Turbine bypass valves are one of the troublesome functions in a power plant. Keeping these valves operating efficiently and avoiding unexpected failures is critical for plant operation.
When engineers are requested to list the most tough control valve purposes, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high stress drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many vegetation tend to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and excessive costs for reactive upkeep. This article offers suggested methods of inspection to anticipate and mitigate points beforehand, and it presents improve options should a valve have to be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in the most tough installations within process vegetation. These installations generally embody cavitating, erosive, corrosive, noisy, excessive stress, high temperature, high strain drop, or excessive velocity media. Turbine bypass valves are exposed to many of these process situations; yet, they have to respond 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, in addition to when a turbine journeys off-line (Figure 1). In normal operation the valves are fully closed, forcing all of the steam through a turbine. During startup, bypass valves divert steam away from the turbine till the properties and conditions of the steam are applicable 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 moist steam, and by making certain only acceptable steam situations and flows make their method to the turbine itself.
1. Depending on the facility plant design, several turbine bypass valves could also be employed to instantly shunt steam around a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to avoid equipment harm as a result of overpressure and excessive temperature, so the turbine bypass valve immediately opens to maintain move through 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 strain, but the exit steam will stay fairly superheated, potentially destroying downstream gear. To keep away from that situation, turbine bypass valves both incorporate a water injection system within the valve body, or employ a separate water injection desuperheater simply downstream, in either case to lower the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of extreme service conditions. While the plant is in operation, these valves must stay tightly closed to keep away from losing power. When a turbine trip occurs, the bypass valves must reply instantly, exposing them to fast temperature changes and requiring them to pass very excessive flows at excessive stress drops, creating excessive noise and potentially 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 in some way. Unfortunately, many of those valves are put in in difficult-to-access places, are sometimes welded in place, and tend to be heavily insulated. As a result, they’re typically ignored until problems begin to floor. Steam leakage by way of the valve is usually the primary symptom seen, but much more significant and probably harmful injury can occur.
The extreme stress drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metal in the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue may end up in catastrophic failure beneath strain.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in both the valve itself and the encircling piping. Courtesy: Emerson
To avoid this problem, every turbine bypass valve and its associated piping must be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect metal fatigue problems that might be creating but usually are not but visually obvious. If a plant lacks the information or gear to carry out these inspections, the valve vendor or authorized representative may be utilized to carry out turbine bypass well being verify companies (Figure 3).
3. A routine turbine bypass valve health check and full inspection is strongly advised. Metal fatigue can develop in various welds located 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 establish any growing issues before 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 also threats to trim parts, similar to seat floor erosion. One common expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This happens more commonly in move down valves, the place the steam accelerates via the cage holes and then strikes the trim/seat area instantly, causing extreme put on and decreasing service life. Over time, this high-velocity steam, which may comprise water throughout startup circumstances and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve physique designs are susceptible to trim injury as the moist, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like soft goods and spray nozzles can usually be replaced comparatively easily and at minimal value. Trim parts that have to be changed because of heavy wear, such as gear toothing, may be fairly pricey, particularly if their delivery needs to be expedited. The worst-case situation for a turbine bypass valve is steel fatigue growing in the valve physique or diffuser, with extreme repair procedures, or even a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service had been put in through the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping systems are exhibiting their age and sometimes have vital signs of steel fatigue. Others have loads of life left in them but may gain advantage from upgrades in technology. Technology and practices have improved, and upgrading to the most recent sealing expertise may 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 detachable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve have to be replaced, plant personnel are strongly inspired to look past a direct alternative and evaluate potential enhancements. A particularly important merchandise to consider when looking at substitute and enchancment opportunities is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a flow 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 simple access, nevertheless it creates numerous long-term operational problems. Gravity tends to result in elevated and uneven wear on trim parts, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the main choice a number of decades in the past for turbine bypass valves. New flow up valve designs (right) may be installed without piping modifications in most situations, offering extended 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 association leads to decreased and more even put on, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that substitute opportunities are sometimes for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead area is out there with present horizontal actuator circulate down designs, a move up design can easily meet the existing face-to-face requirements of the previous valve, while reducing a variety of the put on widespread to flow down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from circulate right down to move up, this change may be made with out modifications to present piping.
ความหมายของเครื่องวัดความดัน up valves (Figure 5, right) significantly prolong seat and trim service life as a result of the steam is moving comparatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates significantly, but that power dissipates into the large body cavity of the valve as an alternative of striking the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower as a outcome of they move over the seat and speed up later as the steam strikes through the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is actually showing erosion, but when in comparability with the flow down trim shown above (Figure four right), the seating floor on the seat ring and plug remain relatively undamaged, permitting the valve to close off fully, the required mode in normal operation.
7. These photos of circulate up trim clearly present the erosive impression of high-velocity steam, however in distinction to the move down trim damage proven beforehand, this valve has no injury to the seating floor and can still shut off tightly. Courtesy: Emerson
The flow up valve fashion design nonetheless provides the same control responsiveness and circulate capability, but the model new move association dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered equipment that must be rigorously specified, put in, and maintained. It is due to this fact clever to be proactive by reaching out to trusted experts prior to buy as they’ll help with valve sizing and specs based on specific operating circumstances.
For existing 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 totally inspected, it would be wise to seek the guidance of your turbine bypass valve vendor for help and assist.
Many of those distributors supply full inspection and well being examine providers to establish areas the place steel fatigue and stress cracking are creating. Ideally the valve may be repaired, but when a valve substitute is warranted, the vendor also can present steerage in evaluating bypass valve design kinds and selecting the finest option for the precise software.