Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most troublesome applications in a power plant. Keeping these valves working effectively and avoiding unexpected failures is crucial for plant operation.
When engineers are requested to listing the most difficult management valve applications, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high stress drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many vegetation tend to ignore these valves until they fail, creating unplanned outages, misplaced production, and excessive costs for reactive maintenance. เกจวัดแรงดันnuovafima offers suggested strategies of inspection to anticipate and mitigate points beforehand, and it provides improve alternate options should a valve have to be repaired or changed.
Strictly Severe Service
Severe service management valves are utilized in the most troublesome installations inside course of crops. These installations commonly include cavitating, erosive, corrosive, noisy, high pressure, high temperature, high stress drop, or excessive velocity media. Turbine bypass valves are exposed to many of those course of situations; but, they must respond flawlessly and stay 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 trips off-line (Figure 1). In normal operation the valves are utterly closed, forcing all the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine till the properties and circumstances of the steam are appropriate 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 making certain only appropriate steam situations and flows make their approach to the turbine itself.
1. Depending on the facility plant design, a number of turbine bypass valves may be employed to immediately shunt steam around a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to keep away from equipment harm due to overpressure and excessive temperature, so the turbine bypass valve instantly opens to take care of circulate by way of the system.
As it operates, a turbine makes use of steam to carry out work, lowering outlet steam temperature and pressure. When a turbine bypass valve opens, it will drop the stress, however the exit steam will stay fairly superheated, probably destroying downstream gear. To avoid that scenario, turbine bypass valves both incorporate a water injection system within the valve physique, or make use of a separate water injection desuperheater simply downstream, in both case to decrease the exit steam temperature.
As a end result, turbine bypass valves face a perfect storm of severe service conditions. While the plant is in operation, these valves must stay tightly closed to keep away from wasting vitality. When a turbine trip happens, the bypass valves must respond instantly, exposing them to speedy temperature modifications and requiring them to move very excessive flows at high pressure drops, creating excessive noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that just about each turbine bypass valve will finally fail indirectly. Unfortunately, many of these valves are put in in difficult-to-access places, are sometimes welded in place, and are typically heavily insulated. As a end result, they are usually ignored until issues begin to floor. Steam leakage by way of the valve is normally the first symptom noticed, however much more significant and doubtlessly harmful injury can happen.
The excessive stress drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the steel in the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can outcome in catastrophic failure underneath pressure.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the metal and create cracks in each the valve itself and the encircling piping. Courtesy: Emerson
To avoid this drawback, each turbine bypass valve and its associated piping must be routinely inspected. Some sort of non-destructive examination ought to be periodically employed to detect metallic fatigue problems that may be creating but usually are not yet visually apparent. If a plant lacks the data or equipment to carry out these inspections, the valve vendor or authorized consultant could additionally be utilized to perform turbine bypass health check services (Figure 3).
3. A routine turbine bypass valve well being verify and full inspection is strongly suggested. Metal fatigue can develop in various welds located on the valve itself, and in the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, buyer connection welds, diffuser welds, and surrounding piping may be inspected to determine any creating problems before the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from issues related to quenching and cracking.
There are also threats to trim parts, corresponding to seat surface erosion. One widespread expression of such erosion, particularly on the plug, is often referred to as “gear toothing.” This happens more generally in circulate down valves, the place the steam accelerates through the cage holes and then strikes the trim/seat area instantly, causing excessive put on and decreasing service life. Over time, this high-velocity steam, which might contain water during startup situations and magnetite throughout any situation, will erode and injury the valve seat and trim (Figure 4).
4. Flow down valve body designs are vulnerable to trim damage as the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like delicate goods and spray nozzles can usually get replaced comparatively simply and at minimal cost. Trim elements that have to be changed because of heavy put on, corresponding to gear toothing, may be quite costly, particularly if their supply needs to be expedited. The worst-case state of affairs for a turbine bypass valve is metallic fatigue creating within the valve body or diffuser, with extreme restore procedures, or maybe a full valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service were put in during the heyday of mixed cycle plant development from 1998 to 2004. Most of those valves and piping methods are displaying their age and often have vital signs of metallic fatigue. Others have plenty of life left in them but may benefit from upgrades in know-how. Technology and practices have improved, and upgrading to the newest sealing technology could inject new life into the valve.
Another example of a know-how upgrade is when there have been repeated failures with a welded diffuser. In this occasion, 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 substitute and consider potential enhancements. A particularly necessary item to contemplate when looking at replacement and enchancment alternatives is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a move down trim design paired with a horizontal actuator (Figure 5, left). This association is right for areas with low overhead clearance, and it locations the actuator closer to the deck or ground for simple access, nevertheless it creates a variety of long-term operational problems. Gravity tends to lead to elevated and uneven wear on trim components, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the primary possibility a quantity of many years ago for turbine bypass valves. New flow up valve designs (right) may be put in with out piping modifications in most cases, offering extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This arrangement results in decreased and more even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that substitute alternatives are usually for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable without important piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead space is available with current horizontal actuator circulate down designs, a flow up design can simply meet the prevailing face-to-face requirements of the old valve, whereas reducing a few of the wear frequent to flow down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from move right down to move up, this change may be made with out modifications to current piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life as a result of the steam is transferring relatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes through the small holes of the cage, it accelerates significantly, however that vitality dissipates into the large physique cavity of the valve instead of striking the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease because they move over the seat and accelerate later as the steam moves via the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is definitely exhibiting erosion, however when compared to the circulate down trim proven above (Figure 4 right), the seating surface on the seat ring and plug remain comparatively undamaged, allowing the valve to close off utterly, the required mode in regular operation.
7. These pictures of move up trim clearly present the erosive impact of high-velocity steam, but not like the flow down trim injury shown beforehand, this valve has no damage to the seating floor and might nonetheless shut off tightly. Courtesy: Emerson
The circulate up valve fashion design nonetheless supplies the identical management responsiveness and flow capability, but the brand new move arrangement dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered equipment that must be carefully specified, put in, and maintained. It is subsequently wise to be proactive by reaching out to trusted experts prior to buy as they will help with valve sizing and specs based on specific working conditions.
For current 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 fully inspected, it will be wise to seek the advice of your turbine bypass valve vendor for assist and support.
Many of those vendors provide full inspection and health verify companies to identify areas where metal fatigue and stress cracking are developing. Ideally the valve could be repaired, but if a valve replacement is warranted, the seller can even present steerage in evaluating bypass valve design types and selecting the most fitted choice for the specific utility.
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