Turbine bypass valves are one of the most troublesome functions in a power plant. Keeping these valves operating efficiently and avoiding surprising failures is critical for plant operation.
When engineers are asked to list essentially the most tough control valve functions, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive strain drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, เครื่องมือที่ใช้วัดความดันเลือด tend to ignore these valves till they fail, creating unplanned outages, lost production, and excessive prices for reactive maintenance. This article offers instructed strategies of inspection to anticipate and mitigate points beforehand, and it provides improve alternatives should a valve must be repaired or changed.
Strictly Severe Service
Severe service management valves are used in essentially the most difficult installations within process vegetation. These installations generally include cavitating, erosive, corrosive, noisy, high strain, high temperature, excessive pressure drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these course of circumstances; but, they have to respond flawlessly and remain leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam turbines throughout plant startup and shutdown, as properly as when a turbine journeys off-line (Figure 1). In normal 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 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 guaranteeing solely applicable steam circumstances and flows make their method 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 journey offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to avoid equipment harm due to overpressure and high temperature, so the turbine bypass valve instantly opens to maintain move through the system.
As it operates, a turbine makes use of steam to carry out work, decreasing outlet steam temperature and pressure. When a turbine bypass valve opens, it will drop the strain, however the exit steam will remain quite superheated, probably destroying downstream tools. To avoid that state of affairs, 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 extreme service circumstances. While the plant is in operation, these valves must remain tightly closed to avoid wasting vitality. When a turbine journey occurs, the bypass valves should reply immediately, exposing them to speedy temperature changes and requiring them to pass very excessive flows at high strain 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 ultimately fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access areas, are typically welded in place, and tend to be closely insulated. As a outcome, they’re typically ignored until problems start to floor. Steam leakage by way of the valve is often the first symptom observed, but rather more significant and doubtlessly dangerous harm can occur.
The excessive pressure 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 metal fatigue may end up in catastrophic failure beneath strain.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in each the valve itself and the surrounding piping. Courtesy: Emerson
To keep away from this problem, each turbine bypass valve and its related piping must be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect metallic fatigue issues that might be growing however usually are not yet visually obvious. If a plant lacks the knowledge or tools to perform these inspections, the valve vendor or approved representative may be utilized to carry out turbine bypass health verify services (Figure 3).
three. A routine turbine bypass valve well being check 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 supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, customer connection welds, diffuser welds, and surrounding piping could be inspected to identify any developing problems earlier than the tools is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from problems associated to quenching and cracking.
There are also threats to trim parts, such as seat surface erosion. One common expression of such erosion, specifically on the plug, is also known as “gear toothing.” This happens extra commonly in circulate down valves, the place the steam accelerates by way of the cage holes and then strikes the trim/seat area instantly, causing extreme put on and decreasing service life. Over time, this high-velocity steam, which may include water throughout startup situations and magnetite during any situation, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve body 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 objects like delicate goods and spray nozzles can often get replaced comparatively easily and at minimal cost. Trim components that need to be replaced due to heavy put on, such as gear toothing, may be fairly expensive, especially if their delivery needs to be expedited. The worst-case scenario for a turbine bypass valve is metal fatigue growing within the valve body or diffuser, with extreme restore procedures, or even a full valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been installed during the heyday of combined cycle plant construction from 1998 to 2004. Most of these valves and piping techniques are exhibiting their age and often have significant signs of metallic fatigue. Others have loads of life left in them however could benefit from upgrades in technology. Technology and practices have improved, and upgrading to the latest sealing know-how could inject new life into the valve.
Another instance of a know-how improve is when there have been repeated failures with a welded diffuser. In this instance, a removable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve have to be replaced, plant personnel are strongly encouraged to look beyond a direct alternative and consider potential enhancements. A significantly important item to contemplate when looking at alternative and enchancment 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 locations with low overhead clearance, and it places the actuator closer to the deck or floor for straightforward entry, however it creates a quantity of long-term operational issues. Gravity tends to lead to elevated and uneven put on on trim elements, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the principle option a number of many years ago for turbine bypass valves. New flow up valve designs (right) may be put in without piping modifications in most situations, offering extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for flow down is vertical mounting of the actuator. This arrangement results in lowered and extra even put on, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that replacement alternatives are usually for horizontally mounted actuators, and the two designs are not readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead house is available with existing horizontal actuator circulate down designs, a move up design can easily meet the prevailing face-to-face necessities of the outdated valve, whereas lowering a few of the wear widespread to flow down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from circulate right down to circulate up, this change can be made with out 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 shifting comparatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes through the small holes of the cage, it accelerates considerably, but that vitality dissipates into the large body cavity of the valve as an alternative of hanging the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease because they cross over the seat and accelerate later as the steam moves via the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is definitely exhibiting erosion, however when in comparison with the move down trim proven above (Figure 4 right), the seating floor on the seat ring and plug stay comparatively undamaged, permitting the valve to close off completely, the required mode in normal operation.
7. These footage of move up trim clearly present the erosive impact of high-velocity steam, however not like the flow down trim harm proven beforehand, this valve has no damage to the seating floor and might nonetheless shut off tightly. Courtesy: Emerson
The move up valve fashion design still supplies the identical control responsiveness and flow capacity, but the model new move association dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised items of engineered tools that have to be carefully specified, put in, and maintained. It is therefore clever to be proactive by reaching out to trusted experts prior to purchase as they can help with valve sizing and specifications based on specific operating situations.
For present 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 would be sensible to consult your turbine bypass valve vendor for assist and assist.
Many of those distributors provide full inspection and health check companies to identify areas where metal fatigue and stress cracking are growing. Ideally the valve can be repaired, but when a valve alternative is warranted, the seller can even provide steerage in evaluating bypass valve design styles and selecting the most fitted choice for the precise utility.
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