Turbine Bypass Valve Challenges

Turbine bypass valves are some of the difficult applications in a power plant. Keeping these valves working efficiently and avoiding unexpected failures is critical 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 strain drops, and the need 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 production, and excessive costs for reactive upkeep. This article offers instructed methods of inspection to anticipate and mitigate points beforehand, and it presents improve alternate options should a valve must be repaired or replaced.
Strictly Severe Service
Severe service management valves are used in essentially the most troublesome installations inside process crops. These installations commonly include cavitating, erosive, corrosive, noisy, excessive stress, excessive temperature, excessive pressure drop, or high velocity media. Turbine bypass valves are uncovered to many of these process situations; yet, they need to reply flawlessly and remain leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam generators during plant startup and shutdown, as properly as when a turbine journeys 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 applicable for sending it to the turbine. This course of occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting probably moist steam, and by making certain only acceptable steam situations and flows make their method to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves could additionally be employed to immediately shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must proceed flowing to avoid equipment injury due to overpressure and excessive temperature, so the turbine bypass valve immediately opens to maintain circulate by way of 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 will drop the pressure, but the exit steam will remain fairly superheated, potentially destroying downstream tools. To avoid that situation, turbine bypass valves both incorporate a water injection system within the valve body, or employ a separate water injection desuperheater just downstream, in either case to decrease the exit steam temperature.
As a result, turbine bypass valves face an ideal 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 instantly, exposing them to fast temperature modifications and requiring them to pass very excessive flows at high strain drops, creating excessive noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that virtually every turbine bypass valve will finally fail ultimately. Unfortunately, many of those valves are installed in difficult-to-access places, are typically welded in place, and are typically heavily insulated. As a end result, they’re typically ignored until problems begin to surface. Steam leakage through the valve is usually the first symptom noticed, but far more significant and potentially harmful damage can occur.
The extreme pressure drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metal in the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can outcome in catastrophic failure under pressure.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metal and create cracks in both the valve itself and the encircling piping. Courtesy: Emerson
To avoid this downside, each turbine bypass valve and its associated piping ought to be routinely inspected. Some sort of non-destructive examination should be periodically employed to detect metallic fatigue problems which could be creating however usually are not but visually apparent. If a plant lacks the knowledge or gear to carry out these inspections, the valve vendor or approved representative may be utilized to carry out turbine bypass health verify services (Figure 3).
3. A routine turbine bypass valve well being examine and full inspection is strongly advised. Metal fatigue can develop in numerous welds situated on the valve itself, and in 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 can be inspected to establish any creating issues before the tools is compromised. Track record and desuperheaters have to be appropriately inspected and maintained to keep away from problems associated to quenching and cracking.
There are also threats to trim parts, corresponding to seat floor erosion. One widespread expression of such erosion, particularly on the plug, is often referred to as “gear toothing.” This occurs more generally in circulate down valves, where the steam accelerates by way of the cage holes and then strikes the trim/seat space directly, causing extreme put on and lowering service life. Over time, this high-velocity steam, which may include water during startup conditions and magnetite throughout any condition, will erode and injury the valve seat and trim (Figure 4).
4. Flow down valve physique designs are prone to trim injury because the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like gentle items and spray nozzles can normally get replaced relatively easily and at minimal price. Trim parts that need to be replaced because of heavy put on, similar to gear toothing, can be quite pricey, especially if their delivery needs to be expedited. The worst-case state of affairs for a turbine bypass valve is metal fatigue developing in the valve body or diffuser, with extreme repair procedures, or perhaps a complete valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service have been installed through the heyday of combined cycle plant development from 1998 to 2004. Most of these valves and piping methods are displaying their age and sometimes have significant signs of metallic fatigue. Others have loads of life left in them however may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing know-how might inject new life into the valve.
Another instance of a know-how upgrade is when there have been repeated failures with a welded diffuser. In this instance, a removable two-in-one seat diffuser will reduce required upkeep.
If a turbine bypass valve must be changed, plant personnel are strongly encouraged to look past a direct replacement and evaluate potential enhancements. A notably important item to suppose about 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 circulate down trim design paired with a horizontal actuator (Figure 5, left). This association is right for places with low overhead clearance, and it locations the actuator nearer to the deck or ground for easy entry, nevertheless it creates numerous long-term operational issues. Gravity tends to result in elevated and uneven put on on trim parts, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the primary possibility a quantity of many years ago for turbine bypass valves. New circulate up valve designs (right) may be put in with out piping modifications in most situations, 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 wear, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that alternative opportunities are sometimes for horizontally mounted actuators, and the two designs usually are not readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead area is available with present horizontal actuator flow down designs, a move up design can simply meet the present face-to-face requirements of the outdated valve, while reducing a few of the wear widespread to move down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from move all the means down to flow up, this alteration can be made with out modifications to present piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life as a result of 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 considerably, but that vitality dissipates into the big body cavity of the valve instead of hanging the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease as a end result of they pass over the seat and accelerate later because the steam moves via the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is certainly displaying erosion, however when compared to the flow down trim shown above (Figure four right), the seating surface on the seat ring and plug stay relatively undamaged, allowing the valve to shut off utterly, the required mode in normal operation.
7. These pictures of circulate up trim clearly show the erosive impression of high-velocity steam, but not like the flow down trim harm proven beforehand, this valve has no damage to the seating surface and might still shut off tightly. Courtesy: Emerson
The move up valve fashion design still offers the same control responsiveness and move capacity, however the new circulate 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 gear that should be rigorously specified, installed, and maintained. It is due to this fact clever to be proactive by reaching out to trusted specialists prior to purchase as they will help with valve sizing and specifications based mostly on particular working 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 totally inspected, it might be wise to consult your turbine bypass valve vendor for assist and assist.
Many of these vendors offer full inspection and health verify services to determine areas the place metal fatigue and stress cracking are growing. Ideally the valve can be repaired, but if a valve alternative is warranted, the vendor also can provide steering in evaluating bypass valve design types and choosing the greatest option for the specific software.
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