Turbine Bypass Valve Challenges

Turbine bypass valves are some of the difficult purposes in an influence plant. Keeping these valves operating efficiently and avoiding surprising failures is critical for plant operation.
When engineers are asked to list probably the most troublesome management valve applications, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high pressure drops, and the need for tight shutoff push these valves to the restrict. Unfortunately, many crops tend to ignore these valves until they fail, creating unplanned outages, lost manufacturing, and high costs for reactive maintenance. This article supplies instructed strategies of inspection to anticipate and mitigate points beforehand, and it provides upgrade alternate options ought to a valve must be repaired or replaced.
Strictly Severe Service
Severe service management valves are utilized in probably the most troublesome installations within course of vegetation. These installations generally include cavitating, erosive, corrosive, noisy, high pressure, excessive temperature, high pressure drop, or excessive velocity media. Turbine bypass valves are exposed to many of these course of situations; but, they have to respond 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 normal operation the valves are utterly closed, forcing all of the steam through a turbine. During startup, bypass valves divert steam away from the turbine till the properties and circumstances of the steam are acceptable 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 probably wet steam, and by making certain solely appropriate steam conditions and flows make their approach to the turbine itself.
1. Depending on the ability plant design, a number of turbine bypass valves may be employed to immediately shunt steam around a turbine should it journey offline. Courtesy: Emerson
Should a turbine journey, the steam should continue flowing to avoid gear damage as a outcome of overpressure and high temperature, so the turbine bypass valve instantly opens to take care of move via the system.
As it operates, a turbine makes use of steam to perform work, reducing outlet steam temperature and pressure. When a turbine bypass valve opens, it’s going to drop the pressure, but the exit steam will stay fairly superheated, probably destroying downstream tools. To avoid that scenario, turbine bypass valves either incorporate a water injection system within the valve physique, or employ a separate water injection desuperheater simply downstream, in either case to decrease the exit steam temperature.
As a result, turbine bypass valves face an ideal storm of extreme service conditions. While the plant is in operation, these valves must stay tightly closed to keep away from wasting power. When a turbine journey occurs, the bypass valves must respond immediately, exposing them to fast temperature changes and requiring them to pass very high flows at excessive stress drops, creating excessive noise and probably extreme vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that virtually every turbine bypass valve will ultimately fail indirectly. Unfortunately, many of those valves are put in in difficult-to-access locations, are usually welded in place, and are usually closely insulated. As a result, they’re typically ignored until issues start to floor. Steam leakage via the valve is often the primary symptom observed, but rather more vital and probably dangerous 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 metal within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue may find yourself in catastrophic failure beneath stress.
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 encircling piping. Courtesy: Emerson
To keep away from this problem, every turbine bypass valve and its associated piping must be routinely inspected. Some kind of non-destructive examination ought to be periodically employed to detect metal fatigue issues that could be developing however usually are not but visually apparent. If a plant lacks the information or tools to carry out these inspections, the valve vendor or approved representative may be utilized to carry out turbine bypass well being verify services (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 can be inspected to identify any growing problems earlier than the tools is compromised. Water injection nozzles and desuperheaters have to be appropriately inspected and maintained to avoid issues associated to quenching and cracking.
There are additionally threats to trim parts, similar to seat surface erosion. One widespread expression of such erosion, specifically on the plug, is often referred to as “gear toothing.” This happens extra commonly in circulate down valves, the place the steam accelerates by way of the cage holes and then strikes the trim/seat space immediately, inflicting excessive put on and lowering service life. Over time, this high-velocity steam, which can include water throughout startup situations and magnetite throughout any condition, will erode and injury the valve seat and trim (Figure 4).
four. Flow down valve physique designs are prone to trim injury as the moist, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson
Common maintenance items like delicate goods and spray nozzles can often get replaced relatively simply and at minimal price. Trim parts that need to be replaced as a result of heavy put on, such as gear toothing, can be quite expensive, particularly if their supply must be expedited. The worst-case scenario for a turbine bypass valve is steel fatigue growing within the valve body or diffuser, with extreme restore procedures, or maybe a complete valve replacement, required.
Upgrade Opportunities
Many of the turbine bypass valves at present in service were installed during the heyday of combined cycle plant development from 1998 to 2004. Most of these valves and piping systems are showing their age and infrequently have vital signs of metal fatigue. Others have plenty of life left in them however may acquire advantage from upgrades in know-how. Technology and practices have improved, and upgrading to the most recent sealing expertise could inject new life into the valve.
Another example of a know-how upgrade is when there have been repeated failures with a welded diffuser. In this occasion, a detachable two-in-one seat diffuser will scale back required maintenance.
If a turbine bypass valve should be replaced, plant personnel are strongly inspired to look beyond a direct alternative and consider potential improvements. A particularly necessary merchandise to assume about when taking a look 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 association is good for places with low overhead clearance, and it places the actuator nearer to the deck or ground for easy access, but it creates numerous long-term operational problems. Gravity tends to result in increased and uneven wear on trim components, and the side-mounted actuator is vulnerable to response problems.
5. Downward flowing valves with horizontal actuators (left) were the principle choice a quantity of a long time in the past for turbine bypass valves. New circulate up valve designs (right) may be put in without piping modifications in most situations, offering prolonged service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior choice for flow down is vertical mounting of the actuator. This arrangement leads to decreased and more even put on, longer runs between maintenance outages, and improved actuator response. No problem with this orientation is that replacement alternatives are usually for horizontally mounted actuators, and the two designs aren’t readily interchangeable with out vital piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead space is available with existing horizontal actuator circulate down designs, a flow up design can easily meet the existing face-to-face necessities of the previous valve, while reducing some of the put on common to circulate down designs with a horizontal actuator, corresponding to gear toothing. Because the trim just switches from circulate right down to circulate up, this modification may be made with out modifications to existing piping.
Flow up valves (Figure 5, right) significantly extend seat and trim service life as a result of the steam is shifting comparatively slowly as 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 energy dissipates into the big body cavity of the valve instead of hanging 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 end result of they pass over the seat and speed up later as the steam strikes through the small trim holes, tremendously extending trim service life. Courtesy: Emerson
Notice how the move up trim (Figure 7) is definitely displaying erosion, however when compared to the circulate down trim proven above (Figure 4 right), the seating floor on the seat ring and plug stay comparatively undamaged, permitting the valve to close off completely, the required mode in normal operation.
7. These footage of move up trim clearly present the erosive influence of high-velocity steam, however in distinction to the flow down trim harm shown previously, this valve has no injury to the seating surface and may nonetheless shut off tightly. Courtesy: Emerson
The move up valve style design still supplies the identical control responsiveness and circulate capability, but the new circulate arrangement dramatically extends the operating life of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are highly specialised pieces of engineered equipment that have to be fastidiously specified, put in, and maintained. It is subsequently wise to be proactive by reaching out to trusted specialists prior to buy as they’ll assist with valve sizing and specifications based mostly on particular operating conditions.
For existing installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while because the plant turbine bypass valves have been fully inspected, it will be clever to seek the advice of your turbine bypass valve vendor for assist and help.
Many of those distributors offer full inspection and well being examine providers to determine areas where metal fatigue and stress cracking are creating. Ideally the valve could be repaired, but when a valve substitute is warranted, the seller can even provide steerage in evaluating bypass valve design types and choosing the finest option for the precise utility.
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