Turbine Bypass Valve Challenges

Turbine bypass valves are some of the tough functions in a power plant. Keeping these valves operating effectively and avoiding sudden failures is critical for plant operation.
When engineers are requested to record essentially the most troublesome control valve purposes, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive pressure drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many vegetation are most likely to ignore these valves till they fail, creating unplanned outages, lost production, and excessive costs for reactive maintenance. This article offers instructed methods of inspection to anticipate and mitigate points beforehand, and it offers upgrade options ought to a valve have to be repaired or replaced.
Strictly Severe Service
Severe service management valves are utilized in probably the most tough installations within course of plants. These installations generally embrace cavitating, erosive, corrosive, noisy, excessive stress, high temperature, high strain drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those process situations; yet, they have to reply 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 regular operation the valves are completely 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 applicable 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 moist steam, and by ensuring solely applicable steam circumstances and flows make their way to the turbine itself.
1. Depending on the power plant design, a quantity of turbine bypass valves could also be employed to immediately shunt steam round a turbine should it journey offline. Courtesy: Emerson
Should a turbine journey, the steam should proceed flowing to avoid tools harm due to overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain flow through the system.
As it operates, a turbine uses steam to carry out work, decreasing outlet steam temperature and strain. When a turbine bypass valve opens, it’ll drop the strain, but the exit steam will remain fairly superheated, doubtlessly destroying downstream gear. To keep away from that scenario, 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 end result, turbine bypass valves face an ideal storm of severe service circumstances. While the plant is in operation, these valves should remain tightly closed to keep away from losing vitality. When a turbine trip occurs, the bypass valves should respond instantly, exposing them to speedy temperature adjustments and requiring them to move very high flows at high stress drops, creating excessive noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that just about each turbine bypass valve will finally fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access areas, are usually welded in place, and tend to be closely insulated. As a end result, they are often ignored until issues start to floor. Steam leakage via the valve is normally the first symptom seen, however much more significant and potentially harmful harm can happen.
เกจ์วัดแก๊สหุงต้ม drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metal in the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can outcome 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 encompassing piping. Courtesy: Emerson
To avoid this downside, each turbine bypass valve and its related piping must be routinely inspected. Some kind of non-destructive examination must be periodically employed to detect metal fatigue problems that may be creating but usually are not but visually apparent. If a plant lacks the knowledge or gear to perform these inspections, the valve vendor or authorized consultant could also be utilized to perform turbine bypass health examine providers (Figure 3).
three. A routine turbine bypass valve health check and full inspection is strongly advised. Metal fatigue can develop in varied welds located on the valve itself, and in 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 can be inspected to identify any developing problems earlier than the gear is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to keep away from issues related to quenching and cracking.
There are also threats to trim parts, similar to seat surface erosion. One widespread expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This happens extra generally in circulate down valves, the place the steam accelerates via the cage holes after which strikes the trim/seat space immediately, inflicting excessive wear and reducing service life. Over time, this high-velocity steam, which may contain water during startup circumstances and magnetite during any condition, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve body designs are prone to trim injury as the wet, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like soft items and spray nozzles can normally be replaced relatively easily and at minimal cost. Trim components that need to be changed because of heavy put on, corresponding to gear toothing, may be fairly pricey, particularly if their delivery must be expedited. The worst-case situation for a turbine bypass valve is steel fatigue creating within the valve physique or diffuser, with excessive repair procedures, or perhaps a full valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were put in through the heyday of combined cycle plant building from 1998 to 2004. Most of those valves and piping methods are showing their age and often have important indicators of metal fatigue. Others have loads of life left in them however may acquire advantage from upgrades in know-how. Technology and practices have improved, and upgrading to the most recent sealing technology might 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 removable two-in-one seat diffuser will scale back required maintenance.
If a turbine bypass valve have to be replaced, plant personnel are strongly inspired to look past a direct replacement and consider potential enhancements. A significantly necessary merchandise to consider when taking a glance at substitute and improvement 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 arrangement is ideal for places with low overhead clearance, and it locations the actuator nearer to the deck or ground for easy access, however it creates a selection of long-term operational problems. Gravity tends to result in increased and uneven wear on trim elements, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) were the primary possibility a few many years ago for turbine bypass valves. New move up valve designs (right) could also be put in without piping modifications in most instances, providing 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 leads to lowered and more even put on, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute alternatives are usually for horizontally mounted actuators, and the 2 designs are not readily interchangeable with out vital piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is available with current horizontal actuator flow down designs, a move up design can easily meet the prevailing face-to-face requirements of the outdated valve, whereas reducing some of the put on common to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from move down to move up, this modification could be made with out modifications to existing piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life as a result of the steam is transferring 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 significantly, however that energy 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 lower as a result of they cross over the seat and accelerate later as the steam moves via the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is certainly showing erosion, but when compared to the flow down trim shown above (Figure four right), the seating floor on the seat ring and plug stay comparatively undamaged, permitting the valve to shut off utterly, the required mode in regular operation.
7. These footage of move up trim clearly show the erosive impact of high-velocity steam, but unlike the move down trim harm shown previously, this valve has no damage to the seating surface and can still shut off tightly. Courtesy: Emerson
The circulate up valve type design still supplies the identical management responsiveness and move capacity, however the brand new circulate arrangement dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered gear that must be carefully specified, put in, and maintained. It is therefore sensible to be proactive by reaching out to trusted experts prior to purchase as they’ll assist with valve sizing and specifications based on particular operating conditions.
For present installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time for the rationale that plant turbine bypass valves have been fully inspected, it would be clever to seek the guidance of your turbine bypass valve vendor for assist and support.
Many of these vendors supply full inspection and well being examine companies to identify areas the place metal fatigue and stress cracking are developing. Ideally the valve can be repaired, but if a valve alternative is warranted, the vendor also can present steering in evaluating bypass valve design kinds and selecting the finest option for the precise utility.
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