Turbine Bypass Valve Challenges

Turbine bypass valves are one of the tough applications in a power plant. Keeping these valves working effectively and avoiding sudden failures is crucial for plant operation.
When engineers are asked to listing the most difficult control valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high pressure drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many vegetation are probably to ignore these valves until they fail, creating unplanned outages, lost production, and excessive prices for reactive maintenance. This article supplies advised methods of inspection to anticipate and mitigate issues beforehand, and it offers improve alternate options ought to a valve have to be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in the most tough installations within process plants. These installations generally include cavitating, erosive, corrosive, noisy, excessive strain, high temperature, excessive pressure drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these process circumstances; yet, they must reply flawlessly and remain leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, as properly as when a turbine trips off-line (Figure 1). In normal operation the valves are utterly closed, forcing all the steam through a turbine. During startup, bypass valves divert steam away from the turbine till the properties and conditions 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 potentially wet steam, and by making certain solely appropriate steam conditions and flows make their method to the turbine itself.
1. Depending on the facility plant design, a number of turbine bypass valves could additionally be employed to immediately shunt steam round a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine trip, the steam should proceed flowing to avoid gear harm as a outcome of overpressure and excessive 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 strain. When a turbine bypass valve opens, it’s going to drop the strain, but the exit steam will stay quite superheated, doubtlessly destroying downstream gear. To keep away from that situation, turbine bypass valves both incorporate a water injection system in the valve physique, or employ 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 severe service situations. While the plant is in operation, these valves should 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 move very high flows at excessive pressure drops, creating high noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that virtually every turbine bypass valve will finally fail in some way. Unfortunately, many of those valves are put in in difficult-to-access areas, are typically welded in place, and tend to be heavily insulated. As a end result, they’re usually ignored till problems start to surface. Steam leakage by way of the valve is often the primary symptom noticed, but much more significant and probably dangerous damage can happen.
The extreme stress drops invariably generate excessive noise and high 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 metal fatigue can lead to catastrophic failure underneath stress.
2. Over time, the high vibrations and thermal shock skilled by turbine bypass valves will fatigue the metallic and create cracks in both the valve itself and the encompassing piping. Courtesy: Emerson
To keep away from this drawback, every turbine bypass valve and its associated piping ought to be routinely inspected. Some kind of non-destructive examination should be periodically employed to detect metal fatigue problems that may be growing but aren’t yet visually apparent. If a plant lacks the knowledge or equipment to perform these inspections, the valve vendor or approved consultant could also be utilized to carry out turbine bypass health verify companies (Figure 3).
3. A routine turbine bypass valve well being examine and full inspection is strongly suggested. 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 determine any developing issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to keep away from problems related to quenching and cracking.
There are also threats to trim parts, such as seat floor erosion. One widespread expression of such erosion, particularly on the plug, is often referred to as “gear toothing.” This occurs more commonly in flow down valves, where the steam accelerates by way of the cage holes after which strikes the trim/seat space immediately, causing excessive put on and decreasing service life. Over time, this high-velocity steam, which might include water throughout startup conditions and magnetite throughout any situation, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve physique designs are vulnerable to trim harm because 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 normally be replaced comparatively simply and at minimal cost. Trim parts that have to be replaced as a result of heavy wear, similar to gear toothing, can be fairly pricey, especially if their delivery must be expedited. The worst-case situation for a turbine bypass valve is metallic fatigue growing in the valve body or diffuser, with extreme repair procedures, or even a full valve substitute, required.
เกจวัดแรงดันออกซิเจนราคา of the turbine bypass valves currently in service have been installed during the heyday of mixed cycle plant building from 1998 to 2004. Most of those valves and piping techniques are exhibiting their age and sometimes have vital signs of steel fatigue. Others have loads of life left in them however could benefit from upgrades in know-how. 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 occasion, a detachable two-in-one seat diffuser will scale back required maintenance.
If a turbine bypass valve should be changed, plant personnel are strongly encouraged to look beyond a direct substitute and consider potential improvements. A notably necessary merchandise to consider when taking a look at replacement 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 right for places with low overhead clearance, and it locations the actuator closer to the deck or floor for straightforward entry, but it creates a number of long-term operational issues. Gravity tends to lead to elevated and uneven wear on trim components, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) were the main option a few decades in the past for turbine bypass valves. New flow up valve designs (right) may be put in without piping modifications in most situations, providing prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for circulate down is vertical mounting of the actuator. This association leads to lowered and more even put on, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that substitute opportunities are usually for horizontally mounted actuators, and the two designs usually are not readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead house is available with current horizontal actuator move down designs, a flow up design can easily meet the prevailing face-to-face necessities of the previous valve, whereas lowering a few of the wear common to circulate down designs with a horizontal actuator, corresponding to gear toothing. Because the trim just switches from move down to circulate up, this change can be made with out modifications to present piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life as a end result of the steam is moving comparatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates considerably, however that vitality dissipates into the large body cavity of the valve as a substitute of hanging the seating surface.
6. Downward flowing valves topic 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 speed up later because the steam moves via the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely showing erosion, but when in comparison with the circulate down trim shown above (Figure four right), the seating surface on the seat ring and plug stay comparatively undamaged, permitting the valve to shut off completely, the required mode in normal operation.
7. These pictures of flow up trim clearly present the erosive influence of high-velocity steam, but unlike the circulate down trim injury shown beforehand, this valve has no damage to the seating surface and can nonetheless shut off tightly. Courtesy: Emerson
The move up valve fashion design still supplies the identical control responsiveness and move capacity, however the new circulate 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 tools that should be rigorously specified, installed, and maintained. It is due to this fact clever to be proactive by reaching out to trusted experts prior to buy as they will help with valve sizing and specifications based mostly on specific operating conditions.
For current installations the place 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 sensible to consult your turbine bypass valve vendor for help and support.
Many of those vendors offer full inspection and health examine providers to establish areas the place steel fatigue and stress cracking are creating. Ideally the valve could be repaired, but if a valve alternative is warranted, the vendor can even present guidance in evaluating bypass valve design types and choosing the best option for the precise application.
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