Turbine bypass valves are one of the most troublesome applications in a power plant. Keeping these valves operating efficiently and avoiding unexpected failures is critical for plant operation.
When engineers are requested to list essentially the most troublesome control valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive stress drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, many plants are inclined to ignore these valves till they fail, creating unplanned outages, misplaced production, and excessive costs for reactive maintenance. This article provides advised strategies of inspection to anticipate and mitigate points beforehand, and it provides improve alternatives should a valve must be repaired or changed.
Strictly Severe Service
Severe service management valves are utilized in essentially the most troublesome installations inside process plants. These installations commonly embrace cavitating, erosive, corrosive, noisy, excessive stress, high temperature, excessive strain drop, or excessive velocity media. Turbine bypass valves are exposed to many of those course of situations; yet, they need to reply flawlessly and stay 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 trips 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 situations of the steam are appropriate for sending it to the turbine. This process happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting potentially wet steam, and by guaranteeing solely applicable steam circumstances and flows make their method to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves may be employed to immediately shunt steam around a turbine should it trip offline. Courtesy: Emerson
Should a turbine trip, the steam must proceed flowing to keep away from gear harm because of overpressure and high temperature, so the turbine bypass valve immediately opens to maintain circulate 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’ll drop the stress, however the exit steam will remain fairly superheated, probably destroying downstream equipment. To avoid that situation, turbine bypass valves either incorporate a water injection system within the valve physique, or make use of a separate water injection desuperheater just downstream, in both case to decrease the exit steam temperature.
As a end result, turbine bypass valves face an ideal storm of severe service conditions. While the plant is in operation, these valves should remain tightly closed to keep away from losing power. When a turbine journey occurs, the bypass valves must respond immediately, exposing them to speedy temperature modifications and requiring them to move very high flows at high stress drops, creating high noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that nearly every turbine bypass valve will in the end fail ultimately. Unfortunately, many of those valves are put in in difficult-to-access places, are typically welded in place, and are typically closely insulated. As a outcome, they are usually ignored until problems begin to floor. Steam leakage via the valve is normally the first symptom observed, however rather more significant and probably harmful injury can occur.
The extreme strain drops invariably generate excessive noise and excessive vibration. Over ราคาเกจวัดแรงดัน , coupled with frequent temperature modifications, fatigue the metal in the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can outcome in catastrophic failure beneath stress.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic 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 associated piping should be routinely inspected. Some type of non-destructive examination ought to be periodically employed to detect metallic fatigue issues that might be developing however are not yet visually obvious. If a plant lacks the information or gear to perform these inspections, the valve vendor or licensed consultant may be utilized to perform turbine bypass health check services (Figure 3).
three. A routine turbine bypass valve well being examine 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 could be inspected to determine any developing issues earlier than the tools is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are additionally threats to trim components, similar to seat floor erosion. เกจวัดแรง of such erosion, specifically on the plug, is also known as “gear toothing.” This happens more commonly in move down valves, where the steam accelerates by way of the cage holes after which strikes the trim/seat space instantly, inflicting extreme wear and decreasing service life. Over time, this high-velocity steam, which might comprise water during startup conditions and magnetite throughout any situation, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve physique designs are susceptible to trim injury because the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like soft items and spray nozzles can often get replaced comparatively easily and at minimal value. Trim parts that need to be changed as a outcome of heavy put on, similar to gear toothing, can be fairly pricey, particularly if their supply needs to be expedited. The worst-case situation for a turbine bypass valve is metal fatigue developing within the valve physique or diffuser, with extreme restore procedures, or even a complete valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service were installed through the heyday of mixed cycle plant building from 1998 to 2004. Most of those valves and piping systems are displaying their age and infrequently have significant signs of metal fatigue. Others have loads of life left in them but may gain advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the latest sealing technology could inject new life into the valve.
Another instance of a technology 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 upkeep.
If a turbine bypass valve have to be replaced, plant personnel are strongly encouraged to look past a direct replacement and evaluate potential enhancements. A notably essential item to contemplate when looking at alternative 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 ideal for locations with low overhead clearance, and it locations the actuator nearer to the deck or floor for straightforward entry, however it creates a variety of long-term operational issues. Gravity tends to result in elevated and uneven put on on trim parts, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the principle option a couple of decades ago for turbine bypass valves. New circulate up valve designs (right) could additionally be installed with out piping modifications in most situations, providing extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for move down is vertical mounting of the actuator. This arrangement leads to lowered and extra even put on, longer runs between maintenance 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 existing horizontal actuator move down designs, a circulate up design can simply meet the prevailing face-to-face necessities of the previous valve, while lowering some of the wear frequent to move down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from flow right down to circulate up, this modification 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 shifting 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 considerably, however that vitality dissipates into the large physique cavity of the valve as an alternative of putting the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower as a outcome of they pass over the seat and speed up later as the steam moves through the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is actually showing erosion, however when in comparability with the flow down trim shown above (Figure four right), the seating surface on the seat ring and plug remain relatively undamaged, permitting the valve to close off fully, the required mode in regular operation.
7. These footage of flow up trim clearly present the erosive impression of high-velocity steam, however unlike the move down trim injury shown beforehand, this valve has no harm to the seating surface and can nonetheless shut off tightly. Courtesy: Emerson
The flow up valve style design still offers the same control responsiveness and circulate capability, however the brand new flow arrangement dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised items of engineered equipment that have to be rigorously specified, put in, and maintained. It is due to this fact wise to be proactive by reaching out to trusted specialists prior to purchase as they will assist with valve sizing and specs based mostly on specific working situations.
For present installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while since the plant turbine bypass valves have been totally inspected, it would be clever to consult your turbine bypass valve vendor for help and assist.
Many of these vendors provide full inspection and health examine providers to determine areas the place steel fatigue and stress cracking are creating. Ideally the valve can be repaired, but if a valve alternative is warranted, the vendor can also present steerage in evaluating bypass valve design types and selecting the best suited choice for the specific application.
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