Turbine bypass valves are one of the most tough purposes in an influence plant. Keeping these valves working efficiently and avoiding unexpected failures is important for plant operation.
When engineers are asked to list essentially the most difficult control valve purposes, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive stress drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many vegetation are inclined to ignore these valves until they fail, creating unplanned outages, lost production, and excessive costs for reactive maintenance. This article provides advised strategies of inspection to anticipate and mitigate issues beforehand, and it provides improve alternate options ought to a valve need to be repaired or changed.
Strictly Severe Service
Severe service control valves are used in essentially the most troublesome installations inside course of plants. These installations generally embody cavitating, erosive, corrosive, noisy, high strain, excessive temperature, excessive strain drop, or high velocity media. Turbine bypass valves are uncovered to many of those course of situations; yet, they want to reply flawlessly and stay leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam turbines during plant startup and shutdown, as nicely 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 circumstances of the steam are acceptable 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 moist steam, and by making certain only acceptable steam circumstances and flows make their approach to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves may be employed to immediately shunt steam around a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to keep away from gear damage because of overpressure and excessive temperature, so the turbine bypass valve instantly opens to hold up flow through the system.
As it operates, a turbine makes use of steam to carry out work, decreasing outlet steam temperature and strain. When a turbine bypass valve opens, it’ll drop the stress, however the exit steam will stay quite superheated, probably destroying downstream equipment. To keep away from that situation, turbine bypass valves either incorporate a water injection system in the valve body, 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 an ideal storm of extreme service circumstances. While the plant is in operation, these valves must stay tightly closed to keep away from wasting vitality. When a turbine journey happens, the bypass valves must respond instantly, exposing them to fast temperature modifications and requiring them to pass very high flows at excessive strain drops, creating high noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that just about every turbine bypass valve will in the end fail in some way. Unfortunately, many of these valves are installed in difficult-to-access areas, are typically welded in place, and are usually closely insulated. As a result, they’re usually ignored until issues start to surface. Steam leakage through the valve is normally the primary symptom noticed, however far more vital and probably harmful harm can occur.
The extreme stress drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metallic within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can lead to catastrophic failure underneath pressure.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in each the valve itself and the encircling piping. Courtesy: Emerson
To avoid this drawback, every turbine bypass valve and its related piping should be routinely inspected. Some type of non-destructive examination ought to be periodically employed to detect metal fatigue issues which may be developing however usually are not but visually apparent. If a plant lacks the knowledge or tools to perform these inspections, the valve vendor or licensed consultant could also be utilized to perform 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 various welds positioned on the valve itself, and in 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 could be inspected to identify any creating issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to keep away from problems related to quenching and cracking.
There are additionally threats to trim elements, corresponding to seat surface erosion. One common expression of such erosion, specifically on the plug, is also recognized as “gear toothing.” This happens extra commonly in move down valves, where the steam accelerates via the cage holes after which strikes the trim/seat area immediately, inflicting extreme wear and decreasing service life. Over time, this high-velocity steam, which can include water throughout startup circumstances and magnetite during any condition, will erode and harm the valve seat and trim (Figure 4).
four. Flow down valve body designs are susceptible to trim injury because the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like soft goods and spray nozzles can normally be replaced comparatively easily and at minimal value. Trim elements that have to be replaced because of heavy put on, such as gear toothing, can be quite expensive, especially if their delivery needs to be expedited. The worst-case state of affairs for a turbine bypass valve is metal fatigue growing within the valve body or diffuser, with excessive restore procedures, or perhaps a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been put in in the course of the heyday of combined cycle plant development from 1998 to 2004. Most of these valves and piping techniques are displaying their age and often have important signs of metallic fatigue. Others have plenty of life left in them but may acquire advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the newest 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 this instance, a removable 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 beyond a direct replacement and evaluate potential improvements. A significantly necessary item to contemplate when looking at replacement and improvement opportunities is valve orientation.
Downward Flowing เกจวัดแรงดัน10bar with Horizontal Actuators. Historically, most turbine bypass valves employed a move down trim design paired with a horizontal actuator (Figure 5, left). This association is good for places with low overhead clearance, and it places the actuator closer to the deck or ground for straightforward access, nevertheless it creates a selection of long-term operational issues. Gravity tends to lead to increased and uneven put on on trim components, and the side-mounted actuator is vulnerable to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the main option a couple of many years ago for turbine bypass valves. New circulate up valve designs (right) could also be put in with out piping modifications in most situations, offering extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for move down is vertical mounting of the actuator. This arrangement leads to reduced and more even put on, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that alternative opportunities are sometimes for horizontally mounted actuators, and the 2 designs aren’t readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead area is available with existing horizontal actuator circulate down designs, a move up design can simply meet the present face-to-face requirements of the previous valve, whereas reducing a few of the wear widespread to circulate down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from circulate down to flow up, this change can be made without modifications to current piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life as a result of the steam is transferring 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 considerably, however that vitality dissipates into the big body cavity of the valve as an alternative of striking the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower because they pass over the seat and accelerate later because the steam moves by way of the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is definitely exhibiting erosion, however when in comparison with the circulate down trim proven above (Figure 4 right), the seating surface on the seat ring and plug stay relatively undamaged, permitting the valve to close off fully, the required mode in regular operation.
7. These photos of move up trim clearly show the erosive influence of high-velocity steam, but not like the circulate down trim injury proven previously, this valve has no harm to the seating surface and might still shut off tightly. Courtesy: Emerson
The circulate up valve type design nonetheless supplies the identical management 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 highly specialised pieces of engineered gear that should be carefully specified, installed, and maintained. It is due to this fact wise to be proactive by reaching out to trusted experts prior to purchase as they will assist with valve sizing and specs primarily based on specific working circumstances.
For current 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 might be sensible to consult your turbine bypass valve vendor for help and assist.
Many of those vendors provide full inspection and health examine providers to identify areas the place metal fatigue and stress cracking are creating. Ideally the valve can be repaired, but if a valve replacement is warranted, the vendor can also present steering in evaluating bypass valve design types and choosing the best option for the precise application.
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