Turbine bypass valves are one of the difficult purposes in a power plant. Keeping these valves working effectively and avoiding surprising failures is important for plant operation.
When engineers are requested to record probably the most tough control valve applications, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive strain drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many plants are inclined to ignore these valves until they fail, creating unplanned outages, misplaced production, and high costs for reactive upkeep. This article offers advised methods of inspection to anticipate and mitigate issues beforehand, and it offers improve options should a valve need to be repaired or replaced.
Strictly Severe Service
Severe service management valves are utilized in essentially the most tough installations within process crops. These installations generally embody cavitating, erosive, corrosive, noisy, excessive stress, excessive temperature, excessive pressure drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those course of situations; but, they need to respond flawlessly and stay leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, as nicely as when a turbine journeys off-line (Figure 1). In normal operation the valves are fully 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 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 wet steam, and by making certain solely appropriate steam situations and flows make their way to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves may be employed to instantly shunt steam round a turbine should it trip offline. Courtesy: Emerson
Should a turbine journey, the steam should proceed flowing to keep away from equipment injury as a end result of overpressure and excessive temperature, so the turbine bypass valve immediately opens to take care of move via the system.
As it operates, a turbine makes use of steam to carry out work, lowering outlet steam temperature and strain. When a turbine bypass valve opens, it’ll drop the strain, however the exit steam will remain fairly superheated, probably destroying downstream equipment. To keep away from that situation, turbine bypass valves both incorporate a water injection system within the valve physique, or employ a separate water injection desuperheater just downstream, in either case to decrease the exit steam temperature.
As a 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 vitality. When a turbine trip happens, the bypass valves must respond instantly, exposing them to fast temperature changes and requiring them to move very excessive flows at high stress drops, creating high noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that just about each turbine bypass valve will in the end fail in some way. Unfortunately, many of those valves are installed in difficult-to-access places, are usually welded in place, and tend to be heavily insulated. As a outcome, they are typically ignored until issues begin to surface. Steam leakage through the valve is normally the first symptom observed, but rather more important and doubtlessly harmful damage can occur.
The extreme pressure drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can outcome in catastrophic failure underneath strain.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in both the valve itself and the encompassing piping. Courtesy: Emerson
To avoid this downside, every turbine bypass valve and its associated piping ought to be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect metallic fatigue issues which may be growing however usually are not yet visually apparent. If a plant lacks the knowledge or gear to carry out these inspections, the valve vendor or licensed representative may be utilized to carry out turbine bypass well being check companies (Figure 3).
three. A routine turbine bypass valve well being check and full inspection is strongly advised. Metal fatigue can develop in varied welds located on the valve itself, and within the inlet, outlet, and water supply 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 determine any creating problems before the tools is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to keep away from problems associated 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 often referred to as “gear toothing.” This occurs extra commonly in circulate down valves, the place the steam accelerates by way of the cage holes and then strikes the trim/seat space directly, inflicting extreme wear and lowering service life. Over time, this high-velocity steam, which can comprise water throughout startup situations and magnetite throughout any situation, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim damage as the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common maintenance items like soft goods and spray nozzles can often be replaced relatively simply and at minimal value. Trim components that must be replaced as a result of heavy wear, such as gear toothing, could be fairly pricey, particularly if their delivery must be expedited. The worst-case situation for a turbine bypass valve is metal fatigue developing in the valve physique or diffuser, with extreme repair procedures, or perhaps a complete valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service had been installed in the course of the heyday of combined cycle plant building from 1998 to 2004. Most of those valves and piping techniques are displaying their age and infrequently have important signs of metal fatigue. เกจวัดแรงดันออกซิเจนราคา have loads of life left in them however may benefit from upgrades in technology. Technology and practices have improved, and upgrading to the newest sealing expertise 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 instance, a detachable two-in-one seat diffuser will scale back required maintenance.
If a turbine bypass valve should be replaced, plant personnel are strongly inspired to look past a direct replacement and evaluate potential improvements. A notably essential merchandise to contemplate when looking at alternative and improvement alternatives 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 arrangement is right for areas with low overhead clearance, and it places the actuator closer to the deck or ground for simple access, but it creates numerous long-term operational issues. 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) have been the main option a quantity of many years ago for turbine bypass valves. New flow up valve designs (right) could also be put in with out piping modifications in most situations, providing prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for move down is vertical mounting of the actuator. This association leads to reduced and more even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement opportunities are sometimes for horizontally mounted actuators, and the two designs aren’t readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead space is on the market with existing horizontal actuator move down designs, a circulate up design can easily meet the prevailing face-to-face requirements of the previous valve, whereas lowering a few of the wear frequent to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from circulate all the means down to flow up, this alteration could be made without modifications to current piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life as a outcome of the steam is transferring 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 vitality dissipates into the large body cavity of the valve instead of hanging 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 cross over the seat and speed up later because 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 displaying erosion, however when compared to the circulate down trim shown above (Figure 4 right), the seating surface on the seat ring and plug stay comparatively undamaged, allowing the valve to shut off utterly, the required mode in normal operation.
7. These footage of circulate up trim clearly present the erosive impact of high-velocity steam, however in distinction to the move down trim harm proven beforehand, this valve has no damage to the seating floor and may nonetheless shut off tightly. Courtesy: Emerson
The flow up valve style design still offers the same control responsiveness and circulate capacity, but 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 pieces of engineered tools that should be fastidiously specified, put in, and maintained. It is therefore clever to be proactive by reaching out to trusted experts prior to buy as they can help with valve sizing and specs primarily based on specific working conditions.
For present installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time for the reason that plant turbine bypass valves have been fully inspected, it would be wise to seek the guidance of your turbine bypass valve vendor for assist and support.
Many of these vendors supply full inspection and health check companies to identify areas where metallic fatigue and stress cracking are creating. Ideally the valve could be repaired, but if a valve substitute is warranted, the vendor can even provide steerage in evaluating bypass valve design kinds and selecting the best suited choice for the particular software.
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