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Turbine bypass valves are some of the tough functions in an influence plant. Keeping these valves operating effectively and avoiding surprising failures is important for plant operation.
When engineers are requested to listing essentially the most tough control valve functions, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive strain 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, misplaced manufacturing, and excessive costs for reactive maintenance. This article supplies instructed strategies of inspection to anticipate and mitigate points beforehand, and it provides improve options ought to a valve have to be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in probably the most difficult installations within course of crops. These installations generally include cavitating, erosive, corrosive, noisy, excessive strain, excessive temperature, excessive stress drop, or high 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 turbines throughout plant startup and shutdown, as well as when a turbine trips off-line (Figure 1). In normal operation the valves are fully closed, forcing all the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine until the properties and situations 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 doubtlessly wet steam, and by guaranteeing solely appropriate steam circumstances and flows make their approach to the turbine itself.
1. Depending on the facility plant design, a quantity of turbine bypass valves may be employed to immediately shunt steam round a turbine should it trip offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to avoid equipment harm as a end result of overpressure and excessive temperature, so the turbine bypass valve instantly opens to take care of circulate via 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 will drop the stress, but the exit steam will remain quite superheated, doubtlessly destroying downstream tools. To keep away from 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 decrease the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of extreme service situations. While the plant is in operation, these valves must stay tightly closed to avoid wasting vitality. When a turbine journey occurs, the bypass valves should respond immediately, exposing them to fast temperature changes and requiring them to pass very excessive flows at excessive strain drops, creating excessive noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that just about every turbine bypass valve will ultimately fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access areas, are usually welded in place, and are typically heavily insulated. As a end result, they’re usually ignored till issues start to surface. Steam leakage by way of the valve is normally the first symptom observed, however rather more significant and doubtlessly harmful harm can happen.
The excessive stress drops invariably generate excessive 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 metallic fatigue can lead to catastrophic failure underneath stress.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in both the valve itself and the encompassing piping. Courtesy: Emerson
To avoid this problem, every turbine bypass valve and its related piping ought to be routinely inspected. Some type of non-destructive examination must be periodically employed to detect metal fatigue problems that could be developing but aren’t yet visually apparent. If a plant lacks the information or tools to carry out these inspections, the valve vendor or licensed consultant could also be utilized to carry out turbine bypass well being examine services (Figure 3).
three. A routine turbine bypass valve health examine and full inspection is strongly advised. Metal fatigue can develop in various welds located 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 may be inspected to identify any creating issues before the equipment is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to avoid issues related to quenching and cracking.
There are additionally threats to trim components, similar to seat surface erosion. One common expression of such erosion, specifically on the plug, is also identified as “gear toothing.” This happens extra generally in move down valves, where the steam accelerates by way of the cage holes and then strikes the trim/seat space instantly, causing extreme wear and lowering service life. Over time, this high-velocity steam, which may comprise water during startup situations and magnetite during any condition, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim damage as the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like soft goods and spray nozzles can usually get replaced comparatively simply and at minimal value. Trim components that have to be changed as a outcome of heavy put on, similar to gear toothing, may be quite expensive, particularly if their delivery must be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue creating in the valve physique or diffuser, with excessive restore procedures, or maybe a full valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service had been installed through the heyday of combined cycle plant development from 1998 to 2004. Most of these valves and piping methods are exhibiting their age and infrequently have important indicators of metal fatigue. Others have loads of life left in them however may achieve advantage from upgrades in technology. pressure gauge น้ำ and practices have improved, and upgrading to the most recent sealing know-how might 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 removable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve have to be replaced, plant personnel are strongly encouraged to look past a direct substitute and consider potential improvements. A particularly essential merchandise to contemplate when looking at alternative and enchancment opportunities 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 good for locations with low overhead clearance, and it places the actuator nearer to the deck or floor for easy access, nevertheless it creates a quantity of long-term operational issues. Gravity tends to end in elevated and uneven wear on trim elements, and the side-mounted actuator is prone to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the principle possibility a few a long time 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 efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for move down is vertical mounting of the actuator. This arrangement ends in decreased and more even wear, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that substitute alternatives are sometimes for horizontally mounted actuators, and the two designs are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead area is on the market with current horizontal actuator flow down designs, a move up design can easily meet the prevailing face-to-face necessities of the old valve, whereas reducing a number of the put on common to circulate down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from flow right down to move up, this alteration can be made with out modifications to existing piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life as a result of the steam is moving relatively 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 significantly, however that energy dissipates into the massive body cavity of the valve instead 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 because the steam moves by way of 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 compared to the circulate down trim proven above (Figure 4 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 circulate up trim clearly present the erosive impact of high-velocity steam, but unlike the flow down trim harm proven beforehand, this valve has no injury to the seating floor and might still shut off tightly. Courtesy: Emerson
The circulate up valve style design nonetheless offers the identical management responsiveness and move capacity, however the model new move association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered equipment that should be rigorously specified, installed, and maintained. It is therefore clever to be proactive by reaching out to trusted experts prior to buy as they’ll assist with valve sizing and specs primarily based on particular working circumstances.
For existing installations where 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 would be wise to seek the assistance of your turbine bypass valve vendor for help and support.
Many of these distributors supply full inspection and well being examine providers to determine areas the place metal fatigue and stress cracking are growing. Ideally the valve may be repaired, but when a valve alternative is warranted, the seller also can present steering in evaluating bypass valve design styles and selecting the best option for the particular software.