Turbine bypass valves are one of the most difficult applications in a power plant. Keeping these valves working efficiently and avoiding unexpected failures is crucial for plant operation.
When engineers are requested to record the most troublesome control valve applications, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive stress drops, and the necessity for tight shutoff push these valves to the limit. Unfortunately, many vegetation are inclined to ignore these valves till they fail, creating unplanned outages, misplaced production, and high prices for reactive maintenance. This article supplies instructed strategies of inspection to anticipate and mitigate points beforehand, and it presents improve alternate options 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 crops. These installations generally include cavitating, erosive, corrosive, noisy, excessive strain, high temperature, excessive stress drop, or excessive velocity media. Turbine bypass valves are exposed to many of these process situations; but, they must 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 trips off-line (Figure 1). In regular operation the valves are fully closed, forcing all the steam through a turbine. During เกจวัดแรงดันปั๊มลมpuma , 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 process happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting potentially moist steam, and by guaranteeing only acceptable steam conditions and flows make their way to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves could also be employed to immediately shunt steam around a turbine should it trip offline. Courtesy: Emerson
Should a turbine trip, the steam should continue flowing to avoid tools harm due to overpressure and high temperature, so the turbine bypass valve instantly opens to maintain up 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 will drop the pressure, but the exit steam will remain quite superheated, doubtlessly destroying downstream gear. To keep away from that situation, turbine bypass valves either incorporate a water injection system within the valve physique, 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 severe service conditions. While the plant is in operation, these valves should stay tightly closed to avoid wasting vitality. When a turbine journey occurs, the bypass valves must respond instantly, exposing them to fast temperature changes and requiring them to move very high flows at excessive strain drops, creating excessive noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the fact is that just about every turbine bypass valve will in the end fail in some way. Unfortunately, many of those valves are put in in difficult-to-access areas, are typically welded in place, and are typically heavily insulated. As a end result, they are often ignored till problems begin to floor. Steam leakage by way of the valve is often the primary symptom noticed, however rather more significant and doubtlessly harmful damage can happen.
The extreme stress drops invariably generate high noise and high 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 metallic fatigue can lead to catastrophic failure underneath stress.
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 drawback, each turbine bypass valve and its related piping ought to be routinely inspected. Some kind of non-destructive examination should be periodically employed to detect metallic fatigue problems that could be creating but aren’t but visually obvious. If a plant lacks the data or tools to carry out these inspections, the valve vendor or licensed consultant may be utilized to perform turbine bypass well being examine companies (Figure 3).
3. A routine turbine bypass valve well being examine and full inspection is strongly advised. Metal fatigue can develop in varied welds positioned on the valve itself, and in 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 could be inspected to identify any developing problems before the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid problems associated to quenching and cracking.
There are additionally threats to trim elements, such as seat surface erosion. One frequent expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This happens more commonly in flow down valves, the place the steam accelerates via the cage holes and then strikes the trim/seat space directly, inflicting extreme wear and lowering service life. Over time, this high-velocity steam, which can include water throughout startup circumstances and magnetite throughout any situation, will erode and injury the valve seat and trim (Figure 4).
4. Flow down valve body designs are vulnerable to trim damage as the wet, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common upkeep items like delicate items and spray nozzles can often be replaced comparatively simply and at minimal cost. Trim elements that must be changed as a result of heavy put on, corresponding to gear toothing, can be quite costly, particularly if their supply needs to be expedited. The worst-case situation for a turbine bypass valve is steel fatigue creating in the valve physique or diffuser, with extreme repair procedures, or even a full valve substitute, required.
Many of the turbine bypass valves at present in service have been put in during the heyday of combined cycle plant building from 1998 to 2004. Most of these valves and piping techniques are showing their age and often have significant signs of metallic fatigue. Others have plenty of life left in them however may benefit from upgrades in expertise. Technology and practices have improved, and upgrading to the most recent sealing technology 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 occasion, a detachable two-in-one seat diffuser will cut back required maintenance.
If a turbine bypass valve should be changed, plant personnel are strongly inspired to look beyond a direct substitute and consider potential enhancements. A particularly essential item to contemplate when taking a look at substitute and enchancment 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 good for locations with low overhead clearance, and it locations the actuator nearer to the deck or ground for simple entry, nevertheless it creates a variety of long-term operational problems. Gravity tends to result in increased and uneven wear on trim parts, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) have been the primary possibility a number of a long time ago for turbine bypass valves. New move up valve designs (right) may be installed without piping modifications in most cases, providing extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for circulate down is vertical mounting of the actuator. This arrangement ends in reduced and more even put on, longer runs between upkeep outages, and improved actuator response. The problem with this orientation is that substitute alternatives are sometimes for horizontally mounted actuators, and the two designs usually are not readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead house is out there with present horizontal actuator move down designs, a circulate up design can easily meet the present face-to-face requirements of the outdated valve, while decreasing a few of the put on widespread to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from circulate right down to circulate up, this alteration could be made without modifications to existing piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life as a outcome of the steam is shifting relatively 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 massive physique cavity of the valve instead of putting the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease as a result of they cross over the seat and speed up later because the steam moves through the small trim holes, greatly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is definitely exhibiting erosion, however when in comparison with the flow down trim proven 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 regular operation.
7. These pictures of flow up trim clearly show the erosive impression of high-velocity steam, but not like the circulate down trim injury proven previously, this valve has no injury to the seating floor and can nonetheless shut off tightly. Courtesy: Emerson
The circulate up valve style design nonetheless provides the same control responsiveness and flow capability, but the brand new circulate association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised items of engineered tools that must be fastidiously specified, installed, and maintained. It is therefore sensible to be proactive by reaching out to trusted experts prior to purchase as they’ll help with valve sizing and specifications primarily based on specific working situations.
For current installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while since the plant turbine bypass valves have been absolutely inspected, it might be clever to consult your turbine bypass valve vendor for assist and support.
Many of those distributors supply full inspection and well being verify services to identify areas the place metal fatigue and stress cracking are growing. Ideally the valve may be repaired, but if a valve substitute is warranted, the vendor can also present steering in evaluating bypass valve design types and selecting the best choice for the precise utility.