Turbine bypass valves are some of the troublesome purposes in an influence plant. Keeping these valves operating effectively and avoiding surprising failures is important for plant operation.
When pressure gauge หลักการ ทํา งาน are asked to list the most tough control valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive stress drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many plants are inclined to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and excessive costs for reactive maintenance. This article supplies suggested strategies of inspection to anticipate and mitigate issues beforehand, and it presents improve options should a valve need to be repaired or changed.
Strictly Severe Service
Severe service management valves are utilized in probably the most difficult installations inside course of vegetation. These installations generally embody cavitating, erosive, corrosive, noisy, excessive pressure, excessive temperature, excessive strain drop, or high velocity media. Turbine bypass valves are exposed to many of these process circumstances; but, they have to reply 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, in addition to when a turbine journeys off-line (Figure 1). In normal operation the valves are fully closed, forcing all the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine till the properties and situations of the steam are applicable for sending it to the turbine. This process occurs 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 solely applicable steam situations and flows make their approach to the turbine itself.
1. Depending on the power plant design, a quantity of turbine bypass valves could additionally be employed to immediately shunt steam round a turbine ought to it trip offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to keep away from gear harm as a end result of overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain up flow by way of the system.
As it operates, a turbine uses steam to carry out work, lowering outlet steam temperature and stress. When a turbine bypass valve opens, it’s going to drop the strain, but the exit steam will stay fairly superheated, doubtlessly destroying downstream tools. To avoid that state of affairs, turbine bypass valves either incorporate a water injection system within 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 an ideal storm of extreme service conditions. While the plant is in operation, these valves should stay tightly closed to avoid wasting vitality. When a turbine journey happens, the bypass valves must reply instantly, exposing them to rapid temperature adjustments and requiring them to pass very high flows at excessive stress drops, creating high noise and probably excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that virtually each turbine bypass valve will in the end fail in some way. Unfortunately, many of those valves are installed in difficult-to-access locations, are typically welded in place, and are usually closely insulated. As a result, they are usually ignored till issues start to surface. Steam leakage via the valve is often the first symptom observed, but far more important and potentially dangerous 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 metal fatigue may end up in catastrophic failure beneath pressure.
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 downside, each turbine bypass valve and its related piping must be routinely inspected. Some sort of non-destructive examination ought to be periodically employed to detect steel fatigue problems that may be creating however are not yet visually apparent. If a plant lacks the knowledge or equipment to carry out these inspections, the valve vendor or approved representative could also be utilized to perform turbine bypass health examine companies (Figure 3).
3. A routine turbine bypass valve health check and full inspection is strongly suggested. Metal fatigue can develop in various welds situated 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 issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters must 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 often referred to as “gear toothing.” This occurs more commonly in move down valves, where the steam accelerates via the cage holes after which strikes the trim/seat space instantly, causing excessive wear and decreasing service life. Over time, this high-velocity steam, which may include water throughout startup circumstances and magnetite throughout any condition, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve body designs are susceptible to trim damage because the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like soft goods and spray nozzles can normally get replaced relatively simply and at minimal cost. Trim components that must be replaced because of heavy wear, corresponding to gear toothing, can be fairly costly, particularly if their supply must be expedited. The worst-case situation for a turbine bypass valve is steel fatigue creating in the valve body or diffuser, with excessive restore procedures, or even a full valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service had been put in through the heyday of combined cycle plant building from 1998 to 2004. Most of these valves and piping methods are displaying their age and sometimes have significant signs of metallic fatigue. Others have loads of life left in them but could benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the latest sealing know-how may inject new life into the valve.
Another instance 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 maintenance.
If a turbine bypass valve should be replaced, plant personnel are strongly inspired to look beyond a direct replacement and consider potential enhancements. A particularly important merchandise to contemplate when taking a look at replacement and improvement 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 places the actuator closer to the deck or ground for simple entry, however it creates a number of long-term operational issues. Gravity tends to end in increased and uneven put on on trim components, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) were the main possibility a couple of decades in the past for turbine bypass valves. New move up valve designs (right) could also be installed without piping modifications in most situations, offering extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for flow down is vertical mounting of the actuator. This association leads to reduced and more even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that substitute opportunities are sometimes for horizontally mounted actuators, and the 2 designs are not readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is out there with existing horizontal actuator move down designs, a move up design can easily meet the present face-to-face requirements of the old valve, whereas lowering a few of the put on common to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from circulate all the means down to flow up, this change can be made without modifications to current piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life as a end result of the steam is shifting comparatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates considerably, however that power dissipates into the big body cavity of the valve instead of striking the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower as a outcome of they move over the seat and speed up later as the steam moves via the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is certainly exhibiting erosion, but when in comparability with the move down trim shown above (Figure four right), the seating surface on the seat ring and plug remain comparatively undamaged, allowing the valve to shut off completely, the required mode in regular operation.
7. These photos of circulate up trim clearly show the erosive impact of high-velocity steam, but in contrast to the circulate down trim damage proven previously, this valve has no damage to the seating floor and can still shut off tightly. Courtesy: Emerson
The circulate up valve type design still offers the same management responsiveness and move capacity, however the new circulate association dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized pieces of engineered tools that should be carefully specified, put in, and maintained. It is subsequently sensible to be proactive by reaching out to trusted consultants prior to purchase as they will help with valve sizing and specifications based on specific working situations.
For present installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time because the plant turbine bypass valves have been totally inspected, it will be wise to seek the assistance of your turbine bypass valve vendor for help and assist.
Many of these vendors provide full inspection and well being examine services to determine areas where metal fatigue and stress cracking are creating. Ideally the valve may be repaired, but if a valve substitute is warranted, the vendor can even present steerage in evaluating bypass valve design kinds and selecting the finest choice for the particular software.
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