1 Indtoduce
Desuperheaters—often called attemperators—in heat-recovery steam generators (HRSGs) are located between the primary and secondary superheaters and reheaters, and sometimes after the final stage of superheating. They are responsible for controlling steam temperature in accordance with start-up and steam-turbine-inlet requirements. The attemperators also prevent thermal damage to superheater and reheater tubes, and to outlet steam piping and downstream equipment.
The attemperator operates by injecting condensate into high-temperature steam to precisely match the steam temperatures required by downstream user. This temperature-matching function is most important during system start-up and shutdown to prevent large temperature gradients in the HRSG or steam turbine steam supply. When the attemperator fails to temper the steam correctly, even a single large overspray excursion can damage steam turbine internals; causing costly tube leaks and significantly reduce the steam turbine efficiency. Wet steam can also quench regions of the steam pipes, causing additional long-term problems. Heavy attemperator spraying during start-up can, over time, initiate cracking in HRSG tube joints or even distort the shape of tube banks. Prolonged operation outside the design-operating envelope will produce accelerated fatigue damage to the pressure parts.
Attemperators are used in power boilers to control (reduce) the temperature of superheated steam to suit the requirements of downstream equipment – frequently a steam turbine. Desuperheaters are a specific type of attemperator that is used to control downstream steam temperature at a point that is very close to saturation temperature. In process plant applications, attemperators are usually incorporated in letdown stations that alter superheated steam conditions to suit process needs. Attemperators can spray close to saturation under certain operating modes but can also be closed-off completely during other modes of operations; called run-dry conditions.
Thermal oscillation is applicable to all attemperators throughout industry and for the 25 attemperators at ESEC; with varying design configurations and risk levels.
ESEC each unit has 6 desuperheater, steam turbine has 3, plant has 2, the major one is HP steam, Superheater steam desuperheater, HP, IP, LP bypass desuperheater, HP to cooling steam, HP, IP to auxiliary steam
For MHI 501G steam cooling gas turbine, refire fig 1 installed HP steam to cooling steam line as bypass to supplementally make up steam to cooling steam if the gas turbine output down which cause IP steam output decrease.
For the plant Auxiliary steam system, also use HP steam provide the back up steam source which use attemperator to control the steam temperature to around 240-degree C.
2, Operation challenge
On March 26, 2019, water was observed dripping from piping insulation in the area of the High Pressure (HP) cooling steam attemperator; downstream of pressure control valve (SHP-PV-02006). On March 27, 2019, the leak rapidly progressed to a steam leak. At this point, the insulation was removed, and the steam leak was confirmed in the area identified. Site Management and Chief Inspector were notified, and plans were initiated to facilitate repair.
On Saturday March 30, 2019, Unit 2 was shut-down, the leak was isolated and the attemperator inspected internally both upstream and downstream from a removed spray nozzle. A significant linear indication/crack was observed on the bottom of the shell directly below the diffuser. Another linear indication/crack was observed at the welding toe attaching the end plate to the diffuser and another area showed a linear indication propagating from the circumferential indication to an adjacent diffuser hole
3, Process introduce
Each HRSG incorporates high pressure steam supply as backup supply for the CT cooling, and an attemperator for the temperature control of this steam. HP boiler feedwater is used for attemperator spray water. The spray flow is controlled by an air operated control valve (1/2BFW-TV-00091) on the HP feedwater supply line.
HP backup supply to CT cooling steam warming line temperature control is provided by controlling the attemperator outlet temperature. Warming line temperature control is proportional/integral action with an auto/manual station. The loop is direct-acting where the spray valve starts to open when warming line temperature rises above setpoint. The analog output to the valve is characterized as described in Setpoints and Functions.
The HP steam header for each of the two units serves as backup steam source for the turbine cooling steam supply system. The HP steam source pressure is reduced with a modulating pressure control valve (1/2SHP-PV-02006). Its temperature is reduced by de-superheating spray control valve (1/2BFW-TV-02008). Cooling steam is supplied and delivered on a “per unit” basis (there is no common supply piping or equipment). Combustion turbine cooling steam primary supply is from the IP steam system, with startup supply coming from the auxiliary boiler.
4 Control Valve test
Operations and Engineering conducted testing on Unit #2 pressure control valve (SHP-PV-02006). The low limit of the valve was slowly increased while the bypass needle valve was closed at base-load. HP cooling steam flow, cooling steam temperature, cooling steam pressure, IP decay valve position, HP cooling steam attemperator water flow, combustor differential pressure and cooling steam outlet were closely monitored. As PV-02006 was opened, the cooling steam increased in temperature until it reached 290°C and TCV-2008 started to open. The HP cooling steam stabilized at 290°C (about 30% warmer than the bypass line). When HP cooling steam combined with 64 T/hr of IP cooling steam (at @302°C), the overall cooling steam temperature increased by 1°C resulting in the cooling steam outlet temperature increasing from 473.5° C to 474.8° C (50+ degrees from runback).
