Turbine bypass valves are some of the tough functions in an influence plant. Keeping these valves operating efficiently and avoiding sudden failures is crucial for plant operation.
When engineers are requested to list probably the most troublesome management valve purposes, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high stress drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, many crops are likely to ignore these valves till they fail, creating unplanned outages, lost production, and high prices for reactive maintenance. This article supplies instructed methods of inspection to anticipate and mitigate issues beforehand, and it presents improve alternatives should a valve need to be repaired or replaced.
Strictly Severe Service
Severe service management valves are utilized in the most troublesome installations inside process crops. These installations generally embody cavitating, erosive, corrosive, noisy, excessive stress, high temperature, excessive strain drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those course of conditions; yet, they must reply flawlessly and stay leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, as nicely as when a turbine trips off-line (Figure 1). In regular operation the valves are completely closed, forcing all of the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine till the properties and conditions of the steam are acceptable for sending it to the turbine. This course of occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting potentially moist steam, and by making certain only applicable steam circumstances and flows make their way to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves could also be employed to immediately shunt steam round a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to keep away from tools harm because of overpressure and excessive temperature, so the turbine bypass valve immediately opens to take care of flow through the system.
As ไดอะแฟรม operates, a turbine makes use of steam to perform work, lowering outlet steam temperature and strain. When a turbine bypass valve opens, it’s going to drop the stress, however the exit steam will remain quite superheated, doubtlessly destroying downstream gear. To keep away from that scenario, turbine bypass valves either incorporate a water injection system in the valve body, or employ a separate water injection desuperheater just downstream, in either case to lower the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of severe service situations. While the plant is in operation, these valves should stay tightly closed to keep away from losing power. When a turbine journey occurs, the bypass valves should respond immediately, exposing them to fast temperature adjustments and requiring them to cross very high flows at excessive strain drops, creating high noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that nearly each turbine bypass valve will ultimately fail in some way. Unfortunately, many of those valves are installed in difficult-to-access places, are usually welded in place, and tend to be heavily insulated. As a result, they are often ignored till issues start to floor. Steam leakage via the valve is usually the first symptom observed, but far more significant and potentially harmful damage can happen.
The extreme stress drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the steel 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 high vibrations and thermal shock skilled by turbine bypass valves will fatigue the metal and create cracks in both the valve itself and the encircling piping. Courtesy: Emerson
To keep away from this drawback, every turbine bypass valve and its associated piping should be routinely inspected. Some type of non-destructive examination ought to be periodically employed to detect metallic fatigue issues that might be growing however aren’t yet visually obvious. If a plant lacks the information or tools to perform these inspections, the valve vendor or approved consultant may be utilized to perform turbine bypass well being check services (Figure 3).
three. A routine turbine bypass valve health check and full inspection is strongly suggested. 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 body and water manifold, customer connection welds, diffuser welds, and surrounding piping can be inspected to identify any growing problems before the gear is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to avoid issues related to quenching and cracking.
There are also threats to trim parts, similar to seat floor erosion. One frequent expression of such erosion, particularly on the plug, is sometimes called “gear toothing.” This happens more commonly in circulate down valves, where the steam accelerates via the cage holes after which strikes the trim/seat space immediately, inflicting excessive put on and lowering service life. Over time, this high-velocity steam, which might comprise water throughout startup conditions and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim damage as the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like delicate items and spray nozzles can often be replaced comparatively easily and at minimal value. Trim parts that need to be changed due to heavy wear, similar to gear toothing, can be fairly expensive, especially if their supply needs to be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue creating within the valve physique or diffuser, with excessive restore procedures, or even a complete valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service have been put in during the heyday of combined cycle plant building from 1998 to 2004. Most of these valves and piping systems are showing their age and infrequently have important indicators of metallic fatigue. Others have loads of life left in them but may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the latest sealing technology could inject new life into the valve.
Another instance of a expertise upgrade 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 must be replaced, plant personnel are strongly inspired to look beyond a direct substitute and consider potential enhancements. A significantly essential merchandise to contemplate when looking at replacement and improvement opportunities is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a flow down trim design paired with a horizontal actuator (Figure 5, left). This association is right for places with low overhead clearance, and it places the actuator nearer to the deck or ground for straightforward entry, nevertheless it creates numerous long-term operational problems. Gravity tends to lead to increased and uneven put on on trim components, and the side-mounted actuator is vulnerable to response problems.
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 installed without piping modifications in most cases, offering prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This association leads to decreased and more even put on, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that replacement alternatives are usually for horizontally mounted actuators, and the two designs aren’t readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead area is on the market with existing horizontal actuator move down designs, a flow up design can easily meet the prevailing face-to-face requirements of the previous valve, whereas lowering a number of the wear widespread to move down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from move all the way down to move up, this change could be made with out modifications to present piping.
Flow up valves (Figure 5, right) considerably prolong seat and trim service life as a end result of the steam is shifting relatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes via 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 a lot decrease as a result of they cross over the seat and speed up later as 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 definitely exhibiting erosion, however when in comparison with the circulate down trim shown above (Figure four right), the seating surface on the seat ring and plug remain relatively undamaged, permitting the valve to close off utterly, the required mode in regular operation.
7. These pictures of flow up trim clearly show the erosive influence of high-velocity steam, but in distinction to the circulate down trim injury proven beforehand, this valve has no harm to the seating surface and can nonetheless shut off tightly. Courtesy: Emerson
The flow up valve fashion design still supplies the same management responsiveness and move capacity, however the brand new flow arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised items of engineered gear that have to be fastidiously specified, installed, and maintained. It is due to this fact wise to be proactive by reaching out to trusted consultants prior to buy as they will assist with valve sizing and specifications based on specific working conditions.
For current installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while for the rationale that plant turbine bypass valves have been absolutely inspected, it might be sensible to seek the guidance of your turbine bypass valve vendor for assist and help.
Many of these distributors supply full inspection and health verify services to identify areas the place metallic fatigue and stress cracking are growing. Ideally the valve can be repaired, but if a valve replacement is warranted, the vendor can also present guidance in evaluating bypass valve design types and choosing the best option for the precise application.
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