Turbine Bypass Valve Challenges

Turbine bypass valves are one of the troublesome functions in an influence plant. Keeping these valves operating effectively and avoiding unexpected failures is critical for plant operation.
When engineers are asked to listing essentially the most tough control valve purposes, 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 vegetation are inclined to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and excessive costs for reactive maintenance. This article offers instructed strategies of inspection to anticipate and mitigate points beforehand, and it presents upgrade alternatives should a valve need to be repaired or changed.
Strictly Severe Service

Severe service control valves are utilized in probably the most troublesome installations inside course of plants. These installations commonly embrace cavitating, erosive, corrosive, noisy, high pressure, excessive temperature, high strain drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these process conditions; yet, they have to respond flawlessly and remain leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, as properly as 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 conditions of the steam are appropriate 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 potentially moist steam, and by ensuring only appropriate steam conditions and flows make their approach to the turbine itself.
1. Depending on the facility plant design, a number of turbine bypass valves could also be employed to instantly shunt steam round a turbine should it trip offline. Courtesy: Emerson

Should a turbine journey, the steam should proceed flowing to avoid equipment injury because of overpressure and high temperature, so the turbine bypass valve instantly opens to take care of flow via the system.
As it operates, a turbine makes use of steam to perform work, decreasing outlet steam temperature and stress. When a turbine bypass valve opens, it’ll drop the stress, however the exit steam will remain quite superheated, probably destroying downstream equipment. To avoid that situation, turbine bypass valves both incorporate a water injection system in the valve physique, or employ a separate water injection desuperheater just downstream, in either case to decrease the exit steam temperature.
As a end result, turbine bypass valves face a perfect storm of extreme service conditions. While the plant is in operation, these valves should remain tightly closed to avoid wasting power. When a turbine journey occurs, the bypass valves must reply immediately, exposing them to rapid temperature modifications and requiring them to move very excessive flows at high pressure drops, creating high noise and doubtlessly extreme vibration.
Getting Ahead of the Game

Given the punishing service, the fact is that just about every turbine bypass valve will finally fail in some way. Unfortunately, many of those valves are installed in difficult-to-access areas, are typically welded in place, and are usually heavily insulated. As a end result, they are typically ignored till issues begin to surface. Steam leakage via the valve is usually the primary symptom noticed, but much more significant and doubtlessly dangerous harm can occur.
The extreme pressure drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can result in catastrophic failure beneath stress.
2. Over เพชเชอร์เกจ , the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the metal and create cracks in both the valve itself and the surrounding piping. Courtesy: Emerson

To avoid this drawback, each turbine bypass valve and its associated piping should be routinely inspected. Some type of non-destructive examination ought to be periodically employed to detect metallic fatigue issues which may be developing but aren’t yet visually apparent. If a plant lacks the information or tools to perform these inspections, the valve vendor or authorized consultant could also be utilized to perform turbine bypass health check 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 positioned on the valve itself, and within the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson

Fabrication welds on the physique and water manifold, buyer connection welds, diffuser welds, and surrounding piping may be inspected to identify any creating issues before the equipment is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to keep away from problems associated to quenching and cracking.
There are additionally threats to trim parts, such as seat surface erosion. One widespread expression of such erosion, particularly on the plug, is sometimes called “gear toothing.” This happens more generally in move down valves, where the steam accelerates via the cage holes after which strikes the trim/seat space directly, causing extreme wear and reducing service life. Over time, this high-velocity steam, which may comprise water throughout startup situations and magnetite throughout any situation, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve body designs are prone to trim damage because the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson

Common upkeep objects like gentle items and spray nozzles can often be replaced relatively easily and at minimal cost. Trim parts that need to be replaced due to heavy put on, such as gear toothing, can be quite expensive, especially if their delivery must be expedited. The worst-case state of affairs for a turbine bypass valve is metallic fatigue creating in the valve physique or diffuser, with excessive repair procedures, or even a complete valve replacement, required.
Upgrade Opportunities

Many of the turbine bypass valves presently in service have been put in in the course of the heyday of combined cycle plant construction from 1998 to 2004. Most of these valves and piping systems are displaying their age and infrequently have important signs of steel fatigue. Others have loads of life left in them however may benefit from upgrades in technology. Technology and practices have improved, and upgrading to the latest sealing expertise could inject new life into the valve.
Another example 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 substitute and consider potential enhancements. A notably important item to contemplate when taking a glance at alternative and improvement 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 areas with low overhead clearance, and it locations the actuator nearer to the deck or floor for easy access, but it creates a number of long-term operational problems. Gravity tends to result in increased and uneven wear on trim elements, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the principle option a number of decades ago for turbine bypass valves. New circulate up valve designs (right) could also be put in without piping modifications in most situations, providing prolonged service life and improved performance. Courtesy: Emerson

Downward Flowing Valves with Vertical Actuators. A superior possibility for circulate down is vertical mounting of the actuator. This arrangement results in decreased and extra even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that alternative opportunities are sometimes for horizontally mounted actuators, and the two designs usually are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead area is available with existing horizontal actuator move down designs, a circulate up design can easily meet the existing face-to-face requirements of the outdated valve, whereas lowering a few of the put on 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 move up, this modification could be made with out modifications to present piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life as a end result of the steam is transferring relatively slowly as 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, but that energy dissipates into the massive physique cavity of the valve as an alternative of hanging the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower as a result of they cross over the seat and speed up later because the steam moves via the small trim holes, significantly extending trim service life. Courtesy: Emerson

Notice how the flow up trim (Figure 7) is definitely exhibiting erosion, but when compared to the circulate down trim proven above (Figure four right), the seating floor on the seat ring and plug stay comparatively undamaged, permitting the valve to close off utterly, the required mode in regular operation.
7. These footage of circulate up trim clearly show the erosive impression of high-velocity steam, however not like the circulate down trim harm shown beforehand, this valve has no harm to the seating surface and may nonetheless shut off tightly. Courtesy: Emerson

The flow up valve type design still supplies the identical management responsiveness and flow capability, but the new move arrangement dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert

Turbine bypass valves are extremely specialized items of engineered tools that should be carefully specified, put in, and maintained. It is due to this fact wise to be proactive by reaching out to trusted experts prior to purchase as they’ll assist with valve sizing and specifications based on specific working circumstances.
For existing 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 clever to consult your turbine bypass valve vendor for help and assist.
Many of these distributors provide full inspection and health verify services to identify areas the place metallic fatigue and stress cracking are creating. Ideally the valve may be repaired, but if a valve alternative is warranted, the seller also can present steering in evaluating bypass valve design styles and selecting the most suitable choice for the particular software.

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