Turbine Bypass Valve Challenges

Turbine bypass valves are one of the tough functions in a power plant. Keeping these valves working effectively and avoiding unexpected failures is crucial for plant operation.
When engineers are requested to list probably the most difficult control valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high stress drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, many crops are inclined to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and high prices for reactive maintenance. This article offers instructed methods of inspection to anticipate and mitigate issues beforehand, and it provides upgrade 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 within process vegetation. These installations generally embrace cavitating, erosive, corrosive, noisy, high strain, high temperature, high strain drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these course of circumstances; but, they must reply flawlessly and remain leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam turbines 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 the steam via a turbine. During startup, bypass valves divert steam away from the turbine until the properties and situations of the steam are acceptable for sending it to the turbine. This process occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting probably moist steam, and by guaranteeing only appropriate steam situations and flows make their approach to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves may be employed to immediately shunt steam round a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to avoid gear damage as a outcome of overpressure and high 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, decreasing outlet steam temperature and stress. When a turbine bypass valve opens, it’ll drop the pressure, but the exit steam will remain fairly superheated, potentially destroying downstream gear. To avoid that scenario, turbine bypass valves either incorporate a water injection system in the valve physique, or make use of a separate water injection desuperheater simply downstream, in both case to lower the exit steam temperature.
As a end result, turbine bypass valves face an ideal storm of extreme service circumstances. While the plant is in operation, these valves must remain tightly closed to keep away from wasting vitality. When a turbine trip occurs, the bypass valves should reply immediately, exposing them to rapid temperature adjustments and requiring them to cross very excessive flows at high pressure drops, creating high noise and probably extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that nearly each turbine bypass valve will ultimately fail indirectly. Unfortunately, many of these valves are put in in difficult-to-access areas, are typically welded in place, and are usually heavily insulated. As a outcome, they are typically ignored until issues begin to surface. Steam leakage by way of the valve is often the primary symptom noticed, however rather more important and doubtlessly dangerous injury can happen.
Recession-proof drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can lead to catastrophic failure under strain.
2. Over time, the high vibrations and thermal shock skilled by turbine bypass valves will fatigue the metallic and create cracks in each the valve itself and the encompassing piping. Courtesy: Emerson
To avoid this problem, each turbine bypass valve and its related piping must be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect metallic fatigue problems that may be growing however aren’t but visually obvious. If a plant lacks the data or gear to carry out these inspections, the valve vendor or licensed consultant could additionally be utilized to carry out turbine bypass health check companies (Figure 3).
3. A routine turbine bypass valve health examine and full inspection is strongly suggested. Metal fatigue can develop in various welds positioned on the valve itself, and within the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, buyer connection welds, diffuser welds, and surrounding piping could be inspected to establish any developing problems before the equipment is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to avoid problems related to quenching and cracking.
There are also threats to trim parts, corresponding to seat surface erosion. One common expression of such erosion, specifically on the plug, is also known as “gear toothing.” This happens more generally in move down valves, where the steam accelerates via the cage holes after which strikes the trim/seat area instantly, causing excessive put on and lowering service life. Over time, this high-velocity steam, which might include water during startup circumstances and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve body designs are susceptible to trim damage as the moist, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep items like gentle goods and spray nozzles can often get replaced relatively simply and at minimal cost. Trim elements that have to be replaced due to heavy wear, similar to gear toothing, may be quite pricey, especially if their supply must be expedited. The worst-case situation for a turbine bypass valve is steel fatigue developing in the valve physique or diffuser, with extreme restore procedures, or even a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been put in through the heyday of combined cycle plant building from 1998 to 2004. Most of those valves and piping techniques are displaying their age and infrequently have significant signs of metallic fatigue. Others have plenty of life left in them but may benefit from upgrades in technology. Technology and practices have improved, and upgrading to the most recent sealing technology may inject new life into the valve.
Another instance of a know-how improve is when there have been repeated failures with a welded diffuser. In this instance, a detachable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve have to be replaced, plant personnel are strongly inspired to look past a direct substitute and evaluate potential enhancements. A particularly essential merchandise to assume about when taking a look at substitute 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 association is ideal for locations with low overhead clearance, and it locations the actuator nearer to the deck or floor for simple entry, but it creates a number of long-term operational issues. Gravity tends to result in increased and uneven put on on trim components, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the principle choice a couple of many years ago for turbine bypass valves. New circulate up valve designs (right) may be put in without piping modifications in most instances, providing prolonged 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 arrangement leads to reduced and extra even put on, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that replacement alternatives are typically for horizontally mounted actuators, and the two designs usually are not readily interchangeable without important piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead space is on the market with existing horizontal actuator flow down designs, a circulate up design can easily meet the prevailing face-to-face necessities of the old valve, whereas decreasing a number of the put on widespread to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from flow all the means down to circulate up, this change may be made without modifications to present piping.
Extra up valves (Figure 5, right) considerably lengthen seat and trim service life as a outcome of the steam is transferring 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, but that vitality dissipates into the big body cavity of the valve as an alternative of hanging the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower as a result of they cross over the seat and accelerate later because the steam moves by way of the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is actually displaying erosion, but when in comparability with the move down trim proven above (Figure 4 right), the seating surface on the seat ring and plug remain relatively undamaged, permitting the valve to shut off fully, the required mode in normal operation.
7. These pictures of flow up trim clearly present the erosive influence of high-velocity steam, however not like the circulate down trim harm shown previously, this valve has no harm to the seating floor and may still shut off tightly. Courtesy: Emerson
The flow up valve style design nonetheless provides the same control responsiveness and flow capacity, but the new circulate arrangement dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized items of engineered gear that should be carefully specified, put in, and maintained. It is therefore wise to be proactive by reaching out to trusted experts prior to purchase as they can help with valve sizing and specifications based mostly on particular operating conditions.
For existing 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 absolutely inspected, it might be wise to consult your turbine bypass valve vendor for help and help.
Many of those distributors offer full inspection and well being examine services to determine areas the place steel fatigue and stress cracking are growing. Ideally the valve can be repaired, but when a valve substitute is warranted, the seller can even provide steerage in evaluating bypass valve design styles and selecting the most suitable choice for the precise software.
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