Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most troublesome functions in a power plant. Keeping these valves working effectively and avoiding sudden failures is crucial for plant operation.
When engineers are asked to list the most difficult management valve functions, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive pressure drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many crops are most likely to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and high costs for reactive upkeep. This article supplies instructed strategies of inspection to anticipate and mitigate points beforehand, and it provides upgrade alternate options ought to a valve need to be repaired or changed.
Strictly Severe Service
Severe service management valves are utilized in probably the most troublesome installations within course of crops. These installations commonly embrace cavitating, erosive, corrosive, noisy, high pressure, excessive temperature, excessive stress drop, or high velocity media. Turbine bypass valves are exposed to many of those course of circumstances; yet, they have to reply flawlessly and stay leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam generators during 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 of the steam via a turbine. During startup, bypass valves divert steam away from the turbine till the properties and conditions of the steam are applicable 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 doubtlessly wet steam, and by guaranteeing only appropriate steam conditions and flows make their way to the turbine itself.
1. Depending on the facility plant design, a quantity of turbine bypass valves may be employed to immediately shunt steam around a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to keep away from gear damage due to overpressure and excessive temperature, so the turbine bypass valve immediately opens to maintain up move through the system.
As it operates, a turbine uses steam to carry out work, reducing outlet steam temperature and strain. When a turbine bypass valve opens, it will drop the pressure, however the exit steam will remain quite superheated, potentially destroying downstream tools. To avoid that situation, turbine bypass valves either incorporate a water injection system within the valve physique, or employ a separate water injection desuperheater simply downstream, in either case to lower the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of extreme service situations. While the plant is in operation, these valves must stay tightly closed to avoid losing vitality. When a turbine journey occurs, the bypass valves should reply instantly, exposing them to speedy temperature adjustments and requiring them to cross very high 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 nearly each turbine bypass valve will in the end fail in some way. Unfortunately, many of these valves are put in in difficult-to-access places, are sometimes welded in place, and tend to be heavily insulated. As a result, they are typically ignored until issues start to floor. Steam leakage through the valve is usually the first symptom observed, but rather more significant and doubtlessly harmful harm can occur.
The extreme strain drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metal in the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can lead to catastrophic failure underneath pressure.
2. Over time, the excessive vibrations and thermal shock skilled 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, each turbine bypass valve and its associated piping must be routinely inspected. Some type of non-destructive examination should be periodically employed to detect steel fatigue problems that could be growing however usually are not yet visually obvious. If Double lacks the information or gear to perform these inspections, the valve vendor or authorized representative could also be utilized to carry out turbine bypass health verify services (Figure 3).
three. A routine turbine bypass valve health check 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 physique and water manifold, customer connection welds, diffuser welds, and surrounding piping could be inspected to identify any growing issues earlier than the gear is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to keep away from problems related to quenching and cracking.
There are also threats to trim parts, such as seat floor erosion. One common expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This happens extra generally in circulate down valves, where the steam accelerates through the cage holes and then strikes the trim/seat space immediately, inflicting excessive wear and decreasing service life. Over time, this high-velocity steam, which can contain water throughout startup circumstances and magnetite throughout any condition, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve physique designs are susceptible to trim harm because the wet, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep items like delicate items and spray nozzles can normally get replaced comparatively simply and at minimal value. Trim parts that must be changed due to heavy put on, similar to gear toothing, could be fairly costly, particularly if their supply needs to be expedited. The worst-case scenario for a turbine bypass valve is metallic fatigue growing in the valve physique or diffuser, with excessive restore procedures, or even a full valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service were installed in the course of the heyday of combined cycle plant construction from 1998 to 2004. Most of these valves and piping systems are displaying their age and often have important indicators of metallic fatigue. Others have loads of life left in them however may benefit from upgrades in know-how. Technology and practices have improved, and upgrading to the most recent sealing expertise could inject new life into the valve.
Another instance of a technology upgrade is when there have been repeated failures with a welded diffuser. In this occasion, a detachable two-in-one seat diffuser will reduce required upkeep.
If a turbine bypass valve should be changed, plant personnel are strongly encouraged to look beyond a direct replacement and evaluate potential improvements. A significantly important merchandise to contemplate when taking a look 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 ideal for places with low overhead clearance, and it locations the actuator nearer to the deck or ground for simple entry, nevertheless it creates a selection of long-term operational issues. 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 primary option a few many years in the past for turbine bypass valves. New move up valve designs (right) may be put in without piping modifications in most cases, offering extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for flow down is vertical mounting of the actuator. This arrangement results in reduced and more even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that alternative opportunities are typically for horizontally mounted actuators, and the two designs aren’t readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead area is out there with current horizontal actuator circulate down designs, a move up design can simply meet the prevailing face-to-face necessities of the previous valve, while reducing a few of the wear widespread to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from circulate right down to move up, this alteration could be made without modifications to present piping.
Flow up valves (Figure 5, right) considerably prolong seat and trim service life as a end result of the steam is moving comparatively 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 significantly, however that vitality dissipates into the large physique cavity of the valve as an alternative of striking the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower as a end result of they pass over the seat and speed up later because the steam moves via the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is actually showing erosion, however when compared to the move down trim proven above (Figure four right), the seating floor on the seat ring and plug remain comparatively undamaged, allowing the valve to shut off fully, the required mode in normal operation.
7. These photos of move up trim clearly present the erosive impact of high-velocity steam, however unlike the circulate down trim injury shown previously, this valve has no injury to the seating surface and might nonetheless shut off tightly. Courtesy: Emerson
The move up valve fashion design nonetheless provides the identical control responsiveness and flow capability, but the model new move arrangement dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered tools that must be fastidiously specified, installed, and maintained. It is therefore wise to be proactive by reaching out to trusted specialists prior to buy as they will help with valve sizing and specs based on specific operating circumstances.
For current installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time since the plant turbine bypass valves have been totally inspected, it would be clever to seek the assistance of your turbine bypass valve vendor for assist and support.
Many of those distributors offer full inspection and health examine providers to determine areas where metal fatigue and stress cracking are growing. Ideally the valve can be repaired, but when a valve alternative is warranted, the vendor also can present steerage in evaluating bypass valve design kinds and selecting the best choice for the particular utility.
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