Turbine Bypass Valve Challenges

Turbine bypass valves are one of the troublesome applications in an influence plant. Keeping these valves working effectively and avoiding sudden failures is critical for plant operation.
When engineers are asked to record essentially the most difficult control valve functions, turbine bypass valves are invariably talked about. Frequent thermal cycles, high strain drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many plants tend to ignore these valves till they fail, creating unplanned outages, lost production, and high costs for reactive upkeep. This article supplies instructed methods of inspection to anticipate and mitigate issues beforehand, and it provides improve options ought to a valve must be repaired or changed.
Strictly Severe Service
Severe service control valves are used in the most troublesome installations inside process plants. These installations commonly embody cavitating, erosive, corrosive, noisy, high strain, excessive temperature, high pressure drop, or excessive velocity media. Turbine bypass valves are exposed to many of these process conditions; but, 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 journeys off-line (Figure 1). In regular 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 process happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting probably wet steam, and by ensuring solely appropriate steam situations and flows make their approach to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves could additionally be employed to immediately shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam should proceed flowing to keep away from gear injury as a outcome of overpressure and excessive temperature, so the turbine bypass valve instantly opens to take care of move by way of the system.
As it operates, a turbine uses steam to perform work, decreasing outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the stress, however the exit steam will stay quite superheated, potentially destroying downstream equipment. To keep away from that state of affairs, turbine bypass valves both incorporate a water injection system in the valve body, or employ a separate water injection desuperheater just downstream, in both case to lower the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of severe service situations. While the plant is in operation, these valves must stay tightly closed to avoid wasting energy. When a turbine journey happens, the bypass valves must reply instantly, exposing them to fast temperature adjustments and requiring them to cross very high flows at excessive pressure drops, creating high noise and probably extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly every turbine bypass valve will in the end fail indirectly. Unfortunately, many of those valves are put in in difficult-to-access locations, are sometimes welded in place, and are typically heavily insulated. As a outcome, they are usually ignored till problems begin to surface. Steam leakage via the valve is normally the first symptom observed, however rather more important and potentially dangerous injury can occur.
The extreme stress drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can end result in catastrophic failure beneath 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. เกจวัดความดันpressuregauge of non-destructive examination ought to be periodically employed to detect steel fatigue problems that may be creating however are not yet visually obvious. If a plant lacks the information or gear to perform these inspections, the valve vendor or approved representative may be utilized to perform turbine bypass health verify services (Figure 3).
3. A routine turbine bypass valve health examine and full inspection is strongly advised. Metal fatigue can develop in various welds positioned on the valve itself, and within the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, buyer connection welds, diffuser welds, and surrounding piping can be inspected to identify any developing issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to keep away from issues related to quenching and cracking.
There are additionally threats to trim parts, similar to seat floor erosion. One common expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This happens extra generally in move down valves, where the steam accelerates through the cage holes after which strikes the trim/seat space directly, causing extreme wear and decreasing service life. Over time, this high-velocity steam, which can comprise water during startup circumstances and magnetite throughout any condition, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim injury as the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like delicate goods and spray nozzles can often get replaced relatively easily and at minimal cost. Trim elements that must be replaced as a result of heavy wear, such as gear toothing, can be fairly costly, particularly if their delivery must be expedited. The worst-case state of affairs for a turbine bypass valve is metallic fatigue developing 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 have been put in through the heyday of mixed cycle plant building from 1998 to 2004. Most of these valves and piping methods are showing their age and infrequently 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 latest sealing know-how could inject new life into the valve.
Another example of a know-how improve is when there have been repeated failures with a welded diffuser. In this occasion, a removable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve must be changed, plant personnel are strongly encouraged to look beyond a direct alternative and evaluate potential improvements. A particularly important merchandise to contemplate 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 association is ideal for locations with low overhead clearance, and it places the actuator closer to the deck or floor for simple entry, nevertheless it creates a number of long-term operational problems. Gravity tends to lead to increased and uneven put on on trim parts, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the primary possibility a few decades in the past for turbine bypass valves. New move up valve designs (right) may be installed with out piping modifications in most instances, providing prolonged 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 association ends in decreased and extra even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute alternatives are sometimes for horizontally mounted actuators, and the two designs usually are not readily interchangeable with out vital piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead house is out there with present horizontal actuator circulate down designs, a move up design can easily meet the present face-to-face necessities of the old valve, whereas decreasing a few of the wear common to flow down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from circulate down to circulate up, this modification 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 transferring comparatively slowly as it passes over the plug and seat (Figure 6). As the steam moves by way of the small holes of the cage, it accelerates significantly, however that vitality dissipates into the big physique cavity of the valve as a substitute of hanging 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 outcome of they pass over the seat and accelerate later as the steam strikes by way of the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is actually exhibiting erosion, but when compared to the circulate down trim proven above (Figure 4 right), the seating floor on the seat ring and plug remain relatively undamaged, allowing the valve to close off completely, the required mode in regular operation.
7. These footage of move up trim clearly present the erosive influence of high-velocity steam, however unlike the flow down trim damage shown previously, this valve has no damage to the seating surface and may still shut off tightly. Courtesy: Emerson
The circulate up valve type design still supplies the same control responsiveness and circulate capacity, but the new circulate arrangement dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered tools that should be rigorously specified, installed, and maintained. It is due to this fact clever to be proactive by reaching out to trusted consultants prior to buy as they will help with valve sizing and specifications based mostly on particular operating circumstances.
For existing 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 sensible to seek the guidance of your turbine bypass valve vendor for assist and support.
Many of those vendors provide full inspection and health examine providers to determine areas where steel fatigue and stress cracking are developing. Ideally the valve could be repaired, but when a valve replacement is warranted, the vendor can even provide guidance in evaluating bypass valve design types and selecting the best choice for the precise application.
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