Turbine bypass valves are one of the most troublesome functions in a power plant. Keeping these valves working efficiently and avoiding surprising failures is important for plant operation.
When engineers are asked to listing probably the most tough control valve functions, turbine bypass valves are invariably talked about. Frequent thermal cycles, high pressure drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many plants are inclined to ignore these valves till they fail, creating unplanned outages, misplaced production, and high costs for reactive maintenance. This article provides suggested strategies of inspection to anticipate and mitigate issues beforehand, and it offers improve options should 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 pressure, excessive temperature, excessive strain drop, or high velocity media. Turbine bypass valves are exposed to many of those course of conditions; but, they must reply flawlessly and stay leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam generators 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 till 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 guard the turbine by diverting probably moist steam, and by guaranteeing solely appropriate steam circumstances and flows make their approach to the turbine itself.
1. Depending on the power plant design, a quantity of turbine bypass valves may be employed to instantly shunt steam around a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine journey, the steam must proceed flowing to keep away from gear injury as a outcome of overpressure and excessive temperature, so the turbine bypass valve immediately opens to maintain flow via 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 strain, but the exit steam will stay fairly superheated, potentially destroying downstream tools. To avoid that state of affairs, turbine bypass valves both incorporate a water injection system in the valve physique, or make use of a separate water injection desuperheater simply downstream, in both case to decrease the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of severe service conditions. While Authentic is in operation, these valves should remain tightly closed to avoid wasting power. When a turbine journey occurs, the bypass valves must respond immediately, exposing them to fast temperature adjustments and requiring them to move very excessive flows at excessive strain drops, creating high noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that nearly each turbine bypass valve will finally fail indirectly. Unfortunately, many of those valves are installed in difficult-to-access places, are typically welded in place, and are typically closely insulated. As a end result, they’re often ignored until issues start to surface. Steam leakage through the valve is normally the first symptom seen, however much more important and doubtlessly harmful harm can happen.
The excessive strain drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metallic in the valve, water connections, and the piping itself (Figure 2). Such metal fatigue may find yourself in catastrophic failure under strain.
2. Over time, the excessive vibrations and thermal shock skilled by turbine bypass valves will fatigue the metallic and create cracks in each the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this downside, each turbine bypass valve and its associated piping should be routinely inspected. Some type of non-destructive examination should be periodically employed to detect steel fatigue issues that might be creating but usually are not yet visually apparent. If a plant lacks the data or tools to carry out these inspections, the valve vendor or approved consultant could additionally be utilized to carry out turbine bypass well being examine providers (Figure 3).
three. A routine turbine bypass valve health check and full inspection is strongly advised. Metal fatigue can develop in numerous welds positioned on the valve itself, and in the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, buyer connection welds, diffuser welds, and surrounding piping may be inspected to establish any growing problems before the equipment is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to keep away from issues associated to quenching and cracking.
There are additionally threats to trim components, such as seat floor erosion. One common expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This happens more commonly in move down valves, where the steam accelerates by way of the cage holes and then strikes the trim/seat space immediately, causing excessive wear and lowering service life. Over time, this high-velocity steam, which may include water during startup situations and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve physique designs are vulnerable to trim damage as the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like gentle items and spray nozzles can usually be replaced comparatively easily and at minimal value. Trim elements that have to be replaced due to heavy wear, such as gear toothing, could be fairly costly, particularly if their supply must be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue developing within the valve physique or diffuser, with extreme restore procedures, or perhaps a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service were installed during the heyday of mixed cycle plant development from 1998 to 2004. Most of these valves and piping methods are showing their age and infrequently have vital indicators of metal fatigue. Others have plenty of life left in them but could benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the most recent sealing expertise might inject new life into the valve.
Another instance of a know-how upgrade is when there have been repeated failures with a welded diffuser. In this occasion, a detachable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve must be replaced, plant personnel are strongly encouraged to look beyond a direct alternative and consider potential enhancements. A significantly important item to think about when taking a glance at alternative and improvement 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 locations with low overhead clearance, and it locations the actuator closer to the deck or ground for easy access, however it creates numerous long-term operational issues. Gravity tends to result in increased and uneven put on on trim parts, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the primary choice a number of many years in the past for turbine bypass valves. New move up valve designs (right) may be installed without piping modifications in most instances, offering extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for move down is vertical mounting of the actuator. This association ends in reduced and more even put on, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that replacement alternatives are typically for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable with out vital piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead house is on the market with existing horizontal actuator flow down designs, a move up design can easily meet the existing face-to-face necessities of the previous valve, whereas decreasing a number of the put on widespread to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from flow right down to flow up, this modification can be made with out modifications to existing piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life because the steam is moving 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 considerably, but that vitality dissipates into the massive body cavity of the valve as a substitute of hanging the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower because they cross over the seat and speed up later as the steam strikes through the small trim holes, tremendously extending trim service life. Double : Emerson
Notice how the circulate up trim (Figure 7) is certainly exhibiting erosion, but when compared to the move down trim shown above (Figure four right), the seating surface on the seat ring and plug stay relatively undamaged, allowing the valve to shut off completely, the required mode in regular operation.
7. These footage of circulate up trim clearly present the erosive influence of high-velocity steam, but in contrast to the circulate down trim damage shown beforehand, this valve has no injury to the seating floor and may nonetheless shut off tightly. Courtesy: Emerson
The move up valve style design still provides the identical management responsiveness and flow capability, but the brand 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 equipment that must be fastidiously specified, put in, and maintained. It is therefore wise to be proactive by reaching out to trusted experts prior to purchase as they can assist with valve sizing and specs primarily 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 absolutely inspected, it will be clever to seek the guidance of your turbine bypass valve vendor for help and assist.
Many of these vendors offer full inspection and well being check companies to establish areas the place metal fatigue and stress cracking are creating. Ideally the valve could be repaired, but when a valve substitute is warranted, the seller can even provide steering in evaluating bypass valve design types and selecting the best option for the particular utility.
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