Turbine Bypass Valve Challenges

Turbine bypass valves are some of the difficult applications in a power plant. Keeping these valves operating efficiently and avoiding surprising failures is crucial for plant operation.
When engineers are requested to listing probably the most tough control valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high 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, misplaced production, and excessive costs for reactive upkeep. This article supplies suggested methods of inspection to anticipate and mitigate points beforehand, and it presents improve alternatives ought to a valve must be repaired or replaced.
Strictly Severe Service
Severe service management valves are utilized in the most troublesome installations inside course of plants. These installations generally include cavitating, erosive, corrosive, noisy, excessive stress, excessive temperature, high strain drop, or excessive velocity media. Turbine bypass valves are exposed to many of these process conditions; but, they want to reply flawlessly and stay leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam turbines throughout plant startup and shutdown, as nicely as when a turbine trips off-line (Figure 1). In normal operation the valves are utterly closed, forcing all the steam by way of a turbine. During เกจวัดแรงดันน้ำ4หุน , bypass valves divert steam away from the turbine till the properties and conditions of the steam are appropriate for sending it to the turbine. This course of happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting doubtlessly 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 quantity of turbine bypass valves could additionally be employed to instantly shunt steam round a turbine should it journey offline. Courtesy: Emerson
Should a turbine journey, the steam should proceed flowing to keep away from tools damage due to overpressure and excessive temperature, so the turbine bypass valve instantly opens to take care of move by way of the system.
As pressure gauge ยี่ห้อ tk operates, a turbine uses steam to carry out work, lowering outlet steam temperature and stress. When a turbine bypass valve opens, it’ll drop the pressure, but the exit steam will stay fairly superheated, potentially destroying downstream gear. To avoid that state of affairs, turbine bypass valves either incorporate a water injection system within the valve physique, or employ a separate water injection desuperheater just downstream, in both case to decrease the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of severe service circumstances. While the plant is in operation, these valves should stay tightly closed to keep away from losing power. When a turbine journey happens, the bypass valves should reply immediately, exposing them to fast temperature adjustments and requiring them to cross very high flows at high strain drops, creating excessive noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that virtually every turbine bypass valve will finally fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access locations, are typically welded in place, and are usually closely insulated. As a end result, they are often ignored till problems start to floor. Steam leakage through the valve is normally the primary symptom seen, however far more important and potentially dangerous harm can happen.
The excessive strain drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metallic within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue may end up in catastrophic failure underneath pressure.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in each the valve itself and the encircling piping. Courtesy: Emerson
To avoid this downside, every turbine bypass valve and its related piping should be routinely inspected. Some sort of non-destructive examination should be periodically employed to detect steel fatigue problems that could be growing however aren’t but visually apparent. If a plant lacks the data or gear to carry out these inspections, the valve vendor or authorized representative may be utilized to perform turbine bypass well being examine companies (Figure 3).
three. A routine turbine bypass valve well being check and full inspection is strongly advised. Metal fatigue can develop in various welds situated on the valve itself, and within the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, customer connection welds, diffuser welds, and surrounding piping may be inspected to establish any developing problems earlier than the equipment is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from issues associated to quenching and cracking.
There are also threats to trim elements, such as seat surface erosion. One frequent expression of such erosion, particularly on the plug, is often referred to as “gear toothing.” This happens extra generally in move down valves, where the steam accelerates via the cage holes after which strikes the trim/seat area immediately, causing excessive put on and lowering service life. Over time, this high-velocity steam, which may include water during startup conditions and magnetite throughout any condition, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve body designs are vulnerable to trim harm because the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like gentle items and spray nozzles can usually be replaced comparatively simply and at minimal price. Trim elements that need to be changed as a outcome of heavy wear, corresponding to gear toothing, can be quite costly, particularly if their delivery needs to be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue creating within the valve body or diffuser, with excessive repair procedures, or perhaps a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service had been installed in the course of the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping techniques are displaying their age and infrequently have vital indicators of metal fatigue. Others have loads of life left in them however may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing technology may 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 detachable two-in-one seat diffuser will cut back required maintenance.
If a turbine bypass valve should be replaced, plant personnel are strongly encouraged to look beyond a direct substitute and evaluate potential enhancements. A particularly necessary 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 move down trim design paired with a horizontal actuator (Figure 5, left). This association is good for locations with low overhead clearance, and it places the actuator nearer to the deck or ground for straightforward entry, but it creates a selection of long-term operational problems. Gravity tends to result in increased and uneven wear on trim elements, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the primary possibility a few many years ago for turbine bypass valves. New circulate up valve designs (right) may be installed with out piping modifications in most situations, offering prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for move down is vertical mounting of the actuator. This arrangement results in decreased and extra even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement opportunities are typically for horizontally mounted actuators, and the two designs usually are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead space is on the market with existing horizontal actuator flow down designs, a circulate up design can simply meet the present face-to-face necessities of the outdated valve, while lowering some of the wear widespread to move down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from circulate down to circulate up, this alteration may be made with out modifications to current piping.
Flow up valves (Figure 5, right) significantly prolong seat and trim service life as a end result of the steam is transferring comparatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes through the small holes of the cage, it accelerates considerably, however that energy dissipates into the big body cavity of the valve as a substitute of hanging the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower as a outcome of they move 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 move up trim (Figure 7) is definitely exhibiting erosion, however when in comparability with the circulate 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 regular operation.
7. These footage of circulate up trim clearly present the erosive influence of high-velocity steam, however not like the circulate down trim harm shown previously, this valve has no harm to the seating floor and may still shut off tightly. Courtesy: Emerson
The circulate up valve type design nonetheless offers the identical control responsiveness and circulate capability, but the brand new flow association dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered tools that should be fastidiously specified, installed, and maintained. It is therefore sensible to be proactive by reaching out to trusted specialists prior to buy as they will help with valve sizing and specifications based on specific working situations.
For existing installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time for the reason that plant turbine bypass valves have been totally inspected, it will be clever to seek the assistance of your turbine bypass valve vendor for help and support.
Many of these vendors offer full inspection and health check companies to identify areas where metallic fatigue and stress cracking are developing. Ideally the valve can be repaired, but if a valve substitute is warranted, the seller can even present steerage in evaluating bypass valve design styles and choosing the greatest choice for the specific application.
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