Turbine bypass valves are one of the most tough applications in a power plant. Keeping these valves working effectively and avoiding surprising failures is crucial for plant operation.
When engineers are asked to list essentially the most tough control valve functions, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high stress drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many plants are most likely to ignore these valves till they fail, creating unplanned outages, misplaced manufacturing, and excessive prices for reactive maintenance. This article provides suggested methods of inspection to anticipate and mitigate points beforehand, and it provides improve alternatives should a valve need to be repaired or replaced.
Strictly Severe Service
Severe service management valves are used in the most troublesome installations within process crops. These installations generally embrace cavitating, erosive, corrosive, noisy, high pressure, high temperature, high strain drop, or excessive velocity media. Turbine bypass valves are exposed to many of these process situations; but, they need 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, as well as when a turbine trips off-line (Figure 1). In regular operation the valves are completely closed, forcing all the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine till the properties and circumstances of the steam are acceptable for sending it to the turbine. This process occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting potentially wet steam, and by ensuring solely appropriate steam situations and flows make their method to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves could also be employed to immediately shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to keep away from tools damage due to overpressure and high temperature, so the turbine bypass valve instantly opens to maintain move by way of the system.
As it operates, a turbine makes use of steam to perform work, lowering outlet steam temperature and strain. When a turbine bypass valve opens, it’s going to drop the pressure, however the exit steam will remain fairly superheated, doubtlessly destroying downstream tools. To keep away from that scenario, 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 end result, turbine bypass valves face an ideal storm of severe service situations. While the plant is in operation, these valves should stay tightly closed to avoid losing energy. When a turbine trip occurs, the bypass valves must reply instantly, exposing them to rapid temperature adjustments and requiring them to cross very excessive flows at excessive pressure drops, creating high noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that nearly each turbine bypass valve will ultimately fail ultimately. Unfortunately, many of these valves are put in in difficult-to-access areas, are typically welded in place, and are usually closely insulated. As a end result, they are usually ignored until issues start to floor. Steam leakage by way of the valve is often the first symptom noticed, however much more important and doubtlessly dangerous injury can occur.
The extreme stress drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can result in catastrophic failure beneath stress.
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 surrounding piping. Courtesy: Emerson
To keep away from this downside, every turbine bypass valve and its associated piping must be routinely inspected. Some type of non-destructive examination must be periodically employed to detect metallic fatigue problems that may be growing however aren’t yet visually apparent. If a plant lacks the data or gear to perform these inspections, the valve vendor or authorized consultant could additionally be utilized to carry out turbine bypass health examine services (Figure 3).
three. A routine turbine bypass valve health verify and full inspection is strongly suggested. Metal fatigue can develop in varied welds positioned on the valve itself, and in the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, buyer connection welds, diffuser welds, and surrounding piping may be inspected to determine any developing problems earlier than the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from problems related to quenching and cracking.
There are additionally threats to trim parts, such as seat floor erosion. One frequent expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This happens extra commonly in circulate down valves, the place the steam accelerates through the cage holes after which strikes the trim/seat space immediately, inflicting extreme put on and reducing service life. Over time, this high-velocity steam, which may include water during startup conditions and magnetite throughout any condition, will erode and harm the valve seat and trim (Figure 4).
four. Flow down valve physique designs are prone to trim damage because the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common upkeep items like soft items and spray nozzles can usually get replaced comparatively easily and at minimal cost. Trim components that have to be changed as a end result of heavy wear, similar to gear toothing, can be quite pricey, especially if their delivery needs to be expedited. The worst-case scenario for a turbine bypass valve is metallic fatigue developing within the valve body or diffuser, with excessive repair procedures, or even a complete valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been put in in the course of the heyday of combined cycle plant building from 1998 to 2004. Most of these valves and piping methods are displaying their age and sometimes have significant signs of steel fatigue. Others have loads of life left in them but may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing technology might 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 occasion, a removable two-in-one seat diffuser will cut back required upkeep.
If เกจวัดไนโตรเจนราคา bypass valve must be changed, plant personnel are strongly encouraged to look past a direct substitute and consider potential improvements. A notably important item to consider when looking at replacement and enchancment opportunities 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 association is right for places with low overhead clearance, and it locations the actuator nearer to the deck or floor for easy access, however it creates a variety of long-term operational problems. Gravity tends to lead to elevated and uneven wear on trim elements, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) were the main possibility a number of a long time in the past for turbine bypass valves. New circulate up valve designs (right) may be put in without piping modifications in most cases, providing prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This association ends in reduced and more even put on, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that alternative alternatives are usually for horizontally mounted actuators, and the 2 designs are not readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is on the market with existing horizontal actuator flow down designs, a circulate up design can simply meet the existing face-to-face necessities of the old valve, whereas reducing some of the wear common to move down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from move right down to move up, this alteration may be made with out modifications to existing piping.
Flow up valves (Figure 5, right) significantly prolong seat and trim service life because the steam is moving comparatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes by way of the small holes of the cage, it accelerates considerably, but that vitality dissipates into the large physique cavity of the valve as a substitute of striking the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease as a result of they move over the seat and speed up later as 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 exhibiting erosion, however when in comparison with the flow down trim proven above (Figure 4 right), the seating floor on the seat ring and plug remain relatively undamaged, permitting the valve to shut off fully, the required mode in regular operation.
7. These pictures of move up trim clearly show the erosive influence of high-velocity steam, but in contrast to the move down trim harm proven previously, this valve has no harm to the seating floor and may nonetheless shut off tightly. Courtesy: Emerson
The move up valve fashion design nonetheless offers the identical management responsiveness and flow capacity, however the new circulate association dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised items of engineered gear that have to be carefully specified, installed, and maintained. It is due to this fact wise to be proactive by reaching out to trusted consultants prior to purchase as they can assist with valve sizing and specifications primarily based on specific operating conditions.
For current installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while for the reason that plant turbine bypass valves have been fully inspected, it would be sensible to seek the advice of your turbine bypass valve vendor for assist and support.
Many of those distributors supply full inspection and well being check services to establish areas where metal fatigue and stress cracking are creating. Ideally the valve can be repaired, but if a valve substitute is warranted, the seller can even provide steering in evaluating bypass valve design styles and selecting the best choice for the precise application.
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