Turbine Bypass Valve Challenges

Turbine bypass valves are some of the troublesome purposes in an influence plant. Keeping these valves operating effectively and avoiding unexpected failures is crucial for plant operation.
When engineers are requested to listing probably the most difficult management valve purposes, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive stress drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many plants are likely to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and excessive prices for reactive upkeep. เกจวัดแรงดันปั๊มลมpuma suggested methods of inspection to anticipate and mitigate issues beforehand, and it presents improve options should a valve need to be repaired or changed.
Strictly Severe Service
Severe service management valves are utilized in the most difficult installations within course of crops. These installations commonly embrace cavitating, erosive, corrosive, noisy, excessive pressure, excessive temperature, excessive pressure drop, or high velocity media. Turbine bypass valves are uncovered to many of those process situations; yet, they have to reply flawlessly and stay leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam turbines throughout plant startup and shutdown, as properly as when a turbine trips off-line (Figure 1). In normal operation the valves are fully closed, forcing all the steam through a turbine. During startup, bypass valves divert steam away from the turbine till the properties and conditions of the steam are applicable for sending it to the turbine. This process occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting potentially wet steam, and by guaranteeing solely acceptable steam situations and flows make their approach 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 ought to it journey offline. Courtesy: Emerson
Should a turbine journey, the steam must proceed flowing to keep away from tools harm as a result of overpressure and high temperature, so the turbine bypass valve instantly opens to take care of circulate through the system.
As it operates, a turbine makes use of steam to carry out work, reducing outlet steam temperature and pressure. When a turbine bypass valve opens, it’ll drop the strain, but the exit steam will remain quite superheated, potentially destroying downstream gear. To avoid that state of affairs, turbine bypass valves both incorporate a water injection system within the valve physique, or employ a separate water injection desuperheater simply downstream, in either case to decrease the exit steam temperature.
As a end result, turbine bypass valves face a perfect storm of severe service situations. While the plant is in operation, these valves must stay tightly closed to keep away from losing energy. When a turbine journey occurs, the bypass valves must respond instantly, exposing them to fast temperature adjustments and requiring them to cross very excessive flows at excessive stress drops, creating excessive noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that just about each turbine bypass valve will in the end fail ultimately. Unfortunately, many of those valves are installed in difficult-to-access locations, are typically welded in place, and are typically closely insulated. As a outcome, they’re often ignored till problems start to floor. Steam leakage via the valve is usually the first symptom seen, but far more important and potentially harmful harm can occur.
The extreme pressure drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metal 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, every turbine bypass valve and its associated piping should be routinely inspected. Some sort of non-destructive examination should be periodically employed to detect metallic fatigue issues which might be developing but usually are not yet visually apparent. If a plant lacks the knowledge or tools to carry out these inspections, the valve vendor or approved consultant could also be utilized to perform turbine bypass health verify companies (Figure 3).
3. A routine turbine bypass valve health check and full inspection is strongly advised. Metal fatigue can develop in various welds located on the valve itself, and in the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, buyer connection welds, diffuser welds, and surrounding piping could be inspected to identify any creating issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid problems associated to quenching and cracking.
There are also 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 occurs extra commonly in flow down valves, the place the steam accelerates via the cage holes and then strikes the trim/seat space directly, causing excessive put on and reducing service life. Over time, this high-velocity steam, which might include water throughout startup circumstances and magnetite during any situation, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve physique designs are prone to trim injury as the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like gentle items and spray nozzles can usually get replaced comparatively easily and at minimal cost. Trim elements that must be changed as a outcome of heavy wear, such as gear toothing, can be fairly pricey, particularly if their supply must be expedited. The worst-case state of affairs for a turbine bypass valve is metal fatigue creating in the valve body or diffuser, with extreme restore procedures, or maybe a complete valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service had been installed through the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping systems are displaying their age and often have significant indicators of steel fatigue. Others have loads of life left in them however may benefit from upgrades in technology. Technology and practices have improved, and upgrading to the latest sealing technology could inject new life into the valve.
Another example of a technology 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 maintenance.
If a turbine bypass valve must be replaced, plant personnel are strongly inspired to look past a direct alternative and consider potential enhancements. A significantly essential merchandise to contemplate when looking at alternative and improvement alternatives is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a circulate down trim design paired with a horizontal actuator (Figure 5, left). This association is right for locations with low overhead clearance, and it locations the actuator nearer to the deck or ground for easy access, however it creates a variety of long-term operational problems. Gravity tends to result in increased and uneven put on on trim components, and the side-mounted actuator is prone to response issues.
5. Downward flowing valves with horizontal actuators (left) were the principle choice a couple of a long time ago for turbine bypass valves. New circulate up valve designs (right) may be put in without piping modifications in most instances, providing extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for move down is vertical mounting of the actuator. This arrangement results in lowered and more even put on, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute alternatives are typically for horizontally mounted actuators, and the two designs usually are not readily interchangeable without important piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is on the market with existing horizontal actuator move down designs, a move up design can simply meet the existing face-to-face necessities of the outdated valve, whereas decreasing a variety of the put on common to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from flow down to move up, this alteration could be made with out modifications to current piping.
Flow up valves (Figure 5, right) significantly prolong seat and trim service life as a outcome of the steam is moving comparatively slowly because it passes over the plug and seat (Figure 6). As the steam moves by way of the small holes of the cage, it accelerates considerably, however that energy dissipates into the massive body cavity of the valve as an alternative of hanging the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease because they pass over the seat and accelerate later because the steam strikes via the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely displaying erosion, however when in comparison with the circulate down trim proven above (Figure four right), the seating floor on the seat ring and plug remain relatively undamaged, permitting the valve to shut off utterly, the required mode in regular operation.
7. These photos of flow up trim clearly present the erosive impression of high-velocity steam, however not like the move down trim injury proven previously, this valve has no harm to the seating surface and can still shut off tightly. Courtesy: Emerson
The circulate up valve style design nonetheless provides the same management responsiveness and flow capability, but the model new move association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized pieces of engineered gear that have to be fastidiously specified, put in, and maintained. It is subsequently wise to be proactive by reaching out to trusted specialists prior to buy as they will help with valve sizing and specifications based mostly on particular operating situations.
For current installations where 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 clever to seek the advice of your turbine bypass valve vendor for help and support.
Many of these vendors supply full inspection and health check companies to establish areas the place metallic fatigue and stress cracking are growing. Ideally the valve could be repaired, but if a valve alternative is warranted, the seller also can present steering in evaluating bypass valve design types and choosing the most suitable choice for the particular utility.
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