Turbine bypass valves are one of the tough functions in a power plant. Keeping these valves operating efficiently and avoiding sudden failures is crucial for plant operation.
When engineers are asked to listing probably the most tough control valve purposes, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high stress drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many vegetation are inclined to ignore these valves until they fail, creating unplanned outages, misplaced production, and excessive prices for reactive upkeep. This article offers instructed strategies of inspection to anticipate and mitigate points beforehand, and it presents upgrade alternate options ought to a valve have to be repaired or replaced.
Strictly Severe Service
Severe service control valves are used in essentially the most troublesome installations inside course of vegetation. These installations generally embody cavitating, erosive, corrosive, noisy, high strain, excessive temperature, high strain drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these course of situations; yet, they have to respond flawlessly and stay leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, in addition to when a turbine journeys off-line (Figure 1). In normal operation the valves are completely closed, forcing all of the steam through 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 happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting probably wet steam, and by ensuring solely acceptable steam circumstances and flows make their method to the turbine itself.
1. Depending on the facility plant design, a quantity of turbine bypass valves may be employed to instantly shunt steam round a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine trip, the steam should proceed flowing to keep away from tools harm as a end result of overpressure and high temperature, so the turbine bypass valve immediately opens to take care of flow through the system.
As it operates, a turbine uses steam to carry out work, decreasing outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the pressure, but the exit steam will remain fairly superheated, potentially destroying downstream gear. To keep away from pressure gauge octa , turbine bypass valves either incorporate a water injection system within the valve body, or employ a separate water injection desuperheater just downstream, in both case to decrease the exit steam temperature.
As a result, turbine bypass valves face an ideal storm of extreme service situations. While the plant is in operation, these valves must stay tightly closed to avoid losing power. When a turbine trip occurs, the bypass valves should respond immediately, exposing them to rapid temperature modifications and requiring them to cross very high flows at excessive strain drops, creating excessive noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that just about every turbine bypass valve will in the end fail indirectly. Unfortunately, many of those valves are installed in difficult-to-access locations, are sometimes welded in place, and tend to be closely insulated. As a end result, they are usually ignored until problems begin to surface. Steam leakage through the valve is usually the primary symptom observed, but much more vital and doubtlessly dangerous injury can occur.
The excessive pressure drops invariably generate high noise and excessive 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 metal fatigue may find yourself in catastrophic failure under stress.
2. Over time, the high vibrations and thermal shock skilled by turbine bypass valves will fatigue the metal and create cracks in both the valve itself and the encircling piping. Courtesy: Emerson
To keep away from this downside, each turbine bypass valve and its associated piping ought to be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect metal fatigue problems that may be creating but usually are not yet visually apparent. If a plant lacks the knowledge or gear to perform these inspections, the valve vendor or licensed representative may be utilized to carry out turbine bypass well being verify services (Figure 3).
three. A routine turbine bypass valve health check and full inspection is strongly advised. Metal fatigue can develop in various welds positioned on the valve itself, and in the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, buyer connection welds, diffuser welds, and surrounding piping could be inspected to establish any creating issues before the tools is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are additionally threats to trim parts, similar to seat floor erosion. One widespread expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This occurs extra generally in circulate down valves, where the steam accelerates through the cage holes and then strikes the trim/seat space directly, inflicting extreme put on and reducing service life. Over time, this high-velocity steam, which can comprise water throughout startup circumstances and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim harm as the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like delicate items and spray nozzles can often get replaced relatively easily and at minimal price. Trim parts that have to be replaced because of heavy wear, similar to gear toothing, can be fairly costly, particularly if their supply needs to be expedited. The worst-case scenario for a turbine bypass valve is metal fatigue growing within the valve physique or diffuser, with extreme repair procedures, or even a complete valve replacement, required.
Many of the turbine bypass valves presently in service have been installed through the heyday of mixed cycle plant building from 1998 to 2004. Most of those valves and piping systems are displaying their age and often have important signs of metal fatigue. Others have loads of life left in them but 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 instance of a expertise upgrade is when there have been repeated failures with a welded diffuser. In เพรสเชอร์เกจ , a detachable two-in-one seat diffuser will scale back required upkeep.
If a turbine bypass valve have to be changed, plant personnel are strongly inspired to look beyond a direct substitute and consider potential improvements. A notably necessary item to think about when looking at alternative and enchancment 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 places with low overhead clearance, and it places the actuator closer to the deck or ground for easy access, nevertheless it creates numerous long-term operational problems. Gravity tends to end in elevated and uneven put on on trim components, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) have been the main option a couple of decades in the past for turbine bypass valves. New flow up valve designs (right) may be put in without piping modifications in most instances, offering extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This association results in decreased and extra even put on, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that alternative opportunities are typically for horizontally mounted actuators, and the two designs aren’t readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead area is on the market with existing horizontal actuator circulate down designs, a circulate up design can simply meet the present face-to-face necessities of the outdated valve, while decreasing a few of the wear widespread to circulate down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from move right down to circulate up, this alteration can be made with out modifications to current piping.
Flow up valves (Figure 5, right) considerably prolong seat and trim service life because the steam is shifting comparatively slowly because it passes over the plug and seat (Figure 6). As the steam moves by way of the small holes of the cage, it accelerates considerably, however that energy dissipates into the large body cavity of the valve instead of hanging the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease as a outcome of they move over the seat and accelerate later because the steam strikes by way of the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely exhibiting erosion, however when in comparison with the circulate down trim shown above (Figure four right), the seating surface on the seat ring and plug remain comparatively undamaged, permitting the valve to close off utterly, the required mode in normal operation.
7. These pictures of flow up trim clearly show the erosive impression of high-velocity steam, however not like the flow down trim harm proven previously, this valve has no damage to the seating surface and can nonetheless shut off tightly. Courtesy: Emerson
The move up valve type design nonetheless offers the identical control responsiveness and move capability, but the new move association dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered equipment that have to be fastidiously specified, put in, and maintained. It is therefore clever to be proactive by reaching out to trusted experts prior to buy as they can help with valve sizing and specifications based on specific operating situations.
For existing installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time since the plant turbine bypass valves have been totally inspected, it might be wise to seek the guidance of your turbine bypass valve vendor for help and help.
Many of those vendors supply full inspection and health examine providers to establish areas where metallic fatigue and stress cracking are growing. Ideally the valve could be repaired, but when a valve replacement is warranted, the seller can even provide guidance in evaluating bypass valve design types and selecting the greatest choice for the precise utility.