เกจวัดแรงดันต่ำ bypass valves are one of the most tough applications in a power plant. Keeping these valves operating efficiently and avoiding unexpected failures is critical for plant operation.
When engineers are asked to record essentially the most tough management valve applications, turbine bypass valves are invariably mentioned. 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, lost production, and high costs for reactive upkeep. This article supplies suggested methods of inspection to anticipate and mitigate points beforehand, and it offers upgrade alternatives should a valve must be repaired or replaced.
Strictly Severe Service
Severe service control valves are utilized in essentially the most difficult installations within process vegetation. These installations generally embrace cavitating, erosive, corrosive, noisy, excessive pressure, high temperature, excessive stress drop, or high velocity media. Turbine bypass valves are uncovered to many of those course of conditions; but, 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 properly as when a turbine trips off-line (Figure 1). In regular operation the valves are fully closed, forcing all of the steam via a turbine. During startup, bypass valves divert steam away from the turbine till the properties and circumstances 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 potentially moist steam, and by making certain solely appropriate steam situations and flows make their method to the turbine itself.
1. Depending on the facility plant design, several turbine bypass valves could also be employed to immediately shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine journey, the steam must continue flowing to keep away from gear injury as a outcome of 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 strain. When a turbine bypass valve opens, it’s going to drop the pressure, however the exit steam will remain quite superheated, potentially destroying downstream tools. To avoid that situation, turbine bypass valves both incorporate a water injection system within the valve body, or employ 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 situations. While the plant is in operation, these valves must stay tightly closed to keep away from wasting energy. When a turbine trip happens, the bypass valves must reply instantly, exposing them to speedy temperature adjustments and requiring them to move very excessive flows at excessive strain drops, creating excessive noise and probably extreme vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly every turbine bypass valve will finally fail ultimately. Unfortunately, many of those valves are installed in difficult-to-access places, are typically welded in place, and are typically closely insulated. As a result, they’re usually ignored till issues begin to floor. Steam leakage via the valve is normally the primary symptom seen, however much more vital and potentially harmful damage can happen.
The extreme strain drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature adjustments, fatigue the metallic in the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue may find yourself in catastrophic failure under pressure.
2. Over time, the high vibrations and thermal shock skilled by turbine bypass valves will fatigue the metallic and create cracks in each the valve itself and the surrounding piping. Courtesy: Emerson
To keep away from this problem, each turbine bypass valve and its related piping must be routinely inspected. Some type of non-destructive examination should be periodically employed to detect steel fatigue problems that might be growing but aren’t yet visually obvious. If a plant lacks the data or gear to perform these inspections, the valve vendor or authorized consultant may be utilized to carry out turbine bypass health check companies (Figure 3).
three. A routine turbine bypass valve health check and full inspection is strongly suggested. Metal fatigue can develop in varied welds positioned on the valve itself, and within the inlet, outlet, and water supply piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, customer connection welds, diffuser welds, and surrounding piping may be inspected to establish any creating problems earlier than the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to keep away from issues associated to quenching and cracking.
There are additionally threats to trim parts, corresponding to seat floor erosion. One common expression of such erosion, particularly on the plug, is often referred to as “gear toothing.” This occurs extra generally in circulate down valves, the place the steam accelerates by way of the cage holes after which strikes the trim/seat space instantly, inflicting excessive wear and reducing service life. Over time, this high-velocity steam, which may contain water throughout startup circumstances and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve physique designs are vulnerable to trim injury as the wet, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common upkeep objects like delicate goods and spray nozzles can often be replaced comparatively easily and at minimal price. Trim parts that need to be changed because of heavy put on, corresponding to gear toothing, may be quite pricey, especially if their supply needs to be expedited. The worst-case situation for a turbine bypass valve is steel fatigue developing in the valve body or diffuser, with extreme restore procedures, or even a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service have been installed in the course of the heyday of combined cycle plant construction from 1998 to 2004. Most of those valves and piping techniques are displaying their age and sometimes have significant indicators of metallic fatigue. Others have loads of life left in them however may acquire advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the most recent 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 scale back required upkeep.
If a turbine bypass valve have to be replaced, plant personnel are strongly inspired to look beyond a direct substitute and evaluate potential improvements. A notably necessary item to consider when looking at replacement 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 places the actuator closer to the deck or ground for easy access, nevertheless it creates a number of long-term operational problems. Gravity tends to lead to elevated and uneven put on on trim components, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the principle choice a few decades ago for turbine bypass valves. New circulate up valve designs (right) could additionally be put in with out piping modifications in most instances, providing prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for flow down is vertical mounting of the actuator. This association leads to reduced and extra even put on, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement opportunities are sometimes for horizontally mounted actuators, and the two designs aren’t readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead space is on the market with current horizontal actuator move down designs, a move up design can easily meet the existing face-to-face requirements of the outdated valve, while decreasing a few of the wear frequent to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from flow right down to move up, this alteration may be made with out modifications to existing piping.
Flow up valves (Figure 5, right) considerably extend seat and trim service life because the steam is moving relatively 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, however that energy dissipates into the massive physique cavity of the valve instead of striking the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower because they pass over the seat and speed up later because the steam moves via the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is certainly exhibiting erosion, however when in comparability with the flow down trim shown above (Figure 4 right), the seating surface on the seat ring and plug stay comparatively undamaged, permitting the valve to close off completely, the required mode in regular operation.
7. These pictures of move up trim clearly show the erosive influence of high-velocity steam, but in distinction to the flow down trim injury proven beforehand, this valve has no damage to the seating surface and might still shut off tightly. Courtesy: Emerson
The circulate up valve type design nonetheless supplies the identical management responsiveness and flow capability, but the new circulate association dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered gear that must be carefully specified, put in, and maintained. It is due to this fact sensible to be proactive by reaching out to trusted specialists prior to purchase as they will assist with valve sizing and specifications primarily based on particular working situations.
For existing installations the place unplanned turbine bypass valve failures are bedeviling your facility, or if it has been some time for the explanation that plant turbine bypass valves have been absolutely inspected, it would be clever to consult your turbine bypass valve vendor for help and assist.
Many of these vendors provide full inspection and well being check companies to determine areas the place metallic fatigue and stress cracking are growing. Ideally the valve could be repaired, but when a valve alternative is warranted, the vendor also can provide steering in evaluating bypass valve design types and choosing the best choice for the specific application.