Turbine bypass valves are some of the tough purposes in a power plant. Keeping these valves operating efficiently and avoiding surprising failures is important for plant operation.
When engineers are requested to record the most troublesome management valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, high strain drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many vegetation tend to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and high prices for reactive upkeep. This article provides suggested strategies of inspection to anticipate and mitigate issues beforehand, and it offers improve options should a valve have to be repaired or replaced.
Strictly Severe Service
Severe service control valves are utilized in the most tough installations inside process crops. These installations commonly include cavitating, erosive, corrosive, noisy, high stress, high temperature, high stress drop, or high velocity media. Turbine bypass valves are uncovered to many of these process situations; yet, they have to reply flawlessly and stay leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, as properly as when a turbine trips off-line (Figure 1). In normal operation the valves are utterly closed, forcing all the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine until the properties and conditions 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 probably moist steam, and by making certain solely appropriate steam situations and flows make their approach to the turbine itself.
1. Depending on the facility plant design, several turbine bypass valves may be employed to immediately shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to keep away from equipment injury because of overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain move by way of the system.
As it operates, a turbine uses steam to carry out work, reducing outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the strain, but the exit steam will remain quite superheated, doubtlessly destroying downstream gear. To keep away from that situation, turbine bypass valves either incorporate a water injection system within the valve body, or employ a separate water injection desuperheater just downstream, in either case to lower the exit steam temperature.
As a outcome, turbine bypass valves face a perfect storm of severe service situations. While the plant is in operation, these valves must stay tightly closed to avoid wasting vitality. When a turbine journey occurs, the bypass valves must reply instantly, exposing them to speedy temperature adjustments and requiring them to cross very high flows at high stress drops, creating excessive noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly each turbine bypass valve will finally fail in some way. Unfortunately, many of these valves are installed in difficult-to-access locations, are usually welded in place, and are typically closely insulated. As a result, they’re usually ignored till problems begin to floor. Steam leakage via the valve is often the primary symptom noticed, however much more significant and doubtlessly dangerous injury can occur.
The excessive pressure drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the steel in the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can outcome in catastrophic failure under strain.
2. Over time, the excessive vibrations and thermal shock skilled by turbine bypass valves will fatigue the steel and create cracks in each the valve itself and the encircling piping. Courtesy: Emerson
To avoid this downside, each turbine bypass valve and its related piping must be routinely inspected. Some sort of non-destructive examination ought to be periodically employed to detect metallic fatigue issues which might be creating however aren’t yet visually apparent. If a plant lacks the information or gear to carry out these inspections, the valve vendor or approved representative could additionally be utilized to perform turbine bypass health examine companies (Figure 3).
3. A routine turbine bypass valve health verify and full inspection is strongly suggested. Metal fatigue can develop in numerous 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, buyer connection welds, diffuser welds, and surrounding piping could be inspected to determine any developing problems before the equipment is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid problems related to quenching and cracking.
There are also threats to trim components, such as seat floor erosion. One widespread expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This happens more generally in flow down valves, where the steam accelerates through the cage holes and then strikes the trim/seat area instantly, inflicting excessive put on and reducing service life. Over time, this high-velocity steam, which might include water throughout startup circumstances and magnetite throughout any situation, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve physique designs are susceptible to trim injury as the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common upkeep items like gentle goods and spray nozzles can often get replaced comparatively simply and at minimal cost. Trim elements that need to be changed because of heavy wear, such as gear toothing, may be fairly costly, particularly if their supply must be expedited. เกจวัดแรงดันปั๊มลมpuma -case scenario for a turbine bypass valve is metallic fatigue creating in the valve body or diffuser, with extreme restore procedures, or even a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been installed through the heyday of combined cycle plant development from 1998 to 2004. Most of these valves and piping techniques are showing their age and often have vital signs of metallic fatigue. Others have loads of life left in them but may achieve advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing expertise might 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 maintenance.
If a turbine bypass valve must be replaced, plant personnel are strongly encouraged to look past a direct replacement and evaluate potential improvements. A significantly necessary merchandise to assume about when looking at replacement and enchancment alternatives 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 association is good for places with low overhead clearance, and it places the actuator nearer to the deck or ground for simple access, nevertheless 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 susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) have been the primary possibility a few many years in the past for turbine bypass valves. New circulate up valve designs (right) could additionally be installed with out piping modifications in most instances, offering prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for flow down is vertical mounting of the actuator. This association leads to decreased and extra even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that replacement alternatives are typically for horizontally mounted actuators, and the 2 designs are not readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is available with present horizontal actuator circulate down designs, a flow up design can simply meet the prevailing face-to-face requirements of the previous valve, whereas decreasing a variety of the wear widespread to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from move right down to flow up, this modification could be made with out modifications to current piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life because the steam is moving comparatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates considerably, however that vitality dissipates into the big body cavity of the valve as a substitute of striking 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 moves via the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely showing erosion, however when compared to the circulate down trim proven above (Figure 4 right), the seating floor on the seat ring and plug remain relatively undamaged, permitting the valve to shut off completely, the required mode in regular operation.
7. These pictures of flow up trim clearly show the erosive impression of high-velocity steam, however in contrast to the move down trim damage proven beforehand, this valve has no damage to the seating surface and might nonetheless shut off tightly. Courtesy: Emerson
The circulate up valve style design still offers the identical control responsiveness and circulate capability, however the model new flow arrangement dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered equipment that should be carefully specified, put in, and maintained. It is therefore wise to be proactive by reaching out to trusted specialists prior to purchase as they’ll help with valve sizing and specs based on specific 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 fully inspected, it would be sensible to consult your turbine bypass valve vendor for assist and help.
Many of these vendors offer full inspection and health examine services to identify areas where steel fatigue and stress cracking are growing. Ideally the valve may be repaired, but if a valve alternative is warranted, the vendor can even provide steering in evaluating bypass valve design types and selecting the finest option for the specific application.
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