While opening SHP-PV-02006, the cooling steam was noted to remain constant as the decay valve opened from 41% to 42%. This sends some of the steam back to the cold reheat, bypassing the cooling steam system. It is not uncommon to have the valve move 2% during normal operation so this may not be related to the changes of SHP-PV-02006. After the system had time to stabilize, PCV-2006 had a demand signal of 11.25% (feedback indicated 10.1%) to have a flow of ~3 T/Hr and TCV-2008 was 57% open with an attemperator water flow of 3.5 T/Hr, controlling to 290°C.
The bypass line has been removed from service with the needle valve closed, attemperator manual isolation valves closed, the bypass desuperheater (TV-00091) valve closed and desuperheater isolation valve (TV-00092) closed.
The new low limit for the pressure control valve (SHP-PV-02006) from 120-400 MW has been set to 11.25%. Based on testing and responses from OEM, the SHP-PV-02006 valve was left about 11% open for a minimum valve position. This has reduced the cycling of metal temperatures but has not completely eliminated it.
Testing was conducted on SHP-PV-02006; The valve was opened slowly to 11.25% demand as the bypass line was closed. The backup HP cooling steam stabilized around 3 T/Hr at full load + ducts
which is the same flow as what was previously through the bypass line only. The bypass desuperheater valve (TV-00091), block valve (TV-00092), and manual isolation valve were all closed.
During the transition from bypass to pressure control valve (SHP-PV-02006), temperatures and pressures into the cooling steam were closely monitored. Pressure was stable and the IP Pressure Decay Valve 2SIPPV00231 moved from 41% to 42%. Temperatures throughout the cooling steam climbed during the transition but stabilized at 1 °C higher than the initial value (298 to 299 ° C). The temperature control valve TV-2008 opened to 55-60% to maintain the backup HP cooling steam temperature at set point ~290. Curve M-L314 has been adjusted to reflect the new low limit for pressure control valve (SHP-PV-02006) of 11.25 between 120-400 MW.
5.0 Determination of Cause
The ESEC unit#2 attemperator/desuperheater was internally inspected for both upstream and downstream after the removal of spray nozzle after the detection of leak from the shell. A significant linear indication was observed on the bottom of the shell directly below the diffuser. The linear indication and the remainder of the immediate area showed visible signs of clustered linear indications that have the appearance of thermal fatigue. The 4” branch line entering the main desuperheater line opposite the spray nozzle approx. 18” downstream shows the bore was assessed to be in good shape as well as the remainder of the line up to the first elbow. There is one area approx. 8” up to the line that appears to have been a butt weld and its root was ground flush at the time of fabrication. The spray nozzle alignment opposite the spray nozzle flange appears to be in good condition
Cracking in the Desuperheater Pipe and the Expander/PR Tube occurred due thermal fatigue. The main factor contributing to the failure is the occurrence of significant temperature fluctuations that produce high cyclic stresses within the materials. Thermal fatigue is a mechanism involving the formation of cracks under conditions of cyclic stress caused by fluctuating temperatures. Damage typically occurs at positions where thermally induced contraction of a component is constrained. This can happen when the surface of a hot component is rapidly cooled, and the shrinkage of the cooler surface is constrained by the hot underlying material. Repeated thermal cycling then results in the formation and growth of cracks over time. During thermal fatigue, typically the cracks form on the surface of components, and can appear as single or multiple cracks, and are typically oxide filled due to exposure to elevated temperatures. Factors that contribute to the formation of the crack include the magnitude and frequency of the temperature fluctuation, cracking occurring more rapidly as the magnitude of the stress variation increases and the number of cycles increase. Thermal fatigue is best prevented by limiting thermal cycling and controlling rates of heating and cooling, avoiding rigid attachments (a position where thermal fatigue cracking commonly occurs), and incorporating design that avoid the pressure of stress concentrators.
Figure 4:Scale-filled multi-directional cracks- as found- under Microscope
Throughout the investigation, the causes and contributing factors that led into the failure of the attemperator were summarized as follows:
Cause #1: Poor attemperator/desuperheater design that allowed rapid temperature oscillation and thermal quenching. Biggest contributor to the huge temperature oscillations was the open and close nature of the valve; which allowed steam flow to be suddenly shut off or placed in service. Attemperator spray water was hitting the upstream surface of the attemperator; particularly under no/low steam flow conditions prevalent when PV-02006 would close. While Valve PV-02006 was cycling based on plant demand, rapid temperature oscillation was produced and resulted in thermal quenching on the piping wall (P22). During the investigation, it was also determined that the risk associated with poor attemperator design in combined cycle power plants was not known to plant operations and/or program owners.
