Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most tough applications in a power plant. Keeping these valves working efficiently and avoiding sudden failures is crucial for plant operation.
When engineers are asked to listing probably the most troublesome management valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive strain drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many crops tend to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and excessive prices for reactive upkeep. This article provides instructed methods of inspection to anticipate and mitigate points beforehand, and it offers improve alternatives ought to a valve need to be repaired or replaced.
Strictly Severe Service
Severe service management valves are utilized in essentially the most tough installations inside course of crops. These installations commonly embody cavitating, erosive, corrosive, noisy, excessive strain, excessive temperature, excessive pressure drop, or high velocity media. Turbine bypass valves are exposed to many of these course of situations; yet, they need to respond flawlessly and stay leak free when closed.
As their name 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 utterly closed, forcing all the steam through a turbine. During startup, bypass valves divert steam away from the turbine until the properties and conditions of the steam are acceptable for sending it to the turbine. This process happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting doubtlessly wet steam, and by guaranteeing only acceptable steam conditions and flows make their approach to the turbine itself.
1. Depending on the ability plant design, a number of turbine bypass valves could also be employed to immediately shunt steam round a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must proceed flowing to keep away from tools injury as a end result of overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain circulate by way of the system.
As it operates, a turbine makes use of steam to carry out work, reducing outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the pressure, however the exit steam will stay quite superheated, probably destroying downstream equipment. To avoid that situation, turbine bypass valves either incorporate a water injection system in 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 a perfect storm of severe service circumstances. While the plant is in operation, these valves must stay tightly closed to keep away from wasting power. When a turbine trip happens, the bypass valves must respond instantly, exposing them to rapid temperature changes and requiring them to move very high flows at high strain drops, creating high noise and probably extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that virtually each turbine bypass valve will ultimately fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access locations, are sometimes welded in place, and are typically closely insulated. As a result, they’re often ignored till problems start to surface. Steam leakage through the valve is normally the first symptom observed, however far more important and potentially dangerous damage can occur.
The extreme stress drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metallic within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can end result in catastrophic failure beneath pressure.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in both the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this drawback, each turbine bypass valve and its related piping should be routinely inspected. Some type of non-destructive examination must be periodically employed to detect metallic fatigue issues which could be developing but aren’t yet visually obvious. If a plant lacks the data or tools to perform these inspections, the valve vendor or licensed consultant may be utilized to carry out turbine bypass health verify companies (Figure 3).
three. A routine turbine bypass valve well being check and full inspection is strongly suggested. Metal fatigue can develop in various welds situated 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, customer connection welds, diffuser welds, and surrounding piping could be inspected to establish any creating problems before the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid problems associated to quenching and cracking.
There are additionally threats to trim components, corresponding to seat surface erosion. One frequent expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This occurs extra commonly in move down valves, the place the steam accelerates via the cage holes and then strikes the trim/seat space instantly, inflicting excessive wear and decreasing service life. Over time, this high-velocity steam, which might contain water during startup conditions and magnetite throughout any situation, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve body designs are vulnerable to trim injury as the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep items like gentle goods and spray nozzles can normally get replaced comparatively easily and at minimal value. Trim parts that have to be replaced due to heavy put on, such as gear toothing, may be quite expensive, especially if their delivery needs to be expedited. The worst-case scenario for a turbine bypass valve is metallic fatigue developing within the valve body 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 have been installed during the heyday of combined cycle plant building from 1998 to 2004. Most of these valves and piping techniques are displaying their age and infrequently have significant indicators of metallic fatigue. Swipe have loads of life left in them but could benefit from upgrades in know-how. Technology and practices have improved, and upgrading to the most recent sealing know-how may inject new life into the valve.
Another instance 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 upkeep.
If a turbine bypass valve have to be replaced, plant personnel are strongly inspired to look beyond a direct replacement and consider potential improvements. A particularly essential merchandise to consider when looking at replacement and improvement opportunities is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a flow down trim design paired with a horizontal actuator (Figure 5, left). This association is ideal for locations with low overhead clearance, and it locations the actuator nearer to the deck or ground for straightforward entry, nevertheless it creates a selection of long-term operational issues. Gravity tends to result in increased 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 main option a few many years in the past for turbine bypass valves. New circulate up valve designs (right) could additionally be put in with out piping modifications in most instances, offering extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for move down is vertical mounting of the actuator. This association results in decreased and extra even wear, longer runs between upkeep outages, and improved actuator response. The challenge 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 sufficient overhead space is on the market with present horizontal actuator move down designs, a flow up design can simply meet the existing face-to-face necessities of the previous valve, while decreasing a few of the wear common to move down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from flow all the way down to move up, this change could be made with out modifications to present piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life as a result of the steam is moving comparatively slowly because it passes over the plug and seat (Figure 6). As the steam moves through the small holes of the cage, it accelerates considerably, however that vitality dissipates into the massive physique cavity of the valve as an alternative of striking the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower as a end result of they pass over the seat and accelerate later as the steam strikes by way of the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is definitely showing erosion, however when in comparability with the move down trim proven above (Figure four right), the seating surface on the seat ring and plug remain comparatively undamaged, allowing the valve to shut off completely, the required mode in normal operation.
7. These footage of circulate up trim clearly show the erosive influence of high-velocity steam, but not like the circulate down trim injury proven beforehand, this valve has no harm to the seating floor and might nonetheless shut off tightly. Courtesy: Emerson
The flow up valve style design nonetheless offers the identical control responsiveness and move capability, however the brand new move association dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized items of engineered tools that have to be rigorously specified, put in, and maintained. It is due to this fact wise to be proactive by reaching out to trusted specialists prior to buy as they will assist with valve sizing and specs based mostly on specific working situations.
For existing installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time because the plant turbine bypass valves have been totally inspected, it would be clever to seek the guidance of your turbine bypass valve vendor for help and support.
Many of those distributors provide full inspection and health check services to identify areas the place steel fatigue and stress cracking are growing. Ideally the valve could be repaired, but if a valve alternative is warranted, the seller can even provide steering in evaluating bypass valve design styles and selecting the finest choice for the precise software.
Share

Leave a Comment