Upgrade of Coal Fired Plant Startup Valves

Transcription

1 Upgrade of Coal Fired Plant Startup Valves... By Stan Miller, CCI; Curtis Sterud, Consultant; Herb Miller, Consultant Electric Power Conference and Exhibition May 2, 2007; Rosemont, Illinois Avenida Empresa Rancho Santa Margarita, CA Fax ccivalve.com / CCI DRAG is a registered trademark of CCI.

2 Upgrade of Coal Fired Plant Startup Valves By Stan Miller, CCI USA; Curtis Sterud, Consultant, USA; Herb Miller, Consultant, USA; Electric Power Conference and Exjibition, May 2, 2007; Rosemont, Illionis Abstract This paper focuses on the benefits of upgrades to the startup system valves. The upgrades described allow quick load changes from minimum boiler flow to 100 percent load. Thus the system can be base loaded but still be responsive to requests for load changes from the dispatcher without imposing undue temperature transients to the turbine. By upgrading the superheater bypass valves the system can be brought up to pressure and temperature smoothly with valve designs that can handle the wide variation in fluid density during the startup process. These valves are designed to throttle the flow with enough resolution to account for the sensitive boiler pressure when the fluids is cool to the escalation of pressure when steam is being produced for heat up of the superheaters and turbine. Resolution is achieved by a unique characterization of the multi-stage multi-path valve trim. Upgrading the stop and control valves ahead of the finishing superheater to designs that provide good control during high pressure drop conditions permits load changes required during startup and higher load to be optimized. These valves allow the turbine to be operated at constant temperature, which in turn permits quick load changes in response to grid demand. The steam turbine valves are held at a constant position and the control valve absorbs the pressure changes. Operation in this manner minimizes transient impact on the boiler as well as the turbine and allows other boiler systems to adapt to the load change compatible with their response capabilities. Actuators can be all pneumatic, which is most cost effective and designs exist that meet all of the performance requirements. Introduction In this paper we are discussing changes to two different sets of valves in the systems associated with sub and supercritical coal fired once through boiler designs. The term sliding pressure will be used in the paper however its meaning is restricted to covering the existing load range of once thru operation for the boiler. Operating at loads below the normal range would require many more changes to the fluid cooling loop than the minimum simplifications than are discussed in this paper. The applicability of the discussion is directed to changes from the original boiler fluid loops and valve configurations. The older plants now need to be upgraded to extend their life and to continue to supply their output. This can be done with fairly minor changes and not only achieve increased life but have good reliability and lower operational costs. The following discussion covers changes to the valves before the secondary superheaters that impact load changes over the existing once-thru operating range and the valves around the primary superheater used for initial startup and shutdown prior to reaching the once-thru operation. It is changes in these valves that can be made to increase plant reliability resulting in minimal thermal transients on the turbine (and other components) while allowing response to quick load changes. Faster startup and shutdown times also result from the better control valves in these two critical locations of the boiler circuit. None of the changes discussed are experimental as they have been successfully implemented either in whole or in part on existing units, References 1-3. Valve operating conditions are well known and the valve duty and special needs clearly understood. The feedback on valve changes has indicated dramatic improvements in system operation with the ability to respond to load change demand effectively while minimizing the wear and tear on the turbine and other boiler components. The Boiler circuits With an older coal fired boiler one of the more onerous tasks is to have to be continually changing load. Although this is not impossible it is difficult and likely involves a number of unique steps by the operators to avoid undue transients on boiler operations while trying to minimize temperature swings on the turbine. Once the boiler turbine is up and running at full load the electrical output is dependent on the constant discharge steam pressure entering the turbine. The turbine is equipped with several valves, known as the turbine throttle valves, which regulate the turbine inlet pressure. As load decreases, the valves may close to reduce turbine inlet pressure, all valves may move closed equal amounts in unison (Full Arc Admission) or they may close sequentially (Partial Arc Admission). This is known as constant pressure control operation. Constant pressure control has two adverse effects when large load changes occur. First the turbine will experience temperature fluctuations that will create fatigue and increase maintenance and reduce life. The second effect is that the net thermal efficiency of the turbine drops as the load is reduced. 2 Upgrade of Coal Fired Plant Startup Valves CCI. All rights reserved.

