Turbine bypass valves are some of the troublesome functions in an influence plant. Keeping these valves operating efficiently and avoiding surprising failures is important for plant operation.
When engineers are requested to record probably the most troublesome control valve functions, turbine bypass valves are invariably talked about. Frequent thermal cycles, high pressure drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many crops tend to ignore these valves until they fail, creating unplanned outages, lost production, and high prices for reactive maintenance. This article supplies instructed strategies of inspection to anticipate and mitigate issues beforehand, and it presents improve alternatives ought to a valve have to be repaired or replaced.
Strictly Severe Service
Severe service management valves are used in probably the most tough installations within process crops. These installations generally embrace cavitating, erosive, corrosive, noisy, high strain, excessive temperature, high pressure drop, or excessive velocity media. Turbine bypass valves are exposed to many of these course of situations; but, they need 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, as well as when a turbine trips off-line (Figure 1). In normal 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 conditions of the steam are applicable 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 probably wet steam, and by making certain only applicable steam situations and flows make their way to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves may be employed to immediately shunt steam around a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to avoid tools damage because of overpressure and high temperature, so the turbine bypass valve immediately opens to maintain move by way of the system.
As it operates, a turbine uses steam to perform work, decreasing outlet steam temperature and stress. When a turbine bypass valve opens, it will drop the pressure, however the exit steam will stay fairly superheated, potentially destroying downstream equipment. To keep away from that situation, turbine bypass valves either incorporate a water injection system in the valve physique, or make use of a separate water injection desuperheater just downstream, in both case to decrease the exit steam temperature.
As a outcome, turbine bypass valves face a perfect storm of extreme service circumstances. While the plant is in operation, these valves should remain tightly closed to avoid losing power. When a turbine journey happens, the bypass valves should respond immediately, exposing them to rapid temperature modifications and requiring them to cross very excessive flows at excessive pressure drops, creating high noise and doubtlessly extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that just about each turbine bypass valve will ultimately fail indirectly. Unfortunately, many of these valves are installed in difficult-to-access areas, are typically welded in place, and tend to be closely insulated. As a outcome, they are typically ignored till issues start to surface. Steam leakage by way of the valve is normally the first symptom seen, but far more vital and potentially harmful damage can happen.
The extreme stress drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metallic within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue may find yourself in catastrophic failure beneath strain.
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 encircling piping. Courtesy: Emerson
To avoid this problem, each turbine bypass valve and its related piping must be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect metal fatigue problems that may be growing however usually are not yet visually obvious. If a plant lacks the information or equipment to perform these inspections, the valve vendor or licensed representative could additionally be utilized to perform turbine bypass health check providers (Figure 3).
3. A routine turbine bypass valve well being examine and full inspection is strongly advised. Metal fatigue can develop in numerous welds positioned on the valve itself, and within 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 issues before the tools 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 additionally threats to trim components, such as seat floor erosion. One frequent expression of such erosion, specifically on the plug, is also identified as “gear toothing.” This occurs extra commonly in flow down valves, the place the steam accelerates via the cage holes after which strikes the trim/seat space directly, inflicting excessive wear and lowering service life. Over time, this high-velocity steam, which might include water during startup conditions and magnetite during any condition, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve physique designs are vulnerable to trim damage because the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like soft items and spray nozzles can often be replaced comparatively simply and at minimal value. Trim elements that have to be replaced because of heavy put on, such as gear toothing, could be quite costly, especially if their supply must be expedited. The worst-case situation for a turbine bypass valve is steel fatigue developing within the valve body or diffuser, with extreme restore procedures, or maybe a full valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service were installed through the heyday of combined cycle plant building from 1998 to 2004. Most of these valves and piping methods are showing their age and infrequently have significant signs of metallic fatigue. Others have loads of life left in them but may benefit from upgrades in know-how. Technology and practices have improved, and upgrading to the latest sealing know-how may inject new life into the valve.
Another instance of a technology 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 have to be changed, plant personnel are strongly inspired to look past a direct substitute and evaluate potential enhancements. A notably important merchandise to think about when taking a look at substitute 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 association is right for areas with low overhead clearance, and it places the actuator nearer to the deck or ground for simple entry, nevertheless it creates a quantity of long-term operational problems. Gravity tends to lead to elevated and uneven put on on trim elements, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) were the main option a couple of decades ago for turbine bypass valves. New move up valve designs (right) could also be installed with out piping modifications in most instances, providing extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for circulate down is vertical mounting of the actuator. This arrangement leads to decreased and extra even put on, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute alternatives are sometimes for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead area is on the market with current horizontal actuator circulate down designs, a circulate up design can easily meet the prevailing face-to-face requirements of the outdated valve, whereas lowering a few of the wear common to flow down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from move all the way down to circulate up, this change can be made without modifications to current piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life as a outcome of the steam is moving relatively slowly as it passes over the plug and seat (Figure 6). As the steam moves via the small holes of the cage, it accelerates significantly, however that power dissipates into the large body cavity of the valve instead of putting the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing เกจวัดแรงดันอาร์กอน are a lot lower as a end result of they cross over the seat and speed up later as the steam strikes through the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely exhibiting erosion, however when compared to the move down trim proven above (Figure four right), the seating surface on the seat ring and plug stay relatively undamaged, permitting the valve to close off utterly, the required mode in regular operation.
7. These photos of circulate up trim clearly show the erosive influence of high-velocity steam, but unlike the flow down trim injury proven previously, this valve has no injury to the seating floor and may nonetheless shut off tightly. Courtesy: Emerson
The move up valve type design still offers the same management responsiveness and flow capacity, but the new flow association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered tools that must be carefully specified, put in, and maintained. It is due to this fact wise to be proactive by reaching out to trusted experts prior to buy as they will help with valve sizing and specifications based mostly on particular working situations.
For current 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 fully inspected, it might be clever to consult your turbine bypass valve vendor for assist and help.
Many of those vendors supply full inspection and well being check providers to identify areas where metallic fatigue and stress cracking are growing. Ideally the valve may be repaired, but when a valve substitute is warranted, the seller also can provide guidance in evaluating bypass valve design types and choosing the most suitable choice for the precise software.