Turbine bypass valves are some of the difficult functions in an influence plant. Keeping these valves operating effectively and avoiding sudden failures is critical for plant operation.
When engineers are asked to record the most tough control valve purposes, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive stress drops, and the need 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 high costs for reactive upkeep. This article provides advised methods of inspection to anticipate and mitigate issues beforehand, and it offers upgrade alternate options should a valve must be repaired or replaced.
Strictly Severe Service
Severe service management valves are utilized in probably the most tough installations inside course of vegetation. These installations commonly include cavitating, erosive, corrosive, noisy, excessive stress, excessive temperature, excessive strain drop, or excessive velocity media. Turbine bypass valves are uncovered to many of those process conditions; yet, 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, as nicely as when a turbine journeys off-line (Figure 1). In normal operation the valves are completely 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 acceptable for sending it to the turbine. This process occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard the turbine by diverting potentially moist steam, and by making certain solely applicable steam circumstances and flows make their method to the turbine itself.
1. Depending on the ability plant design, a quantity of turbine bypass valves could also be employed to immediately shunt steam around a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine trip, the steam should continue flowing to keep away from tools harm because of overpressure and excessive temperature, so the turbine bypass valve immediately opens to keep up circulate through the system.
As it operates, a turbine makes use of steam to carry out work, lowering outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the strain, however the exit steam will stay quite superheated, probably destroying downstream gear. To avoid that situation, turbine bypass valves either incorporate a water injection system within the valve body, or make use of a separate water injection desuperheater simply downstream, in both case to lower the exit steam temperature.
As a result, turbine bypass valves face an ideal storm of extreme service circumstances. While the plant is in operation, these valves must remain tightly closed to avoid wasting energy. When a turbine trip happens, the bypass valves should respond immediately, exposing them to speedy temperature changes and requiring them to cross very high flows at high stress drops, creating high noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that nearly each turbine bypass valve will ultimately fail in some way. Unfortunately, many of these valves are installed in difficult-to-access areas, are typically welded in place, and are usually closely insulated. As a end result, they are usually ignored till problems start to floor. Steam leakage by way of the valve is usually the primary symptom noticed, however rather more significant and potentially dangerous damage can happen.
The extreme strain drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue may find yourself in catastrophic failure beneath pressure.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in each the valve itself and the encircling piping. Courtesy: Emerson
To keep away from this problem, every turbine bypass valve and its related piping should be routinely inspected. Some type of non-destructive examination ought to be periodically employed to detect metal fatigue problems that might be growing however usually are not but visually obvious. If a plant lacks the data or tools to perform these inspections, the valve vendor or approved consultant could also be utilized to perform turbine bypass health verify providers (Figure 3).
three. A routine turbine bypass valve health verify and full inspection is strongly advised. Metal fatigue can develop in varied welds situated on the valve itself, and in the inlet, outlet, and water supply piping (circled areas). diaphragm seal : Emerson
Fabrication welds on the body and water manifold, customer connection welds, diffuser welds, and surrounding piping could be inspected to determine any developing issues earlier than the gear 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 also threats to trim components, such as seat surface erosion. One frequent expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This happens extra commonly in flow down valves, the place the steam accelerates through the cage holes after which strikes the trim/seat area immediately, inflicting extreme wear and reducing service life. Over time, this high-velocity steam, which may comprise water throughout startup situations and magnetite during any condition, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve body designs are susceptible to trim damage because the moist, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like soft goods and spray nozzles can often be replaced relatively simply and at minimal cost. Trim parts that must be changed because of heavy wear, corresponding to gear toothing, can be quite pricey, particularly if their delivery 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 replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service had been installed during the heyday of combined cycle plant construction from 1998 to 2004. Most of those valves and piping techniques are displaying their age and often have significant indicators of metallic 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 newest sealing know-how could inject new life into the valve.
Another example 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 cut back required maintenance.
If a turbine bypass valve must be changed, plant personnel are strongly inspired to look beyond a direct alternative and consider potential improvements. A particularly necessary merchandise to contemplate when looking at alternative 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 places with low overhead clearance, and it locations the actuator closer to the deck or floor for straightforward access, nevertheless it creates numerous long-term operational problems. Gravity tends to lead to increased and uneven put on on trim parts, and the side-mounted actuator is prone to response issues.
5. Downward flowing valves with horizontal actuators (left) have been the principle choice a couple of a long time in the past for turbine bypass valves. New circulate up valve designs (right) could additionally be installed with out piping modifications in most situations, offering prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for move down is vertical mounting of the actuator. This association ends in lowered and extra even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that substitute alternatives are typically for horizontally mounted actuators, and the two designs usually are not readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When adequate overhead space is out there with existing horizontal actuator move down designs, a circulate up design can simply meet the present face-to-face necessities of the previous valve, while lowering some of the put on widespread to flow down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from circulate all the way down to move up, this modification may be made with out modifications to present piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life as a end result 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 considerably, however that energy dissipates into the large body cavity of the valve as a substitute of putting the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower because they pass over the seat and speed up later as the steam strikes by way of the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is certainly 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 relatively undamaged, permitting the valve to shut off fully, the required mode in regular operation.
7. These pictures of flow up trim clearly show the erosive impact of high-velocity steam, however in distinction to the move down trim damage shown beforehand, this valve has no injury to the seating floor and might still shut off tightly. Courtesy: Emerson
The circulate up valve style design nonetheless offers the identical management responsiveness and circulate capacity, but the new circulate arrangement dramatically extends the working life 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 can help with valve sizing and specifications based mostly on specific operating situations.
For current installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while since the plant turbine bypass valves have been totally inspected, it might be clever to seek the assistance of your turbine bypass valve vendor for assist and assist.
Many of these vendors offer full inspection and well being verify companies to establish areas where metallic fatigue and stress cracking are developing. Ideally the valve may be repaired, but if a valve substitute is warranted, the vendor can also provide guidance in evaluating bypass valve design styles and selecting the finest option for the particular utility.
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