Turbine bypass valves are some of the tough applications in an influence plant. Keeping these valves working efficiently and avoiding unexpected failures is important for plant operation.
When engineers are asked to record essentially the most troublesome management valve functions, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive pressure drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, many vegetation tend to ignore these valves till they fail, creating unplanned outages, lost production, and high costs for reactive upkeep. This article provides suggested methods of inspection to anticipate and mitigate points beforehand, and it presents improve alternatives should a valve need to be repaired or changed.
Strictly Severe Service
Severe service management valves are used in the most difficult installations inside course of plants. These installations generally embody cavitating, erosive, corrosive, noisy, excessive pressure, excessive temperature, high strain drop, or high velocity media. Turbine bypass valves are uncovered to many of these course of conditions; yet, they must respond flawlessly and remain leak free when closed.
As their title 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 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 process occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting doubtlessly moist steam, and by ensuring solely appropriate steam conditions and flows make their way 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 trip offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to keep away from equipment injury as a result of overpressure and excessive temperature, so the turbine bypass valve immediately opens to maintain up move via the system.
As it operates, a turbine makes use of steam to perform work, lowering outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the strain, but the exit steam will remain fairly superheated, potentially destroying downstream equipment. To keep away from that state of affairs, turbine bypass valves either incorporate a water injection system within the valve physique, or employ a separate water injection desuperheater just downstream, in either case to decrease the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of severe service situations. While the plant is in operation, these valves must stay tightly closed to avoid wasting energy. When a turbine journey occurs, the bypass valves must reply immediately, exposing them to speedy temperature adjustments and requiring them to pass very high flows at excessive stress drops, creating high noise and potentially extreme 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 typically welded in place, and are usually heavily insulated. As a outcome, they’re typically ignored till issues start to floor. Steam leakage through the valve is often the first symptom seen, but rather more important and doubtlessly dangerous harm can happen.
The extreme strain drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metallic in the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can lead to catastrophic failure under strain.
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 encompassing piping. Courtesy: Emerson
To keep away from this downside, every turbine bypass valve and its related piping should be routinely inspected. เกจแรงดันน้ำ of non-destructive examination ought to be periodically employed to detect metallic fatigue issues that might be growing but usually are not yet visually obvious. If a plant lacks the knowledge or tools to carry out these inspections, the valve vendor or licensed consultant may be utilized to carry out turbine bypass health verify companies (Figure 3).
three. A routine turbine bypass valve health examine and full inspection is strongly suggested. Metal fatigue can develop in numerous welds situated 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, customer connection welds, diffuser welds, and surrounding piping could be inspected to establish any creating issues earlier than the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid problems associated to quenching and cracking.
There are additionally threats to trim elements, corresponding to seat surface erosion. One common expression of such erosion, particularly on the plug, is sometimes called “gear toothing.” This happens more commonly in flow down valves, where the steam accelerates by way of the cage holes and then strikes the trim/seat area directly, causing excessive put on and decreasing service life. Over time, this high-velocity steam, which might comprise water throughout startup conditions and magnetite throughout any condition, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim injury because the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like soft items and spray nozzles can often get replaced relatively simply and at minimal cost. Trim components that must be replaced as a result of heavy wear, corresponding to gear toothing, can be quite pricey, particularly if their delivery needs to be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue developing within the valve body or diffuser, with excessive repair procedures, or even a complete valve replacement, required.
Many of the turbine bypass valves currently in service had been installed through the heyday of mixed cycle plant construction from 1998 to 2004. Most of these valves and piping techniques are displaying their age and infrequently have significant indicators of metallic fatigue. Others have plenty of life left in them however may benefit from upgrades in technology. Technology and practices have improved, and upgrading to the most recent sealing technology may inject new life into the valve.
Another example of a know-how improve is when there have been repeated failures with a welded diffuser. In this instance, a detachable two-in-one seat diffuser will cut back required maintenance.
If a turbine bypass valve have to be changed, plant personnel are strongly encouraged to look beyond a direct substitute and evaluate potential enhancements. A notably necessary item to consider when taking a glance at alternative and enchancment opportunities is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a circulate down trim design paired with a horizontal actuator (Figure 5, left). This arrangement is right for places with low overhead clearance, and it locations the actuator closer to the deck or ground for easy access, nevertheless it creates numerous long-term operational problems. Gravity tends to end in increased and uneven put on on trim components, and the side-mounted actuator is prone to response issues.
5. Downward flowing valves with horizontal actuators (left) were the primary option a couple of many years ago for turbine bypass valves. New flow up valve designs (right) may be put in without piping modifications in most cases, providing prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This arrangement ends in lowered and extra even wear, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that alternative alternatives are usually for horizontally mounted actuators, and the two designs usually are not readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead space is available with existing horizontal actuator move down designs, a circulate up design can simply meet the present face-to-face requirements of the old valve, whereas reducing a few of the put on frequent to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from circulate right down to move up, this change could be made without modifications to present piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life as a end result of the steam is shifting comparatively slowly because it passes over the plug and seat (Figure 6). As the steam moves through the small holes of the cage, it accelerates significantly, however that energy dissipates into the large physique cavity of the valve instead of striking the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease as a end result of they pass over the seat and accelerate later because the steam moves through the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely displaying erosion, however when in comparison with the move 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 footage of flow up trim clearly present the erosive impact of high-velocity steam, but not like the circulate down trim injury proven beforehand, this valve has no harm to the seating surface and might still shut off tightly. Courtesy: Emerson
The flow up valve style design nonetheless provides the identical control responsiveness and circulate capability, however the new circulate association dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered tools that should be rigorously specified, put in, and maintained. It is subsequently sensible to be proactive by reaching out to trusted consultants prior to buy as they will help with valve sizing and specifications primarily based on specific working circumstances.
For present installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time for the reason that plant turbine bypass valves have been totally inspected, it would be sensible to consult your turbine bypass valve vendor for assist and support.
Many of those vendors provide full inspection and health examine providers to identify areas where metallic fatigue and stress cracking are creating. Ideally the valve may be repaired, but when a valve substitute is warranted, the seller also can present steerage in evaluating bypass valve design kinds and selecting the most fitted choice for the precise application.