Turbine Bypass Valve Challenges

Turbine bypass valves are one of the difficult functions in a power plant. Keeping these valves working effectively and avoiding sudden failures is crucial for plant operation.
When engineers are requested to listing essentially the most tough management valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive strain drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many crops are most likely to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and excessive prices for reactive maintenance. This article provides instructed methods of inspection to anticipate and mitigate points beforehand, and it presents upgrade alternate options ought to a valve must be repaired or changed.
Strictly Severe Service

Severe service control valves are utilized in probably the most troublesome installations inside process crops. These installations commonly include cavitating, erosive, corrosive, noisy, high strain, excessive temperature, high pressure drop, or high velocity media. Turbine bypass valves are exposed to many of those process situations; yet, they must reply flawlessly and stay leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam turbines during plant startup and shutdown, in addition to when a turbine trips off-line (Figure 1). In normal operation the valves are fully closed, forcing all of the steam through a turbine. During startup, bypass valves divert steam away from the turbine till 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 protect the turbine by diverting doubtlessly wet steam, and by guaranteeing solely acceptable steam circumstances and flows make their method to the turbine itself.
1. Depending on the ability plant design, a quantity of turbine bypass valves could also be employed to instantly shunt steam round a turbine ought to it journey offline. Courtesy: Emerson

Should a turbine journey, the steam must proceed flowing to avoid equipment injury as a end result of overpressure and excessive temperature, so the turbine bypass valve immediately opens to hold up move through the system.
As it operates, a turbine uses 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 remain fairly superheated, probably destroying downstream gear. To keep away from that state of affairs, turbine bypass valves both incorporate a water injection system within the valve body, or make use of a separate water injection desuperheater simply downstream, in both case to decrease the exit steam temperature.
As a outcome, turbine bypass valves face a perfect storm of severe service circumstances. While the plant is in operation, these valves must remain tightly closed to keep away from losing energy. When a turbine trip occurs, the bypass valves should reply immediately, exposing them to speedy temperature modifications and requiring them to move very excessive flows at excessive pressure drops, creating high noise and potentially extreme vibration.
Getting Ahead of the Game

Given the punishing service, the fact is that virtually every turbine bypass valve will ultimately fail indirectly. Unfortunately, many of these valves are put in in difficult-to-access places, are usually welded in place, and are usually heavily insulated. As a end result, they are typically ignored until problems start to floor. Steam leakage through the valve is often the primary symptom observed, however rather more important and potentially dangerous damage can occur.
The extreme pressure drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metal in the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can outcome in catastrophic failure under stress.
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 avoid this drawback, each turbine bypass valve and its related piping should be routinely inspected. Some sort of non-destructive examination should be periodically employed to detect steel fatigue issues which might be growing however aren’t yet visually obvious. If a plant lacks the data or equipment to perform these inspections, the valve vendor or authorized consultant may be utilized to perform turbine bypass health examine companies (Figure 3).
3. A routine turbine bypass valve health verify and full inspection is strongly suggested. Metal fatigue can develop in varied 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, buyer connection welds, diffuser welds, and surrounding piping may be inspected to determine any growing problems earlier than the tools is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to keep away from issues associated to quenching and cracking.
There are additionally threats to trim elements, such as seat surface erosion. pressure gauge octa of such erosion, particularly on the plug, is often referred to as “gear toothing.” This occurs more generally in flow down valves, where the steam accelerates through the cage holes and then strikes the trim/seat space instantly, inflicting excessive wear and reducing service life. Over time, this high-velocity steam, which might comprise water throughout startup situations and magnetite during any situation, will erode and injury the valve seat and trim (Figure 4).
4. Flow down valve body designs are susceptible to trim injury as the moist, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson

Common maintenance objects like delicate goods and spray nozzles can normally get replaced comparatively easily and at minimal value. Trim parts that need to be replaced because of heavy put on, corresponding to gear toothing, could be fairly pricey, especially if their delivery must be expedited. The worst-case scenario for a turbine bypass valve is metal fatigue growing within the valve body or diffuser, with extreme repair procedures, or maybe a full valve replacement, required.
Upgrade Opportunities

Many of the turbine bypass valves presently in service were put in during the heyday of mixed cycle plant construction from 1998 to 2004. Most of these valves and piping systems are showing their age and infrequently have vital indicators of metallic fatigue. Others have loads of life left in them however could benefit from upgrades in technology. Technology and practices have improved, and upgrading to the most recent sealing know-how may 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 instance, a removable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve have to be changed, plant personnel are strongly inspired to look past a direct alternative and evaluate potential enhancements. A significantly important item to contemplate when looking at alternative and enchancment alternatives 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 association is right for locations with low overhead clearance, and it places the actuator closer to the deck or ground for simple entry, however it creates numerous long-term operational problems. Gravity tends to lead to elevated and uneven wear on trim parts, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the primary possibility a number of decades in the past for turbine bypass valves. New move up valve designs (right) could additionally be put in with out piping modifications in most instances, providing extended service life and improved performance. Courtesy: Emerson

Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This association ends in reduced and more even wear, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that substitute opportunities 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 out there with present horizontal actuator flow down designs, a move up design can easily meet the present face-to-face requirements of the outdated valve, while reducing a few of the wear common to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from move all the method down to circulate up, this modification may be made without modifications to current piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life because the steam is shifting comparatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes by way of the small holes of the cage, it accelerates considerably, however that power dissipates into the large body cavity of the valve as a substitute of hanging the seating surface.
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 because the steam strikes via the small trim holes, significantly extending trim service life. Courtesy: Emerson

Notice how the circulate up trim (Figure 7) is actually exhibiting erosion, however when in comparability with the circulate down trim proven above (Figure four right), the seating surface on the seat ring and plug stay relatively undamaged, permitting the valve to close off fully, the required mode in normal operation.
7. These pictures of flow up trim clearly present the erosive impression of high-velocity steam, but unlike the circulate down trim damage shown beforehand, this valve has no injury to the seating surface and might still shut off tightly. Courtesy: Emerson

The circulate up valve style design still provides the same control responsiveness and move capacity, however the model new move association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert

Turbine bypass valves are extremely specialised items of engineered gear that should be rigorously specified, put in, and maintained. It is due to this fact wise to be proactive by reaching out to trusted consultants prior to purchase as they’ll help with valve sizing and specs primarily based on particular working circumstances.
For current installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while for the rationale that plant turbine bypass valves have been totally inspected, it will be clever to seek the guidance of your turbine bypass valve vendor for help and support.
Many of these vendors provide full inspection and health check providers to identify areas the place steel fatigue and stress cracking are growing. Ideally the valve could be repaired, but when a valve substitute is warranted, the vendor can also provide guidance in evaluating bypass valve design kinds and selecting the greatest choice for the particular application.
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