Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most difficult applications in an influence plant. Keeping these valves working efficiently and avoiding unexpected failures is crucial for plant operation.
When engineers are requested to record probably the most tough management valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high stress drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many crops tend to ignore these valves until they fail, creating unplanned outages, misplaced production, and high prices for reactive maintenance. This article provides suggested strategies of inspection to anticipate and mitigate points beforehand, and it offers improve options should a valve need to be repaired or changed.
Strictly Severe Service
Severe service control valves are used in probably the most troublesome installations within course of vegetation. These installations commonly embody cavitating, erosive, corrosive, noisy, excessive pressure, excessive temperature, excessive stress drop, or high velocity media. Turbine bypass valves are exposed to many of these course of circumstances; yet, they must respond flawlessly and stay leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam generators during plant startup and shutdown, as nicely as when a turbine trips off-line (Figure 1). In normal operation the valves are completely closed, forcing all the steam via 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 happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting doubtlessly wet steam, and by guaranteeing only appropriate steam circumstances and flows make their approach to the turbine itself.
1. Depending on the facility plant design, several turbine bypass valves may be employed to instantly shunt steam round a turbine ought to it journey offline. Spoiler : Emerson
Should a turbine journey, the steam must continue flowing to avoid equipment harm because of overpressure and high temperature, so the turbine bypass valve immediately opens to take care of flow through 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 will drop the stress, but the exit steam will remain quite superheated, probably destroying downstream gear. To keep away from that situation, turbine bypass valves both incorporate a water injection system within the valve body, or employ 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 extreme service conditions. While the plant is in operation, these valves must remain tightly closed to avoid losing power. When a turbine journey happens, the bypass valves should respond immediately, exposing them to speedy temperature modifications and requiring them to cross very excessive flows at high stress drops, creating high noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that just about every turbine bypass valve will ultimately fail ultimately. Unfortunately, many of those valves are put in in difficult-to-access places, are typically welded in place, and are typically heavily insulated. As a end result, they are typically ignored until issues begin to floor. Steam leakage by way of the valve is normally the first symptom observed, however rather more vital and doubtlessly dangerous damage can occur.
The extreme pressure drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue may end up in catastrophic failure underneath strain.
2. Over time, the high vibrations and thermal shock skilled by turbine bypass valves will fatigue the steel and create cracks in both the valve itself and the encircling piping. Courtesy: Emerson
To avoid this drawback, each turbine bypass valve and its related piping must be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect metallic fatigue problems which might be creating but usually are not but visually obvious. If a plant lacks the information or gear to carry out these inspections, the valve vendor or licensed representative may be utilized to perform turbine bypass well being verify services (Figure 3).
three. A routine turbine bypass valve well being examine and full inspection is strongly suggested. Metal fatigue can develop in various welds located on the valve itself, and in 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 could be inspected to identify any developing issues earlier than the tools is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid problems related to quenching and cracking.
There are additionally threats to trim parts, similar to seat surface erosion. One common expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This happens more generally in move down valves, where the steam accelerates via the cage holes and then strikes the trim/seat area immediately, causing extreme wear and decreasing service life. Over time, this high-velocity steam, which can include water during startup situations and magnetite during any condition, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve body designs are susceptible to trim injury as the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like gentle goods and spray nozzles can often get replaced comparatively simply and at minimal value. Trim elements that have to be changed as a outcome of heavy wear, corresponding to gear toothing, may be quite expensive, particularly if their delivery must be expedited. The worst-case situation for a turbine bypass valve is metallic fatigue creating within the valve body or diffuser, with extreme restore procedures, or perhaps a complete valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were installed through the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping techniques are exhibiting their age and sometimes have vital indicators of metallic fatigue. Others have plenty of life left in them but may gain advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing expertise might inject new life into the valve.
Another instance of a expertise upgrade is when there have been repeated failures with a welded diffuser. In this instance, a removable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve must be changed, plant personnel are strongly encouraged to look past a direct alternative and evaluate potential enhancements. A significantly important item to contemplate when taking a glance at substitute and improvement 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). Censored is good for areas with low overhead clearance, and it places the actuator closer to the deck or floor for straightforward access, but it creates a quantity of long-term operational problems. Gravity tends to result in elevated and uneven put on on trim elements, and the side-mounted actuator is prone to response issues.
5. Downward flowing valves with horizontal actuators (left) have been the primary option a couple of many years ago for turbine bypass valves. New flow up valve designs (right) may be installed with out piping modifications in most cases, offering prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for flow down is vertical mounting of the actuator. This arrangement ends in reduced and extra even put on, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement opportunities are typically for horizontally mounted actuators, and the two designs aren’t readily interchangeable without important piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead area is on the market with existing horizontal actuator flow down designs, a circulate up design can simply meet the present face-to-face necessities of the previous valve, while reducing a variety of the wear frequent to flow down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from circulate all the way down to flow up, this alteration may be made without modifications to current piping.
Flow up valves (Figure 5, right) considerably lengthen 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 vitality dissipates into the massive physique cavity of the valve as a substitute of hanging the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower because they cross over the seat and accelerate later as the steam moves via the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is actually displaying erosion, but when compared to the move down trim shown above (Figure 4 right), the seating floor on the seat ring and plug stay comparatively undamaged, permitting the valve to shut off utterly, the required mode in normal operation.
7. These footage of flow up trim clearly present the erosive influence of high-velocity steam, but in distinction to the flow down trim harm shown previously, this valve has no damage to the seating floor and might nonetheless shut off tightly. Courtesy: Emerson
The move up valve fashion design nonetheless provides the identical management responsiveness and flow capability, however the new move association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized items of engineered equipment 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 buy as they will assist with valve sizing and specs based on specific operating conditions.
For current installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while because the plant turbine bypass valves have been absolutely inspected, it will be clever to consult your turbine bypass valve vendor for assist and assist.
Many of those vendors supply full inspection and health verify companies to establish areas where metal fatigue and stress cracking are growing. Ideally the valve could be repaired, but if a valve substitute is warranted, the seller can also present guidance in evaluating bypass valve design styles and selecting the most suitable choice for the specific application.
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