Turbine bypass valves are one of the most difficult functions in an influence plant. Keeping these valves working effectively and avoiding sudden failures is important for plant operation.
When engineers are asked to record probably the most tough control valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive stress drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many crops are probably to ignore these valves till they fail, creating unplanned outages, lost manufacturing, and high costs for reactive upkeep. This article offers suggested methods of inspection to anticipate and mitigate issues beforehand, and it offers improve options should a valve must be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in essentially the most difficult installations within process plants. These installations generally include cavitating, erosive, corrosive, noisy, excessive strain, high temperature, high strain drop, or high velocity media. Turbine bypass valves are uncovered to many of those course of situations; yet, they have to respond flawlessly and remain leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam turbines 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 via a turbine. During startup, bypass valves divert steam away from the turbine till the properties and circumstances 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 probably wet steam, and by ensuring only appropriate steam situations and flows make their approach to the turbine itself.
1. Depending on the ability plant design, a quantity of turbine bypass valves may be employed to instantly shunt steam round a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine journey, the steam should proceed flowing to avoid tools damage due to overpressure and high temperature, so the turbine bypass valve instantly opens to take care of circulate by way of the system.
As it operates, a turbine makes use of steam to carry out work, decreasing outlet steam temperature and stress. When a turbine bypass valve opens, it’ll drop the stress, however the exit steam will remain fairly superheated, probably destroying downstream gear. To avoid that state of affairs, turbine bypass valves either incorporate a water injection system in the valve physique, or make use of a separate water injection desuperheater just downstream, in either case to lower the exit steam temperature.
As a outcome, turbine bypass valves face a perfect storm of severe service situations. While the plant is in operation, these valves must remain tightly closed to keep away from wasting energy. When a turbine trip happens, the bypass valves must respond instantly, exposing them to speedy temperature changes and requiring them to cross very high flows at high stress drops, creating high noise and doubtlessly excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that virtually every turbine bypass valve will finally fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access areas, are usually welded in place, and tend to be heavily insulated. As a outcome, they are often ignored till problems begin to surface. Steam leakage by way of the valve is often the primary symptom seen, but far more important and potentially harmful injury can occur.
The extreme strain drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue may end up in catastrophic failure under pressure.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metal and create cracks in each the valve itself and the encircling piping. Courtesy: Emerson
To avoid this problem, each turbine bypass valve and its associated piping must be routinely inspected. Some kind of non-destructive examination must be periodically employed to detect metal fatigue problems that might be growing however are not but visually apparent. If a plant lacks the information or equipment to carry out these inspections, the valve vendor or authorized consultant may be utilized to perform turbine bypass well being verify companies (Figure 3).
three. A routine turbine bypass valve health verify and full inspection is strongly advised. Metal fatigue can develop in varied welds located on the valve itself, and within the inlet, outlet, and water supply 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 identify any developing problems before the tools is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to keep away from issues related to quenching and cracking.
There are additionally threats to trim components, such as seat surface erosion. One frequent expression of such erosion, specifically on the plug, is also known as “gear toothing.” This occurs more commonly in circulate down valves, where the steam accelerates via the cage holes and then strikes the trim/seat area instantly, causing excessive put on and decreasing service life. Over time, this high-velocity steam, which may comprise water throughout startup situations and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve physique designs are prone to trim harm because the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like gentle items and spray nozzles can usually be replaced comparatively easily and at minimal cost. Trim components that must be replaced as a result of heavy wear, similar to gear toothing, may be fairly costly, particularly if their delivery must be expedited. Thought-provoking -case situation for a turbine bypass valve is metal fatigue developing within the valve physique or diffuser, with extreme restore procedures, or even a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been put in during the heyday of combined cycle plant development from 1998 to 2004. Most of these valves and piping methods are exhibiting their age and often have significant indicators of metal fatigue. Others have plenty of life left in them however could benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing know-how might inject new life into the valve.
Another instance of a know-how improve is when there have been repeated failures with a welded diffuser. In this occasion, a removable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve should be replaced, plant personnel are strongly encouraged to look beyond a direct alternative and evaluate potential improvements. A significantly necessary merchandise to consider when looking at replacement and enchancment 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 arrangement is ideal for locations with low overhead clearance, and it places the actuator closer to the deck or floor for easy access, but it creates numerous long-term operational issues. Gravity tends to result in increased 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) had been the principle possibility a couple of many years in the past for turbine bypass valves. New move up valve designs (right) may be installed without piping modifications in most situations, offering prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for circulate down is vertical mounting of the actuator. This arrangement results in decreased and extra even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute alternatives are typically for horizontally mounted actuators, and the 2 designs aren’t readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead house is out there with existing horizontal actuator flow down designs, a move up design can simply meet the present face-to-face requirements of the previous valve, while decreasing a few of the wear widespread to move down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from circulate right down to circulate up, this alteration could be made with out modifications to present piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life as a outcome of the steam is transferring comparatively slowly because it passes over the plug and seat (Figure 6). As the steam moves via the small holes of the cage, it accelerates considerably, but that power dissipates into the large body cavity of the valve instead of putting 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 accelerate 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 actually exhibiting erosion, however when in comparison with the flow down trim shown above (Figure 4 right), the seating surface on the seat ring and plug stay comparatively undamaged, permitting the valve to shut off completely, the required mode in normal operation.
7. These pictures of move up trim clearly present 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 may nonetheless shut off tightly. Courtesy: Emerson
The move up valve fashion design nonetheless offers the identical control responsiveness and flow capacity, but the new flow association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized pieces of engineered gear that should be rigorously specified, installed, and maintained. It is due to this fact clever to be proactive by reaching out to trusted experts prior to buy as they will assist with valve sizing and specifications based on particular working conditions.
For current 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 totally inspected, it will be sensible to consult your turbine bypass valve vendor for assist and help.
Many of these distributors provide full inspection and health verify companies to determine areas where steel fatigue and stress cracking are creating. Ideally the valve could be repaired, but if a valve substitute is warranted, the vendor also can present steering in evaluating bypass valve design kinds and selecting the best choice for the specific utility.
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