Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most difficult purposes in an influence plant. Keeping these valves operating effectively and avoiding sudden failures is important for plant operation.
When engineers are requested to listing essentially the most tough control valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive pressure drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many crops tend to ignore these valves till they fail, creating unplanned outages, lost production, and excessive costs for reactive maintenance. This article offers instructed methods of inspection to anticipate and mitigate points beforehand, and it provides upgrade alternatives should a valve must be repaired or changed.
Strictly Severe Service
Severe service management valves are utilized in probably the most difficult installations inside course of crops. These installations generally embody cavitating, erosive, corrosive, noisy, excessive strain, high temperature, excessive pressure drop, or high velocity media. Turbine bypass valves are uncovered to many of those course of situations; but, they have to respond flawlessly and remain leak free when closed.
As their identify 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 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 till the properties and conditions of the steam are appropriate for sending it to the turbine. เกจแรงดันสูง occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting doubtlessly wet steam, and by ensuring solely applicable steam conditions and flows make their approach 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 trip, the steam should continue flowing to avoid gear harm due to overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain up circulate via the system.
As it operates, a turbine uses steam to perform work, decreasing outlet steam temperature and pressure. When a turbine bypass valve opens, it’ll drop the pressure, but the exit steam will remain fairly superheated, probably destroying downstream tools. To avoid that situation, turbine bypass valves either incorporate a water injection system within the valve body, 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 should remain tightly closed to keep away from wasting energy. When a turbine trip occurs, the bypass valves must respond immediately, exposing them to fast temperature modifications and requiring them to cross very excessive flows at excessive strain drops, creating high noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that just about every turbine bypass valve will ultimately fail indirectly. Unfortunately, many of these valves are installed in difficult-to-access places, are usually welded in place, and are usually closely insulated. As a outcome, they are typically ignored until issues begin to floor. Steam leakage by way of the valve is usually the primary symptom observed, but far more important and potentially dangerous damage can occur.
The excessive strain drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can lead to catastrophic failure under stress.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in both the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this drawback, every turbine bypass valve and its associated piping ought to be routinely inspected. Some type of non-destructive examination must be periodically employed to detect metallic fatigue problems which could be developing but usually are not yet visually apparent. If a plant lacks the knowledge or tools to perform these inspections, the valve vendor or authorized representative could additionally be utilized to carry out turbine bypass well being check providers (Figure 3).
3. A routine turbine bypass valve well being check and full inspection is strongly advised. Metal fatigue can develop in varied 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, buyer connection welds, diffuser welds, and surrounding piping could be inspected to determine any developing issues before the gear 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 additionally threats to trim parts, corresponding to seat floor erosion. One common expression of such erosion, specifically on the plug, is also recognized as “gear toothing.” This occurs extra commonly 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 lowering service life. Over time, this high-velocity steam, which may contain water throughout startup situations and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve body designs are prone to trim harm as the moist, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common maintenance items like soft items and spray nozzles can usually be replaced relatively simply and at minimal cost. Trim elements that must be changed as a result of heavy put on, such as gear toothing, may be quite costly, particularly if their delivery needs to be expedited. The worst-case scenario for a turbine bypass valve is metallic fatigue developing in the valve body or diffuser, with excessive repair procedures, or perhaps a complete valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been installed in the course of the heyday of mixed cycle plant development from 1998 to 2004. Most of those valves and piping methods are exhibiting their age and sometimes have vital indicators of metal fatigue. Others have loads of life left in them but may gain advantage from upgrades in know-how. Technology and practices have improved, and upgrading to the newest sealing expertise could inject new life into the valve.
Another instance 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 maintenance.
If a turbine bypass valve must be replaced, plant personnel are strongly inspired to look beyond a direct alternative and evaluate potential improvements. A significantly essential merchandise to contemplate when looking at alternative 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 good for places with low overhead clearance, and it places the actuator nearer to the deck or floor for easy access, but it creates a variety of long-term operational issues. Gravity tends to end in elevated and uneven put on on trim elements, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the primary possibility a few many years ago for turbine bypass valves. New circulate 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 decreased and extra even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that alternative alternatives are sometimes for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead area is out there with existing horizontal actuator flow down designs, a circulate up design can easily meet the prevailing face-to-face requirements of the outdated valve, whereas lowering some of the wear widespread to move down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from flow all the way down to move up, this change can be made with out modifications to existing piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life as a outcome of the steam is shifting relatively slowly because it passes over the plug and seat (Figure 6). As the steam moves via the small holes of the cage, it accelerates significantly, but that energy dissipates into the large physique cavity of the valve as an alternative of striking the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease as a result of they pass 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 flow up trim (Figure 7) is certainly displaying erosion, but when compared to the move down trim proven above (Figure 4 right), the seating surface on the seat ring and plug remain comparatively undamaged, permitting the valve to close off utterly, the required mode in regular operation.
7. These footage of flow up trim clearly show the erosive influence of high-velocity steam, but unlike the circulate down trim harm shown beforehand, this valve has no harm to the seating surface and may still shut off tightly. Courtesy: Emerson
The move up valve type design nonetheless supplies the identical control responsiveness and circulate capability, but the model new circulate association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised items of engineered tools that have to be fastidiously specified, installed, and maintained. It is therefore wise to be proactive by reaching out to trusted experts prior to purchase as they will assist with valve sizing and specifications primarily based on particular working circumstances.
For present installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while for the reason that plant turbine bypass valves have been fully inspected, it would be sensible to seek the advice of your turbine bypass valve vendor for help and assist.
Many of these vendors supply full inspection and well being examine companies to determine areas where metallic fatigue and stress cracking are developing. Ideally the valve may be repaired, but when a valve replacement is warranted, the seller can even present steering in evaluating bypass valve design styles and choosing the most suitable choice for the specific application.

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