Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most tough applications in a power plant. Keeping these valves operating effectively and avoiding surprising failures is crucial for plant operation.
When engineers are asked to record probably the most difficult management valve applications, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high strain drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many plants are inclined to ignore these valves till they fail, creating unplanned outages, lost production, and high costs for reactive maintenance. This article provides instructed strategies of inspection to anticipate and mitigate issues beforehand, and it provides improve options should a valve need to be repaired or replaced.
Strictly Severe Service
Severe service control valves are used in probably the most difficult installations inside process vegetation. These installations commonly embrace cavitating, erosive, corrosive, noisy, high strain, excessive temperature, excessive strain drop, or high velocity media. Turbine bypass valves are uncovered to many of these process conditions; but, they need to respond flawlessly and remain leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam generators during plant startup and shutdown, in addition to when a turbine journeys off-line (Figure 1). In normal operation the valves are completely closed, forcing all of the steam via a turbine. During startup, bypass valves divert steam away from the turbine until the properties and situations of the steam are applicable for sending it to the turbine. This course of happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting potentially wet steam, and by ensuring solely acceptable steam situations and flows make their approach to the turbine itself.
1. Depending on the power plant design, a number of turbine bypass valves may be employed to instantly shunt steam round a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam should proceed flowing to avoid tools damage due to overpressure and excessive temperature, so the turbine bypass valve instantly opens to take care of circulate via the system.
As it operates, a turbine uses steam to carry out work, lowering outlet steam temperature and stress. When a turbine bypass valve opens, it will drop the stress, however the exit steam will stay quite superheated, potentially destroying downstream gear. To keep away from that state of affairs, turbine bypass valves both incorporate a water injection system within 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 outcome, turbine bypass valves face a perfect storm of extreme service situations. While the plant is in operation, these valves should stay tightly closed to avoid wasting vitality. When a turbine journey occurs, the bypass valves must reply instantly, exposing them to speedy temperature changes and requiring them to move very excessive flows at high pressure drops, creating excessive noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that nearly each turbine bypass valve will in the end fail indirectly. Unfortunately, many of those valves are put in in difficult-to-access locations, are usually welded in place, and tend to be closely insulated. As a outcome, they are typically ignored till problems start to floor. Steam leakage by way of the valve is often the primary symptom observed, however far more vital and probably harmful damage can happen.
The extreme 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 steel fatigue may end up in catastrophic failure underneath strain.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metal and create cracks in both the valve itself and the encompassing piping. Courtesy: Emerson
To avoid this downside, 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 metal fatigue issues that could be creating however aren’t but visually apparent. If a plant lacks the data or tools to carry out these inspections, the valve vendor or licensed representative could additionally be utilized to carry out turbine bypass health check providers (Figure 3).
three. A routine turbine bypass valve well being examine and full inspection is strongly advised. 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 body and water manifold, buyer connection welds, diffuser welds, and surrounding piping can be inspected to establish any creating problems earlier than the tools is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to keep away from issues associated to quenching and cracking.
There are additionally threats to trim components, such as seat surface erosion. One frequent expression of such erosion, particularly on the plug, is often referred to as “gear toothing.” This occurs extra commonly in flow down valves, where the steam accelerates through the cage holes after which strikes the trim/seat space directly, causing excessive put on and reducing service life. Over time, this high-velocity steam, which may include water during startup conditions and magnetite during any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve body designs are susceptible to trim damage as the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like soft goods and spray nozzles can often get replaced relatively simply and at minimal cost. Trim components that have to be changed as a result of heavy wear, similar to gear toothing, can be quite costly, particularly if their delivery must be expedited. The worst-case situation for a turbine bypass valve is steel fatigue developing within the valve body or diffuser, with extreme restore procedures, or maybe a full valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service have been installed in the course of the heyday of combined cycle plant development from 1998 to 2004. Most of these valves and piping techniques are showing their age and infrequently have vital indicators of metallic fatigue. Others have loads of life left in them however could benefit from upgrades in technology. Technology and practices have improved, and upgrading to the latest 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 reduce required upkeep.
If a turbine bypass valve have to be changed, plant personnel are strongly encouraged to look past a direct replacement and consider potential enhancements. A significantly essential merchandise to contemplate when looking at alternative and improvement opportunities is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a flow down trim design paired with a horizontal actuator (Figure 5, left). This association is ideal for areas with low overhead clearance, and it locations the actuator closer to the deck or floor for simple access, but it creates numerous long-term operational problems. Gravity tends to end in elevated and uneven wear on trim components, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) have been the main possibility a number of many years in the past for turbine bypass valves. New flow up valve designs (right) could additionally be installed without piping modifications in most instances, providing prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for circulate down is vertical mounting of the actuator. This association results in decreased and more even wear, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that substitute alternatives are usually for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead area is on the market with present horizontal actuator circulate down designs, a move up design can simply meet the prevailing face-to-face requirements of the previous valve, while lowering a number of the put on frequent to flow down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from move right down to move up, this modification could be made with out modifications to existing piping.
pressure gauge น้ำ up valves (Figure 5, right) considerably prolong seat and trim service life because the steam is moving relatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes through the small holes of the cage, it accelerates significantly, however that power dissipates into the large physique cavity of the valve instead 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 result of they cross over the seat and speed up later as the steam strikes by way of the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is certainly showing erosion, however when in comparability with the circulate down trim shown above (Figure 4 right), the seating floor on the seat ring and plug remain relatively 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 influence of high-velocity steam, however unlike the move down trim injury shown beforehand, this valve has no injury to the seating floor and might nonetheless shut off tightly. Courtesy: Emerson
The move up valve type design still offers the identical control responsiveness and flow capability, but the new move arrangement dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised items of engineered tools that must be carefully specified, installed, and maintained. It is due to this fact sensible to be proactive by reaching out to trusted consultants prior to buy as they will help with valve sizing and specifications based on specific working situations.
For existing installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time since the plant turbine bypass valves have been absolutely inspected, it would be sensible to consult your turbine bypass valve vendor for help and support.
Many of those vendors provide full inspection and health verify services to determine areas the place metallic fatigue and stress cracking are creating. Ideally the valve can be repaired, but when a valve substitute is warranted, the seller can even provide steering in evaluating bypass valve design types and choosing the finest choice for the particular application.
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