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Turbine bypass valves are one of the most troublesome applications in an influence plant. Keeping these valves working efficiently and avoiding sudden failures is crucial for plant operation.
When engineers are requested to record probably the most difficult management valve purposes, turbine bypass valves are invariably mentioned. Frequent thermal cycles, high pressure drops, and the need for tight shutoff push these valves to the limit. Unfortunately, many plants are likely to ignore these valves till they fail, creating unplanned outages, misplaced production, and high prices for reactive maintenance. This article offers instructed strategies of inspection to anticipate and mitigate issues beforehand, and it presents improve options ought to a valve have to be repaired or replaced.
Strictly Severe Service
Severe service management valves are utilized in the most troublesome installations inside process crops. These installations generally embody cavitating, erosive, corrosive, noisy, excessive strain, excessive temperature, high strain drop, or high velocity media. Turbine bypass valves are uncovered to many of those process circumstances; but, they need to 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, as well as when a turbine journeys off-line (Figure 1). In regular operation the valves are completely closed, forcing all the steam by way of a turbine. During เกจวัดแรงดันsumo , 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 happens in reverse at shutdown. Using bypass valves at startup and shutdown helps to guard 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 facility plant design, a quantity 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 must proceed flowing to avoid equipment injury due to overpressure and high temperature, so the turbine bypass valve immediately opens to take care of 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 will drop the strain, but the exit steam will stay fairly superheated, doubtlessly destroying downstream tools. To keep away from that scenario, turbine bypass valves either incorporate a water injection system in the valve physique, or make use of a separate water injection desuperheater simply downstream, in both case to lower the exit steam temperature.
As a outcome, turbine bypass valves face an ideal storm of severe service conditions. While the plant is in operation, these valves must stay tightly closed to avoid wasting energy. When a turbine trip occurs, the bypass valves must reply immediately, exposing them to speedy temperature modifications and requiring them to cross very excessive flows at high stress drops, creating excessive noise and potentially excessive vibration.
Getting Ahead of the Game
Given the punishing service, the truth is that virtually every turbine bypass valve will finally fail ultimately. Unfortunately, many of those valves are put in in difficult-to-access areas, are typically welded in place, and are typically heavily insulated. As a outcome, they are often ignored till problems start to surface. Steam leakage by way of the valve is usually the first symptom noticed, however rather more vital and potentially harmful injury can happen.
The excessive strain drops invariably generate high noise and high vibration. Over time these vibrations, coupled with frequent temperature modifications, fatigue the metal within the valve, water connections, and the piping itself (Figure 2). Such metallic fatigue can outcome in catastrophic failure under stress.
2. Over time, the excessive vibrations and thermal shock experienced by turbine bypass valves will fatigue the metal and create cracks in each the valve itself and the encircling piping. Courtesy: Emerson
To keep away from this downside, 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 metal fatigue issues that might be developing but are not but visually obvious. If a plant lacks the information or equipment to perform these inspections, the valve vendor or licensed representative could also be utilized to perform turbine bypass health check providers (Figure 3).
three. A routine turbine bypass valve health examine 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 provide piping (circled areas). Courtesy: Emerson
Fabrication welds on the body and water manifold, buyer connection welds, diffuser welds, and surrounding piping may be inspected to establish any creating problems earlier than the equipment is compromised. Water injection nozzles and desuperheaters must be appropriately inspected and maintained to avoid issues related to quenching and cracking.
There are also threats to trim components, similar to seat floor erosion. One widespread expression of such erosion, specifically on the plug, is also recognized as “gear toothing.” This occurs extra generally in flow down valves, the place the steam accelerates via the cage holes and then strikes the trim/seat area directly, inflicting excessive wear and reducing service life. Over time, this high-velocity steam, which might include water during startup conditions and magnetite throughout any condition, will erode and injury the valve seat and trim (Figure 4).
4. Flow down valve body designs are prone to trim harm as the wet, erosive steam passes via the cage and impacts the seat. Courtesy: Emerson
Common maintenance gadgets like delicate items and spray nozzles can usually be replaced relatively easily and at minimal price. Trim parts that have to be changed due to heavy put on, such as gear toothing, can be quite expensive, particularly if their supply must be expedited. The worst-case state of affairs for a turbine bypass valve is steel fatigue growing in the valve body or diffuser, with extreme restore procedures, or even a complete valve alternative, required.