CF #1: Excess Boiler Feed Water (BFW) flow due to excess bypass water from TV-0091 that may have resulted from passing AV-00092 seat protection, inadequate fixed set point (at/near saturation temperature) and/or worn out TV-00091 valve.
CF #2: Excess water from TV-02008 due to incorrect 01-BFW-AV-02009 valve type, seized spray nozzle causing overspray and/or worn out valve TV-02008 (due to over cycling).
Unit 2 HP to SCS attemperator was notably re-assembled without a pressure reducing (PR) tube as the existing PR tube had extensive cracking and a new one had a 17-week delivery time. On the outlet of pressure control valve (SHP-PV-02006), desuperheater 2SHP-DSH-001 was therefore put into service on April 10, 2019 without a PR tube (refer to MOC21247297). The; impact of this action is believed to include:
- Reduced back pressure on pressure control valve 2SHP-PV-02006 will result in the valve providing the same Cv at a lower position; with maximum Cv increasing from 80 to 86. The lowest modeled operating case changed from 27% to 24% valve stroke.
- Noise is expected to increase during operation from a max of 77.9 dBA to 84 dBA based on CCI model. (no noticeable change in noise noted from plant).
- Altered flow pattern and impact to spray water atomization. This impact cannot be accurately quantified by CCI without creating a specific CFD model. However, the recommended downstream straight pipe distance (15 feet) and distance to temperature sensor (36 feet) remain unaffected by the removal of the PR tube.
- Without the PR tube acting as a barrier, there is potential for spray water to reach the grade 91 valve SHP-PV-02006. This is not an intended function of the PR tube, however, based on preliminary RCA discussions (see PI&R 1061310), it is possible that the PR tube prevented damage from extending to the valve body upstream when valve PV-02006 was closed (which is no longer the case). Through the corrective actions of this RCA it is expected that the quenching / thermal fatigue damage mechanisms will be greatly reduced, controlled and closely monitored at this attemperator/desuperheater.
Following the implementation of preventative actions from the RCA, the system operation will be monitored for a period of time, including data from surface-mounted thermocouples. Smart signal monitoring alert was established for any deviation of valve position, flow and temperature parameters. There will be different priority levels for alert depending on severity of issue. This smart signal monitoring will be established for HP and IP attemperators to allow the plant to better predict any upcoming similar events and implement mitigation plans to mitigate the risk consequences.
5 Solution
5.1 HP to cooling steam
- CT1/2 HP to cooling steam warm line temperature control valve (1/2BFWTV00091) change to smaller valve, it is due to at the normal operation and CT start up and shutdown, the temperature control valve maximum open is less than 10% , the valve size it too big, which is not good for accuracy control the attemperator water flow and and control steam temperature, request to Engineering to consider to change a smaller size valve. This is around the bypass line attemperator spray being too high when the valve just comes off its seat. There will be some checking of the flow transmitter that will come into play regarding this initiative. After size decrease, it will provide better flow control, and minimize overshooting
- set up the minimum open for the valve 2006
The minimum valve position for GT1 & 2 has been set at ~11%. This minimum position only becomes active once the HP warming sequence has been completed. This allows the valve to function normally for Startup.
The new setting allows for constant flow through the HP-PV-2006, reducing the severity of thermal cycles/quenching on the desuperheater. Incorporate into logic to keep HP-PV-02006 on each unit at least (roughly) 11% open when the CT is online and above 80MW. This will ensure we do not face quench damage downstream of the valve.
- use the thermocouple for to monitory the temperature change
- OEM provide new valve type to over come the low turndown
HP to CT cooling steam desuperheaters have show signs of inadequate atomization/mixing Inadequate mixing is evident in process data (enthalpy balance), surface-mounted thermocouples, condensate collecting in downstream drip leg, and OEM performance calculations.
Dependent on the operating conditions, the existing nozzles may be susceptible to fouling from debris and/or oxidation (i.e. “baking”) at elevated temperatures. Once fouled/seized, the attemperators will experience insufficient spray patterns resulting in potential quenching damage.
- Replace HP to CT cooling steam desuperheaters 1/2-SHP-DSH-001 with venturi design
Upgrading the desuperheater will improve control of HP backup cooling steam, mitigate thermal quenching damage and reduce impacts to plant reliability. The proposed upgrades will also simplify inventory spares, reducing the number of different spring-loaded CCI nozzle types from 6 to 3.
