable steam situations and flows make their way to the turbine itself.
1. Depending on the facility plant design, several turbine bypass valves may be employed to immediately shunt steam round a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine trip, the steam must proceed flowing to avoid equipment injury because of overpressure and high temperature, so the turbine bypass valve immediately opens to keep up circulate through the system.
As it operates, a turbine uses steam to perform work, lowering outlet steam temperature and pressure. When a turbine bypass valve opens, it’ll drop the stress, however the exit steam will stay fairly superheated, doubtlessly destroying downstream equipment. To keep away from that state of affairs, turbine bypass valves either incorporate a water injection system within the valve body, or make use of a separate water injection desuperheater simply downstream, in both case to lower the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of severe service circumstances. While the plant is in operation, these valves must remain tightly closed to avoid wasting energy. When a turbine trip happens, the bypass valves should respond instantly, exposing them to speedy temperature modifications and requiring them to pass very high flows at excessive stress drops, creating excessive noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that just about each turbine bypass valve will in the end fail indirectly. Unfortunately, many of these valves are installed in difficult-to-access locations, are typically welded in place, and tend to be closely insulated. As a end result, they are often ignored till issues begin to floor. Steam leakage by way of the valve is often the primary symptom noticed, however rather more vital and probably dangerous harm can occur.
The excessive pressure drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the steel in the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can lead to catastrophic failure under 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 encircling piping. Courtesy: Emerson
To avoid this downside, each turbine bypass valve and its associated piping must be routinely inspected. Some sort of non-destructive examination should be periodically employed to detect metallic fatigue problems that could be developing however are not yet visually obvious. If a plant lacks the information or tools to perform these inspections, the valve vendor or authorized representative may be utilized to carry out turbine bypass well being examine companies (Figure 3).
three. A routine turbine bypass valve well being check and full inspection is strongly suggested. Metal fatigue can develop in various welds located on the valve itself, and within the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, customer connection welds, diffuser welds, and surrounding piping may be inspected to determine any developing issues before the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are also threats to trim parts, such as seat floor erosion. One common expression of such erosion, particularly on the plug, is sometimes called “gear toothing.” This occurs extra commonly in circulate down valves, the place the steam accelerates by way of the cage holes after which strikes the trim/seat space directly, inflicting extreme wear and decreasing service life. Over time, this high-velocity steam, which can include water during startup situations and magnetite during any condition, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve body designs are prone to trim damage because the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like delicate goods and spray nozzles can normally get replaced relatively simply and at minimal price. Trim components that need to be changed due to heavy wear, corresponding to gear toothing, may be fairly expensive, particularly if their delivery must be expedited. The worst-case situation for a turbine bypass valve is steel fatigue developing in the valve physique or diffuser, with extreme repair procedures, or maybe a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service had been put in during the heyday of mixed cycle plant development from 1998 to 2004. Most of these valves and piping methods are displaying their age and infrequently have vital indicators of metallic fatigue. เกจวัดแรงดันน้ำประปาราคา have plenty of life left in them but may achieve advantage from upgrades in technology. Technology and practices have improved, and upgrading to the newest sealing know-how could inject new life into the valve.
Another example of a expertise improve is when there have been repeated failures with a welded diffuser. In this occasion, a removable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve must be changed, plant personnel are strongly encouraged to look past a direct alternative and consider potential improvements. A significantly essential merchandise to contemplate when looking at replacement and improvement opportunities is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a move down trim design paired with a horizontal actuator (Figure 5, left). This arrangement is ideal for places with low overhead clearance, and it places the actuator nearer to the deck or ground for simple access, but it creates numerous long-term operational problems. Gravity tends to lead to elevated and uneven wear on trim parts, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) were the principle option a few a long time ago for turbine bypass valves. New flow up valve designs (right) could additionally be installed with out piping modifications in most instances, providing prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for circulate down is vertical mounting of the actuator. This arrangement ends in reduced and extra even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that alternative alternatives 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 out there with current horizontal actuator flow down designs, a move up design can simply meet the present face-to-face necessities of the old valve, whereas decreasing some of the wear common to circulate down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from circulate right down to flow up, this modification may be made with out modifications to present piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life as a 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, but that energy dissipates into the big physique cavity of the valve as an alternative of striking the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower because they cross over the seat and accelerate later as the steam moves by way of the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is actually displaying erosion, however when compared to the flow down trim proven above (Figure four right), the seating surface on the seat ring and plug remain comparatively undamaged, allowing the valve to shut off fully, the required mode in normal operation.
7. These footage of flow up trim clearly present the erosive impression of high-velocity steam, however unlike the move down trim injury proven beforehand, this valve has no injury to the seating surface and can still shut off tightly. Courtesy: Emerson
The flow up valve type design still offers the identical control responsiveness and circulate capacity, however the new circulate arrangement dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised items of engineered equipment that have to be rigorously specified, put in, and maintained. It is subsequently sensible to be proactive by reaching out to trusted consultants prior to buy as they will help with valve sizing and specifications based mostly on particular operating situations.
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 will be wise to consult your turbine bypass valve vendor for help and assist.
Many of these vendors provide full inspection and well being verify companies to determine areas where metal fatigue and stress cracking are developing. Ideally the valve may be repaired, but when a valve substitute is warranted, the seller can even present steering in evaluating bypass valve design styles and selecting the best suited choice for the specific utility.
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