Turbine Bypass Valve Challenges

Turbine bypass valves are some of the tough functions in an influence plant. Keeping these valves operating effectively and avoiding sudden failures is crucial for plant operation.
When engineers are asked to list the most tough management valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive pressure drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many crops are inclined to ignore these valves till they fail, creating unplanned outages, lost production, and high costs for reactive maintenance. This article offers instructed methods of inspection to anticipate and mitigate issues beforehand, and it provides upgrade alternatives ought to a valve have to be repaired or changed.
Strictly Severe Service
Severe service control valves are used in probably the most tough installations inside course of vegetation. These installations commonly include cavitating, erosive, corrosive, noisy, high strain, excessive temperature, high pressure drop, or excessive velocity media. Turbine bypass valves are uncovered to many of these process circumstances; yet, they must respond 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, in addition to when a turbine journeys off-line (Figure 1). In regular operation the valves are completely closed, forcing all the steam through a turbine. During startup, bypass valves divert steam away from the turbine until 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 probably moist steam, and by ensuring only appropriate steam circumstances and flows make their method to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves could also be employed to instantly shunt steam around a turbine should it trip offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to keep away from equipment harm because of overpressure and high temperature, so the turbine bypass valve immediately opens to maintain up flow via the system.
As it operates, a turbine makes use of steam to perform work, lowering outlet steam temperature and strain. When a turbine bypass valve opens, it’ll drop the strain, but the exit steam will stay quite superheated, doubtlessly destroying downstream gear. To keep away from that state of affairs, turbine bypass valves both incorporate a water injection system in the valve physique, or make use of a separate water injection desuperheater just downstream, in both case to lower the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of severe service conditions. While the plant is in operation, these valves must stay tightly closed to keep away from losing vitality. When a turbine journey happens, the bypass valves must respond immediately, exposing them to fast temperature modifications and requiring them to move very excessive flows at excessive pressure drops, creating high noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that virtually every turbine bypass valve will finally fail in some way. Unfortunately, many of those valves are installed in difficult-to-access places, are usually welded in place, and are typically heavily insulated. As a result, they’re usually ignored until issues start to floor. Steam leakage by way of the valve is normally the primary symptom observed, however rather more vital and doubtlessly dangerous injury can occur.
The extreme stress drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metallic within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can outcome in catastrophic failure beneath stress.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in both the valve itself and the encircling piping. Courtesy: Emerson
To keep away from this downside, each turbine bypass valve and its associated piping should be routinely inspected. Some sort of non-destructive examination ought to be periodically employed to detect metal fatigue issues that might be growing however aren’t yet visually apparent. If a plant lacks the information or gear to carry out these inspections, the valve vendor or licensed representative may be utilized to carry out turbine bypass health verify companies (Figure 3).
3. A routine turbine bypass valve well being examine and full inspection is strongly advised. Metal fatigue can develop in numerous welds positioned 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, customer connection welds, diffuser welds, and surrounding piping may be inspected to determine any developing issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid issues related to quenching and cracking.
There are additionally threats to trim components, similar to seat floor erosion. One common expression of such erosion, particularly on the plug, is also recognized as “gear toothing.” This occurs extra commonly in flow down valves, the place the steam accelerates by way of the cage holes after which strikes the trim/seat space immediately, causing excessive put on and decreasing service life. Over time, this high-velocity steam, which might contain water throughout startup circumstances and magnetite throughout any situation, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve body designs are vulnerable to trim injury as the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep items like soft goods and spray nozzles can usually be replaced relatively easily and at minimal price. Trim elements that must be changed because of heavy put on, corresponding to gear toothing, can be fairly expensive, especially if their supply needs to be expedited. The worst-case state of affairs for a turbine bypass valve is metal fatigue developing within the valve physique or diffuser, with excessive restore procedures, or even a complete valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service had been installed in the course of the heyday of mixed cycle plant building from 1998 to 2004. Most of those valves and piping techniques are exhibiting their age and sometimes have significant signs of steel fatigue. Others have plenty of life left in them but may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing technology could inject new life into the valve.
Another example of a know-how upgrade is when there have been repeated failures with a welded diffuser. In this instance, a detachable two-in-one seat diffuser will scale back required maintenance.
If a turbine bypass valve must be changed, plant personnel are strongly encouraged to look beyond a direct replacement and consider potential enhancements. A notably necessary merchandise to contemplate when looking at replacement 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 locations the actuator nearer to the deck or floor for straightforward entry, nevertheless it creates a quantity of long-term operational problems. Gravity tends to lead to elevated and uneven wear on trim components, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the primary choice a few decades in the past for turbine bypass valves. New circulate up valve designs (right) may be installed without piping modifications in most cases, offering prolonged 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 results in reduced and extra even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that replacement opportunities are typically for horizontally mounted actuators, and the two designs are not readily interchangeable with out vital piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is on the market with present horizontal actuator flow down designs, a flow up design can simply meet the existing face-to-face necessities of the outdated valve, whereas decreasing some of the put on frequent to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from move down to move up, this alteration could be made without modifications to current piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life as a outcome of the steam is transferring relatively slowly as it passes over the plug and seat (Figure 6). As the steam moves through the small holes of the cage, it accelerates considerably, however that energy dissipates into the massive body cavity of the valve as an alternative of striking the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower as a outcome of they move over the seat and speed up later as the steam moves by way of the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is actually displaying erosion, but when in comparability with the circulate down trim shown above (Figure four right), the seating surface on the seat ring and plug remain relatively undamaged, allowing the valve to shut off utterly, the required mode in regular operation.
7. These footage of circulate up trim clearly show the erosive influence of high-velocity steam, but in contrast to the move down trim damage shown beforehand, this valve has no injury to the seating floor and might nonetheless shut off tightly. เกจวัดแรงดันออกซิเจนราคา : Emerson
The move up valve style design nonetheless provides the identical management responsiveness and circulate capacity, however the model new move arrangement dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized items of engineered gear that should be fastidiously specified, installed, and maintained. It is due to this fact wise to be proactive by reaching out to trusted consultants prior to purchase as they’ll assist with valve sizing and specs based on specific working conditions.
For present installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time since the plant turbine bypass valves have been absolutely inspected, it will be clever to seek the advice of your turbine bypass valve vendor for assist and assist.
Many of those vendors supply full inspection and well being examine providers to determine areas the place metallic fatigue and stress cracking are developing. Ideally the valve may be repaired, but when a valve substitute is warranted, the seller can even provide guidance in evaluating bypass valve design types and choosing the finest choice for the particular software.

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