Turbine Bypass Valve Challenges

Turbine bypass valves are one of the most troublesome functions in a power plant. Keeping these valves working efficiently and avoiding unexpected failures is critical for plant operation.
When engineers are asked to listing essentially the most difficult control valve applications, turbine bypass valves are invariably mentioned. Frequent thermal cycles, excessive pressure drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, many vegetation are most likely to ignore these valves till they fail, creating unplanned outages, misplaced manufacturing, and excessive prices for reactive maintenance. This article offers instructed strategies of inspection to anticipate and mitigate issues beforehand, and it provides upgrade alternate options ought to a valve have to be repaired or replaced.
Strictly Severe Service
Severe service control valves are utilized in probably the most troublesome installations within course of vegetation. These installations generally embody cavitating, erosive, corrosive, noisy, high stress, excessive temperature, excessive stress drop, or high velocity media. Turbine bypass valves are exposed to many of those process circumstances; but, they have to respond flawlessly and stay leak free when closed.
As their title suggests, turbine bypass valves are used to bypass steam generators during 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 of the steam via a turbine. During startup, bypass valves divert steam away from the turbine until the properties and circumstances of the steam are acceptable 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 doubtlessly wet steam, and by ensuring only applicable steam circumstances and flows make their way to the turbine itself.
1. Depending on the facility plant design, a number of turbine bypass valves could also be employed to instantly shunt steam round a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to keep away from equipment injury as a result of overpressure and high temperature, so the turbine bypass valve instantly opens to maintain up flow through the system.
As it operates, a turbine uses steam to perform work, decreasing outlet steam temperature and strain. When ราคาpressuregauge bypass valve opens, it’ll drop the stress, but the exit steam will stay quite superheated, potentially destroying downstream gear. To avoid 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 either case to decrease the exit steam temperature.
As a result, turbine bypass valves face a perfect storm of severe service conditions. While the plant is in operation, these valves must remain tightly closed to avoid losing energy. When a turbine journey occurs, the bypass valves must respond immediately, exposing them to rapid temperature adjustments and requiring them to move very high flows at excessive strain drops, creating excessive noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that nearly every turbine bypass valve will ultimately fail indirectly. Unfortunately, many of those valves are installed in difficult-to-access areas, are typically welded in place, and tend to be heavily insulated. As a outcome, they are typically ignored till issues begin to surface. Steam leakage through the valve is usually the first symptom noticed, but much more important and probably harmful injury can happen.
The extreme pressure drops invariably generate excessive noise and high vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can outcome in catastrophic failure beneath strain.
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. เกจวัดแรงดันถังแก๊ส : Emerson
To avoid this drawback, each turbine bypass valve and its related piping should be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect metallic fatigue issues that might be developing however aren’t but visually obvious. If a plant lacks the data or equipment to carry out these inspections, the valve vendor or approved consultant may be utilized to perform turbine bypass well being verify providers (Figure 3).
three. A routine turbine bypass valve well being check and full inspection is strongly advised. Metal fatigue can develop in numerous welds located 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 identify any creating problems before the gear is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid issues related to quenching and cracking.
There are additionally threats to trim parts, such as seat floor erosion. One frequent expression of such erosion, particularly on the plug, is also known as “gear toothing.” This happens more generally in circulate down valves, the place the steam accelerates via the cage holes after which strikes the trim/seat area directly, inflicting excessive wear and reducing service life. Over time, this high-velocity steam, which can include water throughout startup conditions and magnetite during any situation, will erode and harm the valve seat and trim (Figure 4).
4. Flow down valve physique designs are susceptible to trim damage because the moist, erosive steam passes through the cage and impacts the seat. Courtesy: Emerson
Common maintenance items like soft goods and spray nozzles can often get replaced comparatively simply and at minimal cost. Trim components that have to be replaced as a end result of heavy wear, corresponding to gear toothing, can be quite pricey, particularly if their delivery must be expedited. The worst-case situation for a turbine bypass valve is steel fatigue developing in the valve physique or diffuser, with excessive repair procedures, or perhaps a full 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 these valves and piping methods are displaying their age and sometimes have important indicators of steel fatigue. Others have loads of life left in them but may gain advantage from upgrades in know-how. Technology and practices have improved, and upgrading to the newest sealing know-how could inject new life into the valve.
Another instance of a know-how upgrade is when there have been repeated failures with a welded diffuser. In this occasion, a removable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve have to be changed, plant personnel are strongly encouraged to look beyond a direct substitute and evaluate potential enhancements. A particularly necessary merchandise to contemplate when looking at alternative 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 arrangement is good for places with low overhead clearance, and it locations the actuator closer to the deck or ground for simple access, nevertheless it creates numerous long-term operational issues. Gravity tends to end in increased and uneven put on on trim parts, and the side-mounted actuator is susceptible to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the principle choice a quantity of a long time in the past for turbine bypass valves. New flow up valve designs (right) may be installed without piping modifications in most instances, 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 ends in reduced and extra even put on, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that alternative opportunities are typically for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is out there with present horizontal actuator flow down designs, a flow up design can easily meet the prevailing face-to-face requirements of the outdated valve, while lowering a number of the put on common to circulate down designs with a horizontal actuator, corresponding to gear toothing. Because the trim simply switches from move down to circulate up, this change may be made without modifications to existing piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life as a outcome of the steam is moving comparatively slowly as it passes over the plug and seat (Figure 6). As the steam moves through the small holes of the cage, it accelerates considerably, but that power dissipates into the massive physique cavity of the valve as a substitute of striking the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are a lot lower as a result of they pass over the seat and speed up later because the steam moves through the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is actually exhibiting erosion, however when compared to the flow down trim proven above (Figure four right), the seating floor on the seat ring and plug stay relatively undamaged, allowing the valve to close off utterly, the required mode in normal operation.
7. These pictures of circulate up trim clearly show the erosive impression of high-velocity steam, however in distinction to the move down trim injury proven beforehand, this valve has no harm to the seating floor and might nonetheless shut off tightly. Courtesy: Emerson
The circulate up valve style design still supplies the same control responsiveness and flow capability, but the brand new move association dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialised pieces of engineered tools that have to be fastidiously specified, put in, and maintained. It is subsequently clever to be proactive by reaching out to trusted specialists prior to buy as they can assist with valve sizing and specifications based on specific operating circumstances.
For existing installations where 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 will be sensible to seek the advice of your turbine bypass valve vendor for assist and help.
Many of these distributors supply full inspection and well being examine services to identify areas where steel fatigue and stress cracking are growing. Ideally the valve can be repaired, but when a valve replacement is warranted, the seller also can provide guidance in evaluating bypass valve design kinds and choosing the best choice for the particular utility.