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Turbine Bypass Valve Challenges

Turbine bypass valves are one of the difficult applications in an influence plant. Keeping these valves operating efficiently and avoiding surprising failures is critical for plant operation.
When engineers are asked to list probably the most difficult control valve functions, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high stress drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many vegetation are inclined to ignore these valves till they fail, creating unplanned outages, lost production, and high costs for reactive maintenance. This article offers suggested strategies of inspection to anticipate and mitigate issues beforehand, and it provides improve alternatives should a valve have to be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in probably the most difficult installations within course of plants. These installations commonly embody cavitating, erosive, corrosive, noisy, excessive strain, excessive temperature, excessive strain drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those process situations; but, they must reply flawlessly and stay leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam turbines during plant startup and shutdown, in addition to when a turbine journeys off-line (Figure 1). In normal operation the valves are utterly closed, forcing all 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 acceptable 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 probably wet steam, and by ensuring solely applicable steam conditions and flows make their approach to the turbine itself.
1. Depending on the power plant design, a quantity of turbine bypass valves may be employed to instantly shunt steam round a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam should continue flowing to avoid equipment damage due to overpressure and high temperature, so the turbine bypass valve instantly opens to take care of flow 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’s going to drop the stress, 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 within the valve body, or make use of a separate water injection desuperheater just downstream, in either case to lower the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of extreme service situations. While the plant is in operation, these valves must stay tightly closed to avoid losing power. When a turbine journey happens, the bypass valves must reply instantly, exposing them to speedy temperature modifications and requiring them to cross very excessive flows at high strain drops, creating excessive noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly every turbine bypass valve will finally fail indirectly. Unfortunately, many of those valves are put in in difficult-to-access places, are typically welded in place, and tend to be heavily insulated. As a end result, they are usually ignored till problems begin to surface. Steam leakage through the valve is normally the primary symptom seen, but much more significant and probably harmful harm can happen.
The excessive stress drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metal in the valve, water connections, and the piping itself (Figure 2). Such steel 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 keep away from this problem, every turbine bypass valve and its associated piping should be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect metal fatigue problems that might be creating but aren’t yet visually apparent. If a plant lacks the information or equipment to carry out these inspections, the valve vendor or authorized representative may be utilized to perform turbine bypass health examine services (Figure 3).
3. A routine turbine bypass valve health examine and full inspection is strongly advised. Metal fatigue can develop in numerous 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, customer connection welds, diffuser welds, and surrounding piping can be inspected to establish any creating problems before the gear is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to keep away from problems associated to quenching and cracking.
There are also threats to trim parts, corresponding to seat surface erosion. One widespread expression of such erosion, particularly on the plug, is also recognized as “gear toothing.” This occurs more commonly in circulate down valves, the place the steam accelerates by way of the cage holes and then strikes the trim/seat space directly, inflicting excessive put on and lowering service life. Over time, this high-velocity steam, which might contain water throughout startup conditions and magnetite during any situation, will erode and injury the valve seat and trim (Figure 4).
4. Flow down valve body designs are prone to trim harm as the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like soft items and spray nozzles can normally get replaced relatively simply and at minimal price. Trim components that must be replaced due to heavy wear, such as gear toothing, may be fairly pricey, especially if their delivery needs to be expedited. The worst-case state of affairs for a turbine bypass valve is metallic fatigue creating in the valve physique or diffuser, with excessive repair procedures, or even a full valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were put in through the heyday of combined cycle plant construction from 1998 to 2004. Most of these valves and piping systems are displaying their age and often have significant signs of steel fatigue. Others have plenty of life left in them however could benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing expertise could inject new life into the valve.
Another example of a know-how improve is when there have been repeated failures with a welded diffuser. In this instance, a detachable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve must be replaced, plant personnel are strongly inspired to look past a direct substitute and evaluate potential enhancements. A significantly important merchandise to contemplate when looking at replacement and improvement alternatives is valve orientation.
Downward Flowing Valves 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 places with low overhead clearance, and it locations the actuator nearer to the deck or ground for simple entry, but it creates a quantity of long-term operational problems. Gravity tends to result in elevated 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) had been the principle possibility a quantity of a long time in the past for turbine bypass valves. New move up valve designs (right) could additionally be put in without piping modifications in most cases, offering prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This arrangement results in decreased and extra even wear, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that alternative alternatives are sometimes for horizontally mounted actuators, and the two designs usually are not readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead area is on the market with current horizontal actuator flow down designs, a flow up design can simply meet the prevailing face-to-face necessities of the previous valve, whereas lowering some of the put on widespread to circulate down designs with a horizontal actuator, corresponding to gear toothing. Because the trim just switches from move all the way down to move up, this modification could be made with out 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 as 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 vitality dissipates into the big body cavity of the valve as an alternative of putting the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease as a end result 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 move up trim (Figure 7) is definitely displaying erosion, however when in comparison with the circulate down trim proven above (Figure four right), the seating floor on the seat ring and plug stay relatively undamaged, permitting the valve to shut off completely, the required mode in regular operation.
7. These pictures of flow up trim clearly show the erosive influence of high-velocity steam, but unlike the circulate down trim injury shown previously, this valve has no injury to the seating floor and might nonetheless shut off tightly. Courtesy: Emerson
The flow up valve style design still provides the identical management responsiveness and flow capacity, however the new move association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised pieces of engineered tools that must be fastidiously specified, installed, and maintained. เกจวัดแรงดันแก๊สlpg is due to this fact sensible to be proactive by reaching out to trusted consultants prior to purchase as they can assist with valve sizing and specs based on specific operating situations.
For present installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while for the reason that plant turbine bypass valves have been fully inspected, it would be clever to consult your turbine bypass valve vendor for assist and help.
Many of these vendors supply full inspection and well being verify companies to establish areas where metal fatigue and stress cracking are developing. Ideally the valve can be repaired, but when a valve substitute is warranted, the seller also can present steering in evaluating bypass valve design types and selecting the most suitable choice for the particular utility.
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