Turbine bypass valves are one of the most troublesome functions in an influence plant. Keeping these valves working effectively and avoiding unexpected failures is important for plant operation.
When engineers are asked to record the most difficult control valve purposes, 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 vegetation are inclined to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and excessive prices for reactive maintenance. This article supplies advised methods of inspection to anticipate and mitigate issues beforehand, and it provides upgrade alternate options ought to a valve need to be repaired or replaced.
Strictly Severe Service
Severe service control valves are utilized in the most tough installations inside course of plants. These installations commonly embody cavitating, erosive, corrosive, noisy, excessive strain, high temperature, high stress drop, or high velocity media. Turbine bypass valves are uncovered to many of these process situations; yet, they need to respond flawlessly and remain leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam turbines during plant startup and shutdown, as properly as when a turbine journeys off-line (Figure 1). In normal operation the valves are completely closed, forcing all the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine till 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 guard the turbine by diverting potentially wet steam, and by making certain solely acceptable steam circumstances and flows make their way to the turbine itself.
1. Depending on the facility plant design, several turbine bypass valves may be employed to instantly shunt steam around a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine trip, the steam should continue flowing to keep away from gear harm because of overpressure and high temperature, so the turbine bypass valve immediately opens to take care of move through the system.
As it operates, a turbine makes use of steam to perform work, decreasing outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the stress, however the exit steam will remain quite superheated, probably destroying downstream equipment. To avoid that state of affairs, turbine bypass valves either incorporate a water injection system in the valve physique, or employ a separate water injection desuperheater simply downstream, in either case to lower the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of extreme service conditions. While เกจวัดแรงดันถังลม is in operation, these valves must stay tightly closed to keep away from wasting power. When a turbine journey occurs, the bypass valves should reply instantly, exposing them to speedy temperature modifications and requiring them to pass very high flows at high pressure drops, creating high noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that virtually each turbine bypass valve will finally fail indirectly. Unfortunately, many of these valves are put in in difficult-to-access places, are sometimes welded in place, and are usually closely insulated. As a end result, they’re often ignored until issues start to surface. Steam leakage through the valve is often the primary symptom seen, however far more significant and potentially dangerous harm can occur.
The excessive stress drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the steel in the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue may end up in catastrophic failure under strain.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in each 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 sort of non-destructive examination ought to be periodically employed to detect steel fatigue problems that could be growing however are not but visually obvious. If a plant lacks the information or equipment to carry out these inspections, the valve vendor or licensed consultant could also be utilized to carry out turbine bypass health verify providers (Figure 3).
3. 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 within the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, customer connection welds, diffuser welds, and surrounding piping could be inspected to determine any growing issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to keep away from problems associated to quenching and cracking.
There are also threats to trim elements, corresponding to seat floor erosion. One frequent expression of such erosion, specifically on the plug, is also recognized as “gear toothing.” This happens extra generally in circulate down valves, where the steam accelerates via the cage holes and then strikes the trim/seat area directly, inflicting excessive wear and reducing service life. Over time, this high-velocity steam, which may contain water throughout startup conditions and magnetite throughout any condition, will erode and harm the valve seat and trim (Figure 4).
four. Flow down valve physique designs are susceptible to trim damage because the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like gentle items and spray nozzles can often be replaced relatively simply and at minimal price. Trim parts that must be changed due to heavy put on, such as gear toothing, may be quite expensive, particularly if their supply must be expedited. The worst-case scenario for a turbine bypass valve is metal fatigue developing within the valve physique or diffuser, with extreme repair procedures, or even a complete valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were installed during the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping systems are displaying their age and infrequently have significant indicators of steel 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 newest 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 this instance, a detachable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve should be changed, plant personnel are strongly encouraged to look beyond a direct substitute and consider potential enhancements. A significantly necessary merchandise to suppose about when looking at replacement 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 arrangement is right for places with low overhead clearance, and it locations the actuator closer to the deck or ground for easy entry, but it creates numerous long-term operational problems. Gravity tends to result in elevated and uneven put on on trim elements, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the principle possibility a number of many years in the past for turbine bypass valves. New circulate up valve designs (right) could also be put in with out piping modifications in most instances, offering prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for flow down is vertical mounting of the actuator. This association leads to reduced and more even put on, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement alternatives are sometimes for horizontally mounted actuators, and the 2 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 move up design can simply meet the prevailing face-to-face necessities of the old valve, while reducing some of the put on widespread to circulate down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from circulate all the means down to circulate up, this alteration can be made with out modifications to present piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life as a outcome of the steam is moving relatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes by way of the small holes of the cage, it accelerates considerably, but that energy dissipates into the massive physique cavity of the valve as an alternative of putting 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 through the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is certainly exhibiting erosion, however when in comparability with the flow down trim shown above (Figure four right), the seating floor on the seat ring and plug remain comparatively undamaged, permitting the valve to shut off fully, the required mode in regular operation.
7. These pictures of flow up trim clearly present the erosive influence of high-velocity steam, but unlike the move down trim injury shown beforehand, this valve has no damage to the seating surface and might still shut off tightly. Courtesy: Emerson
The flow up valve style design still offers the identical management responsiveness and flow capability, however the new circulate arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered gear that have to be fastidiously specified, put in, and maintained. It is due to this fact sensible to be proactive by reaching out to trusted consultants prior to buy as they will assist with valve sizing and specifications primarily based on specific working conditions.
For existing 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 fully inspected, it will be sensible to seek the advice of your turbine bypass valve vendor for help and assist.
Many of those vendors provide full inspection and health examine services to establish areas the place metallic fatigue and stress cracking are developing. Ideally the valve may be repaired, but when a valve substitute is warranted, the vendor can even provide steering in evaluating bypass valve design kinds and selecting the finest choice for the particular software.
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