Turbine Bypass Valve Challenges

Turbine bypass valves are one of the tough functions in a power plant. Keeping these valves operating effectively and avoiding sudden failures is critical for plant operation.
When engineers are requested to list essentially the most difficult control valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive stress drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, ตัววัดแรงดันน้ำมัน tend to ignore these valves till they fail, creating unplanned outages, lost manufacturing, and high prices for reactive maintenance. This article supplies suggested strategies of inspection to anticipate and mitigate points beforehand, and it provides upgrade alternatives ought to a valve need to be repaired or changed.
Strictly Severe Service
Severe service control valves are used in the most difficult installations inside process plants. These installations commonly embrace cavitating, erosive, corrosive, noisy, excessive stress, excessive temperature, excessive strain drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these course of circumstances; yet, they must respond flawlessly and stay leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam turbines throughout plant startup and shutdown, in addition to when a turbine journeys off-line (Figure 1). In regular operation the valves are completely closed, forcing all the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine till the properties and conditions 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 protect the turbine by diverting potentially wet steam, and by guaranteeing only appropriate steam circumstances and flows make their approach to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves may be employed to immediately shunt steam round a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to avoid gear harm because of overpressure and high temperature, so the turbine bypass valve instantly opens to maintain flow by way of the system.
As it operates, a turbine makes use of steam to perform work, lowering outlet steam temperature and stress. When a turbine bypass valve opens, it’ll drop the strain, however the exit steam will remain quite superheated, doubtlessly destroying downstream tools. To avoid that situation, turbine bypass valves both incorporate a water injection system in 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 conditions. While the plant is in operation, these valves should remain tightly closed to keep away from losing power. When a turbine trip occurs, the bypass valves should respond instantly, exposing them to speedy temperature adjustments and requiring them to cross very high flows at excessive strain drops, creating high noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that virtually every turbine bypass valve will in the end fail indirectly. Unfortunately, many of these valves are installed in difficult-to-access places, are usually welded in place, and are typically heavily insulated. As a result, they’re typically ignored until problems begin to surface. Steam leakage via the valve is usually the primary symptom seen, however much more vital and doubtlessly harmful injury can happen.
The extreme stress drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can result in catastrophic failure under pressure.
2. Over time, the high vibrations and thermal shock skilled by turbine bypass valves will fatigue the metallic and create cracks in each the valve itself and the surrounding piping. Courtesy: Emerson
To keep away from this problem, every turbine bypass valve and its associated piping ought to be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect metal fatigue problems that could be developing however are not yet visually apparent. If a plant lacks the information or equipment to carry out these inspections, the valve vendor or approved representative could also be utilized to perform turbine bypass health examine companies (Figure 3).
3. A routine turbine bypass valve health examine and full inspection is strongly suggested. Metal fatigue can develop in varied welds situated on the valve itself, and in the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, customer connection welds, diffuser welds, and surrounding piping can be inspected to establish any creating problems before the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are additionally threats to trim components, such as seat surface erosion. One common expression of such erosion, particularly on the plug, is also identified as “gear toothing.” This happens extra generally in move down valves, the place the steam accelerates through the cage holes after which strikes the trim/seat space immediately, inflicting excessive put on and decreasing service life. Over time, this high-velocity steam, which might include water during startup conditions and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve body designs are susceptible to trim harm because the moist, 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 cost. Trim components that must be replaced due to heavy put on, corresponding to gear toothing, can be quite expensive, particularly if their delivery must be expedited. The worst-case situation for a turbine bypass valve is steel fatigue creating within the valve body or diffuser, with excessive repair procedures, or even a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service had been installed through the heyday of mixed cycle plant development from 1998 to 2004. Most of these valves and piping methods are exhibiting their age and often have important indicators of metallic fatigue. Others have plenty of life left in them but could benefit from upgrades in know-how. Technology and practices have improved, and upgrading to the latest sealing technology may inject new life into the valve.
Another instance of a technology improve is when there have been repeated failures with a welded diffuser. In this instance, a removable two-in-one seat diffuser will scale back required upkeep.
If a turbine bypass valve have to be replaced, plant personnel are strongly inspired to look beyond a direct substitute and evaluate potential enhancements. A significantly necessary merchandise to think about when taking a glance at substitute and enchancment alternatives 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 right for locations with low overhead clearance, and it places the actuator closer to the deck or ground for simple entry, nevertheless it creates a selection of long-term operational problems. Gravity tends to result in elevated and uneven wear on trim parts, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the main choice a number of a long time in the past for turbine bypass valves. New circulate up valve designs (right) may be put in without piping modifications in most situations, offering extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for move down is vertical mounting of the actuator. This arrangement leads to decreased and extra even put on, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that substitute opportunities are usually for horizontally mounted actuators, and the two designs are not readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead house is on the market with current horizontal actuator flow down designs, a flow up design can simply meet the present face-to-face necessities of the previous valve, while decreasing a variety of the put on common to move down designs with a horizontal actuator, corresponding to gear toothing. Because the trim just switches from flow all the means down to move up, this modification may be made with out modifications to present piping.
Flow up valves (Figure 5, right) significantly prolong seat and trim service life as a outcome of 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 considerably, but that energy dissipates into the massive physique cavity of the valve as a substitute of putting the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease because they cross over the seat and speed up later as the steam strikes through the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is definitely exhibiting erosion, but when in comparability with the move down trim proven above (Figure four right), the seating surface on the seat ring and plug stay comparatively undamaged, allowing the valve to shut off utterly, the required mode in normal operation.
7. These footage of move up trim clearly show the erosive impression of high-velocity steam, but not like the circulate down trim damage shown previously, this valve has no injury to the seating surface and may nonetheless shut off tightly. Courtesy: Emerson
The move up valve fashion design nonetheless supplies the same control responsiveness and flow capacity, but the model new flow arrangement dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised items of engineered gear that have to be rigorously specified, put in, and maintained. It is due to this fact clever to be proactive by reaching out to trusted specialists prior to buy as they can help with valve sizing and specs based on specific operating circumstances.
For present installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time because the plant turbine bypass valves have been fully inspected, it might be sensible to seek the assistance of your turbine bypass valve vendor for assist and help.
Many of these vendors supply full inspection and well being examine providers to establish areas the place metallic fatigue and stress cracking are developing. Ideally the valve could be repaired, but if a valve alternative is warranted, the vendor can also present steerage in evaluating bypass valve design styles and selecting the best option for the specific application.

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