Turbine bypass valves are some of the difficult applications in an influence plant. Keeping these valves working effectively and avoiding sudden failures is critical for plant operation.
When engineers are requested to record the most troublesome control valve applications, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high stress drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, many plants are probably to ignore these valves until they fail, creating unplanned outages, misplaced production, and excessive prices for reactive maintenance. This article provides advised methods of inspection to anticipate and mitigate points beforehand, and it provides upgrade options should a valve need to be repaired or changed.
Strictly Severe Service
Severe service management valves are utilized in the most troublesome installations within course of plants. These installations commonly embody cavitating, erosive, corrosive, noisy, high stress, high temperature, high strain drop, or high velocity media. Turbine bypass valves are exposed to many of these course of situations; yet, 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, in addition to when a turbine journeys off-line (Figure 1). In normal operation the valves are fully closed, forcing all the steam through 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 guard the turbine by diverting doubtlessly wet steam, and by ensuring solely acceptable steam circumstances and flows make their way to the turbine itself.
1. Depending on the power plant design, a number of turbine bypass valves may be employed to instantly shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine trip, the steam must continue flowing to keep away from gear injury as a result of overpressure and high temperature, so the turbine bypass valve immediately opens to maintain flow via the system.
As it operates, a turbine makes use of steam to carry out work, decreasing outlet steam temperature and strain. When a turbine bypass valve opens, it will drop the stress, however the exit steam will stay quite superheated, doubtlessly destroying downstream equipment. To keep away from that scenario, turbine bypass valves both incorporate a water injection system within the valve physique, 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 a perfect storm of extreme service conditions. While the plant is in operation, these valves should remain tightly closed to avoid wasting energy. When a turbine trip happens, the bypass valves should respond instantly, exposing them to fast temperature modifications and requiring them to cross very excessive flows at high pressure drops, creating excessive noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the reality is that nearly each turbine bypass valve will ultimately fail ultimately. Unfortunately, many of those 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 problems begin to floor. Steam leakage through the valve is often the primary symptom noticed, but far more significant and doubtlessly harmful harm can happen.
The excessive pressure drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature changes, fatigue the metal in the valve, water connections, and the piping itself (Figure 2). Such steel fatigue may end up in catastrophic failure under pressure.
2. Over time, the high vibrations and thermal shock experienced by turbine bypass valves will fatigue the steel and create cracks in each the valve itself and the surrounding piping. Courtesy: Emerson
To avoid this drawback, each turbine bypass valve and its associated piping should be routinely inspected. Some type of non-destructive examination must be periodically employed to detect steel fatigue issues which may be growing but usually are not but visually apparent. If a plant lacks the knowledge or gear to carry out these inspections, the valve vendor or approved consultant could additionally be utilized to carry out turbine bypass health check companies (Figure 3).
3. A routine turbine bypass valve health examine and full inspection is strongly suggested. Metal fatigue can develop in varied welds situated on the valve itself, and in the inlet, outlet, and water provide 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 issues earlier than the equipment 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 components, corresponding to seat floor erosion. One frequent expression of such erosion, specifically on the plug, is sometimes called “gear toothing.” This happens extra generally in circulate down valves, the place the steam accelerates by way of the cage holes and then strikes the trim/seat space directly, inflicting excessive wear and reducing service life. Over time, this high-velocity steam, which can contain water throughout startup circumstances and magnetite throughout any situation, will erode and damage the valve seat and trim (Figure 4).
4. Flow down valve body designs are susceptible to trim injury because the wet, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance objects like soft goods and spray nozzles can usually be replaced relatively easily and at minimal value. Trim parts that need to be changed because of heavy put on, corresponding to gear toothing, may be quite pricey, particularly if their supply must be expedited. The worst-case scenario for a turbine bypass valve is metal fatigue developing within the valve physique or diffuser, with excessive restore procedures, or even a full valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service have been put in in the course of the heyday of combined cycle plant construction from 1998 to 2004. Most of these valves and piping techniques are exhibiting their age and often have important indicators of metal fatigue. Others have plenty of life left in them but may gain advantage from upgrades in expertise. Technology and practices have improved, and upgrading to the newest sealing technology might 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 instance, a removable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve should be replaced, plant personnel are strongly encouraged to look beyond a direct alternative and evaluate potential enhancements. A notably necessary item to consider when taking a look at substitute and enchancment opportunities 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 areas with low overhead clearance, and it locations the actuator closer to the deck or ground for simple access, but it creates a quantity of long-term operational problems. Gravity tends to result in elevated and uneven put on on trim parts, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) were the primary option a few many years ago for turbine bypass valves. New move up valve designs (right) could additionally be put in with out piping modifications in most instances, providing prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for move down is vertical mounting of the actuator. This association leads to decreased and extra even wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement alternatives are typically for horizontally mounted actuators, and the 2 designs aren’t readily interchangeable with out important piping modifications.
Upward Flowing Turbine Bypass Valves. When sufficient overhead area is on the market with existing horizontal actuator move down designs, a circulate up design can simply meet the prevailing face-to-face requirements of the previous valve, whereas reducing a few of the put on widespread to circulate down designs with a horizontal actuator, similar to gear toothing. Because the trim simply switches from move down to flow up, this alteration may be made with out modifications to existing piping.
Flow up valves (Figure 5, right) significantly prolong seat and trim service life as a result of 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 considerably, but that power dissipates into the big physique cavity of the valve as a substitute of striking the seating surface.
6. Downward flowing valves topic the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease because they pass over the seat and speed up later because the steam moves through the small trim holes, significantly extending trim service life. เกจวัดแรงดันปั๊มลม : Emerson
Notice how the move up trim (Figure 7) is certainly exhibiting erosion, but when in comparison with the move down trim shown above (Figure 4 right), the seating floor on the seat ring and plug stay relatively undamaged, permitting the valve to shut off fully, the required mode in normal operation.
7. These footage of flow up trim clearly present the erosive influence of high-velocity steam, but unlike the circulate down trim injury proven previously, this valve has no harm to the seating floor and can still shut off tightly. Courtesy: Emerson
The move up valve style design nonetheless provides the identical control responsiveness and circulate capacity, but the brand new circulate association dramatically extends the operating lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialized pieces of engineered equipment that have to be carefully specified, put in, and maintained. It is subsequently clever to be proactive by reaching out to trusted consultants prior to buy as they will assist with valve sizing and specifications based mostly on specific working circumstances.
For current 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 sensible to seek the advice of your turbine bypass valve vendor for assist and support.
Many of these vendors offer full inspection and health verify providers to establish areas where metal fatigue and stress cracking are growing. Ideally the valve may be repaired, but when a valve substitute is warranted, the seller can even provide steering in evaluating bypass valve design types and choosing the best option for the particular application.
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