Turbine bypass valves are one of the most tough applications in a power plant. Keeping these valves working effectively and avoiding sudden failures is crucial for plant operation.
When engineers are requested to listing probably the most troublesome control valve functions, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive pressure drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many plants are probably to ignore these valves till they fail, creating unplanned outages, misplaced manufacturing, and high costs for reactive maintenance. This article supplies advised methods of inspection to anticipate and mitigate points beforehand, and it offers upgrade options should a valve have to be repaired or changed.
Strictly Severe Service

Severe service control valves are utilized in the most tough installations inside course of vegetation. These installations commonly include cavitating, erosive, corrosive, noisy, high stress, excessive temperature, excessive stress drop, or high velocity media. Turbine bypass valves are uncovered to many of those process conditions; yet, they must respond flawlessly and stay leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam turbines throughout plant startup and shutdown, as properly as when a turbine trips 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 till the properties and conditions of the steam are acceptable 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 potentially wet steam, and by making certain only appropriate steam conditions and flows make their method to the turbine itself.
1. Depending on the facility plant design, a number of turbine bypass valves could also be employed to immediately shunt steam around a turbine ought to it trip offline. Courtesy: Emerson

Should a turbine journey, the steam should proceed flowing to avoid equipment harm because of overpressure and excessive temperature, so the turbine bypass valve immediately opens to maintain up move through the system.
As it operates, a turbine uses steam to carry out work, reducing outlet steam temperature and pressure. When a turbine bypass valve opens, it’ll drop the strain, however the exit steam will stay fairly superheated, probably destroying downstream tools. To keep away from that state of affairs, turbine bypass valves either incorporate a water injection system within the valve body, or make use of a separate water injection desuperheater just downstream, in both case to decrease the exit steam temperature.
As a result, turbine bypass valves face an ideal storm of extreme service circumstances. While เพรสเชอร์เกจ is in operation, these valves must stay tightly closed to avoid losing energy. When a turbine trip happens, the bypass valves must reply instantly, exposing them to rapid temperature modifications and requiring them to cross very high 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 just about every turbine bypass valve will ultimately fail ultimately. Unfortunately, many of these valves are installed in difficult-to-access places, are typically welded in place, and are usually closely insulated. As a result, they are usually ignored till issues begin to floor. Steam leakage through the valve is normally the primary symptom seen, however far more important and potentially dangerous injury can happen.
The excessive stress drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the steel in the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can lead to catastrophic failure beneath strain.
2. Over time, the excessive vibrations and thermal shock skilled 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 must be routinely inspected. Some type of non-destructive examination should be periodically employed to detect metal fatigue issues which could be growing but usually are not but visually apparent. If a plant lacks the knowledge or equipment to perform these inspections, the valve vendor or approved consultant may be utilized to carry out turbine bypass well being examine services (Figure 3).
three. A routine turbine bypass valve health verify and full inspection is strongly suggested. Metal fatigue can develop in varied welds located 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, customer connection welds, diffuser welds, and surrounding piping could be inspected to establish any developing problems before the gear is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are additionally threats to trim components, corresponding to seat floor erosion. One common expression of such erosion, particularly on the plug, is also known as “gear toothing.” This occurs extra commonly in circulate down valves, the place the steam accelerates by way of the cage holes after which strikes the trim/seat space immediately, inflicting extreme put on and reducing service life. Over time, this high-velocity steam, which may comprise water throughout startup situations and magnetite during any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve physique designs are susceptible to trim injury as the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson

Common maintenance items like delicate items and spray nozzles can often be replaced comparatively simply and at minimal cost. Trim elements that have to be changed because of heavy put on, similar to gear toothing, can be fairly costly, particularly if their delivery must be expedited. The worst-case situation for a turbine bypass valve is metal fatigue growing within the valve physique or diffuser, with extreme repair procedures, or maybe a complete 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 building from 1998 to 2004. Most of those valves and piping techniques are showing their age and often have vital indicators of metal fatigue. Others have plenty of life left in them but may benefit from upgrades in know-how. Technology and practices have improved, and upgrading to the most recent sealing expertise could inject new life into the valve.
Another example of a know-how improve is when there have been repeated failures with a welded diffuser. In this occasion, a detachable two-in-one seat diffuser will scale back required upkeep.
If a turbine bypass valve must be changed, plant personnel are strongly inspired to look beyond a direct substitute and evaluate potential improvements. A notably necessary merchandise to consider when looking at substitute and enchancment alternatives 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 arrangement is ideal for areas with low overhead clearance, and it places the actuator nearer to the deck or floor for simple access, however it creates numerous long-term operational problems. Gravity tends to lead to elevated and uneven wear on trim components, and the side-mounted actuator is prone to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the main option a couple of decades ago for turbine bypass valves. New circulate up valve designs (right) could also be installed without piping modifications in most instances, 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 arrangement results in lowered and extra even put on, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement alternatives are usually for horizontally mounted actuators, and the two designs usually are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead space is out there with present horizontal actuator move down designs, a flow up design can easily meet the existing face-to-face necessities of the previous valve, while lowering a number of the put on widespread to circulate down designs with a horizontal actuator, corresponding to gear toothing. Because the trim just switches from move down to flow up, this alteration could be made with out modifications to present piping.
Flow up valves (Figure 5, right) considerably prolong seat and trim service life as a outcome of the steam is transferring relatively 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 massive physique cavity of the valve as an alternative of hanging the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower as a end result of they cross over the seat and speed up 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 certainly exhibiting erosion, however when compared to the flow down trim shown above (Figure four right), the seating floor on the seat ring and plug remain relatively undamaged, permitting the valve to shut off completely, the required mode in regular operation.
7. These pictures of circulate up trim clearly present the erosive influence of high-velocity steam, but unlike the move down trim injury proven previously, this valve has no damage to the seating floor and can nonetheless shut off tightly. Courtesy: Emerson

The circulate up valve style design still supplies the same control responsiveness and flow capability, however the new flow arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert

Turbine bypass valves are highly specialised pieces of engineered equipment that should be rigorously specified, put in, and maintained. It is therefore wise to be proactive by reaching out to trusted specialists prior to buy as they will help with valve sizing and specs based on specific working conditions.
For current installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time since the plant turbine bypass valves have been fully inspected, it might be wise to consult your turbine bypass valve vendor for help and assist.
Many of these vendors supply full inspection and health check providers to identify areas where metallic fatigue and stress cracking are creating. Ideally the valve can be repaired, but when a valve replacement is warranted, the seller can even provide steering in evaluating bypass valve design types and choosing the greatest choice for the precise software.
Share