Turbine bypass valves are one of the troublesome applications in a power plant. Keeping these valves operating effectively and avoiding unexpected failures is important for plant operation.
When engineers are requested to list essentially the most tough control valve applications, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive strain drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many plants tend to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and excessive costs for reactive maintenance. This article provides advised strategies of inspection to anticipate and mitigate points beforehand, and it offers improve alternate options ought to a valve have to be repaired or changed.
Strictly Severe Service
Severe service management valves are utilized in the most difficult installations within process vegetation. These installations commonly embody cavitating, erosive, corrosive, noisy, high stress, excessive temperature, excessive strain drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those process conditions; but, they must respond flawlessly and remain leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam generators during plant startup and shutdown, as properly as when a turbine trips off-line (Figure 1). In regular operation the valves are fully 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 circumstances of the steam are appropriate 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 doubtlessly wet steam, and by making certain only acceptable steam conditions and flows make their approach to the turbine itself.
1. Depending on the power plant design, a number of turbine bypass valves could additionally be employed to instantly shunt steam round a turbine should it journey offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to keep away from gear injury because of overpressure and high temperature, so the turbine bypass valve immediately opens to keep up flow via the system.
As it operates, a turbine makes use of steam to perform work, reducing outlet steam temperature and pressure. When a turbine bypass valve opens, it will drop the stress, however the exit steam will remain quite superheated, doubtlessly destroying downstream equipment. To avoid that state of affairs, turbine bypass valves either incorporate a water injection system within the valve physique, or make use of a separate water injection desuperheater simply downstream, in both case to lower the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of extreme service situations. While the plant is in operation, these valves must remain tightly closed to avoid losing energy. When a turbine journey occurs, the bypass valves should reply instantly, exposing them to rapid temperature adjustments and requiring them to cross very high flows at excessive strain drops, creating high noise and doubtlessly excessive vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that virtually every turbine bypass valve will in the end fail in some way. Unfortunately, many of these valves are installed in difficult-to-access locations, are sometimes welded in place, and are typically heavily insulated. As a result, they’re often ignored until problems start to floor. Steam leakage by way of the valve is usually the first symptom seen, however rather more significant and potentially harmful harm can happen.
เกจวัดแรงดันไนโตรเจน drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metallic in the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can outcome in catastrophic failure under pressure.
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 drawback, every turbine bypass valve and its related piping ought to be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect steel fatigue issues that may be developing but are not yet visually obvious. If a plant lacks the knowledge or equipment to perform these inspections, the valve vendor or authorized representative could additionally be utilized to perform turbine bypass health examine services (Figure 3).
3. A routine turbine bypass valve well being examine and full inspection is strongly advised. 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, buyer connection welds, diffuser welds, and surrounding piping can be inspected to determine any developing issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are additionally threats to trim components, similar to seat surface erosion. One common expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This happens extra commonly in circulate down valves, the place the steam accelerates by way of the cage holes after which strikes the trim/seat area instantly, causing excessive put on and lowering service life. Over time, this high-velocity steam, which may comprise water throughout startup conditions and magnetite during any situation, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve body designs are vulnerable to trim harm because the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep items like gentle items and spray nozzles can often be replaced comparatively easily and at minimal price. Trim components that have to be replaced due to heavy wear, such as gear toothing, can be fairly pricey, especially if their delivery needs to be expedited. The worst-case state of affairs for a turbine bypass valve is metal fatigue developing within the valve physique or diffuser, with excessive restore procedures, or perhaps a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service had been installed during the heyday of combined cycle plant building from 1998 to 2004. Most of those valves and piping systems are displaying their age and often have vital indicators of metallic fatigue. Others have plenty of life left in them but may gain advantage from upgrades in technology. Technology and practices have improved, and upgrading to the latest sealing technology may inject new life into the valve.
Another instance of a technology upgrade is when there have been repeated failures with a welded diffuser. In this instance, a removable two-in-one seat diffuser will scale back required maintenance.
If a turbine bypass valve must be replaced, plant personnel are strongly encouraged to look past a direct alternative and consider potential improvements. A notably essential merchandise to consider when taking a look at alternative and improvement 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 arrangement is right for places with low overhead clearance, and it places the actuator closer to the deck or ground for straightforward access, however it creates a number of long-term operational issues. Gravity tends to lead to increased and uneven wear on trim components, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) were the main choice a couple of a long time in the past for turbine bypass valves. New move up valve designs (right) could also be installed without piping modifications in most cases, offering extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for flow down is vertical mounting of the actuator. This arrangement ends in decreased and more even put on, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that alternative alternatives are usually for horizontally mounted actuators, and the 2 designs are not readily interchangeable without important piping modifications.
Upward Flowing ราคาเกจวัดแรงดันลม . When enough overhead area is available with present horizontal actuator flow down designs, a move up design can simply meet the present face-to-face requirements of the outdated valve, whereas reducing a few of the put on frequent to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from circulate down to flow up, this modification may be made with out modifications to existing piping.
Flow up valves (Figure 5, right) significantly prolong seat and trim service life as a outcome of the steam is shifting 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 significantly, however that vitality dissipates into the big physique cavity of the valve instead 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 result of they cross over the seat and accelerate later because the steam strikes by way of the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely showing erosion, but when compared to the flow down trim shown above (Figure 4 right), the seating surface on the seat ring and plug stay relatively undamaged, allowing the valve to close off fully, the required mode in regular operation.
7. These footage of move up trim clearly present the erosive influence of high-velocity steam, however in contrast to the move down trim harm proven beforehand, this valve has no injury to the seating surface and can nonetheless shut off tightly. Courtesy: Emerson
The move up valve style design nonetheless provides the identical management responsiveness and circulate capacity, but the new move association dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized items of engineered gear that have to be rigorously specified, installed, and maintained. It is due to this fact sensible to be proactive by reaching out to trusted experts prior to buy as they will help with valve sizing and specifications based on particular working situations.
For existing installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while because the plant turbine bypass valves have been fully inspected, it will be wise to consult your turbine bypass valve vendor for assist and assist.
Many of these distributors offer full inspection and health examine providers to establish areas where metal fatigue and stress cracking are creating. Ideally the valve can be repaired, but if a valve replacement is warranted, the vendor also can present steerage in evaluating bypass valve design types and choosing the best suited choice for the specific application.