Turbine Bypass Valve Challenges

Turbine bypass valves are one of the difficult applications in an influence plant. Keeping these valves operating effectively and avoiding surprising failures is critical for plant operation.
When engineers are requested to listing probably the most tough control valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, high pressure drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many plants are inclined to ignore these valves until they fail, creating unplanned outages, lost production, and high costs for reactive upkeep. This article offers suggested strategies of inspection to anticipate and mitigate points beforehand, and it presents upgrade alternate options should a valve need to be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in probably the most tough installations inside course of crops. These installations commonly include cavitating, erosive, corrosive, noisy, high strain, excessive temperature, excessive stress drop, or high velocity media. Turbine bypass valves are uncovered to many of these process circumstances; yet, they must respond flawlessly and remain leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam turbines during plant startup and shutdown, as well as when a turbine trips off-line (Figure 1). In normal operation the valves are completely closed, forcing all 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 appropriate for sending it to the turbine. This process occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting potentially moist steam, and by ensuring solely acceptable steam situations and flows make their way to the turbine itself.
1. Depending on the facility plant design, a number of turbine bypass valves may be employed to instantly shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to avoid equipment harm as a result of overpressure and excessive temperature, so the turbine bypass valve instantly opens to keep up move through the system.
As it operates, a turbine uses steam to carry out work, lowering outlet steam temperature and strain. When a turbine bypass valve opens, it will drop the stress, however the exit steam will remain quite superheated, probably destroying downstream tools. To keep away from that state of affairs, 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 end result, turbine bypass valves face an ideal storm of severe service circumstances. While the plant is in operation, these valves should remain tightly closed to avoid losing vitality. When a turbine trip happens, the bypass valves must reply immediately, exposing them to fast temperature changes and requiring them to cross very excessive flows at high strain drops, creating high noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that just about each turbine bypass valve will in the end fail in some way. Unfortunately, many of these valves are installed in difficult-to-access places, are typically welded in place, and are typically closely insulated. As a result, they are often ignored till problems start to floor. Steam leakage via the valve is usually the first symptom observed, but much more important and probably dangerous harm can happen.
The extreme stress drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can outcome in catastrophic failure underneath strain.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in both the valve itself and the encircling piping. Courtesy: Emerson
To keep away from this problem, every turbine bypass valve and its associated piping should be routinely inspected. Some sort of non-destructive examination should be periodically employed to detect metal fatigue issues that might be creating but aren’t but visually apparent. If a plant lacks the data or equipment to carry out these inspections, the valve vendor or approved representative could also be utilized to perform turbine bypass health verify companies (Figure 3).
three. A routine turbine bypass valve health verify and full inspection is strongly suggested. Metal fatigue can develop in various welds located on the valve itself, and within 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 establish any developing problems earlier than the equipment 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 components, such as seat floor erosion. One frequent expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This occurs extra generally in flow down valves, where the steam accelerates via the cage holes after which strikes the trim/seat space directly, causing excessive wear and decreasing service life. Over time, this high-velocity steam, which can comprise water during startup situations and magnetite during any condition, will erode and injury the valve seat and trim (Figure 4).
4. Flow down valve body designs are vulnerable to trim harm 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 relatively easily and at minimal price. Trim elements that must be replaced because of heavy put on, such as gear toothing, may be quite costly, especially if their supply needs to be expedited. The worst-case situation for a turbine bypass valve is metallic fatigue developing in the valve body or diffuser, with excessive repair procedures, or perhaps a full valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service had been installed through the heyday of mixed cycle plant development from 1998 to 2004. ตัววัดแรงดันน้ำมัน of these valves and piping systems are exhibiting their age and sometimes have vital signs of metallic fatigue. Others have plenty of life left in them but could benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the most recent sealing technology 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 reduce required maintenance.
If a turbine bypass valve have to be changed, plant personnel are strongly inspired to look beyond a direct substitute and evaluate potential improvements. A notably necessary merchandise to contemplate when taking a glance at alternative and improvement opportunities 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 areas with low overhead clearance, and it places the actuator nearer to the deck or ground for easy entry, but it creates a number of long-term operational problems. Gravity tends to result in elevated and uneven put on on trim parts, and the side-mounted actuator is vulnerable to response issues.
5. Downward flowing valves with horizontal actuators (left) have been the principle choice a quantity of decades ago for turbine bypass valves. New circulate up valve designs (right) could also be put in with out piping modifications in most instances, providing extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for move down is vertical mounting of the actuator. This arrangement results in reduced and extra even wear, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that substitute alternatives are sometimes for horizontally mounted actuators, and the 2 designs aren’t readily interchangeable without important piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead area is on the market with existing horizontal actuator move down designs, a move up design can easily meet the present face-to-face requirements of the old valve, while decreasing a variety of the put on widespread to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from flow all the means down to move up, this modification could be made with out modifications to current piping.
Flow up valves (Figure 5, right) considerably prolong seat and trim service life as a end result of the steam is shifting relatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes by way of the small holes of the cage, it accelerates considerably, but that vitality dissipates into the big physique cavity of the valve as an alternative of hanging the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease as a result of they move over the seat and accelerate later because the steam moves via the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is definitely showing erosion, but when in comparability with the flow down trim proven above (Figure 4 right), the seating floor on the seat ring and plug stay relatively undamaged, allowing the valve to shut off fully, the required mode in normal operation.
7. These photos of flow up trim clearly present the erosive influence of high-velocity steam, however unlike the move down trim harm shown previously, this valve has no damage to the seating floor and might still shut off tightly. Courtesy: Emerson
The circulate up valve type design nonetheless provides the same control responsiveness and move capability, but the new move association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised items of engineered gear that have to be carefully specified, installed, and maintained. It is due to this fact sensible to be proactive by reaching out to trusted consultants prior to purchase as they can assist with valve sizing and specifications based on particular operating conditions.
For existing 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 fully inspected, it might be clever to consult your turbine bypass valve vendor for assist and support.
Many of those vendors provide full inspection and well being check services to establish areas where steel fatigue and stress cracking are growing. Ideally the valve could be repaired, but if a valve substitute is warranted, the vendor can even present guidance in evaluating bypass valve design styles and selecting the most suitable choice for the specific application.
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