Turbine bypass valves are one of the most tough applications in an influence plant. Keeping these valves operating effectively and avoiding sudden failures is important for plant operation.
When engineers are asked to record essentially the most difficult management valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, high strain drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many crops are inclined to ignore these valves till they fail, creating unplanned outages, lost production, and excessive costs for reactive maintenance. This article offers advised methods of inspection to anticipate and mitigate points beforehand, and it provides upgrade alternatives ought to a valve must be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in the most difficult installations inside process plants. These installations commonly include cavitating, erosive, corrosive, noisy, high stress, high temperature, excessive pressure drop, or excessive velocity media. Turbine bypass valves are exposed to many of those course of circumstances; yet, they want to reply flawlessly and stay leak free when closed.
As their identify 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 regular 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 situations of the steam are applicable 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 probably moist steam, and by ensuring solely appropriate steam situations and flows make their method to the turbine itself.
1. Depending on the power plant design, a quantity of turbine bypass valves could additionally be employed to instantly shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must proceed flowing to keep away from gear harm due to overpressure and high temperature, so the turbine bypass valve instantly opens to maintain flow 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’s going to drop the pressure, however the exit steam will remain fairly superheated, probably destroying downstream tools. To keep away from that scenario, turbine bypass valves both incorporate a water injection system within the valve physique, 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 should remain tightly closed to avoid wasting energy. When a turbine journey occurs, the bypass valves must respond instantly, exposing them to speedy temperature modifications and requiring them to pass very excessive flows at excessive strain drops, creating high noise and doubtlessly excessive vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that virtually every turbine bypass valve will finally fail ultimately. Unfortunately, many of these valves are put in in difficult-to-access areas, are usually welded in place, and are typically heavily insulated. As a result, they’re often ignored till problems begin to floor. Steam leakage by way of the valve is often the first symptom seen, however far more important and potentially dangerous harm can happen.
The excessive strain drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the steel in the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can lead to catastrophic failure beneath stress.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the metallic and create cracks in each the valve itself and the encircling piping. Courtesy: Emerson
To keep away from this problem, each turbine bypass valve and its associated piping ought to be routinely inspected. Some type of non-destructive examination must be periodically employed to detect metal fatigue problems that could be growing however aren’t yet visually apparent. If a plant lacks the knowledge or tools to carry out these inspections, the valve vendor or licensed representative may be utilized to perform turbine bypass health examine companies (Figure 3).
3. A routine turbine bypass valve health verify and full inspection is strongly advised. Metal fatigue can develop in various welds situated on the valve itself, and within the inlet, outlet, and water provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the physique and water manifold, buyer connection welds, diffuser welds, and surrounding piping may be inspected to identify any growing issues earlier than the equipment is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from problems associated to quenching and cracking.
There are also threats to trim elements, such as seat floor erosion. One widespread expression of such erosion, specifically on the plug, is also known as “gear toothing.” This happens extra commonly in circulate down valves, the place the steam accelerates via the cage holes after which strikes the trim/seat area instantly, causing extreme wear and lowering service life. Over time, this high-velocity steam, which can contain water throughout startup situations and magnetite during any situation, will erode and harm the valve seat and trim (Figure 4).
four. Flow down valve body designs are susceptible to trim damage as the wet, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like soft goods and spray nozzles can often get replaced comparatively easily and at minimal price. Trim components that must be replaced due to heavy wear, such as gear toothing, may be quite costly, especially if their supply needs to be expedited. The worst-case state of affairs for a turbine bypass valve is metallic fatigue developing in the valve physique or diffuser, with extreme repair procedures, or maybe a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service were installed during the heyday of combined cycle plant building from 1998 to 2004. Most of these valves and piping techniques are showing their age and infrequently have important indicators of steel fatigue. Others have loads of life left in them however may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the latest sealing expertise could 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 instance, a detachable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve have to be replaced, plant personnel are strongly inspired to look beyond a direct substitute and consider potential enhancements. A significantly necessary merchandise to contemplate when looking at substitute and enchancment opportunities is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a flow down trim design paired with a horizontal actuator (Figure 5, left). This association is good for places with low overhead clearance, and it locations the actuator closer to the deck or ground for straightforward access, however it creates a selection of long-term operational issues. Gravity tends to result in increased and uneven put on on trim parts, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the principle option a couple of a long time in the past for turbine bypass valves. New flow up valve designs (right) may be put in with out piping modifications in most cases, providing extended service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for flow down is vertical mounting of the actuator. This arrangement results in reduced and more even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that replacement opportunities are typically for horizontally mounted actuators, and the 2 designs are not readily interchangeable without significant piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead house is available with existing horizontal actuator flow down designs, a flow up design can simply meet the present face-to-face requirements of the old valve, while decreasing some of the wear widespread to move down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from move down to flow up, this modification may be made with out modifications to existing piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life as a result of the steam is transferring comparatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes by way of the small holes of the cage, it accelerates considerably, however that vitality dissipates into the big body cavity of the valve as an alternative of hanging the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease because they cross over the seat and speed up later because the steam strikes by way of the small trim holes, significantly extending trim service life. หลักการทํางานของpressuregauge : Emerson
Notice how the move up trim (Figure 7) is actually displaying erosion, however when compared to the move down trim shown above (Figure 4 right), the seating floor on the seat ring and plug stay relatively undamaged, permitting the valve to shut off completely, the required mode in normal operation.
7. These footage of move up trim clearly show the erosive impact of high-velocity steam, however in distinction to the circulate down trim harm shown beforehand, this valve has no injury to the seating floor and might still shut off tightly. Courtesy: Emerson
The circulate up valve type design still provides the same management responsiveness and circulate capacity, however the new flow association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised pieces of engineered tools that must be carefully specified, installed, and maintained. It is subsequently clever to be proactive by reaching out to trusted specialists prior to buy as they’ll help with valve sizing and specifications primarily based on specific operating circumstances.
For existing 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 would be wise to seek the guidance of your turbine bypass valve vendor for help and help.
Many of those distributors supply full inspection and well being check services to establish areas where steel fatigue and stress cracking are growing. Ideally the valve can be repaired, but when a valve alternative is warranted, the seller can even provide guidance in evaluating bypass valve design types and selecting the finest option for the particular application.
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