Turbine bypass valves are some of the tough applications in an influence plant. Keeping these valves working effectively and avoiding unexpected failures is critical for plant operation.
When engineers are requested to record essentially the most difficult control valve applications, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive strain drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many plants are probably to ignore these valves until they fail, creating unplanned outages, lost production, and high costs for reactive upkeep. This article provides instructed strategies of inspection to anticipate and mitigate issues beforehand, and it presents upgrade alternatives ought to a valve have to be repaired or replaced.
Strictly Severe Service
Severe service control valves are utilized in probably the most difficult installations inside course of vegetation. These installations commonly embrace cavitating, erosive, corrosive, noisy, excessive stress, high temperature, high pressure drop, or high velocity media. Turbine bypass valves are uncovered to many of those course of conditions; yet, they want to reply flawlessly and remain leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam turbines during plant startup and shutdown, as nicely as when a turbine trips off-line (Figure 1). In regular operation the valves are utterly closed, forcing all the steam through a turbine. During startup, bypass valves divert steam away from the turbine until the properties and circumstances of the steam are appropriate for sending it to the turbine. This process 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 applicable steam circumstances and flows make their way to the turbine itself.
1. Depending on the power plant design, a number of turbine bypass valves may be employed to immediately shunt steam round a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine journey, the steam should proceed flowing to keep away from equipment injury due to overpressure and high temperature, so the turbine bypass valve instantly opens to take care of move by way of the system.
As it operates, a turbine uses steam to perform work, lowering outlet steam temperature and pressure. When a turbine bypass valve opens, it’ll drop the strain, but the exit steam will stay fairly superheated, doubtlessly destroying downstream equipment. To keep away from that situation, turbine bypass valves either incorporate a water injection system in the valve physique, or employ a separate water injection desuperheater simply 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 must stay tightly closed to avoid losing power. When a turbine trip occurs, the bypass valves should reply immediately, exposing them to fast temperature modifications and requiring them to move very high flows at high pressure drops, creating high noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that virtually each turbine bypass valve will finally 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 typically ignored until issues begin to floor. Steam leakage through the valve is often the first symptom noticed, however rather more vital and probably harmful injury can happen.
The extreme stress drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such metal fatigue can end result in catastrophic failure underneath stress.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in both the valve itself and the encompassing piping. Courtesy: Emerson
To keep away from this downside, every turbine bypass valve and its associated piping ought to be routinely inspected. Some sort of non-destructive examination must be periodically employed to detect metal fatigue issues that may be creating however usually are not but visually apparent. If a plant lacks the data or gear to carry out these inspections, the valve vendor or authorized representative may be utilized to perform turbine bypass health check companies (Figure 3).
three. A routine turbine bypass valve health verify and full inspection is strongly advised. Metal fatigue can develop in varied welds positioned on the valve itself, and in the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, buyer connection welds, diffuser welds, and surrounding piping could be inspected to establish any growing issues before the gear is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to keep away from problems related to quenching and cracking.
There are also threats to trim elements, such as seat floor erosion. One common expression of such erosion, particularly on the plug, is also known as “gear toothing.” This happens more generally in circulate down valves, the place the steam accelerates via the cage holes and then strikes the trim/seat space immediately, inflicting excessive wear and lowering service life. Over time, this high-velocity steam, which can comprise water during startup situations and magnetite during any condition, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve body designs are prone to trim damage as 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 relatively simply and at minimal value. Trim components that must be replaced because of heavy wear, corresponding to gear toothing, could be quite costly, especially if their delivery needs to be expedited. The worst-case situation for a turbine bypass valve is metal fatigue developing within the valve body or diffuser, with excessive restore procedures, or perhaps a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service had been put in through the heyday of mixed cycle plant construction from 1998 to 2004. Most of those valves and piping techniques are exhibiting their age and sometimes have vital signs of metal fatigue. Others have loads of life left in them but could benefit from upgrades in know-how. Technology and practices have improved, and upgrading to the most recent sealing technology might inject new life into the valve.
Another example of a expertise improve is when there have been repeated failures with a welded diffuser. In this occasion, a detachable two-in-one seat diffuser will reduce required upkeep.
If a turbine bypass valve should be replaced, plant personnel are strongly inspired to look beyond a direct replacement and evaluate potential improvements. A significantly essential merchandise to contemplate when taking a look at replacement and improvement opportunities is valve orientation.
Downward Flowing เกจวัดแรงดันไนโตรเจน 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 right for locations with low overhead clearance, and it places the actuator nearer to the deck or ground for simple access, however it creates a number of long-term operational problems. Gravity tends to end in increased and uneven put on on trim elements, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the primary possibility a number of decades ago 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 option for move down is vertical mounting of the actuator. This arrangement ends in reduced and more even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that alternative alternatives are typically for horizontally mounted actuators, and the two designs usually are not readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead space 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 old valve, whereas lowering some of the put on frequent to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from move down to circulate up, this change may be made with out modifications to current piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life as a end result of the steam is moving comparatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes by way of the small holes of the cage, it accelerates significantly, however that power dissipates into the large body cavity of the valve as a substitute of striking the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease because they cross over the seat and speed up later because the steam strikes by way of the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is actually showing erosion, but when in comparability with the flow down trim shown above (Figure four right), the seating floor on the seat ring and plug stay comparatively undamaged, permitting the valve to close off fully, the required mode in regular operation.
7. These photos of move up trim clearly present the erosive impact of high-velocity steam, however not like the circulate down trim harm proven beforehand, this valve has no injury to the seating surface and can still shut off tightly. Courtesy: Emerson
The circulate up valve fashion design nonetheless offers the identical management responsiveness and move capacity, but the new move association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized items of engineered equipment that should be fastidiously specified, put in, and maintained. It is subsequently sensible to be proactive by reaching out to trusted consultants prior to buy as they can help with valve sizing and specifications based mostly on particular working conditions.
For present installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while because the plant turbine bypass valves have been totally inspected, it would be clever to seek the guidance of your turbine bypass valve vendor for help and support.
Many of those vendors offer full inspection and health examine services to determine areas where steel fatigue and stress cracking are growing. Ideally the valve can be repaired, but when a valve replacement is warranted, the seller also can present steering in evaluating bypass valve design kinds and choosing the finest choice for the particular utility.
Share