Turbine bypass valves are one of the most difficult purposes in a power plant. Keeping these valves operating efficiently and avoiding sudden failures is crucial 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, excessive pressure drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many plants tend to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and excessive prices for reactive upkeep. This article supplies advised strategies of inspection to anticipate and mitigate points beforehand, and it offers improve alternate options should a valve need to be repaired or changed.
Strictly Severe Service
Severe service management valves are used in essentially the most difficult installations within process vegetation. These installations commonly embrace cavitating, erosive, corrosive, noisy, excessive stress, high temperature, excessive stress drop, or excessive velocity media. Turbine bypass valves are exposed to many of those process conditions; yet, they must reply flawlessly and remain leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, as nicely as when a turbine journeys off-line (Figure 1). In normal operation the valves are fully closed, forcing all of the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine until the properties and conditions of the steam are appropriate for sending it to the turbine. This process happens in reverse at shutdown. Using หลักการทำงานของเกจ์วัดแก๊ส at startup and shutdown helps to guard the turbine by diverting potentially wet steam, and by ensuring solely applicable steam conditions and flows make their approach to the turbine itself.
1. Depending on the ability plant design, a number of turbine bypass valves may be employed to immediately shunt steam round a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to avoid tools damage because of overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain up move via the system.
As it operates, a turbine makes use of steam to perform work, decreasing outlet steam temperature and strain. When a turbine bypass valve opens, it’ll drop the pressure, however the exit steam will stay quite superheated, doubtlessly destroying downstream gear. To keep away from that scenario, 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 extreme service situations. While the plant is in operation, these valves must remain tightly closed to avoid wasting energy. When a turbine journey happens, the bypass valves should reply instantly, exposing them to speedy temperature changes and requiring them to pass very excessive flows at excessive stress drops, creating high noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that nearly every turbine bypass valve will in the end fail indirectly. Unfortunately, many of those valves are installed in difficult-to-access areas, are usually welded in place, and are typically heavily insulated. As a outcome, they’re often ignored until issues start to surface. Steam leakage via the valve is usually the primary symptom seen, however much more vital and doubtlessly harmful harm can happen.
The excessive strain drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metallic within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue may end up in catastrophic failure beneath stress.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in both the valve itself and the surrounding piping. Courtesy: Emerson
To keep away from this problem, each turbine bypass valve and its related piping ought to be routinely inspected. Some kind of non-destructive examination should be periodically employed to detect metallic fatigue problems that may be developing but usually are not yet visually apparent. If a plant lacks the knowledge or tools to perform these inspections, the valve vendor or authorized representative could also be utilized to perform turbine bypass health check companies (Figure 3).
3. A routine turbine bypass valve well being check and full inspection is strongly suggested. Metal fatigue can develop in numerous welds positioned 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 determine any growing problems before the equipment is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid problems associated to quenching and cracking.
There are additionally threats to trim components, such as seat floor erosion. One common expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This occurs extra generally in move down valves, the place the steam accelerates by way of the cage holes and then strikes the trim/seat area immediately, causing excessive put on and reducing service life. Over time, this high-velocity steam, which may comprise water throughout startup conditions and magnetite during any condition, will erode and harm the valve seat and trim (Figure 4).
four. Flow down valve body designs are susceptible to trim damage because the moist, erosive steam passes by way of 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 cost. Trim components that have to be changed as a end result of heavy put on, corresponding to gear toothing, may be fairly pricey, particularly if their supply needs to be expedited. The worst-case situation for a turbine bypass valve is metallic fatigue developing within the valve body or diffuser, with extreme restore procedures, or even a full valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been put in through the heyday of mixed cycle plant development from 1998 to 2004. Most of those valves and piping techniques are showing their age and sometimes have significant indicators of metal fatigue. Others have plenty of life left in them however could benefit from upgrades in technology. Technology and practices have improved, and upgrading to the most recent sealing know-how might 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 occasion, a removable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve have to be changed, plant personnel are strongly inspired to look past a direct substitute and evaluate potential enhancements. A notably necessary merchandise to consider when taking a look at replacement and enchancment alternatives is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a circulate down trim design paired with a horizontal actuator (Figure 5, left). This arrangement is ideal for locations with low overhead clearance, and it places the actuator closer to the deck or ground for simple entry, but 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 prone to response problems.
5. Downward flowing valves with horizontal actuators (left) were the primary choice a couple of decades ago for turbine bypass valves. New circulate up valve designs (right) may be put in without piping modifications in most instances, providing prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for move down is vertical mounting of the actuator. This arrangement ends in reduced and more even put on, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute opportunities are sometimes for horizontally mounted actuators, and the 2 designs are not readily interchangeable without important piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead house is on the market with present horizontal actuator flow down designs, a move up design can easily meet the present face-to-face requirements of the previous valve, while decreasing a few of the wear common to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from flow down to circulate up, this modification can be made without modifications to present piping.
Flow up valves (Figure 5, right) significantly lengthen 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 moves through the small holes of the cage, it accelerates significantly, but that energy dissipates into the massive body 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 decrease as a result of they pass over the seat and accelerate later as the steam strikes by way of the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is certainly showing erosion, however when compared to the move down trim proven above (Figure four right), the seating surface on the seat ring and plug remain comparatively undamaged, allowing the valve to shut off completely, the required mode in normal operation.
7. These footage of flow up trim clearly show the erosive impression of high-velocity steam, however not like the move down trim harm proven previously, this valve has no injury to the seating floor and might still shut off tightly. Courtesy: Emerson
The move up valve type design still supplies the same management responsiveness and move capability, but the model new circulate association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized items of engineered equipment that must be rigorously specified, installed, and maintained. It is therefore wise to be proactive by reaching out to trusted specialists prior to purchase as they’ll help with valve sizing and specs primarily based on particular working conditions.
For existing installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while because the plant turbine bypass valves have been absolutely inspected, it might be sensible to seek the assistance of your turbine bypass valve vendor for assist and support.
Many of those vendors offer full inspection and well being examine services to identify areas the place steel fatigue and stress cracking are creating. Ideally the valve can be repaired, but when a valve substitute is warranted, the seller also can provide guidance in evaluating bypass valve design kinds and choosing the best choice for the precise utility.
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