- Upgrade valve trim for spray water control valves 1/2-BFW-TV-02008
- Decommissioning the bypass/warming desuperheater 1/2-SHP-DSH-002
- HP to auxiliary steam
Investigate desuperheater control options to improve stability and longevity of HP to Aux steam attemperator Investigate desuperheater control options to improve stability and longevity of HP to Aux steam attemperator Ideas such as were implemented on 2006 for minimum position (which would translate to minimum aux steam demand of just the air ejectors’ useage), PLUS potential functionality such as spray valve interlock to ensure martyr closes before root AND sprays close if demand on 21019B is <X% (eg. 1%)
- Install a isolation valve at header which avoid the plant shut down for the HP to auxiliary steam system
6, Attemperator Design requirements
The service requirements for interstage desuperheating are extremely demanding. As the HRSG cycles, attemperator hardware can remain for extended periods at elevated temperatures without spray water flowing through it. Adding insult to injury, the hardware then is quenched instantaneously by relatively cool spray water.
Attemperator designs with flow-control elements in the steam path—the multinozzle probe style shown in Figure 2, for example—are particularly susceptible to such damage. Cycling causes fatigue and thermal cracks in critical components, including the nozzle holder, individual nozzles, the lower body, and piston rings. Vulnerable to damage. Multinozzle, probe-type attemperators generally are not recommended when the difference in temperature between the steam and spray water exceeds 450F.
Multinozzle designs also are prone to internal flashing, which can occur when the flow of spraywater is extremely low and the water is allowed to heat up to saturation temperature before exiting the nozzles. Flashing fosters erosion of nozzles and the nozzle holder. Galling of piston rings and related components also is a possibility when temperature swings are large. This design also has the control element within the hot steam, making it subject to wide temperature variations.
Probe-style attemperators of any type are prone to vibration created by vortex shedding and the high-velocity head (kinetic energy) of the steam passing the probe assembly. The vibration induced by the vortices, in combination with the high temperature, can cause cracking of the weld joint between the probe’s mounting flange and its lower body. Thermal cycling can initiate cracking of the seal welds connecting the lower probe body with the nozzle head, thereby loosening the nozzle head and changing spray-angle orientation.
The turndown required for attemperation is quite high and often underestimated. Note that a 20:1 turndown in attemperation water flow does not necessarily equate to a 20:1 turndown in capacity for the flow-control element, or Cv. The spraywater control-element turndown requirement is influenced by variations in supply water pressure, steam pressure, and nozzle backpressure, which in combined-cycle power plants can be extreme. The last varies with flow demand.
In some cases, the difference between supply-water pressure and interstage steam pressure (Dp) at low flow is much higher than at high steam flow. In other cases, constant-speed boiler feedpumps provide spray water at relatively constant pressure, but interstage steam pressure slides during start-up—particularly when multiple HRSGs serve one steam turbine. For both situations, the variation in differential pressure across the operating range may require a spraywater flow-control element with extremely high turndown capability.
The turndown requirement of the spray-water flow-control element also is influenced by variations in pressure drop across the attemperator’s spray nozzles. This influence is much less pronounced in attemperators with spring-loaded nozzles than in those with fixed-area nozzles.
In addition to providing high turndown, the spray-water control element may experience high Dp at low flow and a low Dp at high flow and, therefore, must be able to handle these conditions.
Repeatable, tight shut-off of the control element is necessary. This calls for a high plug-to-seat thrust. Specify a trim exit velocity of less than 100 ft/sec to prevent cavitation and erosion damage. In short, users should look for a control valve that offers equal-percentage-characterized trim to maximize resolution during low Cv requirements, thereby ensuring tight control of spraywater and of outlet temperature.
Of course, proper installation is important to the success of every attemperator. Here are three cardinal rules to remember:
- Provide a straight run of pipe upstream of the attemperator of no less than three diameters. Installation of a liner in the inlet piping is recommended to ensure uniform geometry of the steam flow at the point of spraywater injection.
- Provide a straight run of pipe downstream of the attemperator. Insufficient distance between the attemperator and the first downstream elbow can cause the agglomeration of water droplets along the elbow wall, a phenomenon conducive to water fallout, thermal shock, inaccurate feedback from instrumentation to the flow-control element, and erosion.
- Install the temperature sensor downstream of the attemperator at a point where all the spraywater has been evaporated to avoid false readings and inaccurate feedback to the flow-control element.
Figure 6 indicates the recommended installation distances when attemperator spray—in conjunction with appropriate liner—is perpendicular to steam flow and nozzles are circumferentially mounted.
Temperature Measure carefully. The recommended installation distance is quite specific when the attemperator spray is perpendicular to steam flow.
7 Conclusion
ESEC overcome a seriously challenge regards the desuperheater, upon the OEM help to redesign the attemperator and operation successfully for new design desuperheater.