Turbine bypass valves are one of the most difficult applications in an influence plant. Keeping these valves working effectively and avoiding surprising failures is important for plant operation.
When engineers are asked to record probably the most troublesome management 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 limit. Unfortunately, many vegetation are inclined to ignore these valves till they fail, creating unplanned outages, lost manufacturing, and high costs for reactive upkeep. This article offers advised methods of inspection to anticipate and mitigate points beforehand, and it provides upgrade options 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 vegetation. These installations commonly embrace cavitating, erosive, corrosive, noisy, high pressure, high temperature, high strain drop, or excessive velocity media. Turbine bypass valves are exposed to many of these course of conditions; yet, they must reply flawlessly and stay leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam generators during 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 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 process occurs in reverse at shutdown. Using bypass valves at startup and shutdown helps to protect the turbine by diverting potentially wet steam, and by ensuring solely appropriate steam circumstances and flows make their approach to the turbine itself.
1. Depending on the power plant design, several turbine bypass valves may be employed to immediately shunt steam around a turbine ought to it journey offline. Courtesy: Emerson
Should a turbine trip, the steam should proceed flowing to keep away from gear harm as a result of overpressure and excessive temperature, so the turbine bypass valve immediately opens to take care of flow via the system.
As it operates, a turbine uses steam to perform work, decreasing outlet steam temperature and strain. When a turbine bypass valve opens, it will drop the pressure, but the exit steam will stay quite superheated, probably destroying downstream tools. To avoid that state of affairs, turbine bypass valves either incorporate a water injection system within the valve body, or employ a separate water injection desuperheater just downstream, in either case to lower the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of extreme service conditions. While the plant is in operation, these valves should stay tightly closed to keep away from wasting energy. When a turbine journey happens, the bypass valves should respond instantly, exposing them to fast temperature adjustments and requiring them to cross very high flows at excessive strain drops, creating excessive noise and potentially extreme vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that just about each turbine bypass valve will in the end fail indirectly. Unfortunately, many of these valves are put in in difficult-to-access places, are usually welded in place, and are usually heavily insulated. As a result, they’re typically ignored till issues start to surface. Steam leakage via the valve is normally the first symptom noticed, but much more important and doubtlessly harmful damage can occur.
The extreme pressure drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the steel in the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can lead to catastrophic failure beneath pressure.
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 encompassing piping. Courtesy: Emerson
To avoid this problem, every turbine bypass valve and its associated piping ought to be routinely inspected. Some type of non-destructive examination ought to be periodically employed to detect steel fatigue issues which could be growing however usually are not but visually apparent. If a plant lacks the knowledge or gear to carry out these inspections, the valve vendor or licensed representative could additionally be utilized to perform turbine bypass well being verify companies (Figure 3).
three. A routine turbine bypass valve well being 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). Courtesy: Emerson
Fabrication welds on the physique and water manifold, customer connection welds, diffuser welds, and surrounding piping may be inspected to determine any developing problems before the equipment is compromised. Water injection nozzles and desuperheaters should be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are additionally threats to trim elements, corresponding to seat surface erosion. One frequent expression of such erosion, particularly on the plug, is also known as “gear toothing.” This happens extra commonly in flow down valves, where the steam accelerates through the cage holes and then strikes the trim/seat area immediately, inflicting excessive wear and reducing service life. Over time, this high-velocity steam, which may comprise water during startup circumstances and magnetite during any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve body designs are vulnerable to trim harm 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 get replaced comparatively easily and at minimal cost. Trim parts that must be changed because of heavy put on, similar to gear toothing, may be quite expensive, particularly if their supply needs to be expedited. The worst-case situation for a turbine bypass valve is steel fatigue creating within the valve body or diffuser, with extreme repair procedures, or even a complete valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves presently in service have been put in through the heyday of combined cycle plant building from 1998 to 2004. Most of these valves and piping techniques are displaying their age and often have significant signs of steel 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 might inject new life into the valve.
Another instance 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 reduce required upkeep.
If a turbine bypass valve must be replaced, plant personnel are strongly inspired to look past a direct alternative and consider potential enhancements. A notably necessary merchandise to consider when taking a look at substitute and enchancment alternatives is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, pressure gauge ยี่ห้อ tk bypass valves employed a circulate down trim design paired with a horizontal actuator (Figure 5, left). This arrangement is right for locations with low overhead clearance, and it places the actuator nearer to the deck or ground for easy access, however it creates a variety of long-term operational problems. Gravity tends to result in elevated and uneven put on on trim elements, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) have been the primary option a few decades ago for turbine bypass valves. New flow up valve designs (right) could also be put in without piping modifications in most cases, providing prolonged service life and improved efficiency. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior possibility for move down is vertical mounting of the actuator. This arrangement results in lowered and extra even put on, longer runs between maintenance outages, and improved actuator response. The problem with this orientation is that substitute opportunities are typically for horizontally mounted actuators, and the two designs are not readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is available with existing horizontal actuator circulate down designs, a move up design can easily meet the prevailing face-to-face necessities of the outdated valve, while lowering some of the wear widespread to flow down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from move down to circulate up, this change can be made without modifications to existing piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life because the steam is transferring comparatively slowly as it passes over the plug and seat (Figure 6). As the steam strikes via the small holes of the cage, it accelerates significantly, but that power dissipates into the massive physique cavity of the valve instead of striking the seating floor.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot decrease as a end result of they pass over the seat and speed up 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 actually displaying erosion, but when in comparison with the move down trim proven above (Figure 4 right), the seating floor on the seat ring and plug stay relatively undamaged, allowing the valve to shut off completely, the required mode in regular operation.
7. These pictures of circulate up trim clearly present the erosive impact of high-velocity steam, however unlike the flow down trim harm proven previously, this valve has no injury to the seating floor and may nonetheless shut off tightly. Courtesy: Emerson
The circulate up valve style design still provides the identical management responsiveness and move capability, however the brand new flow arrangement dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised pieces of engineered equipment that should be fastidiously specified, put in, and maintained. It is therefore sensible to be proactive by reaching out to trusted specialists prior to purchase as they’ll help with valve sizing and specifications based mostly on specific working situations.
For present 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 will be clever to seek the assistance of your turbine bypass valve vendor for help and assist.
Many of those distributors offer full inspection and well being check companies to establish areas where metallic fatigue and stress cracking are developing. Ideally the valve may be repaired, but if a valve replacement is warranted, the seller also can present steerage in evaluating bypass valve design kinds and selecting the finest choice for the precise software.