pressure gauge bypass valves are some of the difficult applications in a power plant. Keeping these valves working efficiently and avoiding surprising failures is important for plant operation.
When engineers are asked to record essentially the most tough control valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high strain drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many crops are most likely to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and high costs for reactive upkeep. This article offers suggested methods of inspection to anticipate and mitigate points beforehand, and it offers upgrade options should a valve must be repaired or replaced.
Strictly Severe Service
Severe service control valves are used in essentially the most troublesome installations within process plants. These installations generally include cavitating, erosive, corrosive, noisy, high stress, high temperature, excessive pressure drop, or high velocity media. Turbine bypass valves are uncovered to many of those course of circumstances; yet, they need 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, in addition to when a turbine trips off-line (Figure 1). In regular operation the valves are completely closed, forcing all the steam via 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 process happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting doubtlessly wet 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 instantly shunt steam round a turbine should it trip offline. Courtesy: Emerson
Should a turbine journey, the steam must proceed flowing to avoid gear damage because of overpressure and excessive temperature, so the turbine bypass valve immediately opens to hold up move through the system.
As it operates, a turbine uses steam to carry out work, lowering outlet steam temperature and pressure. When a turbine bypass valve opens, it’ll drop the stress, however the exit steam will stay fairly superheated, doubtlessly destroying downstream tools. To keep away from that situation, 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 circumstances. While the plant is in operation, these valves must remain tightly closed to avoid losing vitality. When a turbine journey happens, the bypass valves should respond instantly, exposing them to speedy temperature changes and requiring them to cross very high flows at excessive pressure drops, creating high noise and probably extreme 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 indirectly. Unfortunately, many of those valves are installed in difficult-to-access areas, are sometimes welded in place, and tend to be heavily insulated. As a outcome, they are typically ignored until issues start to surface. Steam leakage through the valve is often the primary symptom observed, but rather more significant and potentially dangerous harm can happen.
The extreme strain drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metallic in the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can result 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 each the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this downside, each turbine bypass valve and its associated piping ought to be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect metal fatigue problems that may be growing however usually are not but visually obvious. If a plant lacks the data or gear to perform these inspections, the valve vendor or approved consultant may be utilized to carry out turbine bypass well being examine providers (Figure 3).
3. A routine turbine bypass valve health verify and full inspection is strongly suggested. Metal fatigue can develop in various 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, customer connection welds, diffuser welds, and surrounding piping can be inspected to identify any growing issues earlier than the gear is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to keep away from problems related to quenching and cracking.
There are also threats to trim components, such as seat floor erosion. One widespread expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This occurs extra generally in flow down valves, the place the steam accelerates through the cage holes after which strikes the trim/seat space directly, inflicting extreme put on and reducing service life. Over time, this high-velocity steam, which might comprise water during startup situations and magnetite throughout any condition, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve physique designs are susceptible to trim injury as the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like gentle items and spray nozzles can usually get replaced relatively easily and at minimal value. Trim parts that need to be changed as a result of heavy put on, 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 steel fatigue developing within the valve body or diffuser, with excessive repair procedures, or maybe a complete valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service had been put in during the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping techniques are showing their age and sometimes have significant indicators of metallic fatigue. Others have loads of life left in them but could benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing technology may inject new life into the valve.
Another instance of a expertise upgrade is when there have been repeated failures with a welded diffuser. In this instance, a removable two-in-one seat diffuser will cut back required maintenance.
If a turbine bypass valve should be replaced, plant personnel are strongly encouraged to look beyond a direct alternative and evaluate potential improvements. A significantly necessary merchandise to consider when taking a look at substitute and improvement alternatives 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 arrangement is good for locations with low overhead clearance, and it places the actuator nearer to the deck or floor for straightforward entry, nevertheless it creates a variety of long-term operational problems. Gravity tends to lead to increased and uneven put on on trim components, and the side-mounted actuator is vulnerable to response issues.
5. Downward flowing valves with horizontal actuators (left) were the primary possibility a couple of a long time ago for turbine bypass valves. New move up valve designs (right) may be put in with out piping modifications in most instances, offering prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for circulate down is vertical mounting of the actuator. This arrangement leads to decreased and extra even put on, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that substitute alternatives are sometimes for horizontally mounted actuators, and the two designs usually are not readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead house is on the market with existing horizontal actuator circulate down designs, a flow up design can easily meet the present face-to-face necessities of the previous valve, whereas decreasing a variety of the wear common to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from circulate all the way down to circulate up, this change could be made with out modifications to existing piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life because the steam is transferring 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 considerably, however that energy dissipates into the large body cavity of the valve as a substitute of striking the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease as a result of they pass over the seat and speed up later because the steam moves via the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is certainly displaying erosion, but when in comparability with the move down trim shown above (Figure four right), the seating surface on the seat ring and plug remain relatively undamaged, allowing the valve to close off fully, the required mode in regular operation.
7. These footage of circulate up trim clearly present the erosive impression of high-velocity steam, but in distinction to the circulate down trim harm proven previously, this valve has no harm to the seating floor and might still shut off tightly. Courtesy: Emerson
The move up valve type design still provides the identical control responsiveness and flow capacity, but the new move association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised pieces of engineered equipment that should be rigorously specified, installed, and maintained. It is therefore sensible to be proactive by reaching out to trusted consultants prior to purchase as they’ll help with valve sizing and specifications primarily based on particular working conditions.
For existing installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while for the explanation that plant turbine bypass valves have been absolutely inspected, it would be wise to consult your turbine bypass valve vendor for help and support.
Many of these distributors provide full inspection and health verify services to determine areas the place metallic fatigue and stress cracking are creating. Ideally the valve could be repaired, but if a valve alternative is warranted, the seller can even present steerage in evaluating bypass valve design styles and selecting the best choice for the specific utility.
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