Turbine bypass valves are some of the troublesome applications in a power plant. Keeping these valves operating effectively and avoiding unexpected failures is important for plant operation.
When engineers are requested to list the most tough control valve applications, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive pressure drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, many crops are inclined to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and high prices for reactive maintenance. This article offers suggested methods of inspection to anticipate and mitigate issues beforehand, and it provides upgrade alternate options should a valve have to be repaired or changed.
Strictly Severe Service
Severe service control valves are used in essentially the most tough installations within course of plants. These installations generally include cavitating, erosive, corrosive, noisy, high strain, excessive temperature, high stress drop, or high velocity media. Turbine bypass valves are uncovered to many of these process circumstances; but, they have to reply flawlessly and stay leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, in addition to when a turbine trips off-line (Figure 1). In regular operation the valves are fully closed, forcing all of 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 process happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting doubtlessly wet steam, and by ensuring only acceptable steam circumstances and flows make their approach to the turbine itself.
1. Depending on the facility plant design, a number of turbine bypass valves could also be employed to instantly shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam should continue flowing to keep away from gear damage as a outcome of overpressure and high temperature, so the turbine bypass valve instantly opens to maintain move by way of the system.
As it operates, a turbine uses steam to perform work, decreasing outlet steam temperature and pressure. When a turbine bypass valve opens, it’ll drop the stress, however the exit steam will remain fairly superheated, probably destroying downstream tools. To keep away from that situation, turbine bypass valves either incorporate a water injection system in the valve body, or employ a separate water injection desuperheater simply downstream, in either case to decrease the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of extreme service conditions. While the plant is in operation, these valves must remain tightly closed to keep away from losing vitality. When a turbine trip happens, the bypass valves should respond immediately, exposing them to speedy temperature modifications and requiring them to cross very excessive flows at high stress drops, creating excessive noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that just about every turbine bypass valve will ultimately fail in some way. Unfortunately, many of those valves are installed in difficult-to-access areas, are sometimes welded in place, and are typically closely insulated. As a result, they are often ignored until issues begin to surface. Steam leakage via the valve is often the primary symptom seen, but rather more important and potentially dangerous damage can happen.
The extreme pressure drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metal in the valve, water connections, and the piping itself (Figure 2). Such metal fatigue may end up in catastrophic failure underneath stress.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metal and create cracks in both the valve itself and the encircling piping. Courtesy: Emerson
To avoid this problem, every turbine bypass valve and its associated piping should be routinely inspected. Some type of non-destructive examination must be periodically employed to detect metallic fatigue issues that might be growing but are not but visually apparent. If a plant lacks the knowledge or equipment to carry out these inspections, the valve vendor or licensed consultant could also be utilized to perform turbine bypass health examine companies (Figure 3).
three. A routine turbine bypass valve well being verify and full inspection is strongly suggested. Metal fatigue can develop in numerous welds situated 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 can be inspected to identify any developing problems earlier than the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from issues associated to quenching and cracking.
There are additionally threats to trim elements, such as seat surface erosion. One frequent expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This happens more commonly in move down valves, where the steam accelerates through the cage holes after which strikes the trim/seat space instantly, causing excessive put on and reducing service life. Over time, this high-velocity steam, which may comprise water throughout startup situations and magnetite throughout any situation, will erode and harm the valve seat and trim (Figure 4).
four. Flow down valve physique designs are prone to trim injury because the moist, erosive steam passes through 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 value. Trim parts that need to be changed because of heavy put on, similar to gear toothing, may be quite expensive, particularly if their supply must be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue creating within the valve body or diffuser, with excessive repair procedures, or even a full valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service had been put in in the course of the heyday of combined cycle plant development from 1998 to 2004. Most of these valves and piping techniques are showing their age and often have important signs of steel fatigue. Others have plenty of life left in them however may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the latest sealing know-how could inject new life into the valve.
Another example 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 cut back required maintenance.
If a turbine bypass valve must be changed, plant personnel are strongly encouraged to look beyond a direct replacement and consider potential improvements. A particularly essential item to suppose about when looking at replacement and enchancment alternatives 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 arrangement is ideal for locations with low overhead clearance, and it locations the actuator nearer to the deck or ground for simple access, but it creates numerous long-term operational problems. Gravity tends to result in increased and uneven put on on trim elements, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) were the principle option a number of decades in the past for turbine bypass valves. New circulate up valve designs (right) could also be installed without piping modifications in most situations, providing prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for move down is vertical mounting of the actuator. This association leads to lowered and more even put on, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that replacement opportunities are sometimes for horizontally mounted actuators, and the two designs usually are not readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead area is on the market with present horizontal actuator move down designs, a circulate up design can simply meet the prevailing face-to-face necessities of the old valve, while reducing a few of the wear common to move down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from flow all the method down to circulate up, this alteration may be made without modifications to current piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life as a end result of the steam is shifting 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, but that vitality dissipates into the massive physique 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 accelerate later because the steam strikes through the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is actually displaying erosion, however when in comparison with the circulate down trim shown above (Figure 4 right), the seating floor on the seat ring and plug stay relatively undamaged, allowing the valve to close off fully, the required mode in normal operation.
7. These footage of move up trim clearly present the erosive impact of high-velocity steam, however in distinction to the move down trim damage shown previously, this valve has no harm to the seating floor and might nonetheless shut off tightly. Courtesy: Emerson
The move up valve style design nonetheless supplies the same management responsiveness and flow capacity, however the new move arrangement dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered equipment that must be carefully specified, installed, and maintained. It is subsequently sensible to be proactive by reaching out to trusted specialists prior to buy as they’ll help with valve sizing and specs primarily based on particular working circumstances.
For current installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while since the plant turbine bypass valves have been totally inspected, it would be wise to consult your turbine bypass valve vendor for assist and support.
Many of these vendors provide full inspection and well being check companies to determine areas where metal fatigue and stress cracking are growing. Ideally เครื่องมือตรวจวัดความดันเลือดเรียกว่า could be repaired, but when a valve replacement is warranted, the vendor can even present guidance in evaluating bypass valve design kinds and selecting the finest option for the particular software.
Share