Turbine bypass valves are some of the tough purposes in an influence plant. Keeping these valves working efficiently and avoiding surprising failures is critical for plant operation.
When engineers are requested to record probably the most troublesome control valve functions, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive stress drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, many plants are probably to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and high costs for reactive maintenance. This article provides advised strategies of inspection to anticipate and mitigate issues beforehand, and it offers improve options should a valve need to be repaired or replaced.
Strictly Severe Service
Severe service management valves are utilized in essentially the most troublesome installations within course of plants. These installations commonly embrace cavitating, erosive, corrosive, noisy, excessive strain, excessive temperature, high stress drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those course of situations; but, they want 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 nicely as when a turbine journeys off-line (Figure 1). In normal operation the valves are fully closed, forcing all of the steam by way of 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 happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting potentially moist steam, and by making certain solely acceptable steam situations and flows make their method to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves could additionally be employed to instantly shunt steam around a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to keep away from tools harm as a end result of overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain move via the system.
As it operates, a turbine makes use of steam to perform work, lowering outlet steam temperature and pressure. When a turbine bypass valve opens, it’ll drop the stress, but the exit steam will stay quite superheated, potentially destroying downstream gear. To avoid that scenario, turbine bypass valves both incorporate a water injection system in the valve body, or make use of a separate water injection desuperheater just downstream, in either case to decrease the exit steam temperature.
As a end result, turbine bypass valves face an ideal storm of extreme service circumstances. While the plant is in operation, these valves must remain tightly closed to avoid wasting power. When a turbine trip happens, the bypass valves must reply immediately, exposing them to fast temperature changes and requiring them to move very high flows at excessive strain drops, creating high noise and doubtlessly excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that just about each turbine bypass valve will finally fail indirectly. Unfortunately, many of these valves are put in in difficult-to-access places, are typically welded in place, and are usually closely insulated. As a end result, they are typically ignored till issues begin to floor. Steam leakage by way of the valve is often the primary symptom noticed, however far more significant and potentially dangerous harm can happen.
The extreme strain 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 metallic fatigue may end up in catastrophic failure under stress.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in each the valve itself and the encompassing piping. Courtesy: Emerson
To keep away from this downside, every turbine bypass valve and its associated piping should be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect steel fatigue problems which may be growing however aren’t but visually obvious. If a plant lacks the knowledge or gear to carry out these inspections, the valve vendor or licensed representative could additionally be utilized to carry out turbine bypass health check companies (Figure 3).
three. A routine turbine bypass valve health check and full inspection is strongly advised. Metal fatigue can develop in numerous welds positioned on the valve itself, and within the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, buyer connection welds, diffuser welds, and surrounding piping could be inspected to determine any creating issues earlier than the gear is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to keep away from issues related to quenching and cracking.
There are additionally threats to trim elements, similar to seat floor erosion. One widespread expression of such erosion, particularly on the plug, is often referred to as “gear toothing.” This happens extra commonly in circulate down valves, where the steam accelerates through the cage holes and then strikes the trim/seat area immediately, causing excessive put on and decreasing service life. Over time, this high-velocity steam, which can contain water throughout startup circumstances and magnetite during any situation, will erode and harm the valve seat and trim (Figure 4).
4. เกจวัดแรงดันดิจิตอลราคา down valve body designs are prone to trim damage because the wet, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like soft goods and spray nozzles can often get replaced relatively simply and at minimal value. Trim parts that must be replaced due to heavy put on, corresponding to gear toothing, may be fairly expensive, particularly if their delivery must be expedited. The worst-case state of affairs for a turbine bypass valve is metal fatigue growing in the valve body or diffuser, with extreme restore procedures, or perhaps a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service have been put in during the heyday of mixed cycle plant building from 1998 to 2004. Most of these valves and piping techniques are exhibiting their age and sometimes have vital indicators of steel fatigue. Others have plenty of life left in them however may acquire advantage from upgrades in technology. Technology and practices have improved, and upgrading to the most recent sealing expertise might inject new life into the valve.
Another instance of a expertise improve is when there have been repeated failures with a welded diffuser. In this occasion, a detachable two-in-one seat diffuser will scale back required upkeep.
If a turbine bypass valve should be changed, plant personnel are strongly encouraged to look beyond a direct substitute and consider potential improvements. A particularly necessary merchandise to contemplate when looking at alternative and enchancment opportunities is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a move down trim design paired with a horizontal actuator (Figure 5, left). This association is ideal for areas with low overhead clearance, and it places the actuator closer to the deck or ground for straightforward entry, nevertheless it creates a selection 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) were the main choice a couple of many years in the past for turbine bypass valves. New move up valve designs (right) could also be installed without piping modifications in most cases, offering prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for circulate down is vertical mounting of the actuator. This association results in decreased and extra even put on, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that alternative alternatives are typically for horizontally mounted actuators, and the two designs aren’t readily interchangeable with out vital piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead space is available with present horizontal actuator move down designs, a move up design can simply meet the prevailing face-to-face requirements of the old valve, whereas reducing a variety of the put on widespread to flow down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from flow right down to flow up, this modification could be made without modifications to existing 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 because it passes over the plug and seat (Figure 6). As the steam moves via the small holes of the cage, it accelerates considerably, but that power dissipates into the massive physique cavity of the valve instead of hanging the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower because they move over the seat and speed up later as the steam strikes via the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is actually exhibiting 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 comparatively undamaged, allowing the valve to close off utterly, the required mode in normal operation.
7. These pictures of circulate up trim clearly present the erosive influence of high-velocity steam, however in distinction to the move down trim injury proven previously, this valve has no damage to the seating surface and can still shut off tightly. Courtesy: Emerson
The move up valve style design still supplies the same management responsiveness and flow capacity, however the new circulate arrangement dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered tools that must be rigorously specified, installed, and maintained. It is subsequently sensible to be proactive by reaching out to trusted consultants prior to buy as they’ll help with valve sizing and specs based on particular 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 fully inspected, it will be wise to seek the advice of your turbine bypass valve vendor for assist and support.
Many of these vendors offer full inspection and health verify companies to determine areas the place metal fatigue and stress cracking are growing. Ideally the valve can be repaired, but when a valve alternative is warranted, the vendor also can provide steerage in evaluating bypass valve design types and choosing the greatest option for the particular application.
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