Turbine bypass valves are one of the tough applications in an influence plant. Keeping these valves operating efficiently 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 talked about. Frequent thermal cycles, excessive pressure drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many vegetation are most likely to ignore these valves until they fail, creating unplanned outages, lost production, and excessive prices for reactive upkeep. This article provides advised strategies of inspection to anticipate and mitigate issues beforehand, and it presents improve alternatives should a valve need to be repaired or replaced.
Strictly Severe Service
Severe service management valves are used in probably the most troublesome installations within course of plants. pressure gauge octa embrace cavitating, erosive, corrosive, noisy, high pressure, excessive temperature, high pressure drop, or high velocity media. Turbine bypass valves are uncovered to many of those course of conditions; yet, they have to respond flawlessly and remain leak free when closed.
As their identify suggests, turbine bypass valves are used to bypass steam turbines during 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 the steam via a turbine. During startup, bypass valves divert steam away from the turbine till the properties and circumstances of the steam are acceptable 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 potentially wet steam, and by guaranteeing solely applicable steam situations and flows make their method to the turbine itself.
1. Depending on the ability plant design, several turbine bypass valves may be employed to instantly shunt steam around a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must proceed flowing to avoid tools harm as a end result of overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain circulate via the system.
As it operates, a turbine makes use of steam to carry out work, reducing outlet steam temperature and strain. When a turbine bypass valve opens, it’ll drop the stress, but the exit steam will remain quite superheated, doubtlessly destroying downstream gear. To keep away from that situation, turbine bypass valves both incorporate a water injection system within the valve body, or employ a separate water injection desuperheater just downstream, in either case to decrease the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of severe service circumstances. While the plant is in operation, these valves must stay tightly closed to avoid wasting power. When a turbine trip occurs, the bypass valves should respond instantly, exposing them to speedy temperature modifications and requiring them to pass very high flows at excessive strain drops, creating high noise and probably extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly every turbine bypass valve will in the end fail in some way. Unfortunately, many of these valves are put in in difficult-to-access locations, are usually welded in place, and are typically heavily insulated. As a outcome, they’re usually ignored till problems begin to surface. Steam leakage through the valve is normally the first symptom observed, but far more vital and probably dangerous harm can happen.
The extreme strain drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can end result in catastrophic failure underneath strain.
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 keep away from this drawback, every turbine bypass valve and its associated piping ought to be routinely inspected. Some sort of non-destructive examination should be periodically employed to detect metallic fatigue issues that could be developing but usually are not but visually apparent. If a plant lacks the information or tools to perform these inspections, the valve vendor or licensed consultant could also be utilized to carry out turbine bypass well being examine companies (Figure 3).
three. A routine turbine bypass valve health examine and full inspection is strongly suggested. Metal fatigue can develop in numerous welds located on the valve itself, and in the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, customer connection welds, diffuser welds, and surrounding piping can be inspected to establish any creating problems earlier than the tools is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from issues related to quenching and cracking.
There are additionally threats to trim elements, similar to seat floor erosion. One widespread expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This occurs more commonly in circulate down valves, the place the steam accelerates through the cage holes after which strikes the trim/seat space immediately, causing excessive wear and lowering service life. Over time, this high-velocity steam, which might contain water during startup conditions and magnetite during any situation, 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 by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like gentle items and spray nozzles can often be replaced comparatively simply and at minimal price. Trim parts that must be changed as a result of heavy wear, corresponding to gear toothing, may be fairly expensive, especially if their supply needs to be expedited. The worst-case situation for a turbine bypass valve is metal fatigue growing within the valve body or diffuser, with extreme restore procedures, or maybe a full valve substitute, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service have been put in during the heyday of combined cycle plant construction from 1998 to 2004. Most of those valves and piping techniques are displaying their age and sometimes have significant indicators of metallic fatigue. Others have plenty of life left in them however may gain advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the latest sealing expertise might inject new life into the valve.
Another example of a know-how upgrade is when there have been repeated failures with a welded diffuser. In this instance, a detachable two-in-one seat diffuser will reduce required maintenance.
If a turbine bypass valve must be changed, plant personnel are strongly inspired to look past a direct replacement and evaluate potential improvements. A significantly essential merchandise to consider when looking at substitute and improvement opportunities is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, pressure gauge 10 bar bypass valves employed a flow down trim design paired with a horizontal actuator (Figure 5, left). This arrangement is good for places with low overhead clearance, and it locations the actuator nearer to the deck or ground for straightforward entry, but it creates a number of long-term operational issues. Gravity tends to lead to elevated and uneven put on on trim components, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the principle option a quantity of a long time ago for turbine bypass valves. New move up valve designs (right) may be put in without piping modifications in most instances, providing prolonged 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 leads to reduced and more even wear, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that replacement alternatives are usually for horizontally mounted actuators, and the two designs usually are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead space is on the market with existing horizontal actuator circulate down designs, a flow up design can easily meet the prevailing face-to-face necessities of the old valve, while decreasing a variety of the wear frequent to circulate down designs with a horizontal actuator, corresponding to gear toothing. Because the trim just switches from flow right down to move up, this alteration may be made without modifications to current piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life as a outcome of the steam is transferring 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 significantly, however that vitality dissipates into the large physique cavity of the valve as a substitute of putting 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 moves by way of the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely exhibiting erosion, but when compared to the circulate down trim shown above (Figure four right), the seating surface on the seat ring and plug stay comparatively undamaged, permitting the valve to close off utterly, the required mode in normal operation.
7. These pictures of move up trim clearly show the erosive impression of high-velocity steam, however unlike the move down trim injury shown previously, this valve has no harm to the seating surface and may nonetheless shut off tightly. Courtesy: Emerson
The flow up valve fashion design still offers the same control responsiveness and move capacity, but the new move association dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized items of engineered tools that have to be rigorously specified, installed, and maintained. It is therefore clever to be proactive by reaching out to trusted specialists prior to buy as they will assist with valve sizing and specs based on particular operating conditions.
For existing installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time because the plant turbine bypass valves have been absolutely inspected, it might be sensible to seek the guidance of your turbine bypass valve vendor for help and help.
Many of those distributors offer full inspection and health check companies to identify areas the place steel fatigue and stress cracking are creating. Ideally the valve can be repaired, but when a valve substitute is warranted, the vendor can also provide steering in evaluating bypass valve design types and selecting the best choice for the particular utility.
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