Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most tough functions in a power plant. Keeping these valves working effectively and avoiding sudden failures is critical for plant operation.
When engineers are asked to listing the most tough control valve functions, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive stress drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many plants are probably to ignore these valves till they fail, creating unplanned outages, lost manufacturing, and high costs for reactive maintenance. This article supplies suggested strategies of inspection to anticipate and mitigate issues beforehand, and it presents improve alternate options should a valve must be repaired or replaced.
Strictly Severe Service
Severe service control valves are used in the most troublesome installations within process crops. These installations generally embody cavitating, erosive, corrosive, noisy, high strain, high temperature, high stress drop, or high velocity media. Turbine bypass valves are uncovered to many of these course of situations; yet, they want to respond flawlessly and remain leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, as properly as when a turbine journeys off-line (Figure 1). In normal operation the valves are fully closed, forcing all of the steam through a turbine. During startup, bypass valves divert steam away from the turbine until the properties and circumstances of the steam are acceptable 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 probably moist steam, and by ensuring only appropriate steam conditions and flows make their method to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves could additionally be employed to instantly shunt steam around a turbine should it trip offline. Courtesy: Emerson
Should a turbine trip, the steam should proceed flowing to avoid gear harm because of overpressure and high temperature, so the turbine bypass valve instantly opens to maintain flow by way of the system.
As it operates, a turbine uses steam to carry out work, decreasing outlet steam temperature and strain. When a turbine bypass valve opens, it will drop the pressure, however the exit steam will stay quite superheated, doubtlessly destroying downstream equipment. To keep away from that situation, turbine bypass valves either incorporate a water injection system in the valve physique, or make use of a separate water injection desuperheater just downstream, in both case to lower the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of extreme service circumstances. While the plant is in operation, these valves should stay tightly closed to keep away from wasting vitality. When a turbine journey occurs, the bypass valves must respond instantly, exposing them to rapid temperature modifications and requiring them to move very high flows at high stress drops, creating excessive noise and doubtlessly excessive vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly every turbine bypass valve will in the end fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access locations, are usually welded in place, and are typically heavily insulated. As a end result, they are often ignored until issues start to surface. Steam leakage via the valve is usually the first symptom seen, however much more important and doubtlessly dangerous injury can occur.
The extreme pressure drops invariably generate high 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 metal fatigue can lead to catastrophic failure beneath pressure.
2. Over time, the excessive vibrations and thermal shock skilled by turbine bypass valves will fatigue the metal and create cracks in both the valve itself and the encircling piping. Courtesy: Emerson
To keep away from this downside, each turbine bypass valve and its associated piping must be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect metallic fatigue issues that could be creating but aren’t yet visually obvious. If a plant lacks the data or equipment to carry out these inspections, the valve vendor or approved consultant could additionally be utilized to carry out turbine bypass health verify services (Figure 3).
3. A routine turbine bypass valve health check 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 supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, customer connection welds, diffuser welds, and surrounding piping can be inspected to establish any creating issues before the tools is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from problems related to quenching and cracking.
There are also threats to trim components, 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 circulate down valves, the place the steam accelerates through the cage holes after which strikes the trim/seat area immediately, causing extreme wear and reducing service life. Over time, this high-velocity steam, which might contain water throughout startup situations and magnetite during any condition, will erode and injury the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim damage because the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like gentle items and spray nozzles can usually be replaced relatively simply and at minimal value. Trim components that have to be changed because of heavy wear, similar to gear toothing, can be fairly pricey, especially if their delivery needs to be expedited. The worst-case scenario for a turbine bypass valve is metallic fatigue creating within the valve physique or diffuser, with extreme restore procedures, or even a full valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service had been put in through the heyday of combined cycle plant development from 1998 to 2004. Most of these valves and piping methods are displaying their age and often have vital indicators of metallic fatigue. Others have loads of life left in them but could benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the latest sealing technology may inject new life into the valve.
Another example of a expertise improve is when there have been repeated failures with a welded diffuser. In this instance, a detachable two-in-one seat diffuser will scale back required upkeep.
If a turbine bypass valve have to be changed, plant personnel are strongly inspired to look past a direct replacement and consider potential improvements. A notably important item to consider when looking at alternative and enchancment 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 association is right for locations with low overhead clearance, and it places the actuator nearer to the deck or floor for simple entry, nevertheless it creates a variety of long-term operational issues. Gravity tends to lead to 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) had been the principle possibility a few a long time ago for turbine bypass valves. New circulate up valve designs (right) may be installed with out piping modifications in most instances, providing extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for move down is vertical mounting of the actuator. This association leads to decreased and more even put on, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that alternative 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 adequate overhead area is available with existing horizontal actuator circulate down designs, a move up design can simply meet the prevailing face-to-face necessities of the outdated valve, whereas decreasing a number of the put on frequent to flow down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from move all the means down to move up, this alteration can be made without modifications to present piping.
Flow up valves (Figure 5, right) considerably extend 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 เพรสเชอร์เกจวัดแรงดันน้ำ moves by way of the small holes of the cage, it accelerates considerably, however that vitality dissipates into the big 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 much lower as a result of they move over the seat and speed up later because the steam moves through the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is actually showing erosion, but when compared to 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 close off completely, the required mode in regular operation.
7. These photos of circulate up trim clearly show the erosive impact of high-velocity steam, but in contrast to the move down trim harm shown previously, this valve has no harm to the seating floor and may still shut off tightly. Courtesy: Emerson
The flow up valve fashion design still offers the identical control responsiveness and circulate capacity, but the new circulate arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered gear that have to be carefully specified, installed, and maintained. It is therefore clever to be proactive by reaching out to trusted consultants prior to purchase as they will help with valve sizing and specs primarily based on specific operating situations.
For present installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while for the rationale that plant turbine bypass valves have been fully inspected, it would be clever to consult your turbine bypass valve vendor for help and support.
Many of these distributors provide full inspection and well being examine services to establish areas where steel fatigue and stress cracking are growing. Ideally the valve may be repaired, but if a valve replacement is warranted, the vendor can even provide steering in evaluating bypass valve design types and choosing the most fitted choice for the particular software.
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