Turbine bypass valves are one of the most difficult functions in an influence plant. Keeping these valves operating efficiently and avoiding unexpected failures is critical for plant operation.
When engineers are asked to record essentially the most troublesome control valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, high strain drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many crops are likely to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and excessive prices for reactive maintenance. This article offers suggested methods of inspection to anticipate and mitigate issues beforehand, and it provides upgrade options ought to a valve have to be repaired or replaced.
Strictly Severe Service
Severe service management valves are used in probably the most tough installations within process plants. These installations generally embrace cavitating, erosive, corrosive, noisy, high strain, high temperature, excessive strain drop, or high velocity media. Turbine bypass valves are uncovered to many of these course of situations; yet, they have to respond flawlessly and remain leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam turbines during plant startup and shutdown, in addition to when a turbine trips off-line (Figure 1). In normal operation the valves are utterly closed, forcing all of the steam through a turbine. During startup, bypass valves divert steam away from the turbine until the properties and circumstances 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 protect the turbine by diverting doubtlessly moist steam, and by making certain solely applicable steam conditions and flows make their way to the turbine itself.
1. Depending on the facility plant design, a quantity of turbine bypass valves could additionally be employed to immediately shunt steam around a turbine should it trip offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to avoid equipment harm due to overpressure and high temperature, so the turbine bypass valve instantly opens to hold up flow by way of the system.
As it operates, a turbine uses steam to perform work, reducing outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the strain, however the exit steam will remain quite superheated, probably destroying downstream tools. To avoid that situation, 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 result, turbine bypass valves face an ideal storm of severe service conditions. While the plant is in operation, these valves should stay tightly closed to keep away from wasting vitality. When a turbine journey happens, the bypass valves should respond immediately, exposing them to rapid temperature adjustments and requiring them to pass very high flows at high strain drops, creating high noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that just about each turbine bypass valve will ultimately fail in some way. Unfortunately, many of those valves are put in in difficult-to-access locations, are usually welded in place, and are typically heavily insulated. As a outcome, they’re typically ignored until issues start to surface. Steam leakage via the valve is usually the primary symptom noticed, however much more significant and potentially harmful harm can occur.
The excessive pressure drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the steel in the valve, water connections, and the piping itself (Figure 2). เกจวัดแรงดัน may find yourself in catastrophic failure beneath strain.
2. Over time, the excessive vibrations and thermal shock skilled by turbine bypass valves will fatigue the metal and create cracks in each the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this drawback, each turbine bypass valve and its associated piping ought to be routinely inspected. Some type of non-destructive examination should be periodically employed to detect steel fatigue issues that might be developing however usually are not but visually obvious. If a plant lacks the information or gear to perform these inspections, the valve vendor or authorized consultant could additionally be utilized to carry out turbine bypass well being verify providers (Figure 3).
three. A routine turbine bypass valve well being verify and full inspection is strongly suggested. Metal fatigue can develop in various welds located on the valve itself, and in the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, customer connection welds, diffuser welds, and surrounding piping could be inspected to determine any creating problems before the equipment is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are also threats to trim elements, similar to seat floor erosion. One common expression of such erosion, specifically on the plug, is also recognized as “gear toothing.” This occurs extra commonly in flow down valves, where the steam accelerates by way of the cage holes and then strikes the trim/seat space instantly, inflicting extreme put on and reducing service life. Over time, this high-velocity steam, which can include water throughout startup situations and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve physique designs are susceptible to trim harm as the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like gentle goods and spray nozzles can often be replaced comparatively easily and at minimal value. Trim parts that must be replaced because of heavy put on, corresponding to gear toothing, may be quite pricey, particularly if their supply must be expedited. The worst-case scenario for a turbine bypass valve is steel fatigue developing in the valve body or diffuser, with excessive repair procedures, or even a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were put in in the course of the heyday of combined cycle plant development from 1998 to 2004. Most of those valves and piping methods are showing their age and sometimes have significant indicators of metallic fatigue. Others have loads of life left in them however may gain advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing technology could inject new life into the valve.
ไดอะแฟรม ซีล 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 scale back required upkeep.
If a turbine bypass valve should be changed, plant personnel are strongly inspired to look beyond a direct substitute and consider potential improvements. A particularly important merchandise to suppose about when looking at substitute 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 ideal for places with low overhead clearance, and it places the actuator nearer to the deck or ground for simple access, nevertheless it creates a number of long-term operational problems. Gravity tends to lead to elevated and uneven wear on trim elements, and the side-mounted actuator is vulnerable to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the principle possibility a few many years in the past for turbine bypass valves. New flow up valve designs (right) could also be put in without piping modifications in most situations, providing extended 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 arrangement ends in reduced and extra even wear, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that alternative alternatives are typically for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable with out vital piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead house 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 decreasing a few of the put on frequent to circulate down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from move down to flow up, this modification could be made without modifications to current piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life as a end result of the steam is shifting comparatively slowly because 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 energy dissipates into the large body cavity of the valve as a substitute of hanging 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 result of they move over the seat and accelerate later because the steam moves by way of the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is certainly exhibiting erosion, but when in comparability with the circulate down trim shown above (Figure 4 right), the seating surface on the seat ring and plug remain relatively undamaged, allowing the valve to shut off utterly, the required mode in normal operation.
7. These footage of circulate up trim clearly present the erosive impact of high-velocity steam, however unlike the move down trim damage proven previously, this valve has no damage to the seating surface and may nonetheless shut off tightly. Courtesy: Emerson
The move up valve fashion design nonetheless provides the identical management responsiveness and circulate capacity, but the brand new move association dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised items of engineered gear that should be carefully specified, installed, and maintained. It is therefore clever to be proactive by reaching out to trusted specialists prior to purchase as they’ll help with valve sizing and specs based on specific operating circumstances.
For current installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while since the plant turbine bypass valves have been fully inspected, it would be sensible to consult your turbine bypass valve vendor for assist and assist.
Many of those distributors offer full inspection and health verify providers to identify areas the place metal fatigue and stress cracking are developing. Ideally the valve may be repaired, but when a valve replacement is warranted, the vendor can also provide guidance in evaluating bypass valve design kinds and selecting the most suitable choice for the specific utility.
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