Turbine Bypass Valve Challenges

Turbine bypass valves are some of the difficult applications in a power plant. Keeping these valves operating efficiently and avoiding sudden failures is crucial for plant operation.
When engineers are requested to list 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 limit. Unfortunately, many vegetation tend to ignore these valves till they fail, creating unplanned outages, lost manufacturing, and excessive prices for reactive upkeep. This article supplies instructed strategies of inspection to anticipate and mitigate issues beforehand, and it provides upgrade alternate options should a valve must be repaired or replaced.
Strictly Severe Service
Severe service management valves are used in the most troublesome installations inside course of crops. These installations generally embrace cavitating, erosive, corrosive, noisy, excessive pressure, excessive temperature, high strain drop, or excessive velocity media. Turbine bypass valves are exposed to many of these course of conditions; but, they need to reply flawlessly and stay leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam turbines during plant startup and shutdown, in addition to when a turbine journeys off-line (Figure 1). In regular operation the valves are utterly closed, forcing all the steam by way of 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 protect the turbine by diverting doubtlessly moist steam, and by guaranteeing solely appropriate steam situations and flows make their approach to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves may be employed to immediately shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam should continue flowing to keep away from equipment injury because 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 carry out work, decreasing outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the stress, however the exit steam will stay quite superheated, probably destroying downstream gear. To avoid that situation, turbine bypass valves either incorporate a water injection system within the valve body, or make use of a separate water injection desuperheater simply downstream, in both case to decrease the exit steam temperature.
As a end result, turbine bypass valves face a perfect storm of severe service circumstances. While the plant is in operation, these valves must remain tightly closed to avoid losing vitality. When a turbine trip happens, the bypass valves should reply immediately, exposing them to fast temperature adjustments and requiring them to move very high flows at high stress drops, creating high noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that just about each turbine bypass valve will finally fail indirectly. Unfortunately, many of these valves are installed in difficult-to-access locations, are sometimes welded in place, and tend to be heavily insulated. As a result, they are typically ignored till problems start to floor. Steam leakage by way of the valve is normally the primary symptom observed, but rather more significant and potentially harmful injury can occur.
เกจ์วัดแรงดันน้ำ drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the steel in the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can end result in catastrophic failure under pressure.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in both the valve itself and the encompassing piping. Courtesy: Emerson
To avoid this problem, 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 steel fatigue problems which may be creating but aren’t but visually obvious. If a plant lacks the data or gear to perform these inspections, the valve vendor or approved consultant could also be utilized to perform turbine bypass health verify companies (Figure 3).
3. A routine turbine bypass valve well being examine and full inspection is strongly advised. Metal fatigue can develop in various welds positioned on the valve itself, and within the inlet, outlet, and water supply 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 identify any creating issues before the tools is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are also threats to trim parts, similar to seat surface erosion. One common expression of such erosion, specifically on the plug, is also identified as “gear toothing.” This happens extra commonly in flow down valves, where the steam accelerates by way of the cage holes after which strikes the trim/seat space directly, inflicting excessive wear and lowering service life. Over time, this high-velocity steam, which can contain water during startup situations and magnetite throughout any situation, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve physique designs are prone to trim harm as the wet, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like delicate items and spray nozzles can normally be replaced relatively easily and at minimal cost. Trim elements that have to be changed as a result of heavy put on, corresponding to gear toothing, could be quite expensive, especially if their supply needs to be expedited. The worst-case situation for a turbine bypass valve is metal fatigue developing in the valve physique or diffuser, with excessive repair procedures, or maybe a full valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service had been put in in the course of the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping systems are displaying their age and often have vital signs of metallic fatigue. Others have plenty of life left in them but may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the most recent sealing technology might 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 occasion, a removable two-in-one seat diffuser will scale back required maintenance.
If a turbine bypass valve have to be replaced, plant personnel are strongly encouraged to look past a direct replacement and consider potential enhancements. A notably important item to contemplate when taking a look 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 places with low overhead clearance, and it locations the actuator closer to the deck or ground for straightforward access, nevertheless it creates a selection of long-term operational problems. Gravity tends to result in elevated and uneven wear on trim parts, and the side-mounted actuator is vulnerable to response issues.
5. Downward flowing valves with horizontal actuators (left) have been the principle possibility a couple of a long time ago for turbine bypass valves. New move up valve designs (right) could additionally be put in with out piping modifications in most situations, providing extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for flow down is vertical mounting of the actuator. This arrangement leads to decreased and extra even put on, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that alternative alternatives are usually 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 flow down designs, a circulate up design can simply meet the existing face-to-face requirements of the old valve, while decreasing a variety of the put on widespread to move down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from move all the method down to circulate up, this change can be made with out modifications to existing piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life because the steam is transferring relatively slowly as 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 floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower because they cross over the seat and speed up later because the steam strikes via the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is actually showing erosion, but when in comparability with the circulate down trim shown above (Figure four right), the seating floor on the seat ring and plug stay comparatively undamaged, allowing the valve to shut off fully, the required mode in regular operation.
7. These footage of circulate up trim clearly present the erosive impact of high-velocity steam, but unlike the circulate down trim harm proven beforehand, this valve has no damage to the seating surface and can still shut off tightly. Courtesy: Emerson
The flow up valve fashion design nonetheless offers the identical management responsiveness and flow capacity, however the brand new circulate association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized items of engineered equipment that have to be rigorously specified, put in, and maintained. It is therefore sensible to be proactive by reaching out to trusted specialists prior to buy as they will assist with valve sizing and specs based on particular operating conditions.
For existing installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time because 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 assist.
Many of these distributors offer full inspection and health check services to identify areas the place metallic fatigue and stress cracking are developing. Ideally the valve can be repaired, but when a valve replacement is warranted, the vendor also can provide guidance in evaluating bypass valve design kinds and choosing the most fitted choice for the particular utility.
Share

Leave a Comment