Turbine bypass valves are one of the difficult purposes in a power plant. Keeping these valves working effectively and avoiding sudden failures is critical for plant operation.
When engineers are asked to record probably the most tough management valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high strain drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many plants tend to ignore these valves till they fail, creating unplanned outages, lost manufacturing, and high costs for reactive upkeep. This article supplies instructed methods of inspection to anticipate and mitigate issues beforehand, and it presents improve alternatives ought to a valve must be repaired or changed.
Strictly Severe Service
Severe service management valves are used in the most troublesome installations within course of vegetation. These installations commonly include cavitating, erosive, corrosive, noisy, excessive stress, high temperature, excessive pressure drop, or high velocity media. Turbine bypass valves are uncovered to many of those process situations; yet, they have to 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, in addition to when a turbine trips off-line (Figure 1). In regular operation the valves are utterly closed, forcing all the steam by way of 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 process occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting probably wet steam, and by ensuring only applicable steam conditions and flows make their approach to the turbine itself.
1. Depending on the facility plant design, a number of turbine bypass valves may be employed to instantly shunt steam round a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine trip, the steam should proceed flowing to avoid gear damage due to overpressure and high temperature, so the turbine bypass valve instantly opens to take care of circulate via the system.
As it operates, a turbine uses steam to perform work, reducing outlet steam temperature and stress. When a turbine bypass valve opens, it will drop the strain, however the exit steam will remain quite superheated, probably destroying downstream gear. To keep away from that scenario, turbine bypass valves both incorporate a water injection system within the valve physique, or employ a separate water injection desuperheater simply downstream, in both case to lower the exit steam temperature.
As a end result, turbine bypass valves face an ideal storm of extreme service conditions. While the plant is in operation, these valves must stay tightly closed to avoid wasting energy. When a turbine trip occurs, the bypass valves should reply immediately, exposing them to speedy temperature adjustments and requiring them to cross very high flows at high stress drops, creating excessive noise and doubtlessly excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that nearly every turbine bypass valve will finally fail indirectly. Unfortunately, many of these valves are installed in difficult-to-access areas, are typically welded in place, and are typically heavily insulated. As a end result, they’re usually ignored till problems start to floor. Steam leakage via the valve is usually the primary symptom noticed, however much more vital and potentially harmful injury can occur.
The excessive strain drops invariably generate excessive 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 metal fatigue can result in catastrophic failure underneath pressure.
2. Over time, the excessive vibrations and thermal shock skilled by turbine bypass valves will fatigue the metallic and create cracks in both the valve itself and the surrounding piping. Courtesy: Emerson
To keep away from this downside, every turbine bypass valve and its associated piping must be routinely inspected. Some type of non-destructive examination must be periodically employed to detect metallic fatigue problems which might be developing but are not but visually apparent. If a plant lacks the information or equipment to perform these inspections, the valve vendor or licensed consultant could additionally be utilized to carry out turbine bypass well being examine services (Figure 3).
3. A routine turbine bypass valve health examine and full inspection is strongly advised. Metal fatigue can develop in numerous welds positioned 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, buyer connection welds, diffuser welds, and surrounding piping may be inspected to establish any creating issues earlier than the tools is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid problems associated to quenching and cracking.
There are also threats to trim components, such as seat floor erosion. One common expression of such erosion, particularly on the plug, is also known as “gear toothing.” This happens more generally in move down valves, where the steam accelerates through the cage holes after which strikes the trim/seat area directly, causing excessive wear and reducing service life. Over time, this high-velocity steam, which might include water during startup conditions and magnetite during any condition, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve body designs are susceptible to trim harm as the wet, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like gentle goods and spray nozzles can usually get replaced comparatively simply and at minimal cost. Trim elements that must be changed due to heavy wear, corresponding to gear toothing, can be fairly costly, especially if their delivery needs to be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue creating in the valve body or diffuser, with excessive restore procedures, or maybe a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were installed during the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping systems are exhibiting their age and often have significant signs of metal fatigue. Others have plenty of life left in them however may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the most recent sealing expertise might inject new life into the valve.
Another instance of a expertise upgrade is when there have been repeated failures with a welded diffuser. In เกจวัดแรงดันอาร์กอน , a detachable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve have to be replaced, plant personnel are strongly encouraged to look beyond a direct replacement and consider potential enhancements. A significantly necessary merchandise to assume about when looking at alternative and enchancment 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 areas with low overhead clearance, and it locations the actuator nearer to the deck or floor for straightforward entry, nevertheless it creates a number of long-term operational issues. Gravity tends to end in increased and uneven wear on trim components, and the side-mounted actuator is prone to response issues.
5. Downward flowing valves with horizontal actuators (left) were the main choice a couple of a long time in the past for turbine bypass valves. New flow up valve designs (right) could also be put in with out piping modifications in most cases, offering 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 arrangement leads to reduced and extra even put on, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement opportunities are usually for horizontally mounted actuators, and the 2 designs aren’t readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is available with current horizontal actuator circulate down designs, a flow up design can simply meet the prevailing face-to-face requirements of the previous valve, whereas decreasing some of the put on frequent to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from circulate all the way down to circulate up, this alteration 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 moving comparatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes through the small holes of the cage, it accelerates considerably, but that energy dissipates into the big physique cavity of the valve as a substitute of striking the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease because they move over the seat and speed up later because 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, but when compared to the flow down trim shown above (Figure 4 right), the seating floor on the seat ring and plug stay comparatively undamaged, permitting the valve to close off fully, the required mode in normal operation.
7. These footage of circulate up trim clearly present the erosive influence of high-velocity steam, however in contrast to the flow down trim harm shown previously, this valve has no injury to the seating floor and can still shut off tightly. Courtesy: Emerson
The circulate up valve fashion design still offers the identical control responsiveness and move capability, however the brand new move association dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized items of engineered equipment that must be carefully specified, put in, and maintained. It is subsequently sensible to be proactive by reaching out to trusted experts prior to buy as they can assist with valve sizing and specs primarily based on particular working situations.
For current 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 would be clever to seek the guidance of your turbine bypass valve vendor for help and assist.
Many of these distributors provide full inspection and well being verify companies to identify areas where metallic fatigue and stress cracking are growing. Ideally the valve can be repaired, but if a valve replacement is warranted, the seller also can provide guidance in evaluating bypass valve design styles and selecting the best option for the particular utility.
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