Turbine bypass valves are some of the tough purposes in an influence plant. Keeping these valves operating efficiently and avoiding sudden failures is crucial for plant operation.
When engineers are asked to listing the most difficult management valve functions, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high stress drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many crops tend to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and high costs for reactive upkeep. เกจวัดแรงดัน4บาร์ provides suggested strategies of inspection to anticipate and mitigate issues beforehand, and it offers upgrade options ought to a valve have to be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in the most tough installations within process vegetation. These installations commonly include cavitating, erosive, corrosive, noisy, excessive pressure, excessive temperature, high stress drop, or high velocity media. Turbine bypass valves are uncovered to many of these course of conditions; yet, they must respond flawlessly and stay leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam generators during plant startup and shutdown, as nicely as when a turbine trips off-line (Figure 1). In normal operation the valves are completely 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 process happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting doubtlessly moist steam, and by ensuring only appropriate steam situations and flows make their method to the turbine itself.
1. Depending on the facility plant design, a quantity of turbine bypass valves may be employed to instantly shunt steam round a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to keep away from tools damage as a outcome of overpressure and high temperature, so the turbine bypass valve immediately opens to take care of move by way of the system.
As it operates, a turbine uses steam to perform work, reducing outlet steam temperature and stress. When a turbine bypass valve opens, it will drop the stress, but the exit steam will stay quite superheated, probably destroying downstream equipment. To avoid that situation, turbine bypass valves either incorporate a water injection system in the valve physique, or employ a separate water injection desuperheater just downstream, in both case to decrease the exit steam temperature.
As a outcome, turbine bypass valves face a perfect storm of extreme service circumstances. While the plant is in operation, these valves should remain tightly closed to avoid losing power. When a turbine journey occurs, the bypass valves should reply immediately, exposing them to rapid temperature changes and requiring them to pass very excessive flows at excessive 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 finally fail ultimately. Unfortunately, many of those valves are installed in difficult-to-access areas, are usually welded in place, and are usually heavily insulated. As a result, they are usually ignored till issues begin to surface. Steam leakage via the valve is normally the primary symptom seen, but rather more important and probably harmful harm can occur.
The extreme stress drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the steel in the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can lead to 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 each the valve itself and the encompassing piping. Courtesy: Emerson
To avoid this downside, each turbine bypass valve and its related piping ought to be routinely inspected. Some sort of non-destructive examination should be periodically employed to detect metallic fatigue issues that may be developing but usually are not yet visually obvious. If a plant lacks the data or tools to carry out these inspections, the valve vendor or approved representative could also 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 situated 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 developing problems before the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid problems associated to quenching and cracking.
There are additionally threats to trim components, similar to seat surface erosion. One widespread expression of such erosion, particularly on the plug, is often referred to as “gear toothing.” This occurs extra commonly in flow down valves, where the steam accelerates by way of the cage holes after which strikes the trim/seat area directly, inflicting extreme put on and reducing service life. Over time, this high-velocity steam, which may contain water during startup conditions and magnetite throughout any condition, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve physique designs are vulnerable to trim harm as the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like delicate goods and spray nozzles can often get replaced relatively easily and at minimal cost. Trim elements that must be changed because of heavy put on, corresponding to gear toothing, can be quite expensive, particularly if their delivery needs to be expedited. The worst-case situation for a turbine bypass valve is steel fatigue creating in the valve physique or diffuser, with excessive repair procedures, or even a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were installed in the course of the heyday of mixed cycle plant development from 1998 to 2004. Most of those valves and piping methods are displaying their age and sometimes have vital signs of steel fatigue. Others have loads of life left in them however may benefit from upgrades in know-how. Technology and practices have improved, and upgrading to the newest sealing technology could inject new life into the valve.
Another instance of a technology 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 have to be changed, plant personnel are strongly encouraged to look past a direct alternative and consider potential enhancements. A notably important item to assume about when taking a glance at replacement 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 ideal for locations with low overhead clearance, and it locations the actuator nearer to the deck or ground for straightforward entry, but it creates numerous long-term operational problems. Gravity tends to lead to elevated and uneven put on on trim parts, and the side-mounted actuator is susceptible to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the main option a couple of a long time in the past for turbine bypass valves. New flow up valve designs (right) may be installed without piping modifications in most cases, providing prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for flow down is vertical mounting of the actuator. This association leads to lowered and extra even wear, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that replacement opportunities 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 sufficient overhead area is available with current horizontal actuator move down designs, a flow up design can simply meet the present face-to-face necessities of the previous valve, while lowering some of the wear frequent to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from circulate down to move up, this modification can be made with out modifications to existing piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life as a end result of the steam is transferring comparatively slowly because it passes over the plug and seat (Figure 6). As the steam moves through the small holes of the cage, it accelerates significantly, however that energy dissipates into the large body cavity of the valve as an alternative of hanging the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower as a outcome of they pass over the seat and accelerate later because the steam strikes via the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is actually showing erosion, but when compared to the move down trim proven above (Figure four right), the seating floor on the seat ring and plug stay comparatively undamaged, allowing the valve to close off fully, the required mode in normal operation.
7. These photos of circulate up trim clearly present the erosive influence of high-velocity steam, however unlike the move down trim injury proven beforehand, this valve has no harm to the seating surface and might nonetheless shut off tightly. Courtesy: Emerson
The circulate up valve style design nonetheless provides the same control responsiveness and flow capacity, however the brand new flow association dramatically extends the working life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized items of engineered tools that must be carefully specified, put in, and maintained. It is therefore sensible to be proactive by reaching out to trusted consultants prior to purchase as they’ll assist with valve sizing and specifications based on particular working situations.
For existing installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time for the reason that plant turbine bypass valves have been fully inspected, it might be clever to consult your turbine bypass valve vendor for assist and help.
Many of these vendors supply full inspection and health examine services to identify areas where metallic fatigue and stress cracking are developing. Ideally the valve can be repaired, but if a valve replacement is warranted, the seller can also present guidance in evaluating bypass valve design types and selecting the most suitable choice for the particular utility.
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