Turbine bypass valves are one of the most tough functions in an influence plant. Keeping these valves working efficiently and avoiding unexpected failures is important for plant operation.
When engineers are requested to record probably the most difficult management valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high pressure drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many vegetation are probably to ignore these valves until they fail, creating unplanned outages, misplaced production, and excessive prices for reactive maintenance. This article provides instructed methods of inspection to anticipate and mitigate points beforehand, and it presents improve alternate options ought to a valve must be repaired or changed.
Strictly Severe Service
Severe service management valves are used in the most difficult installations inside process crops. These installations commonly embody cavitating, erosive, corrosive, noisy, excessive pressure, excessive temperature, excessive stress drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those course of conditions; yet, they want to respond flawlessly and remain leak free when closed.
As their identify 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 utterly 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 occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting doubtlessly moist steam, and by guaranteeing solely acceptable steam circumstances and flows make their method to the turbine itself.
1. Depending on the facility plant design, several turbine bypass valves could additionally be employed to instantly shunt steam around a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to avoid gear damage because of overpressure and high temperature, so the turbine bypass valve instantly opens to maintain move via the system.
As it operates, a turbine makes use of steam to carry out work, reducing outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the pressure, however the exit steam will remain quite superheated, potentially destroying downstream tools. To avoid that scenario, turbine bypass valves both incorporate a water injection system within the valve physique, or make use of 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 severe service circumstances. While the plant is in operation, these valves must stay tightly closed to avoid losing vitality. When a turbine journey happens, the bypass valves should reply instantly, exposing them to rapid temperature changes and requiring them to cross very excessive flows at excessive stress drops, creating excessive noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that virtually each turbine bypass valve will finally fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access places, are usually welded in place, and tend to be heavily insulated. As a result, they are typically ignored till problems begin to floor. Steam leakage by way of the valve is usually the primary symptom noticed, but much more important and doubtlessly harmful damage can happen.
The excessive stress drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can lead to catastrophic failure beneath stress.
2. Over time, the high vibrations and thermal shock skilled by turbine bypass valves will fatigue the metal and create cracks in both the valve itself and the encircling piping. Courtesy: Emerson
To keep away from this problem, every turbine bypass valve and its related piping ought to be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect steel fatigue problems that might be developing however are not yet visually apparent. If เกจ์วัดแรงดันน้ำมันเครื่อง lacks the knowledge or gear to carry out these inspections, the valve vendor or approved representative may be utilized to perform turbine bypass well being verify companies (Figure 3).
3. A routine turbine bypass valve well being verify and full inspection is strongly advised. Metal fatigue can develop in numerous welds located on the valve itself, and in the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, buyer connection welds, diffuser welds, and surrounding piping can be inspected to determine any developing issues earlier than the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from problems associated to quenching and cracking.
There are additionally threats to trim components, such as seat floor erosion. One widespread expression of such erosion, specifically on the plug, is also recognized as “gear toothing.” This occurs more generally in flow down valves, the place the steam accelerates by way of the cage holes and then strikes the trim/seat space instantly, causing excessive put on and decreasing service life. Over time, this high-velocity steam, which might comprise water throughout startup situations and magnetite throughout any condition, will erode and harm the valve seat and trim (Figure 4).
four. Flow down valve physique designs are vulnerable to trim damage as the wet, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like gentle goods and spray nozzles can normally get replaced comparatively easily and at minimal value. Trim parts that need to be changed due to heavy wear, similar to gear toothing, can be fairly expensive, especially if their delivery needs to be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue growing in the valve body or diffuser, with excessive repair procedures, or maybe a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service had been installed during the heyday of combined cycle plant construction from 1998 to 2004. Most of these valves and piping techniques are displaying their age and sometimes have important signs of steel fatigue. Others have plenty of life left in them but may achieve 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 upgrade is when there have been repeated failures with a welded diffuser. In this instance, a removable two-in-one seat diffuser will reduce required upkeep.
If a turbine bypass valve should be replaced, plant personnel are strongly inspired to look past a direct alternative and evaluate potential enhancements. A particularly important item to assume about 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 arrangement is ideal for places with low overhead clearance, and it locations the actuator nearer to the deck or floor for simple access, nevertheless it creates numerous long-term operational problems. Gravity tends to result in elevated and uneven put on on trim elements, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) were the primary option a couple of a long time ago for turbine bypass valves. New flow up valve designs (right) may be installed with out piping modifications in most cases, offering extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for circulate down is vertical mounting of the actuator. This association ends in decreased and more even put on, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that replacement alternatives are typically for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead house is on the market with current horizontal actuator flow down designs, a move up design can simply meet the prevailing face-to-face requirements of the previous valve, while decreasing a few of the wear widespread to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from move all the means down to move up, this change could be made without modifications to present piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life because the steam is moving relatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates considerably, however that power dissipates into the large body cavity of the valve as an alternative of putting the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease because they pass over the seat and accelerate later because the steam moves via the small trim holes, greatly 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 shown above (Figure 4 right), the seating floor on the seat ring and plug remain relatively undamaged, allowing the valve to shut off completely, the required mode in normal operation.
7. These footage of circulate up trim clearly show the erosive impression of high-velocity steam, but in contrast to the circulate down trim damage proven beforehand, this valve has no harm to the seating surface and can nonetheless shut off tightly. Courtesy: Emerson
The flow up valve fashion design nonetheless provides the identical control responsiveness and flow capability, but the new flow arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised items of engineered gear that must be rigorously specified, put in, and maintained. It is due to this fact sensible to be proactive by reaching out to trusted experts prior to purchase as they’ll help with valve sizing and specs based mostly on particular working circumstances.
For present installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time because the plant turbine bypass valves have been fully inspected, it might be sensible to consult your turbine bypass valve vendor for help and support.
Many of these vendors provide full inspection and well being examine services to identify areas the place steel fatigue and stress cracking are developing. Ideally the valve could be repaired, but if a valve alternative is warranted, the vendor also can provide steering in evaluating bypass valve design kinds and selecting the greatest choice for the particular software.
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