Turbine bypass valves are one of the most difficult functions in an influence plant. Keeping these valves working efficiently and avoiding sudden failures is critical for plant operation.
When engineers are requested to list probably the most tough 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 restrict. Unfortunately, many crops tend to ignore these valves till they fail, creating unplanned outages, misplaced production, and excessive prices for reactive upkeep. This article supplies suggested methods of inspection to anticipate and mitigate points beforehand, and it presents upgrade options ought to a valve have to be repaired or changed.
Strictly Severe Service
Severe service management valves are utilized in probably the most troublesome installations within process plants. These installations commonly embrace cavitating, erosive, corrosive, noisy, high stress, high temperature, high pressure drop, or high velocity media. Turbine bypass valves are exposed to many of those process conditions; but, they have to reply 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, as well as when a turbine journeys off-line (Figure 1). In regular operation the valves are completely closed, forcing all 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 course of occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting probably wet steam, and by making certain solely appropriate steam circumstances and flows make their way to the turbine itself.
1. Depending on the facility plant design, a number of turbine bypass valves may be employed to immediately shunt steam round a turbine should it trip offline. Courtesy: Emerson
Should a turbine journey, the steam must proceed flowing to avoid gear injury because of overpressure and high temperature, so the turbine bypass valve instantly opens to take care of move by way of the system.
As it operates, a turbine makes use of steam to carry out work, reducing outlet steam temperature and stress. When a turbine bypass valve opens, it will drop the strain, however the exit steam will stay quite superheated, doubtlessly destroying downstream tools. To avoid that state of affairs, turbine bypass valves either incorporate a water injection system in the valve physique, or employ a separate water injection desuperheater just downstream, in either case to lower the exit steam temperature.
As a end result, turbine bypass valves face an ideal storm of severe service conditions. While the plant is in operation, these valves must remain tightly closed to avoid wasting vitality. When a turbine journey occurs, the bypass valves should respond instantly, exposing them to speedy temperature changes and requiring them to move very high flows at high strain drops, creating excessive noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly each turbine bypass valve will finally fail indirectly. Unfortunately, many of those valves are put in in difficult-to-access locations, are usually welded in place, and are usually closely insulated. As a result, they are usually ignored till issues start to floor. Steam leakage through the valve is normally the primary symptom noticed, however rather more important and potentially harmful injury can happen.
The extreme stress drops invariably generate high noise and high 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 steel and create cracks in both the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this downside, every turbine bypass valve and its associated piping ought to be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect metallic fatigue issues that might be creating but usually are not but visually apparent. If a plant lacks the information or equipment to perform these inspections, the valve vendor or licensed consultant may be utilized to perform turbine bypass health examine providers (Figure 3).
three. A routine turbine bypass valve health verify and full inspection is strongly suggested. Metal fatigue can develop in varied welds positioned 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, buyer connection welds, diffuser welds, and surrounding piping could be inspected to determine any creating problems earlier than the gear is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid issues related to quenching and cracking.
There are additionally threats to trim components, corresponding to seat surface erosion. One widespread expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This occurs extra generally in move 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 throughout startup conditions and magnetite during any condition, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve physique designs are prone to trim injury as the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like soft goods and spray nozzles can often be replaced comparatively easily and at minimal value. Trim components that have to be changed due to heavy wear, such as gear toothing, may be fairly pricey, especially if their supply needs to be expedited. The worst-case situation for a turbine bypass valve is metal 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 currently in service have been installed through the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping techniques are showing their age and infrequently have significant signs of steel fatigue. Others have loads of life left in them but may benefit from upgrades in know-how. Technology and practices have improved, and upgrading to the newest sealing know-how may inject new life into the valve.
Another instance of a technology improve is when there have been repeated failures with a welded diffuser. In this instance, a detachable two-in-one seat diffuser will cut back required maintenance.
If a turbine bypass valve have to be replaced, plant personnel are strongly inspired to look past a direct replacement and evaluate potential improvements. A notably necessary item to think about when looking at substitute and improvement alternatives 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 right for areas with low overhead clearance, and it places the actuator nearer to the deck or ground for simple access, however it creates a quantity of long-term operational issues. Gravity tends to end in increased and uneven put on on trim parts, and the side-mounted actuator is vulnerable to response issues.
5. Downward flowing valves with horizontal actuators (left) have been the main choice a few decades ago for turbine bypass valves. New flow up valve designs (right) could additionally be put in with out piping modifications in most cases, 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 association leads to lowered and extra even wear, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that alternative alternatives are sometimes for horizontally mounted actuators, and the 2 designs aren’t readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead space is on the market with current horizontal actuator circulate down designs, a move up design can easily meet the present face-to-face requirements of the outdated valve, while lowering some of the wear widespread to move down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from flow all the means down to flow up, this alteration may be made without modifications to current piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life because the steam is moving 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 power dissipates into the big body cavity of the valve instead of striking the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower because they cross over the seat and accelerate later as the steam strikes via the small trim holes, tremendously 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 four right), the seating surface on the seat ring and plug stay comparatively undamaged, permitting the valve to shut off completely, the required mode in regular operation.
7. These photos of circulate up trim clearly show the erosive influence of high-velocity steam, but unlike the circulate down trim harm proven previously, this valve has no harm to the seating surface and may nonetheless shut off tightly. Courtesy: Emerson
The flow up valve style design nonetheless provides the same management responsiveness and circulate capability, but the new circulate arrangement dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized items of engineered tools that must be rigorously specified, put in, and maintained. It is due to this fact sensible to be proactive by reaching out to trusted consultants prior to buy as they can assist with valve sizing and specifications based on specific working circumstances.
For current installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while since the plant turbine bypass valves have been totally inspected, it would be clever to consult your turbine bypass valve vendor for help and assist.
เกจแรงดันลม of those distributors offer full inspection and health check companies to determine areas where metallic fatigue and stress cracking are developing. Ideally the valve could be repaired, but when a valve substitute is warranted, the vendor also can provide guidance in evaluating bypass valve design styles and selecting the finest option for the particular utility.
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