Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most tough functions in an influence plant. Keeping these valves operating efficiently and avoiding surprising failures is crucial for plant operation.
When engineers are asked to listing essentially the most troublesome management valve applications, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive stress drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many vegetation tend to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and high prices for reactive upkeep. This article supplies instructed methods of inspection to anticipate and mitigate issues beforehand, and it offers upgrade options should a valve must be repaired or changed.
Strictly Severe Service
Severe service management valves are utilized in the most tough installations inside course of crops. These installations commonly embrace cavitating, erosive, corrosive, noisy, excessive strain, excessive temperature, high strain drop, or high velocity media. Turbine bypass valves are exposed to many of those process conditions; yet, they must reply flawlessly and remain leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam generators throughout 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 the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine until the properties and situations of the steam are applicable for sending it to the turbine. This process happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting probably wet steam, and by ensuring only appropriate steam conditions and flows make their method to the turbine itself.
1. Depending on the facility plant design, a quantity of turbine bypass valves could also be employed to instantly shunt steam round a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to keep away from gear injury because of overpressure and high temperature, so the turbine bypass valve immediately opens to keep up flow by way of the system.
As it operates, a turbine uses steam to carry out work, lowering outlet steam temperature and strain. When a turbine bypass valve opens, it’s going to drop the strain, however the exit steam will remain quite superheated, probably destroying downstream tools. To avoid that situation, turbine bypass valves either incorporate a water injection system in the valve physique, or employ a separate water injection desuperheater just downstream, in both case to decrease the exit steam temperature.
As a outcome, turbine bypass valves face a perfect storm of severe service conditions. While the plant is in operation, these valves should remain tightly closed to keep away from losing vitality. When a turbine trip occurs, the bypass valves should reply instantly, exposing them to fast temperature changes and requiring them to move very high flows at high pressure 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 ultimately fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access places, are typically welded in place, and are typically heavily insulated. As a outcome, they are usually ignored until problems begin to floor. Steam leakage by way of the valve is normally the primary symptom noticed, but rather more vital and doubtlessly dangerous harm can occur.
The extreme pressure drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metallic in the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can lead to catastrophic failure beneath pressure.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in each the valve itself and the encompassing piping. Courtesy: Emerson
To keep away from this downside, each turbine bypass valve and its associated piping ought to be routinely inspected. Some sort of non-destructive examination ought to be periodically employed to detect steel fatigue issues that might be growing but usually are not but visually obvious. If a plant lacks the knowledge or gear to perform these inspections, the valve vendor or licensed representative could additionally be utilized to perform turbine bypass health verify services (Figure 3).
three. A routine turbine bypass valve health check 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 supply 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 before the equipment 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 pressure gauge หลักการ ทํา งาน , similar to seat floor erosion. One common expression of such erosion, particularly on the plug, is sometimes called “gear toothing.” This occurs extra commonly in circulate down valves, where the steam accelerates by way of the cage holes after which strikes the trim/seat space immediately, inflicting excessive wear and decreasing service life. Over time, this high-velocity steam, which may include water throughout startup situations and magnetite during any situation, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim harm because the wet, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like soft goods and spray nozzles can often get replaced comparatively simply and at minimal cost. Trim parts that have to be replaced as a result of heavy put on, such as gear toothing, could be quite expensive, especially if their delivery needs to be expedited. The worst-case situation for a turbine bypass valve is metal fatigue growing in the valve body or diffuser, with excessive repair procedures, or even a complete valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service were installed through the heyday of combined cycle plant construction from 1998 to 2004. Most of those valves and piping systems are exhibiting their age and sometimes have significant indicators of metal fatigue. Others have loads of life left in them but may achieve advantage from upgrades in know-how. Technology and practices have improved, and upgrading to the most recent sealing expertise could inject new life into the valve.
Another example 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 maintenance.
If a turbine bypass valve have to be changed, plant personnel are strongly encouraged to look past a direct substitute and evaluate potential enhancements. A particularly important item to consider when taking a look 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 association is right for areas with low overhead clearance, and it locations the actuator nearer to the deck or floor for easy access, but it creates a variety of long-term operational issues. Gravity tends to end in elevated and uneven put on on trim elements, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the primary choice a few decades in the past for turbine bypass valves. New flow up valve designs (right) may be put in without piping modifications in most cases, providing 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 association leads to decreased and more even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute opportunities are typically for horizontally mounted actuators, and the two designs are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead space is on the market with existing horizontal actuator move down designs, a move up design can easily meet the present face-to-face requirements of the previous valve, whereas decreasing some of the wear common to flow down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from flow right down to flow up, this change can be made with out modifications to existing piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life because the steam is moving 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 significantly, but that power dissipates into the massive body cavity of the valve as a substitute of striking the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower because they cross over the seat and accelerate later as the steam strikes via the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is actually showing erosion, but when in comparability with the circulate down trim shown above (Figure four right), the seating floor on the seat ring and plug remain comparatively undamaged, allowing the valve to close off completely, the required mode in normal operation.
7. These photos of circulate up trim clearly show the erosive impression of high-velocity steam, however not like the flow down trim injury proven beforehand, this valve has no injury to the seating surface and may nonetheless shut off tightly. Courtesy: Emerson
The move up valve type design nonetheless provides the identical management responsiveness and circulate capacity, but the brand new circulate association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered equipment that have to be carefully specified, installed, and maintained. It is subsequently 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 specific operating circumstances.
For current 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 absolutely inspected, it would be clever to seek the advice of your turbine bypass valve vendor for assist and help.
Many of those vendors supply full inspection and well being examine companies to determine areas where metallic fatigue and stress cracking are developing. Ideally the valve could be repaired, but if a valve alternative is warranted, the seller can even present steerage in evaluating bypass valve design styles and selecting the most suitable choice for the specific utility.
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