Turbine bypass valves are one of the most tough functions in an influence plant. Keeping these valves working efficiently and avoiding unexpected failures is critical for plant operation.
When engineers are asked to record the most troublesome 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, misplaced production, and excessive costs for reactive upkeep. This article provides suggested strategies of inspection to anticipate and mitigate points beforehand, and it offers improve alternate options ought to a valve have to be repaired or replaced.
Strictly Severe Service
Severe service control valves are utilized in essentially the most difficult installations within course of crops. These installations generally embrace cavitating, erosive, corrosive, noisy, excessive stress, excessive temperature, excessive pressure drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those process conditions; but, they want to respond flawlessly and remain leak free when closed.
As their name 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 regular operation the valves are utterly closed, forcing all of the steam via a turbine. During startup, bypass valves divert steam away from the turbine till the properties and circumstances 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 probably moist steam, and by making certain only appropriate steam conditions and flows make their method to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves could also be employed to immediately shunt steam round a turbine should it trip offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to keep away from gear harm because of overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain move by way of the system.
As it operates, a turbine makes use of steam to carry out work, lowering outlet steam temperature and stress. When a turbine bypass valve opens, it’ll drop the pressure, however the exit steam will remain quite superheated, doubtlessly destroying downstream equipment. To avoid that situation, 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 either case to decrease the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of extreme service circumstances. While the plant is in operation, these valves must stay tightly closed to keep away from wasting energy. When a turbine trip happens, the bypass valves must respond instantly, exposing them to fast temperature changes and requiring them to pass very high flows at excessive stress drops, creating high noise and doubtlessly excessive vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly each turbine bypass valve will ultimately fail ultimately. Unfortunately, many of these valves are put in in difficult-to-access locations, are usually welded in place, and are usually heavily insulated. As a outcome, they are typically ignored till problems start to surface. Steam leakage via the valve is often the first symptom observed, but far more significant and doubtlessly dangerous harm can happen.
The excessive pressure drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can outcome in catastrophic failure under stress.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in each the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this drawback, every turbine bypass valve and its related piping ought to be routinely inspected. Some kind of non-destructive examination must be periodically employed to detect metal fatigue issues that may be creating but aren’t but visually apparent. If a plant lacks the information or gear to carry out these inspections, the valve vendor or licensed consultant may be utilized to perform turbine bypass well being check providers (Figure 3).
3. A routine turbine bypass valve health verify and full inspection is strongly suggested. Metal fatigue can develop in various welds located 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 identify any developing problems earlier than the gear 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 elements, corresponding to seat floor erosion. One common expression of such erosion, particularly on the plug, is sometimes called “gear toothing.” This happens more generally in circulate down valves, the place the steam accelerates via the cage holes after which strikes the trim/seat area instantly, inflicting excessive put on and decreasing service life. Over time, this high-velocity steam, which may comprise water during startup circumstances and magnetite throughout any situation, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim damage because the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like delicate goods and spray nozzles can usually get replaced comparatively easily and at minimal price. Trim elements that have to be replaced due to heavy wear, similar to gear toothing, could be quite costly, especially if their delivery must be expedited. The worst-case situation for a turbine bypass valve is metallic fatigue growing in the valve body or diffuser, with excessive restore procedures, or even a complete valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service were put in through the heyday of combined cycle plant building from 1998 to 2004. Most of these valves and piping systems are showing their age and infrequently have important indicators of steel fatigue. Others have loads of life left in them but may gain advantage from upgrades in technology. Technology and practices have improved, and upgrading to the latest sealing expertise might inject new life into the valve.
Another instance of a technology upgrade is when there have been repeated failures with a welded diffuser. In this occasion, a removable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve should be replaced, plant personnel are strongly encouraged to look past a direct replacement and consider potential improvements. A notably necessary item to think about when taking a look at replacement and improvement 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 good for areas with low overhead clearance, and it places the actuator closer to the deck or ground for easy access, nevertheless it creates a variety of long-term operational problems. Gravity tends to end in elevated and uneven wear on trim elements, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) were the principle option a few decades in the past for turbine bypass valves. New circulate up valve designs (right) may be put in with out piping modifications in most instances, offering prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for flow down is vertical mounting of the actuator. This arrangement leads to decreased and more even put on, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that replacement opportunities are sometimes for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is available with current horizontal actuator flow down designs, a flow up design can easily meet the existing face-to-face necessities of the outdated valve, whereas decreasing a few of the put on widespread to flow down designs with a horizontal actuator, corresponding to gear toothing. Because the trim just switches from move down to circulate up, this alteration can be made with out modifications to current piping.
Flow up valves (Figure 5, right) significantly prolong seat and trim service life because the steam is moving comparatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates significantly, but that power dissipates into the big body cavity of the valve as a substitute of hanging the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease as a end result of they move over the seat and speed up later as the steam moves by way of the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is actually showing erosion, but when in comparability with the circulate down trim proven above (Figure four right), the seating surface on the seat ring and plug remain relatively undamaged, allowing the valve to close off fully, the required mode in regular operation.
7. These footage of circulate up trim clearly show the erosive influence of high-velocity steam, but not like the move down trim harm shown beforehand, this valve has no injury to the seating surface and can still shut off tightly. Courtesy: Emerson
The move up valve fashion design nonetheless supplies the identical control responsiveness and move capacity, however the model new circulate association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised items of engineered gear that must be fastidiously specified, installed, and maintained. It is subsequently wise to be proactive by reaching out to trusted experts prior to purchase as they can assist with valve sizing and specifications based mostly on particular operating circumstances.
For ร้านซ่อมเครื่องวัดความดันomron bypass valve failures are bedeviling your facility, or if it has been a while since the plant turbine bypass valves have been absolutely inspected, it will be wise to seek the advice of your turbine bypass valve vendor for help and assist.
Many of those distributors supply full inspection and well being verify services to establish areas the place metallic fatigue and stress cracking are developing. Ideally the valve may be repaired, but when a valve replacement is warranted, the seller can also provide steering in evaluating bypass valve design types and selecting the greatest choice for the particular utility.
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