Turbine Bypass Valve Challenges

Turbine bypass valves are one of the difficult applications in an influence plant. Keeping these valves operating effectively and avoiding surprising failures is critical for plant operation.
When engineers are requested to listing the most tough control valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high strain drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many vegetation tend to ignore these valves until they fail, creating unplanned outages, misplaced manufacturing, and excessive costs for reactive maintenance. This article offers suggested strategies of inspection to anticipate and mitigate issues beforehand, and it offers improve alternatives ought to a valve need to be repaired or replaced.
Strictly Severe Service
Severe service control valves are used in the most troublesome installations within process crops. These installations generally include cavitating, erosive, corrosive, noisy, high pressure, high temperature, high stress drop, or excessive velocity media. Turbine bypass valves are exposed to many of these process circumstances; but, they need to respond flawlessly and remain leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam generators during plant startup and shutdown, as well as when a turbine journeys off-line (Figure 1). In regular operation the valves are utterly closed, forcing all the steam via a turbine. During startup, bypass valves divert steam away from the turbine till 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 protect the turbine by diverting potentially moist steam, and by guaranteeing only acceptable steam situations and flows make their method to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves could additionally be employed to immediately shunt steam around a turbine should it trip offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to keep away from gear harm because of 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 makes use of steam to carry out work, decreasing outlet steam temperature and pressure. When a turbine bypass valve opens, it’ll drop the strain, but the exit steam will stay fairly superheated, potentially destroying downstream tools. To avoid 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 both case to decrease the exit steam temperature.
As a result, turbine bypass valves face an ideal storm of severe service situations. While the plant is in operation, these valves should stay tightly closed to avoid losing energy. When a turbine journey happens, the bypass valves should respond immediately, exposing them to fast temperature modifications and requiring them to cross very excessive flows at high strain 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 ultimately fail indirectly. Unfortunately, many of these valves are put in in difficult-to-access places, are sometimes welded in place, and tend to be closely insulated. As a end result, they’re often ignored till problems start to floor. Steam leakage through the valve is usually the first symptom noticed, but rather more vital and doubtlessly dangerous damage can happen.
The extreme pressure drops invariably generate excessive noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can result in catastrophic failure underneath strain.
2. Over time, the excessive vibrations and thermal shock skilled by turbine bypass valves will fatigue the metallic and create cracks in each the valve itself and the encircling piping. Courtesy: Emerson
To avoid this drawback, 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 could be creating but are not but visually apparent. If a plant lacks the knowledge or equipment to perform these inspections, the valve vendor or licensed consultant may be utilized to carry out turbine bypass well being check services (Figure 3).
three. A routine turbine bypass valve health examine and full inspection is strongly advised. Metal fatigue can develop in varied welds located on the valve itself, and in 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 could be inspected to establish any developing problems before the equipment is compromised. Water injection nozzles and desuperheaters have to 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 surface erosion. One frequent expression of such erosion, particularly on the plug, is often referred to as “gear toothing.” This occurs extra generally in move down valves, the place the steam accelerates via the cage holes and then strikes the trim/seat space immediately, causing extreme put on and lowering service life. Over time, this high-velocity steam, which may include water during startup circumstances and magnetite throughout any condition, will erode and harm the valve seat and trim (Figure 4).
four. Flow down valve physique designs are vulnerable to trim damage because the moist, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep gadgets like delicate goods and spray nozzles can normally get replaced relatively simply and at minimal price. Trim elements that must be changed due to heavy wear, similar to gear toothing, may be fairly expensive, especially if their supply must be expedited. The worst-case situation for a turbine bypass valve is metal fatigue creating in the valve physique or diffuser, with excessive restore procedures, or maybe a full valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service have been put in through the heyday of mixed cycle plant construction from 1998 to 2004. Most of these valves and piping techniques are exhibiting their age and often have important signs of steel fatigue. Others have plenty of life left in them but may achieve advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the latest sealing technology may inject new life into the valve.
Another example of a technology 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 replaced, plant personnel are strongly inspired to look past a direct replacement and evaluate potential improvements. A particularly important merchandise to consider when taking a glance at substitute and enchancment opportunities 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 right for locations with low overhead clearance, and it locations the actuator nearer to the deck or floor for straightforward entry, nevertheless it creates a selection of long-term operational problems. Gravity tends to end in elevated and uneven put on on trim parts, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the main option a few a long time ago for turbine bypass valves. New flow up valve designs (right) may be put in without piping modifications in most situations, offering prolonged service life and improved efficiency. Astonishing : Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for move down is vertical mounting of the actuator. This arrangement leads to lowered and more even put on, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement alternatives are typically 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 out there with existing horizontal actuator flow down designs, a move up design can simply meet the existing face-to-face necessities of the previous valve, while lowering some of the wear frequent to flow down designs with a horizontal actuator, such as gear toothing. Because the trim simply switches from circulate all the way down to flow up, this modification may be made without modifications to present piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life because 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, but that power dissipates into the big physique cavity of the valve as an alternative of putting the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower as a outcome of they move over the seat and speed up later because the steam strikes through the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the flow up trim (Figure 7) is actually exhibiting erosion, however when compared to the flow down trim shown above (Figure 4 right), the seating floor on the seat ring and plug remain comparatively undamaged, allowing the valve to close off utterly, the required mode in regular operation.
7. These footage of circulate up trim clearly present the erosive influence of high-velocity steam, however unlike the move down trim injury proven beforehand, this valve has no harm to the seating surface and may still shut off tightly. Courtesy: Emerson
The flow up valve style design nonetheless provides the identical management responsiveness and flow capability, however the new move arrangement dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised items of engineered tools that must be fastidiously specified, installed, and maintained. It is therefore wise to be proactive by reaching out to trusted experts prior to purchase as they will assist with valve sizing and specs primarily based on specific operating circumstances.
For present installations the place 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 might be wise to seek the guidance of your turbine bypass valve vendor for assist and help.
Many of these distributors supply full inspection and well being examine services to establish areas the place steel fatigue and stress cracking are growing. Ideally the valve could be repaired, but if a valve replacement is warranted, the seller can also present steerage in evaluating bypass valve design types and choosing the most fitted choice for the particular application.
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