Turbine Bypass Valve Challenges

Turbine bypass valves are one of the tough purposes in a power plant. Keeping these valves working effectively and avoiding sudden failures is critical for plant operation.
When engineers are asked to list probably the most tough management valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high pressure drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many vegetation are likely to ignore these valves till they fail, creating unplanned outages, misplaced manufacturing, and excessive prices for reactive upkeep. This article supplies advised methods of inspection to anticipate and mitigate points beforehand, and it provides improve alternatives ought to a valve must be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in essentially the most tough installations within course of vegetation. These installations generally embrace cavitating, erosive, corrosive, noisy, excessive strain, excessive temperature, excessive strain drop, or high velocity media. Turbine bypass valves are exposed to many of these course of conditions; yet, they have to respond flawlessly and stay leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, in addition to when a turbine journeys off-line (Figure 1). In regular 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 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 wet steam, and by ensuring only applicable steam conditions and flows make their method 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 trip offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to keep away from gear harm due to overpressure and high temperature, so the turbine bypass valve immediately opens to hold up flow by way of the system.
As it operates, a turbine makes use of steam to carry out work, decreasing outlet steam temperature and pressure. When a turbine bypass valve opens, it will drop the strain, however the exit steam will remain quite superheated, potentially destroying downstream equipment. To keep away from that situation, turbine bypass valves both incorporate a water injection system in the valve physique, or employ a separate water injection desuperheater simply downstream, in either case to decrease the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of severe service situations. While the plant is in operation, these valves should remain tightly closed to keep away from wasting power. When a turbine trip occurs, the bypass valves must reply instantly, exposing them to speedy temperature adjustments and requiring them to cross very high flows at high pressure drops, creating excessive noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that virtually every turbine bypass valve will finally fail in some way. Unfortunately, many of these valves are put in in difficult-to-access locations, are sometimes welded in place, and are typically heavily insulated. As a result, they’re often ignored till issues begin to surface. Steam leakage via the valve is often the first symptom seen, but far more important and probably harmful harm can occur.
The extreme strain drops invariably generate excessive noise and excessive 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 steel fatigue can lead to catastrophic failure underneath strain.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in both the valve itself and the encircling piping. เกจวัดแรงดันแก๊สlpgรถยนต์ : Emerson
To keep away from this downside, every turbine bypass valve and its associated piping ought to be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect steel fatigue problems which might be creating but aren’t but visually obvious. If a plant lacks the information or tools to perform these inspections, the valve vendor or approved representative could additionally be utilized to perform turbine bypass well being verify services (Figure 3).
3. A routine turbine bypass valve health examine and full inspection is strongly advised. Metal fatigue can develop in various welds positioned 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, customer connection welds, diffuser welds, and surrounding piping could be inspected to establish any developing issues before the gear is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid problems related to quenching and cracking.
There are additionally threats to trim elements, similar to seat surface erosion. One frequent expression of such erosion, particularly on the plug, is also identified as “gear toothing.” This happens extra generally in flow down valves, where the steam accelerates by way of the cage holes and then strikes the trim/seat space instantly, inflicting excessive put on and lowering service life. Over time, this high-velocity steam, which can include water throughout startup circumstances and magnetite throughout any situation, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim harm as the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like delicate goods and spray nozzles can often be replaced relatively easily and at minimal price. Trim components that need to be replaced as a result of heavy wear, similar to gear toothing, can be quite pricey, particularly if their supply needs to be expedited. The worst-case situation for a turbine bypass valve is steel fatigue creating within the valve body or diffuser, with extreme repair procedures, or perhaps a full valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been installed during the heyday of combined cycle plant construction from 1998 to 2004. Most of those valves and piping methods are displaying their age and infrequently have important signs of metallic fatigue. Others have plenty of life left in them however could benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the latest sealing expertise could inject new life into the valve.
Another example of a technology improve 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 encouraged to look beyond a direct alternative and consider potential improvements. A notably important item to consider when looking at substitute and enchancment 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 right for places with low overhead clearance, and it locations the actuator closer to the deck or floor for easy access, however it creates a variety of long-term operational problems. Gravity tends to result in elevated and uneven put on on trim elements, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) were the principle possibility a couple of a long time in the past for turbine bypass valves. New flow up valve designs (right) could additionally be installed with out piping modifications in most cases, providing prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for move down is vertical mounting of the actuator. This arrangement results in decreased and extra even put on, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute opportunities 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 house is out there with current horizontal actuator circulate down designs, a circulate up design can easily meet the prevailing face-to-face requirements of the previous valve, whereas reducing some of the put on widespread to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from move all the way down to move up, this change may be made with out modifications to existing piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life as a end result of the steam is transferring relatively slowly because it passes over the plug and seat (Figure 6). As the steam moves through the small holes of the cage, it accelerates significantly, however that power dissipates into the massive body cavity of the valve as an alternative of putting the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower as a end result of they move over the seat and speed up later because the steam moves via the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is actually showing erosion, however when compared to the move down trim shown above (Figure 4 right), the seating surface on the seat ring and plug stay comparatively undamaged, permitting the valve to shut off completely, the required mode in normal operation.
7. These pictures of move up trim clearly show the erosive influence of high-velocity steam, however in distinction to the move down trim harm proven beforehand, this valve has no harm to the seating surface and may still shut off tightly. Courtesy: Emerson
The move up valve style design still offers the identical management responsiveness and circulate capability, but the model new flow arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized items of engineered gear that have to be rigorously specified, put in, and maintained. It is therefore clever to be proactive by reaching out to trusted experts prior to buy as they can assist with valve sizing and specs primarily based on particular 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 totally inspected, it would be clever to consult your turbine bypass valve vendor for help and help.
Many of these distributors offer full inspection and well being verify providers to identify areas where metallic fatigue and stress cracking are creating. Ideally the valve could be repaired, but when a valve alternative is warranted, the vendor also can provide guidance in evaluating bypass valve design styles and choosing the best suited choice for the precise utility.

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