Turbine Bypass Valve Challenges

Turbine bypass valves are one of the difficult functions in an influence plant. Keeping these valves operating efficiently and avoiding unexpected failures is crucial for plant operation.
When engineers are requested to record essentially the most difficult control valve functions, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high pressure drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many plants are most likely to ignore these valves till they fail, creating unplanned outages, misplaced manufacturing, and excessive costs for reactive maintenance. This article provides instructed methods of inspection to anticipate and mitigate issues beforehand, and it offers upgrade options ought to a valve must be repaired or changed.
Strictly Severe Service
Severe service management valves are used in probably the most difficult installations within process plants. These installations generally include cavitating, erosive, corrosive, noisy, excessive stress, high temperature, high stress drop, or high velocity media. Turbine bypass valves are exposed to many of these course of conditions; yet, they want to respond flawlessly and stay leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam turbines throughout plant startup and shutdown, in addition to when a turbine journeys off-line (Figure 1). In normal 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 situations of the steam are appropriate for sending it to the turbine. This course of happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting probably moist steam, and by guaranteeing only applicable steam circumstances and flows make their approach to the turbine itself.
1. Depending on the ability plant design, a quantity of turbine bypass valves could also be employed to immediately shunt steam around a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine trip, the steam should proceed flowing to keep away from tools harm as a outcome of overpressure and high temperature, so the turbine bypass valve instantly opens to maintain up circulate via the system.
As it operates, a turbine uses steam to carry out work, reducing outlet steam temperature and strain. When a turbine bypass valve opens, it’ll drop the stress, but the exit steam will remain quite superheated, doubtlessly destroying downstream equipment. To avoid 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 simply downstream, in both case to lower the exit steam temperature.
As a end result, turbine bypass valves face an ideal storm of severe service circumstances. While the plant is in operation, these valves should remain tightly closed to avoid losing energy. When a turbine journey happens, the bypass valves should respond immediately, exposing them to speedy temperature modifications and requiring them to move very excessive flows at high pressure drops, creating excessive noise and doubtlessly excessive vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that virtually every turbine bypass valve will in the end fail indirectly. Unfortunately, many of these valves are installed in difficult-to-access places, are typically welded in place, and are usually closely insulated. As a end result, they’re typically ignored till issues begin to floor. Steam leakage through the valve is normally the primary symptom observed, however far more significant and doubtlessly harmful injury can occur.
The extreme pressure drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can result in catastrophic failure under strain.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic 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 ought to be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect steel fatigue problems that may be developing but aren’t yet visually obvious. If a plant lacks the data or equipment to perform these inspections, the valve vendor or licensed representative could also be utilized to carry out turbine bypass well being examine companies (Figure 3).
three. A routine turbine bypass valve health check and full inspection is strongly advised. Metal fatigue can develop in varied welds located 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, customer 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 avoid issues associated to quenching and cracking.
There are additionally threats to trim parts, corresponding to seat floor erosion. เกจวัดแรงดัน of such erosion, specifically on the plug, is sometimes called “gear toothing.” This occurs extra commonly in flow down valves, where the steam accelerates through the cage holes and then strikes the trim/seat area instantly, causing extreme put on and decreasing service life. Over time, this high-velocity steam, which can include water during startup circumstances and magnetite during any condition, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve body designs are susceptible to trim harm because the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common upkeep items like soft items and spray nozzles can usually get replaced relatively easily and at minimal cost. Trim components that must be changed because of heavy wear, similar to gear toothing, could be quite costly, particularly if their supply needs to be expedited. The worst-case state of affairs for a turbine bypass valve is metallic fatigue creating in the valve body or diffuser, with excessive restore procedures, or maybe a complete valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service have been put in in the course of the heyday of combined cycle plant building from 1998 to 2004. Most of those valves and piping systems are displaying their age and infrequently have important signs of steel fatigue. Others have loads of life left in them but may achieve advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the most recent sealing know-how 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 detachable two-in-one seat diffuser will reduce required upkeep.
If a turbine bypass valve have to be replaced, plant personnel are strongly inspired to look past a direct alternative and consider potential improvements. A particularly necessary item to assume about when looking at alternative and enchancment alternatives 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 locations with low overhead clearance, and it locations the actuator closer to the deck or floor for straightforward entry, but it creates a number of long-term operational problems. Gravity tends to lead to increased and uneven put on on trim components, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the main option a few decades in the past for turbine bypass valves. New move up valve designs (right) could also be installed with out 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 lowered and more even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement alternatives are sometimes for horizontally mounted actuators, and the two designs usually are not readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead space is on the market with current horizontal actuator circulate down designs, a flow up design can simply meet the existing face-to-face necessities of the old valve, whereas lowering some of the wear common 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 can be made with out modifications to present piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life because the steam is transferring comparatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates significantly, but 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 a lot decrease as a end result of they cross over the seat and speed up later because the steam strikes via the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is certainly showing erosion, however when in comparison with the flow down trim proven above (Figure 4 right), the seating surface on the seat ring and plug stay relatively undamaged, permitting the valve to close off completely, the required mode in normal operation.
7. These footage of flow up trim clearly present the erosive impression of high-velocity steam, however not like the flow down trim harm proven previously, this valve has no damage to the seating floor and can nonetheless shut off tightly. Courtesy: Emerson
The flow up valve fashion design still provides the same control responsiveness and flow capacity, however the model new circulate arrangement dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered equipment that should be rigorously specified, put in, and maintained. It is subsequently clever to be proactive by reaching out to trusted experts prior to buy as they can help 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 a while for the rationale that plant turbine bypass valves have been fully inspected, it might be clever to consult your turbine bypass valve vendor for help and help.
Many of those vendors supply full inspection and well being check services to determine areas the place metallic fatigue and stress cracking are growing. Ideally the valve may be repaired, but when a valve alternative is warranted, the vendor can also present guidance in evaluating bypass valve design styles and choosing the finest option for the particular application.
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