Turbine bypass valves are one of the difficult purposes in an influence plant. Keeping these valves operating efficiently and avoiding sudden failures is critical for plant operation.
When engineers are requested to listing essentially the most tough management valve purposes, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive pressure drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many crops are inclined to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and excessive prices for reactive upkeep. This article offers advised methods of inspection to anticipate and mitigate issues beforehand, and it offers upgrade alternate options should a valve have to be repaired or changed.
Strictly Severe Service
Severe service management valves are used in essentially the most troublesome installations within course of vegetation. These installations commonly embrace cavitating, erosive, corrosive, noisy, high strain, high temperature, excessive stress drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these course of situations; yet, they need to respond flawlessly and remain leak free when closed.
As their title 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 regular operation the valves are fully closed, forcing all of the steam via 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 process happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting potentially wet steam, and by making certain only applicable steam conditions and flows make their method to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves may be employed to immediately shunt steam around a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to keep away from gear injury due to overpressure and excessive temperature, so the turbine bypass valve immediately opens to maintain move through the system.
As it operates, a turbine uses steam to carry out work, reducing outlet steam temperature and strain. When a turbine bypass valve opens, it’s going to drop the strain, however the exit steam will remain quite superheated, doubtlessly destroying downstream tools. To avoid that state of affairs, turbine bypass valves either incorporate a water injection system within the valve body, or make use of a separate water injection desuperheater simply downstream, in either case to lower the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of extreme service conditions. While the plant is in operation, these valves must remain tightly closed to keep away from losing vitality. When a turbine journey happens, the bypass valves must respond instantly, exposing them to rapid temperature adjustments and requiring them to cross very excessive flows at excessive strain drops, creating high noise and doubtlessly 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 ultimately. Unfortunately, many of those valves are installed in difficult-to-access areas, are typically welded in place, and are typically closely insulated. As a outcome, they’re often ignored till problems begin to surface. Steam leakage via the valve is often the first symptom seen, but far more vital and probably dangerous damage can happen.
The excessive strain drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metallic within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue may end up in catastrophic failure beneath pressure.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in each the valve itself and the encircling piping. เกจวัดแรงดันน้ำ10บาร์ : Emerson
To keep away from this problem, each turbine bypass valve and its associated piping should be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect metallic fatigue problems that could be growing but usually are not but visually apparent. If a plant lacks the knowledge or tools to perform these inspections, the valve vendor or authorized consultant could additionally be utilized to carry out turbine bypass health examine providers (Figure 3).
three. A routine turbine bypass valve well being verify and full inspection is strongly suggested. Metal fatigue can develop in varied welds situated on the valve itself, and within the inlet, outlet, and water provide 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 establish any developing problems before the tools 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 additionally threats to trim parts, such as seat floor erosion. One widespread expression of such erosion, specifically on the plug, is also recognized as “gear toothing.” This happens more generally in flow down valves, the place the steam accelerates by way of the cage holes after which strikes the trim/seat space immediately, causing excessive wear and reducing service life. Over time, this high-velocity steam, which might include water throughout startup circumstances and magnetite during any situation, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve body designs are susceptible to trim damage because the wet, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like delicate items and spray nozzles can normally be replaced relatively simply and at minimal cost. Trim parts that need to be replaced due to heavy wear, similar to gear toothing, can be fairly costly, especially if their delivery needs to be expedited. The worst-case situation for a turbine bypass valve is metallic fatigue developing within the valve physique or diffuser, with excessive repair procedures, or even a complete valve substitute, required.
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 these valves and piping methods are displaying their age and infrequently have vital signs of metal 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 most recent sealing know-how 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 instance, a detachable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve should be replaced, plant personnel are strongly encouraged to look past a direct substitute and consider potential enhancements. A notably essential merchandise to consider when looking at substitute and enchancment 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 association is ideal for places with low overhead clearance, and it locations the actuator closer to the deck or floor for simple entry, but it creates a variety of long-term operational issues. Gravity tends to result in increased and uneven put on on trim parts, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) were the primary option a few many years ago for turbine bypass valves. New move up valve designs (right) may be put in with out piping modifications in most cases, providing extended 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 extra even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute opportunities are usually for horizontally mounted actuators, and the 2 designs aren’t readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead house is out there with existing horizontal actuator flow down designs, a move up design can simply meet the present face-to-face requirements of the previous valve, whereas reducing a variety of the wear common to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from flow all the way down to flow up, this change could be made without modifications to present piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life as a result of the steam is shifting relatively slowly as it passes over the plug and seat (Figure 6). As the steam moves via the small holes of the cage, it accelerates considerably, but that power dissipates into the large physique cavity of the valve as a substitute of putting 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 outcome of they cross over the seat and speed up later as the steam strikes via the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is actually exhibiting erosion, however when in comparison with the move down trim shown above (Figure 4 right), the seating surface on the seat ring and plug remain comparatively undamaged, permitting the valve to shut off utterly, the required mode in normal operation.
7. These footage of move up trim clearly show the erosive impact of high-velocity steam, but not like the move down trim injury shown previously, this valve has no injury to the seating surface and can nonetheless shut off tightly. Courtesy: Emerson
The circulate up valve fashion design nonetheless supplies the same control responsiveness and flow capability, but the new circulate association dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised pieces of engineered tools that should be fastidiously specified, installed, and maintained. It is due to this fact wise to be proactive by reaching out to trusted consultants prior to buy as they will help with valve sizing and specifications based on specific operating conditions.
For present installations the place 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 assist and help.
Many of these vendors supply full inspection and health check companies to determine areas where metallic fatigue and stress cracking are creating. Ideally the valve may be repaired, but when a valve substitute is warranted, the seller can also present steerage in evaluating bypass valve design kinds and choosing the best option for the specific application.