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Turbine Bypass Valve Challenges

Turbine bypass valves are one of the troublesome applications in a power plant. Keeping these valves working efficiently and avoiding sudden failures is critical for plant operation.
When engineers are requested to listing probably the most troublesome control valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high stress drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many crops tend to ignore these valves till they fail, creating unplanned outages, lost production, and excessive costs for reactive upkeep. This article supplies advised methods of inspection to anticipate and mitigate points beforehand, and it presents improve alternate options ought to a valve need to be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in essentially the most troublesome installations inside process crops. These installations generally embrace cavitating, erosive, corrosive, noisy, excessive strain, high temperature, excessive strain drop, or high velocity media. Turbine bypass valves are uncovered to many of these course of conditions; yet, they need to reply flawlessly and stay leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam turbines throughout plant startup and shutdown, as nicely as 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 circumstances of the steam are acceptable for sending it to the turbine. This course of happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting probably wet steam, and by ensuring solely applicable steam situations and flows make their approach to the turbine itself.
1. Depending on the ability plant design, a number of turbine bypass valves may be employed to instantly shunt steam round a turbine should it trip offline. Courtesy: Emerson
Should a turbine trip, the steam should continue flowing to avoid tools injury because of overpressure and excessive temperature, so the turbine bypass valve immediately opens to take care of flow by way of the system.
As it operates, a turbine makes use of steam to perform work, lowering outlet steam temperature and stress. When a turbine bypass valve opens, it’s going to drop the pressure, however the exit steam will stay quite superheated, potentially destroying downstream tools. To avoid that scenario, 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 outcome, turbine bypass valves face a perfect storm of extreme service circumstances. While the plant is in operation, these valves must remain tightly closed to avoid wasting energy. When a turbine trip happens, the bypass valves must reply immediately, exposing them to speedy temperature modifications and requiring them to cross very excessive flows at high stress drops, creating high noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that virtually every turbine bypass valve will in the end fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access areas, are usually welded in place, and are usually heavily insulated. As a outcome, they are typically ignored till problems start to surface. Steam leakage through the valve is often the primary symptom observed, however far more vital and doubtlessly harmful injury can occur.
The extreme stress drops invariably generate excessive noise and high 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 steel fatigue can result in catastrophic failure underneath stress.
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 surrounding piping. Courtesy: Emerson
To keep away from this drawback, each turbine bypass valve and its associated piping ought to be routinely inspected. Some kind of non-destructive examination must be periodically employed to detect metallic fatigue problems which may be growing but are not yet visually obvious. If a plant lacks the knowledge or tools to carry out these inspections, the valve vendor or licensed representative could also be utilized to carry out turbine bypass health verify companies (Figure 3).
three. เครื่องมือตรวจวัดความดันเลือดเรียกว่า bypass valve well being examine and full inspection is strongly suggested. Metal fatigue can develop in numerous welds situated on the valve itself, and within 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 establish any creating issues before the tools is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to keep away from issues related to quenching and cracking.
There are also threats to trim components, such as seat surface erosion. One frequent expression of such erosion, particularly on the plug, is sometimes called “gear toothing.” This occurs extra generally in move down valves, where the steam accelerates by way of the cage holes and then strikes the trim/seat area instantly, causing extreme wear and reducing service life. Over time, this high-velocity steam, which might contain water throughout startup circumstances and magnetite throughout any situation, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve body designs are prone to trim harm as the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like gentle goods and spray nozzles can usually get replaced comparatively simply and at minimal value. Trim elements that need to be changed due to heavy put on, such as gear toothing, can be quite costly, especially if their delivery must be expedited. The worst-case situation for a turbine bypass valve is steel fatigue developing in the valve physique or diffuser, with extreme repair procedures, or even a full valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service had been put in in the course of the heyday of combined cycle plant construction from 1998 to 2004. Most of those valves and piping methods are displaying their age and often have significant signs of metallic fatigue. Others have loads of life left in them however may acquire advantage from upgrades in technology. Technology and practices have improved, and upgrading to the latest sealing expertise could inject new life into the valve.
Another example 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 reduce required upkeep.
If a turbine bypass valve must be changed, plant personnel are strongly encouraged to look beyond a direct alternative and evaluate potential enhancements. A notably important merchandise to consider when looking at alternative 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 ideal for areas with low overhead clearance, and it places the actuator nearer to the deck or ground for easy access, nevertheless it creates a selection of long-term operational issues. Gravity tends to end in elevated and uneven wear on trim elements, and the side-mounted actuator is prone to response issues.
5. Downward flowing valves with horizontal actuators (left) were the primary choice a couple of decades in the past for turbine bypass valves. New circulate up valve designs (right) may be put in without piping modifications in most cases, offering prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for circulate down is vertical mounting of the actuator. This arrangement results in lowered and more even wear, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that alternative opportunities are usually for horizontally mounted actuators, and the 2 designs aren’t readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead house is out there with current horizontal actuator move down designs, a circulate up design can easily meet the prevailing face-to-face necessities of the outdated valve, while lowering a few of the put on frequent to move down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from move down to move up, this alteration may be made with out modifications to current piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life because the steam is transferring relatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates significantly, but that vitality dissipates into the massive physique cavity of the valve as a substitute of putting the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower as a end result of they cross over the seat and accelerate later because the steam strikes through the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is certainly showing erosion, but when in comparability with the flow down trim proven 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 pictures of circulate up trim clearly present the erosive impact of high-velocity steam, however in distinction to the circulate down trim injury shown beforehand, this valve has no harm to the seating surface and can still shut off tightly. Courtesy: Emerson
The move up valve style design still offers the identical management responsiveness and circulate capacity, however the new flow 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 gear that must be fastidiously specified, put in, and maintained. It is therefore wise to be proactive by reaching out to trusted specialists prior to buy as they’ll help with valve sizing and specifications primarily based on specific operating circumstances.
For current installations where 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 might be clever to seek the assistance of your turbine bypass valve vendor for assist and support.
Many of those vendors provide full inspection and health verify providers to identify areas where steel fatigue and stress cracking are growing. Ideally the valve could be repaired, but when a valve substitute is warranted, the vendor can even present steering in evaluating bypass valve design kinds and choosing the finest choice for the precise application.
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