Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most troublesome applications in an influence plant. Keeping these valves operating efficiently and avoiding surprising failures is crucial for plant operation.
When engineers are asked to record probably the most troublesome control valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive pressure drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many vegetation are most likely to ignore these valves till they fail, creating unplanned outages, misplaced production, and excessive costs for reactive upkeep. This article offers instructed strategies of inspection to anticipate and mitigate issues beforehand, and it offers upgrade alternatives ought to a valve must be repaired or replaced.
Strictly Severe Service
Severe service control valves are used in the most difficult installations within process plants. These installations generally include cavitating, erosive, corrosive, noisy, excessive strain, high temperature, high stress drop, or excessive velocity media. Turbine bypass valves are exposed to many of these course of situations; but, they must respond flawlessly and stay leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam generators during plant startup and shutdown, as properly as when a turbine trips off-line (Figure 1). In normal operation the valves are utterly closed, forcing all of 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 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 potentially moist steam, and by ensuring only appropriate steam circumstances and flows make their approach to the turbine itself.
1. Depending on the power plant design, a quantity of turbine bypass valves may be employed to immediately shunt steam around a turbine should it trip offline. Courtesy: Emerson
Should a turbine journey, the steam must proceed flowing to keep away from gear damage because of overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain up circulate through the system.
As it operates, a turbine uses steam to carry out work, reducing outlet steam temperature and strain. When a turbine bypass valve opens, it’ll drop the pressure, however the exit steam will stay quite superheated, doubtlessly destroying downstream equipment. To avoid that state of affairs, turbine bypass valves both incorporate a water injection system within the valve physique, or employ a separate water injection desuperheater simply downstream, in either case to lower the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of extreme service circumstances. While the plant is in operation, these valves must remain tightly closed to avoid wasting vitality. When a turbine journey happens, the bypass valves must respond instantly, exposing them to rapid temperature changes and requiring them to pass very high flows at excessive stress drops, creating excessive noise and probably extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly every turbine bypass valve will ultimately fail indirectly. Unfortunately, many of these valves are put in in difficult-to-access areas, are usually welded in place, and are typically closely insulated. As a outcome, they’re often ignored until problems begin to floor. Steam leakage by way of the valve is usually the primary symptom observed, but much more vital and potentially dangerous injury can happen.
The extreme pressure drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue may find yourself in catastrophic failure underneath pressure.
2. Over time, the excessive vibrations and thermal shock skilled by turbine bypass valves will fatigue the metal and create cracks in both the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this problem, each turbine bypass valve and its associated piping ought to be routinely inspected. Some type of non-destructive examination ought to be periodically employed to detect metallic fatigue issues which might be creating however aren’t but visually obvious. If a plant lacks the knowledge or equipment to perform these inspections, the valve vendor or approved representative could additionally be utilized to perform turbine bypass health examine services (Figure 3).
3. A routine turbine bypass valve health examine and full inspection is strongly suggested. Metal fatigue can develop in various welds positioned on the valve itself, and in the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, customer connection welds, diffuser welds, and surrounding piping could be inspected to determine any growing issues before the tools is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to keep away from problems associated to quenching and cracking.
There are additionally threats to trim components, corresponding to seat floor erosion. One common expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This happens more commonly in circulate down valves, the place the steam accelerates by way of the cage holes after which strikes the trim/seat area instantly, inflicting extreme put on and lowering service life. Over time, this high-velocity steam, which may comprise water throughout startup circumstances and magnetite throughout any situation, will erode and injury the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim harm because the wet, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance items like soft items and spray nozzles can normally be replaced relatively easily and at minimal price. Trim parts that need to be replaced because of heavy put on, such as gear toothing, may be fairly costly, especially if their supply needs to be expedited. Bonanza -case situation for a turbine bypass valve is steel fatigue growing within the valve body or diffuser, with excessive restore procedures, or even a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were installed in the course of the heyday of mixed cycle plant development from 1998 to 2004. Most of these valves and piping methods are showing their age and infrequently have important indicators of steel fatigue. Others have plenty of life left in them but may gain advantage from upgrades in technology. Technology and practices have improved, and upgrading to the latest sealing know-how may 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 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 inspired to look past a direct replacement and evaluate potential enhancements. A significantly important item to assume about when taking a look at alternative and improvement opportunities 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 ideal for places with low overhead clearance, and it places the actuator closer to the deck or floor for easy entry, nevertheless it creates numerous long-term operational issues. Gravity tends to result in elevated and uneven put on on trim elements, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the main choice a couple of decades in the past for turbine bypass valves. New flow up valve designs (right) may be installed with out piping modifications in most cases, offering extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This association ends in reduced and extra even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that alternative alternatives are typically for horizontally mounted actuators, and the 2 designs are not readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead area is out there with current horizontal actuator circulate down designs, a move up design can easily meet the existing face-to-face requirements of the previous valve, while decreasing a variety of the put on frequent to circulate down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from move right down to flow up, this change can be made with out modifications to existing piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life as a end result of the steam is moving comparatively slowly as it passes over the plug and seat (Figure 6). As the steam moves via the small holes of the cage, it accelerates significantly, however that energy dissipates into the massive body cavity of the valve as a substitute of striking the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower because they pass over the seat and speed up later as the steam strikes through the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is definitely displaying erosion, but when compared to the move down trim proven above (Figure 4 right), the seating surface on the seat ring and plug stay comparatively undamaged, allowing the valve to close off utterly, the required mode in normal operation.
7. These photos of circulate up trim clearly show the erosive impression of high-velocity steam, however in contrast to the flow down trim harm proven beforehand, this valve has no harm to the seating surface and might still shut off tightly. Courtesy: Emerson
The circulate up valve style design still supplies the same management responsiveness and circulate capacity, however the new circulate association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised items of engineered gear that have to be fastidiously specified, installed, and maintained. It is therefore sensible to be proactive by reaching out to trusted consultants prior to buy as they can help with valve sizing and specs based on particular working situations.
For present installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while since the plant turbine bypass valves have been fully inspected, it will be clever to seek the assistance of your turbine bypass valve vendor for help and help.
Many of those vendors offer full inspection and health check providers to identify areas the place metal fatigue and stress cracking are developing. Ideally the valve can be repaired, but if a valve substitute is warranted, the vendor can even provide steering in evaluating bypass valve design types and choosing the best suited choice for the particular software.
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