Turbine bypass valves are one of the difficult applications in a power plant. Keeping these valves working effectively and avoiding surprising failures is important for plant operation.
When engineers are asked to list the most tough management valve functions, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high pressure drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many vegetation are inclined to ignore these valves till they fail, creating unplanned outages, lost manufacturing, and high costs for reactive upkeep. This article supplies suggested strategies of inspection to anticipate and mitigate points beforehand, and it presents upgrade alternate options should a valve have to be repaired or changed.
Strictly Severe Service
Severe service control valves are used in the most difficult installations within process crops. These installations generally embrace cavitating, erosive, corrosive, noisy, high strain, high temperature, excessive pressure drop, or high velocity media. Turbine bypass valves are exposed to many of these course of situations; but, they have to reply 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, as well as when a turbine trips off-line (Figure 1). In regular operation the valves are completely 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 applicable for sending it to the turbine. This course of occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting potentially moist steam, and by ensuring solely applicable steam situations and flows make their method to the turbine itself.
1. Depending on the power plant design, a quantity of turbine bypass valves may be employed to immediately shunt steam round a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to keep away from gear damage as a outcome of overpressure and high temperature, so the turbine bypass valve immediately opens to maintain up move via the system.
As it operates, a turbine uses steam to perform work, decreasing outlet steam temperature and pressure. When a turbine bypass valve opens, it will drop the pressure, however the exit steam will remain quite superheated, doubtlessly destroying downstream equipment. To avoid that scenario, turbine bypass valves both incorporate a water injection system in the valve body, or make use of a separate water injection desuperheater just downstream, in either case to lower the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of extreme service circumstances. While the plant is in operation, these valves should remain tightly closed to avoid wasting energy. When a turbine journey occurs, the bypass valves must reply instantly, exposing them to fast temperature modifications and requiring them to move very high flows at excessive pressure drops, creating high noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that virtually every turbine bypass valve will ultimately fail indirectly. Unfortunately, many of those valves are installed in difficult-to-access areas, are sometimes welded in place, and are typically closely insulated. As ราคาเกจวัดแรงดันน้ำ , they’re typically ignored until issues begin to floor. Steam leakage through the valve is normally the primary symptom noticed, however rather more vital and potentially harmful damage can occur.
The excessive pressure drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue may find yourself in catastrophic failure beneath strain.
2. Over time, the high vibrations and thermal shock skilled by turbine bypass valves will fatigue the metal and create cracks in each the valve itself and the encompassing piping. Courtesy: Emerson
To avoid this problem, each turbine bypass valve and its associated piping should be routinely inspected. Some kind of non-destructive examination must be periodically employed to detect metallic fatigue issues that could be growing but are not but visually apparent. If a plant lacks the information or gear to perform these inspections, the valve vendor or authorized consultant could additionally be utilized to perform turbine bypass well being verify services (Figure 3).
3. A routine turbine bypass valve well being verify and full inspection is strongly advised. Metal fatigue can develop in varied welds situated 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, buyer connection welds, diffuser welds, and surrounding piping could be inspected to determine any growing issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are also threats to trim elements, corresponding to seat surface erosion. pressure gauge nuova fima ราคา of such erosion, specifically on the plug, is sometimes called “gear toothing.” This happens extra generally in move down valves, the place the steam accelerates through the cage holes after which strikes the trim/seat space instantly, causing extreme put on and lowering service life. Over time, this high-velocity steam, which may comprise water during startup conditions and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve physique designs are prone to trim damage as the wet, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like delicate goods and spray nozzles can often get replaced comparatively easily and at minimal value. Trim elements that have to be changed because of heavy put on, such as gear toothing, could be quite costly, especially if their supply must be expedited. The worst-case situation for a turbine bypass valve is metal fatigue creating within the valve physique or diffuser, with extreme restore procedures, or perhaps a complete valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were installed during the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping techniques are exhibiting their age and sometimes have vital signs of metallic fatigue. Others have loads of life left in them but may acquire advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the latest sealing know-how might inject new life into the valve.
Another instance of a know-how upgrade is when there have been repeated failures with a welded diffuser. In this occasion, a detachable 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 alternative and consider potential improvements. A significantly important merchandise 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 areas with low overhead clearance, and it places the actuator nearer to the deck or ground for simple entry, but it creates a variety of long-term operational problems. Gravity tends to lead to elevated and uneven wear on trim parts, and the side-mounted actuator is vulnerable to response issues.
5. Downward flowing valves with horizontal actuators (left) were the principle choice a couple of many years ago for turbine bypass valves. New move up valve designs (right) could also be put in with out piping modifications in most cases, offering extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for circulate down is vertical mounting of the actuator. This arrangement ends in reduced and more even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that substitute alternatives are typically for horizontally mounted actuators, and the 2 designs aren’t readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead house is on the market with present horizontal actuator circulate down designs, a circulate up design can easily meet the existing face-to-face requirements of the previous valve, whereas decreasing some of the wear widespread to circulate down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from flow down to flow up, this modification can be made without modifications to existing piping.
Flow up valves (Figure 5, right) significantly prolong seat and trim service life as a end result of the steam is moving comparatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates considerably, but that vitality dissipates into the massive physique cavity of the valve as a substitute of hanging 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 speed up later as the steam strikes through the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is actually exhibiting erosion, however when in comparison with the flow down trim shown above (Figure four right), the seating floor on the seat ring and plug remain relatively undamaged, allowing the valve to close off fully, the required mode in regular operation.
7. These pictures of move up trim clearly present the erosive influence of high-velocity steam, but unlike the flow 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 fashion design nonetheless supplies the identical management responsiveness and circulate capacity, but the model new circulate arrangement dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized items of engineered equipment that have to be fastidiously specified, installed, and maintained. It is therefore sensible to be proactive by reaching out to trusted experts prior to buy as they will help with valve sizing and specs based mostly on particular working conditions.
For present installations where 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 might be wise to consult your turbine bypass valve vendor for assist and help.
Many of those distributors supply full inspection and well being examine companies to establish areas the place steel fatigue and stress cracking are developing. Ideally the valve could be repaired, but when a valve alternative is warranted, the vendor also can provide guidance in evaluating bypass valve design kinds and selecting the best choice for the specific application.