Turbine Bypass Valve Challenges

Turbine bypass valves are some of the troublesome applications in an influence plant. Keeping these valves operating effectively and avoiding sudden failures is important for plant operation.
When engineers are requested to list the most troublesome management valve functions, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive pressure drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many plants are inclined to ignore these valves until they fail, creating unplanned outages, lost production, and excessive costs for reactive upkeep. This article supplies instructed methods of inspection to anticipate and mitigate points beforehand, and it offers improve options should a valve need to be repaired or changed.
Strictly Severe Service
Severe service control valves are used in essentially the most difficult installations within process plants. These installations commonly include cavitating, erosive, corrosive, noisy, high pressure, high temperature, excessive stress drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those course of conditions; but, they need to respond 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 conditions of the steam are applicable for sending it to the turbine. This course of occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting doubtlessly moist steam, and by guaranteeing only appropriate steam circumstances and flows make their method to the turbine itself.
1. Depending on the facility plant design, several turbine bypass valves may be employed to immediately shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to avoid equipment injury as a result of overpressure and excessive temperature, so the turbine bypass valve immediately opens to take care of move via the system.
As it operates, a turbine uses steam to perform work, lowering outlet steam temperature and pressure. When a turbine bypass valve opens, it’ll drop the pressure, however the exit steam will remain quite superheated, probably destroying downstream tools. To keep away from that scenario, turbine bypass valves either incorporate a water injection system within 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 extreme service conditions. While the plant is in operation, these valves should stay tightly closed to avoid losing energy. When a turbine journey occurs, the bypass valves must reply instantly, exposing them to fast temperature modifications and requiring them to pass very excessive flows at excessive pressure drops, creating high noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that nearly every turbine bypass valve will ultimately fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access places, are usually welded in place, and are typically heavily insulated. As a result, they’re usually ignored until issues begin to floor. Steam leakage through the valve is normally the first symptom noticed, however rather more significant and probably dangerous damage can happen.
The extreme pressure drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can lead to catastrophic failure underneath 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 encircling piping. Courtesy: Emerson
To avoid this downside, each turbine bypass valve and its associated piping ought to be routinely inspected. Some kind of non-destructive examination must be periodically employed to detect metal fatigue problems that might be developing however usually are not but visually obvious. If a plant lacks the knowledge or tools to carry out these inspections, the valve vendor or approved representative could additionally be utilized to perform turbine bypass health examine companies (Figure 3).
three. A routine turbine bypass valve health check and full inspection is strongly advised. Metal fatigue can develop in various welds situated on the valve itself, and within the inlet, outlet, and water supply piping (circled areas). เครื่องวัดแรงดันเกจที่นิยมใช้ : Emerson
Fabrication welds on the physique and water manifold, customer connection welds, diffuser welds, and surrounding piping may be inspected to identify any developing issues earlier than the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from problems associated to quenching and cracking.
There are additionally threats to trim elements, similar to seat floor erosion. One frequent expression of such erosion, specifically on the plug, is also identified as “gear toothing.” This occurs more generally in circulate down valves, where the steam accelerates through the cage holes after which strikes the trim/seat area instantly, inflicting extreme wear and lowering service life. Over time, this high-velocity steam, which may comprise water throughout 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 prone to trim injury because the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common upkeep items like soft items and spray nozzles can usually be replaced relatively simply and at minimal value. Trim components that have to be replaced because of heavy put on, corresponding to gear toothing, could be quite pricey, especially if their supply must be expedited. The worst-case scenario for a turbine bypass valve is steel fatigue creating within the valve physique or diffuser, with excessive restore procedures, or even a complete valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service were put in during the heyday of mixed cycle plant building from 1998 to 2004. Most of those valves and piping systems are displaying their age and sometimes have important indicators of metal fatigue. Others have plenty of life left in them but could benefit from upgrades in know-how. Technology and practices have improved, and upgrading to the newest sealing expertise might inject new life into the valve.
Another example of a expertise improve is when there have been repeated failures with a welded diffuser. In this instance, a removable two-in-one seat diffuser will scale back required maintenance.
If a turbine bypass valve must be changed, plant personnel are strongly encouraged to look beyond a direct replacement and consider potential improvements. A particularly essential item to consider when taking a glance at substitute and enchancment 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 association is ideal for locations with low overhead clearance, and it locations the actuator closer to the deck or floor for straightforward access, but it creates numerous long-term operational problems. Gravity tends to lead to elevated and uneven put on on trim parts, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the primary choice a couple of many years ago for turbine bypass valves. New circulate up valve designs (right) may be installed with out piping modifications in most cases, providing prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for move down is vertical mounting of the actuator. This arrangement results in reduced and extra even put on, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that alternative alternatives are usually for horizontally mounted actuators, and the two designs are not readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead house is available with current horizontal actuator circulate down designs, a flow up design can easily meet the existing face-to-face requirements of the outdated valve, whereas decreasing some of the wear frequent to move down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from flow all the means down to flow up, this alteration could be made without modifications to current piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life as a outcome of the steam is shifting relatively slowly as it passes over the plug and seat (Figure 6). As the steam moves through the small holes of the cage, it accelerates considerably, but that power dissipates into the big body cavity of the valve as an alternative of putting the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower because they move over the seat and speed up later as the steam strikes via the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is actually exhibiting erosion, however when in comparison with the circulate down trim proven above (Figure four right), the seating floor on the seat ring and plug stay comparatively undamaged, allowing the valve to close off fully, the required mode in regular operation.
7. These photos of move up trim clearly present the erosive impact of high-velocity steam, but in contrast to the move down trim harm shown beforehand, this valve has no damage to the seating surface and may still shut off tightly. Courtesy: Emerson
The flow up valve fashion design nonetheless offers the identical management responsiveness and move capacity, but the new flow association dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised items of engineered tools that should be carefully specified, put in, and maintained. It is subsequently clever to be proactive by reaching out to trusted consultants prior to purchase as they can assist with valve sizing and specifications based mostly on particular operating circumstances.
For present installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while because the plant turbine bypass valves have been totally inspected, it would be wise to consult your turbine bypass valve vendor for assist and help.
Many of those vendors supply full inspection and health examine companies to identify areas where metallic fatigue and stress cracking are developing. Ideally the valve may be repaired, but if a valve replacement is warranted, the vendor can also present steering in evaluating bypass valve design styles and selecting the most suitable choice for the specific software.
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