Turbine Bypass Valve Challenges

Turbine bypass valves are one of the tough applications in an influence plant. Keeping these valves working efficiently and avoiding sudden failures is important for plant operation.
When engineers are asked to listing the most troublesome management valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, high pressure drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, many crops are likely to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and high prices for reactive maintenance. This article provides instructed methods of inspection to anticipate and mitigate points beforehand, and it offers improve alternatives should a valve have to be repaired or changed.
Strictly Severe Service
Severe service control valves are used in essentially the most difficult installations inside course of plants. These installations commonly embrace cavitating, erosive, corrosive, noisy, excessive pressure, high temperature, high pressure drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these process situations; yet, they must reply flawlessly and remain leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam turbines during plant startup and shutdown, in addition to when a turbine trips off-line (Figure 1). In regular operation the valves are completely closed, forcing all of the steam through a turbine. During startup, bypass valves divert steam away from the turbine till the properties and situations of the steam are appropriate 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 probably wet steam, and by guaranteeing only appropriate steam circumstances and flows make their way to the turbine itself.
1. Depending on the power plant design, a quantity of turbine bypass valves could additionally be employed to immediately shunt steam round a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine trip, the steam should proceed flowing to avoid tools damage due to overpressure and high temperature, so the turbine bypass valve instantly opens to maintain circulate through the system.
As it operates, a turbine uses steam to perform work, lowering outlet steam temperature and pressure. When เกวัดแรงดันน้ำ bypass valve opens, it’ll drop the pressure, but the exit steam will remain quite superheated, doubtlessly destroying downstream gear. To keep away from 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 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 should stay tightly closed to keep away from wasting energy. When a turbine trip happens, the bypass valves should reply instantly, exposing them to speedy temperature modifications and requiring them to pass very high flows at high strain drops, creating high noise and probably extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly every turbine bypass valve will finally fail ultimately. Unfortunately, many of those valves are installed in difficult-to-access areas, are sometimes welded in place, and are typically heavily insulated. As a result, they are usually ignored until issues begin to surface. Steam leakage via the valve is often the first symptom seen, however far more vital and probably harmful harm can happen.
The excessive pressure drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metallic within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can lead to 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 both the valve itself and the encompassing piping. Courtesy: Emerson
To avoid this problem, every turbine bypass valve and its related piping must be routinely inspected. Some sort of non-destructive examination should be periodically employed to detect steel fatigue issues that might be growing but usually are not but visually apparent. If a plant lacks the knowledge or gear to perform these inspections, the valve vendor or authorized consultant may be utilized to carry out turbine bypass well being verify services (Figure 3).
three. A routine turbine bypass valve well being check and full inspection is strongly advised. Metal fatigue can develop in varied welds located on the valve itself, and within 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 can be inspected to establish any creating issues earlier than the gear is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid problems related to quenching and cracking.
There are also threats to trim parts, similar to seat surface erosion. One frequent expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This happens extra generally in circulate down valves, where the steam accelerates through the cage holes after which strikes the trim/seat space immediately, inflicting excessive put on and reducing service life. Over time, this high-velocity steam, which might comprise water during startup situations and magnetite during any situation, will erode and harm the valve seat and trim (Figure 4).
four. Flow down valve body designs are susceptible to trim harm as the moist, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like delicate goods and spray nozzles can usually get replaced comparatively easily and at minimal cost. Trim parts that need to be replaced as a outcome of heavy wear, similar to 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 perhaps a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service had been installed through the heyday of mixed cycle plant development from 1998 to 2004. Most of those valves and piping systems are showing their age and infrequently have vital signs of steel fatigue. Others have plenty of life left in them however may benefit from upgrades in technology. Technology and practices have improved, and upgrading to the most recent sealing technology might inject new life into the valve.
Another example of a know-how upgrade is when there have been repeated failures with a welded diffuser. In this instance, a detachable two-in-one seat diffuser will scale back required upkeep.
If a turbine bypass valve should be changed, plant personnel are strongly inspired to look beyond a direct alternative and evaluate potential improvements. A particularly important item 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 right for locations with low overhead clearance, and it locations the actuator closer to the deck or ground for simple entry, nevertheless it creates a variety of long-term operational issues. Gravity tends to result in elevated and uneven put on on trim elements, and the side-mounted actuator is vulnerable to response issues.
5. Downward flowing valves with horizontal actuators (left) have been the primary option a couple of a long time in the past for turbine bypass valves. New flow up valve designs (right) could also be installed with out piping modifications in most instances, providing extended 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 association ends in reduced and more even put on, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that replacement alternatives are usually for horizontally mounted actuators, and the 2 designs aren’t readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead house is available with current horizontal actuator flow down designs, a circulate up design can easily meet the existing face-to-face requirements of the previous valve, while decreasing some of the put on widespread to circulate down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from move all the means down to flow up, this modification could be made with out modifications to present piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life because the steam is transferring relatively slowly as 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 vitality dissipates into the large physique cavity of the valve as a substitute of striking the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower because they cross over the seat and speed up later because the steam strikes through the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is certainly showing erosion, but when compared to the move down trim shown above (Figure four right), the seating floor on the seat ring and plug remain relatively undamaged, allowing the valve to shut off fully, the required mode in normal operation.
7. These photos of flow up trim clearly show the erosive impact of high-velocity steam, but unlike the circulate down trim injury proven beforehand, this valve has no damage to the seating floor and can nonetheless shut off tightly. Courtesy: Emerson
The flow up valve type design nonetheless supplies the identical management responsiveness and move capacity, but the new flow arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised pieces of engineered gear that have to be fastidiously 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 working situations.
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 absolutely inspected, it will be clever to seek the assistance of your turbine bypass valve vendor for help and assist.
Many of those distributors supply full inspection and well being examine providers to establish areas where steel fatigue and stress cracking are developing. Ideally the valve may be repaired, but if a valve substitute is warranted, the vendor also can present steerage in evaluating bypass valve design kinds and choosing the best suited choice for the particular application.
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