Turbine Bypass Valve Challenges

Turbine bypass valves are one of the tough functions in an influence plant. Keeping these valves operating efficiently and avoiding surprising failures is crucial for plant operation.
When engineers are asked to list probably the most difficult control valve functions, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive stress drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many vegetation tend to ignore these valves till they fail, creating unplanned outages, lost manufacturing, and excessive costs for reactive maintenance. This article offers instructed methods of inspection to anticipate and mitigate points beforehand, and it offers upgrade alternatives should a valve must be repaired or changed.
Strictly Severe Service
Severe service control valves are used in the most troublesome installations within course of plants. These installations generally embrace cavitating, erosive, corrosive, noisy, high stress, high temperature, excessive strain drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these process circumstances; yet, they must reply flawlessly and stay leak free when closed.
As their identify 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 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 situations of the steam are acceptable for sending it to the turbine. This course of occurs in reverse at shutdown. Using เกจ์วัดแรงดัน at startup and shutdown helps to guard the turbine by diverting doubtlessly wet steam, and by making certain solely 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 instantly shunt steam round a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must proceed flowing to avoid equipment harm due to overpressure and excessive temperature, so the turbine bypass valve instantly opens to take care of flow via the system.
As it operates, a turbine makes use of steam to carry out work, reducing outlet steam temperature and stress. When a turbine bypass valve opens, it’s going to drop the pressure, but the exit steam will remain quite superheated, probably destroying downstream tools. To keep away from that scenario, turbine bypass valves both incorporate a water injection system in the valve physique, or make use of a separate water injection desuperheater just downstream, in either case to lower the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of extreme service situations. While the plant is in operation, these valves should stay tightly closed to keep away from wasting power. When a turbine trip happens, the bypass valves must respond immediately, exposing them to fast temperature adjustments and requiring them to move very high flows at high pressure drops, creating high noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly each turbine bypass valve will ultimately fail in some way. Unfortunately, many of these valves are put in in difficult-to-access places, are sometimes welded in place, and are typically closely insulated. As a outcome, they are usually ignored until issues start to surface. Steam leakage by way of the valve is often the primary symptom observed, but far more important and doubtlessly dangerous damage can occur.
The excessive pressure drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metallic within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue may find yourself in catastrophic failure under stress.
2. Over time, the high 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 drawback, every turbine bypass valve and its associated piping should be routinely inspected. Some sort of non-destructive examination ought to be periodically employed to detect metallic fatigue problems that might be growing however usually are not yet visually apparent. If a plant lacks the knowledge or equipment to perform these inspections, the valve vendor or licensed consultant could also be utilized to perform turbine bypass well being examine providers (Figure 3).
three. A routine turbine bypass valve well being verify 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 provide 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 determine any growing issues earlier than the tools is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid problems associated to quenching and cracking.
There are additionally threats to trim elements, corresponding to seat surface erosion. One common expression of such erosion, particularly on the plug, is also identified as “gear toothing.” This happens extra commonly in circulate down valves, the place the steam accelerates by way of the cage holes after which strikes the trim/seat space instantly, inflicting excessive put on and decreasing service life. Over time, this high-velocity steam, which can comprise water during startup conditions and magnetite throughout any situation, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve physique designs are vulnerable to trim injury as the moist, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common maintenance items like delicate goods and spray nozzles can normally get replaced relatively simply and at minimal price. Trim parts that must be replaced as a result of heavy wear, such as gear toothing, could be quite costly, especially if their supply must be expedited. The worst-case scenario for a turbine bypass valve is metal fatigue growing in the valve physique or diffuser, with extreme repair procedures, or even a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service had been put in through the heyday of combined cycle plant construction from 1998 to 2004. Most of those valves and piping systems are showing their age and often have important signs of metal fatigue. Others have loads of life left in them but may acquire advantage from upgrades in know-how. Technology and practices have improved, and upgrading to the most recent sealing know-how might inject new life into the valve.
Another instance of a know-how improve 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 have to be replaced, plant personnel are strongly encouraged to look past a direct alternative and evaluate potential enhancements. A notably important item to consider when taking a look at substitute 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 arrangement is good for places with low overhead clearance, and it locations the actuator nearer to the deck or floor for simple access, but it creates a quantity of long-term operational issues. Gravity tends to lead to increased and uneven wear on trim elements, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the principle choice a couple of decades in the past for turbine bypass valves. New flow up valve designs (right) could also be put in with out piping modifications in most cases, offering prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for flow down is vertical mounting of the actuator. This arrangement ends in decreased and extra even put on, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that substitute opportunities are usually for horizontally mounted actuators, and the two designs aren’t readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is out there with current horizontal actuator move down designs, a flow up design can easily meet the prevailing face-to-face necessities of the old valve, while decreasing a variety of the put on common to move down designs with a horizontal actuator, corresponding to gear toothing. Because the trim just switches from circulate right down to move up, this alteration could be made without modifications to existing piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life as a end result of the steam is moving comparatively slowly as it passes over the plug and seat (Figure 6). As the steam moves via the small holes of the cage, it accelerates considerably, however that vitality dissipates into the massive physique cavity of the valve as an alternative of striking the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease as a end result of they pass over the seat and speed up later as the steam moves by way of the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is certainly showing erosion, but when compared to the flow down trim proven above (Figure four right), the seating floor on the seat ring and plug remain relatively undamaged, allowing the valve to shut off fully, the required mode in regular operation.
7. These footage of circulate up trim clearly present the erosive impact of high-velocity steam, but not like the flow down trim damage proven previously, this valve has no harm to the seating surface and can nonetheless shut off tightly. เกจวัดแรงดันต่ำ : Emerson
The flow up valve fashion design still provides the same management responsiveness and circulate capability, but the brand new circulate arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized pieces of engineered equipment that have to be rigorously specified, installed, and maintained. It is subsequently sensible to be proactive by reaching out to trusted specialists prior to buy as they will assist with valve sizing and specifications 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 absolutely inspected, it might be sensible to consult your turbine bypass valve vendor for help and assist.
Many of these distributors offer full inspection and health check services to identify areas the place steel fatigue and stress cracking are growing. Ideally the valve may be repaired, but when a valve substitute is warranted, the vendor also can present steering in evaluating bypass valve design types and selecting the greatest option for the precise application.

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