Upgrade Opportunities
Many of the turbine bypass valves currently in service were installed through the heyday of mixed cycle plant building from 1998 to 2004. Most of these valves and piping techniques are showing their age and infrequently have important signs of steel fatigue. Others have loads of life left in them however may benefit from upgrades in technology. Technology and practices have improved, and upgrading to the most recent sealing know-how 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 occasion, a detachable two-in-one seat diffuser will cut back required upkeep.
If a turbine bypass valve must be replaced, plant personnel are strongly inspired to look past a direct replacement and evaluate potential improvements. A notably important merchandise to consider when taking a look at substitute and enchancment alternatives is valve orientation.
Downward Flowing Valves with Horizontal Actuators. Historically, most turbine bypass valves employed a move down trim design paired with a horizontal actuator (Figure 5, left). This association is ideal for places with low overhead clearance, and it locations the actuator nearer to the deck or floor for straightforward entry, but it creates a quantity of long-term operational problems. Gravity tends to result in increased and uneven put on on trim elements, and the side-mounted actuator is prone to response issues.
5. Downward flowing valves with horizontal actuators (left) had been the main possibility a few a long time ago for turbine bypass valves. New flow up valve designs (right) could additionally be put in without piping modifications in most situations, providing extended service life and improved performance. Courtesy: Emerson
Downward Flowing Valves with Vertical Actuators. A superior option for circulate down is vertical mounting of the actuator. This association leads to lowered and more even wear, longer runs between upkeep outages, and improved actuator response. The challenge with this orientation is that substitute alternatives are sometimes for horizontally mounted actuators, and the two designs aren’t readily interchangeable without vital piping modifications.
Upward Flowing Turbine Bypass Valves. When enough overhead area is on the market with current horizontal actuator circulate down designs, a circulate up design can easily meet the present face-to-face necessities of the old valve, while lowering a number of the wear frequent to circulate down designs with a horizontal actuator, such as gear toothing. Because the trim just switches from flow down to circulate up, this modification could be made without modifications to current piping.
Flow up valves (Figure 5, right) considerably lengthen seat and trim service life as a outcome of the steam is transferring comparatively slowly because it passes over the plug and seat (Figure 6). As the steam moves via the small holes of the cage, it accelerates considerably, however that power dissipates into the big body cavity of the valve instead of striking the seating floor.
6. Downward flowing valves subject the seat to erosive high-velocity steam. Upward flowing steam velocities are much decrease because they cross over the seat and accelerate later as the steam moves by way of the small trim holes, significantly extending trim service life. Courtesy: Emerson
Notice how the circulate up trim (Figure 7) is actually displaying erosion, however when compared to the move down trim shown above (Figure 4 right), the seating floor on the seat ring and plug stay comparatively undamaged, permitting the valve to close off fully, the required mode in normal operation.
7. These photos of circulate up trim clearly show the erosive impact of high-velocity steam, however not like the move down trim harm shown beforehand, this valve has no damage to the seating floor and can nonetheless shut off tightly. Courtesy: Emerson
The move up valve fashion design still offers the same management responsiveness and circulate capability, however the model new flow association dramatically extends the working lifetime of the seals, seat, and valve internals.
Consult an Expert
Turbine bypass valves are extremely specialized items of engineered gear that have to be rigorously specified, put in, and maintained. It is subsequently sensible to be proactive by reaching out to trusted consultants prior to purchase as they can help with valve sizing and specs primarily based on specific operating situations.
For current installations where unplanned turbine bypass valve failures are bedeviling your facility, or if it has been a while since the plant turbine bypass valves have been absolutely inspected, it will be clever to seek the assistance of your turbine bypass valve vendor for assist and help.
Many of these distributors supply full inspection and health verify companies to establish areas where metal fatigue and stress cracking are growing. Ideally the valve may be repaired, but when a valve replacement is warranted, the vendor can also present steering in evaluating bypass valve design styles and selecting the best choice for the specific software.

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