Turbine Bypass Valve Challenges

Turbine bypass valves are one of the troublesome purposes in an influence plant. Keeping these valves working effectively and avoiding surprising failures is important for plant operation.
When engineers are requested to list probably the most tough management valve functions, turbine bypass valves are invariably talked about. Frequent thermal cycles, high stress drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many crops are likely to ignore these valves till they fail, creating unplanned outages, misplaced production, and excessive prices for reactive maintenance. This article supplies instructed methods of inspection to anticipate and mitigate issues beforehand, and it presents improve alternatives ought to a valve need to be repaired or changed.
Strictly Severe Service
Severe service control valves are used in the most difficult installations inside process vegetation. These installations commonly include cavitating, erosive, corrosive, noisy, high stress, excessive temperature, high strain drop, or excessive velocity media. Turbine bypass valves are exposed to many of those course of situations; yet, they have to respond flawlessly and remain 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 trips off-line (Figure 1). In normal operation the valves are completely closed, forcing all the steam through 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 happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting doubtlessly moist steam, and by guaranteeing solely applicable steam situations and flows make their way 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 trip offline. Courtesy: Emerson
Should a turbine trip, the steam should proceed flowing to keep away from equipment injury due to overpressure and excessive temperature, so the turbine bypass valve immediately opens to hold up move through the system.
As it operates, a turbine uses steam to perform work, lowering outlet steam temperature and stress. When a turbine bypass valve opens, it will drop the strain, but the exit steam will stay fairly superheated, doubtlessly destroying downstream equipment. To keep away from that situation, 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 extreme service conditions. While the plant is in operation, these valves should stay tightly closed to avoid losing energy. When a turbine journey happens, the bypass valves should respond immediately, exposing them to rapid temperature modifications and requiring them to cross very excessive flows at excessive pressure drops, creating excessive noise and doubtlessly excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that just about every turbine bypass valve will finally fail in some way. Unfortunately, many of those valves are installed in difficult-to-access locations, are usually welded in place, and tend to be heavily insulated. As a result, they are typically ignored till issues begin to surface. Steam leakage via the valve is often the first symptom observed, however far more important and probably dangerous injury can happen.
เกจวัดแรงดันดิจิตอลราคา drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue may end up in catastrophic failure beneath pressure.
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 encompassing piping. Courtesy: Emerson
To keep away from this drawback, every turbine bypass valve and its related piping should be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect steel fatigue problems which might be developing however usually are not but visually obvious. If a plant lacks the information or tools to perform these inspections, the valve vendor or authorized representative could also be utilized to perform turbine bypass well being check companies (Figure 3).
three. A routine turbine bypass valve well being examine 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 may be inspected to identify any creating problems before the tools is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are also threats to trim elements, corresponding to seat floor erosion. One common expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This happens more generally in flow down valves, the place the steam accelerates via the cage holes after which strikes the trim/seat space immediately, causing extreme put on and lowering service life. Over time, this high-velocity steam, which may contain water during startup conditions and magnetite during any condition, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve physique designs are prone to trim harm because the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like delicate items and spray nozzles can often get replaced relatively simply and at minimal price. Trim components that must be changed as a end result of heavy wear, such as gear toothing, may be quite pricey, especially if their delivery must be expedited. pressure gauge น้ำมัน -case scenario for a turbine bypass valve is metallic fatigue creating in the valve physique or diffuser, with excessive restore procedures, or even a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were installed in the course of the heyday of combined cycle plant development from 1998 to 2004. Most of these valves and piping systems are showing their age and often have significant indicators of steel fatigue. Others have plenty of life left in them however may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing technology might 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 scale back required upkeep.
If a turbine bypass valve should be replaced, plant personnel are strongly inspired to look past a direct replacement and evaluate potential improvements. A significantly essential item to suppose about when looking at replacement and improvement 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 good for areas with low overhead clearance, and it locations the actuator closer to the deck or floor for straightforward access, nevertheless it creates a number of long-term operational problems. Gravity tends to end in increased and uneven wear on trim elements, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the primary option a couple of a long time ago for turbine bypass valves. New move up valve designs (right) could additionally be put in without piping modifications in most cases, offering prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This arrangement results in reduced and extra even put on, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute opportunities are usually for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable with out vital piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead house is on the market with existing horizontal actuator flow down designs, a circulate up design can easily meet the existing face-to-face requirements of the previous valve, while lowering a few of the put on widespread to circulate down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from flow down to move up, this change could be made without modifications to existing piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life as a result of the steam is shifting comparatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes through the small holes of the cage, it accelerates significantly, however that power dissipates into the large physique 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 move over the seat and speed up later as the steam strikes through the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is actually displaying erosion, but when compared to the flow down trim shown above (Figure four right), the seating surface on the seat ring and plug stay comparatively undamaged, allowing the valve to close off utterly, the required mode in normal operation.
7. These footage of flow up trim clearly show the erosive impact of high-velocity steam, but in contrast to the flow down trim injury proven beforehand, this valve has no harm to the seating floor and might still shut off tightly. Courtesy: Emerson
The move up valve type design nonetheless provides the same control responsiveness and circulate capability, but the new flow association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered tools that have to be fastidiously specified, put in, and maintained. It is due to this fact clever to be proactive by reaching out to trusted experts prior to buy as they will help with valve sizing and specs based mostly on particular working circumstances.
For current installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while because the plant turbine bypass valves have been totally inspected, it will be sensible to consult your turbine bypass valve vendor for assist and assist.
Many of those distributors provide full inspection and well being verify companies to identify areas where steel fatigue and stress cracking are developing. Ideally the valve may be repaired, but if a valve alternative is warranted, the vendor also can present steerage in evaluating bypass valve design types and choosing the greatest choice for the particular utility.
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