Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most tough applications in an influence plant. Keeping these valves operating efficiently and avoiding sudden failures is critical for plant operation.
When engineers are requested to list probably the most tough management 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 plants tend to ignore these valves till they fail, creating unplanned outages, misplaced manufacturing, and excessive costs for reactive upkeep. This article supplies advised strategies of inspection to anticipate and mitigate issues beforehand, and it presents upgrade options should a valve must be repaired or changed.
Strictly Severe Service
Severe service control valves are utilized in the most troublesome installations within process crops. These installations commonly include cavitating, erosive, corrosive, noisy, excessive stress, high temperature, excessive strain drop, or high velocity media. Turbine bypass valves are uncovered to many of those course of circumstances; but, they have to reply flawlessly and stay leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam generators throughout plant startup and shutdown, in addition to when a turbine journeys off-line (Figure 1). In regular operation the valves are completely closed, forcing all of the steam by way of a turbine. During startup, bypass valves divert steam away from the turbine till the properties and conditions of the steam are appropriate for sending it to the turbine. This course of occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting doubtlessly moist steam, and by making certain only applicable steam situations and flows make their way to the turbine itself.
1. Depending on the facility plant design, several turbine bypass valves could additionally be employed to instantly shunt steam round a turbine should it journey offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to keep away from gear damage due to overpressure and high temperature, so the turbine bypass valve instantly opens to maintain move by way of the system.
As it operates, a turbine uses steam to perform work, reducing outlet steam temperature and pressure. When a turbine bypass valve opens, it will drop the stress, however the exit steam will remain quite superheated, probably destroying downstream tools. To avoid that state of affairs, turbine bypass valves either incorporate a water injection system within the valve physique, or make use of a separate water injection desuperheater just downstream, in both case to decrease the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of severe service situations. While the plant is in operation, these valves should stay tightly closed to keep away from wasting power. When a turbine trip occurs, the bypass valves must reply immediately, exposing them to speedy temperature adjustments and requiring them to pass very high flows at high stress drops, creating high noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that nearly each turbine bypass valve will finally fail in some way. Unfortunately, many of those valves are installed in difficult-to-access places, are usually welded in place, and are typically closely insulated. As a result, they are typically ignored till problems begin to surface. Steam leakage through the valve is often the primary symptom observed, but rather more important and probably dangerous harm can occur.
The excessive strain drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the steel within the valve, water connections, and the piping itself (Figure 2). Such steel fatigue can outcome in catastrophic failure beneath strain.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in both the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this drawback, every turbine bypass valve and its associated piping must be routinely inspected. Some type of non-destructive examination must be periodically employed to detect steel fatigue problems that could be developing but aren’t but visually obvious. If a plant lacks the data or tools to carry out these inspections, the valve vendor or authorized representative could also be utilized to perform turbine bypass health check companies (Figure 3).
three. A routine turbine bypass valve well being examine and full inspection is strongly advised. Metal fatigue can develop in various welds positioned on the valve itself, and within 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 identify any creating problems before the tools is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to avoid issues 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 sometimes called “gear toothing.” This happens more generally in move down valves, the place the steam accelerates via the cage holes and then strikes the trim/seat area instantly, inflicting excessive put on and decreasing service life. Over time, this high-velocity steam, which can contain water during startup circumstances and magnetite throughout any situation, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve body designs are vulnerable to trim injury as the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like soft items and spray nozzles can normally get replaced comparatively simply and at minimal price. Trim elements that need to be changed due to heavy put on, similar to gear toothing, could be quite expensive, especially if their delivery needs to be expedited. The worst-case state of affairs for a turbine bypass valve is metal fatigue creating within the valve body or diffuser, with excessive restore procedures, or even a full valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service had been put in through the heyday of mixed cycle plant building from 1998 to 2004. Most of those valves and piping systems are exhibiting their age and sometimes have important indicators of metal fatigue. Others have plenty of life left in them however may acquire advantage from upgrades in technology. Technology and practices have improved, and upgrading to the latest sealing technology could 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 reduce required upkeep.
If a turbine bypass valve must be replaced, plant personnel are strongly inspired to look beyond a direct alternative and evaluate potential enhancements. A particularly essential merchandise to consider when looking at replacement and enchancment alternatives 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 arrangement is ideal for areas with low overhead clearance, and it places the actuator closer to the deck or ground for easy entry, however it creates a quantity of long-term operational issues. Gravity tends to result in increased and uneven put on on trim elements, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) were the primary option a couple of a long time in the past for turbine bypass valves. ร้านซ่อมเครื่องวัดความดันโลหิต up valve designs (right) could also be put in with out piping modifications in most situations, providing 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 association ends in decreased and more even wear, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that substitute alternatives are typically for horizontally mounted actuators, and the 2 designs are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is out there with existing horizontal actuator flow down designs, a circulate up design can easily meet the prevailing face-to-face necessities of the outdated valve, while reducing a few of the put on frequent to move down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from flow all the means down to flow up, this modification could be made with out modifications to current piping.
Flow up valves (Figure 5, right) considerably prolong 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 moves by way of the small holes of the cage, it accelerates significantly, but that energy dissipates into the large body cavity of the valve as a substitute of putting the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease as a result of they move over the seat and accelerate later because the steam moves by way of the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is definitely exhibiting erosion, however when in comparability with the flow down trim shown above (Figure four right), the seating floor on the seat ring and plug remain relatively undamaged, permitting the valve to close off completely, the required mode in regular operation.
7. These photos of move up trim clearly show the erosive influence of high-velocity steam, however in contrast to the circulate down trim harm shown beforehand, this valve has no damage to the seating surface and may still shut off tightly. Courtesy: Emerson
The flow up valve type design still provides the same management responsiveness and move capability, however the model new move arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised items of engineered equipment that have to be fastidiously specified, installed, and maintained. It is subsequently sensible to be proactive by reaching out to trusted consultants prior to purchase as they’ll help with valve sizing and specs primarily based on specific working conditions.
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 clever to seek the assistance of your turbine bypass valve vendor for assist and help.
Many of those vendors supply full inspection and health check companies to identify areas the place steel fatigue and stress cracking are developing. Ideally the valve could be repaired, but when a valve substitute is warranted, the seller can also present guidance in evaluating bypass valve design kinds and selecting the finest choice for the specific software.
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