Turbine Bypass Valve Challenges

Turbine bypass valves are one of the troublesome purposes in a power plant. Keeping these valves working efficiently and avoiding sudden failures is important for plant operation.
When engineers are asked to record essentially the most tough management valve purposes, turbine bypass valves are invariably talked about. Frequent thermal cycles, excessive pressure drops, and the necessity for tight shutoff push these valves to the restrict. Unfortunately, many plants are inclined to ignore these valves until they fail, creating unplanned outages, misplaced production, and excessive costs for reactive upkeep. This article supplies suggested methods of inspection to anticipate and mitigate points beforehand, and it presents upgrade alternatives ought to a valve have to be repaired or replaced.
Strictly Severe Service

Severe service control valves are used in probably the most tough installations inside course of crops. These installations generally embody cavitating, erosive, corrosive, noisy, excessive pressure, excessive temperature, excessive strain drop, or high velocity media. Turbine bypass valves are exposed to many of these course of circumstances; yet, they want to reply flawlessly and stay leak free when closed.
As their name suggests, turbine bypass valves are used to bypass steam turbines 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 the steam through a turbine. During startup, bypass valves divert steam away from the turbine until the properties and circumstances of the steam are applicable 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 potentially wet steam, and by ensuring only applicable steam conditions and flows make their approach to the turbine itself.
1. Depending on ไดอะแฟรม ซีล , a quantity of turbine bypass valves could additionally be employed to instantly shunt steam round a turbine should it journey offline. Courtesy: Emerson

Should a turbine journey, the steam must proceed flowing to avoid equipment damage because of overpressure and excessive temperature, so the turbine bypass valve instantly opens to maintain move 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’ll drop the stress, however the exit steam will stay fairly superheated, doubtlessly destroying downstream tools. To keep away from that situation, turbine bypass valves both incorporate a water injection system in the valve body, or employ a separate water injection desuperheater just downstream, in either case to decrease the exit steam temperature.
As a end 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 keep away from wasting power. When a turbine trip occurs, the bypass valves should reply immediately, exposing them to fast temperature adjustments and requiring them to move very excessive flows at high pressure drops, creating high noise and doubtlessly extreme vibration.
Getting Ahead of the Game

Given the punishing service, the fact is that just about each turbine bypass valve will finally fail in some way. Unfortunately, many of these valves are put in in difficult-to-access locations, are typically welded in place, and are typically heavily insulated. As a end result, they’re usually ignored till problems start to floor. Steam leakage via the valve is often the first symptom observed, but much more vital and doubtlessly harmful harm can occur.
The excessive pressure drops invariably generate high noise and excessive vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metallic 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 high vibrations and thermal shock skilled by turbine bypass valves will fatigue the steel and create cracks in both the valve itself and the surrounding piping. Courtesy: Emerson

To keep away from this problem, each turbine bypass valve and its related piping ought to be routinely inspected. Some type of non-destructive examination ought to be periodically employed to detect metallic fatigue issues which may be creating however aren’t yet visually obvious. If a plant lacks the data or tools to carry out these inspections, the valve vendor or licensed representative could also be utilized to perform turbine bypass well being verify providers (Figure 3).
three. A routine turbine bypass valve well being examine and full inspection is strongly advised. Metal fatigue can develop in various welds located 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, customer connection welds, diffuser welds, and surrounding piping may be inspected to establish any creating issues earlier than the tools is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to keep away from problems related to quenching and cracking.
There are additionally threats to trim elements, similar to seat surface erosion. One frequent expression of such erosion, particularly on the plug, is also recognized as “gear toothing.” This occurs more commonly in flow down valves, the place the steam accelerates by way of the cage holes and then strikes the trim/seat space instantly, inflicting extreme put on and reducing service life. Over time, this high-velocity steam, which can contain water during startup situations and magnetite throughout any condition, will erode and damage the valve seat and trim (Figure 4).
four. Flow down valve body designs are prone to trim damage because the wet, erosive steam passes by way of the cage and impacts the seat. Courtesy: Emerson

Common upkeep items like delicate goods and spray nozzles can often be replaced relatively easily and at minimal cost. Trim components that need to be changed because of heavy put on, such as gear toothing, can be fairly pricey, particularly if their delivery needs to be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue developing within the valve physique or diffuser, with excessive repair procedures, or perhaps a complete valve alternative, required.
Upgrade Opportunities

Many of the turbine bypass valves at present in service had been put in in the course of the heyday of combined cycle plant construction from 1998 to 2004. Most of those valves and piping systems are displaying their age and often have vital signs of metallic 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 expertise 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 instance, a removable two-in-one seat diffuser will cut back required maintenance.
If a turbine bypass valve must be replaced, plant personnel are strongly inspired to look past a direct substitute and consider potential enhancements. A significantly essential merchandise to contemplate when taking a look at replacement 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 association is right for locations with low overhead clearance, and it places the actuator nearer to the deck or floor for easy entry, but it creates a quantity of long-term operational problems. Gravity tends to result in elevated and uneven wear on trim elements, and the side-mounted actuator is prone to response problems.
5. Downward flowing valves with horizontal actuators (left) had been the primary possibility a number of many years ago for turbine bypass valves. pressure gauge up valve designs (right) could additionally be installed without piping modifications in most situations, offering 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 wear, longer runs between maintenance outages, and improved actuator response. The challenge with this orientation is that replacement alternatives are usually for horizontally mounted actuators, and the 2 designs usually are not readily interchangeable with out significant piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead house is out there with present horizontal actuator move down designs, a circulate up design can easily meet the existing face-to-face requirements of the old valve, while decreasing some of the put on widespread to move down designs with a horizontal actuator, similar to gear toothing. Because the trim just switches from circulate down to flow up, this modification can be made with out modifications to existing piping.
Flow up valves (Figure 5, right) significantly lengthen seat and trim service life because the steam is transferring relatively slowly because it passes over the plug and seat (Figure 6). As the steam strikes by way of the small holes of the cage, it accelerates considerably, however that power dissipates into the large body cavity of the valve as an alternative of striking the seating surface.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much lower as a result of they move over the seat and speed up later as the steam strikes by way of the small trim holes, tremendously extending trim service life. Courtesy: Emerson

Notice how the flow up trim (Figure 7) is definitely exhibiting erosion, however when compared to the move down trim proven above (Figure 4 right), the seating floor on the seat ring and plug remain relatively undamaged, permitting the valve to shut off fully, the required mode in regular operation.
7. These pictures of circulate up trim clearly present the erosive impression of high-velocity steam, however in contrast to the circulate down trim injury shown beforehand, this valve has no injury to the seating surface and can nonetheless shut off tightly. Courtesy: Emerson

The flow up valve type design nonetheless provides the identical management responsiveness and flow 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 extremely specialized pieces of engineered gear that have to be fastidiously specified, put in, and maintained. ไดอะแฟรม is therefore wise to be proactive by reaching out to trusted consultants prior to buy as they’ll help with valve sizing and specifications based on particular working situations.
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 fully inspected, it will be clever to consult your turbine bypass valve vendor for assist and help.
Many of those vendors provide full inspection and health check services to determine areas the place metallic fatigue and stress cracking are growing. Ideally the valve could be repaired, but when a valve replacement is warranted, the vendor also can present steering in evaluating bypass valve design kinds and choosing the greatest option for the specific application.

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