Turbine Bypass Valve Challenges

Turbine bypass valves are one of the tough purposes in a power plant. Keeping these valves working effectively and avoiding sudden failures is critical for plant operation.
When engineers are asked to listing the most tough 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 inclined to ignore these valves till they fail, creating unplanned outages, misplaced production, and high costs for reactive upkeep. This article provides suggested strategies of inspection to anticipate and mitigate points beforehand, and it presents improve alternate options should a valve must be repaired or replaced.
Strictly Severe Service
Severe service management valves are used in probably the most troublesome installations within process vegetation. These installations generally embrace cavitating, erosive, corrosive, noisy, high pressure, excessive temperature, high strain drop, or high velocity media. Turbine bypass valves are exposed to many of these process conditions; but, they must respond flawlessly and remain leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam turbines throughout plant startup and shutdown, in addition to when a turbine trips off-line (Figure 1). In normal operation the valves are fully closed, forcing all the steam via a turbine. During startup, bypass valves divert steam away from the turbine until the properties and situations of the steam are acceptable for sending it to the turbine. This process happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting potentially wet steam, and by making certain solely acceptable steam circumstances and flows make their way to the turbine itself.
1. Depending on the power plant design, a quantity of turbine bypass valves may be employed to immediately shunt steam round a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to avoid equipment damage because of overpressure and high temperature, so the turbine bypass valve immediately opens to maintain flow by way of the system.
As เกจวัดแรงดันsumo operates, a turbine makes use of steam to carry out work, reducing outlet steam temperature and strain. When a turbine bypass valve opens, it’ll drop the stress, but the exit steam will stay fairly superheated, potentially destroying downstream equipment. To keep away from that state of affairs, turbine bypass valves either 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 a perfect storm of extreme service conditions. While the plant is in operation, these valves should stay tightly closed to avoid wasting vitality. When a turbine journey happens, the bypass valves should respond instantly, exposing them to speedy temperature changes and requiring them to cross very excessive flows at high strain drops, creating excessive noise and probably extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that virtually every turbine bypass valve will ultimately fail ultimately. Unfortunately, many of these valves are put in in difficult-to-access areas, are usually welded in place, and tend to be heavily insulated. As a result, they are often ignored till issues begin to surface. Steam leakage by way of the valve is usually the primary symptom noticed, however much more vital and probably dangerous harm can happen.
The excessive stress drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue may find yourself in catastrophic failure under stress.
2. Over time, the high vibrations and thermal shock skilled 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, each turbine bypass valve and its associated piping ought to be routinely inspected. Some kind of non-destructive examination must be periodically employed to detect metallic fatigue issues that could be growing however usually are not but visually obvious. If a plant lacks the knowledge or equipment to perform these inspections, the valve vendor or authorized consultant could additionally be utilized to carry out turbine bypass health examine services (Figure 3).
3. A routine turbine bypass valve health examine and full inspection is strongly advised. Metal fatigue can develop in various welds positioned on the valve itself, and in the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, customer connection welds, diffuser welds, and surrounding piping can be inspected to establish any developing issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from problems related to quenching and cracking.
There are also threats to trim parts, similar to seat floor erosion. One widespread expression of such erosion, particularly on the plug, is sometimes called “gear toothing.” This happens more commonly in flow down valves, where the steam accelerates by way of the cage holes after which strikes the trim/seat area immediately, inflicting excessive wear and lowering service life. Over time, this high-velocity steam, which may include water throughout startup situations and magnetite throughout any condition, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve body designs are vulnerable to trim injury because the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like soft goods and spray nozzles can often be replaced comparatively simply and at minimal cost. Trim components that have to be replaced as a result of heavy put on, similar to gear toothing, may be fairly pricey, especially if their delivery must be expedited. The worst-case situation for a turbine bypass valve is steel fatigue developing within the valve body or diffuser, with excessive restore procedures, or maybe a complete valve substitute, required.
pressure gauge ราคา of the turbine bypass valves presently in service were put in in the course of the heyday of combined cycle plant construction from 1998 to 2004. Most of these valves and piping systems are showing their age and often have significant indicators of metal fatigue. Others have loads of life left in them however may benefit from upgrades in technology. Technology and practices have improved, and upgrading to the most recent sealing know-how might inject new life into the valve.
Another instance of a know-how 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 should be replaced, plant personnel are strongly inspired to look beyond a direct substitute and evaluate potential improvements. A significantly necessary merchandise to suppose about 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 circulate down trim design paired with a horizontal actuator (Figure 5, left). This association is good for locations with low overhead clearance, and it locations the actuator nearer to the deck or ground for easy entry, nevertheless it creates a number of long-term operational issues. Gravity tends to lead to increased and uneven wear on trim elements, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) were the main possibility a quantity of many years ago for turbine bypass valves. New circulate up valve designs (right) may be put in without piping modifications in most cases, providing extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for flow down is vertical mounting of the actuator. This association results in reduced and more even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that substitute opportunities are typically for horizontally mounted actuators, and the two designs aren’t readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is available with existing horizontal actuator circulate down designs, a circulate up design can easily meet the prevailing face-to-face requirements of the old valve, whereas decreasing a few of the put on widespread to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from flow all the way down to move up, this change can be made with out modifications to existing piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life because the steam is moving comparatively slowly as it passes over the plug and seat (Figure 6). As the steam moves through the small holes of the cage, it accelerates considerably, however that energy dissipates into the large physique cavity of the valve instead of putting the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease as a outcome of they pass 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 flow up trim (Figure 7) is actually exhibiting erosion, however when compared to 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 close off fully, the required mode in normal operation.
7. These footage of flow up trim clearly present the erosive impact of high-velocity steam, but unlike the flow down trim harm proven previously, this valve has no damage to the seating floor and can still shut off tightly. Courtesy: Emerson
The circulate up valve style design still supplies the identical control responsiveness and circulate capability, however the new flow association dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered tools that have to be carefully specified, installed, and maintained. It is subsequently wise to be proactive by reaching out to trusted consultants prior to buy as they’ll assist with valve sizing and specs based on particular operating circumstances.
For present installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time because the plant turbine bypass valves have been totally inspected, it will be wise to consult your turbine bypass valve vendor for help and help.
Many of these distributors supply full inspection and health verify services to establish areas where metallic fatigue and stress cracking are growing. Ideally the valve can be repaired, but if a valve replacement is warranted, the vendor also can provide guidance in evaluating bypass valve design styles and choosing the finest choice for the precise software.

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