3 Sliding pressure operation is designed to mitigate the negative influences of constant pressure operation. In the sliding pressure operation the pressure to the turbine is allowed to reduce while the steam temperature to the turbine is held constant. As the load is reduced, the pressure to the turbine is maintained by control valves located either between the superheaters as shown in Figure 1a (B&W 200 and 201 valves) or before the superheaters as shown in Figure 2b (CE BT and BTB valves and Foster Wheeler W and Y valves). The turbine throttle valves are held at a near full open position. The steam temperature is adjusted at the superheaters by attemperation control so that the temperature to the turbine is held constant at all loads. The original steam boiler designs have valves in the locations noted by Figure 1a and/or 1b. However, in the original units these valves have been selected primarily to achieve good shut off during boiler hydrostatic pressure testing and at the low loads on the steam separator (flash tank). In most cases the large capacity block/stop valves with a small control valve in parallel are jogged to assist in getting the unit up to full load. The small control valves do not have the capacity and are not equipped to provide good control for the high pressure drop conditions over the full once through operation range. The once through boiler designs have valves either before the superheaters or between the primary and secondary superheater that bypass fluid around the secondary superheater to a separator tank. This is necessary for system control while raising the fluid temperature during a cold start and then the escalation of load up to the minimum before once thru operation. The hot high pressure water in the early stages of heat up must be letdown to a separator (flash tank) to produce steam for controlled heating of the superheaters and turbine. The control valves for this service see a unique set of fluid conditions that were not fully accounted for in the original designs. For an extended period in the heat up the fluid density and temperature are changing without a change in flow rate. This occurs when the pressure is held constant in the furnace walls and heat is added in order to produce steam in the separators to raise the superheater and turbine temperature. Because the valves cannot control for small change in fluid properties there is continuous oscillations in pressure both up and down stream of the valves. In some cases these pressure swings are excessive and can only be controlled by putting the valves into manual control. The net result is a very uneven heat up and load escalation that over time results in fatigue and frequent maintenance of the equipment. The labels for these start up valves are the 202 & 207, BE, and W & P for the Babcock & Wilcox, Combustion Engineering and Foster Wheeler designs, respectively. Typical flow loop schematics and valves for these three systems are shown in Figures 2 through 4. In the Combustion systems the circuit to the separator is initiated between the furnace wall circuit and the primary superheater. For the Babcock & Wilcox and Foster Wheeler systems the bypass to the separator occurs between the superheaters. The Babcock & Wilcox system also has valves (202 s) between the furnace wall circuit and the primary superheater (for boiler pressure control during startup) 2007 CCI. All rights reserved. 890 Upgrade of Coal Fired Plant Startup Valves 3

4 Figure 2, Babcock & Wilcox Boiler flow loop schematic. The current valves The original control valves in all of the locations discussed were designed as ruggedly as possible at the time. The valves needed to have some control for the high pressure drop conditions and have minimum leakage through the valves during operation as well as for hydrostatic pressure testing. Block valve designs were the main high capacity conduits with as small a control valve as possible in parallel to provide what was felt to be sufficient control. With small control valves leakage would be minimized. Actuation options included pneumatic, electric and hydraulic. Figure 3, Combustion Engineering flow loop schematic. 4 Upgrade of Coal Fired Plant Startup Valves CCI. All rights reserved.

5 Figure 4, Foster Wheeler flow loop schematic. Typical valves are shown in Figure 5. All are single stage pressure drop devices, which are not very effective in providing the flow control needed in these critical locations. Frequently sufficient control can only be achieved by either putting the control of these valves in manual or continually jogging the block valves as flow is increased in the start up or shut down sequence. With these single seated valves in high pressure drop applications the service life is limited and the violent flow through these valves cause vibration that damages actuators and results in poor control. The valves are very noisy. Velocities up to sonic levels lead to rapid seat and plug erosion with subsequent leakage when the valves are closed. Multiple valves of the Figure 5 valve type are used in the start up systems. Multiples range to as high as seven to ten valves in parallel on a single unit. This results in added complexity in the control systems, extensive maintenance on each shut down and many different leak paths when the valve trim is just slightly worn. Figures 6 and 7 show examples of the reduction in the number of valves. In Figure 6, nine valves were used in the original design and control is now achieved with 5. Figure 7, shows the combination of the 200 and 201 valves with an elimination of the 202 valves and flow circuit. Studies of just the impact of leakage in the original number and designs in these valve locations has indicated losses on the order of 3 to 5 percent of the unit output with absolute values as high as 35 Megawatts. Figure 5, Typical single stage start up valve designs CCI. All rights reserved. 890 Upgrade of Coal Fired Plant Startup Valves 5

6 Figure 6, Showing reduction of number of valves on a CE unit. The new circuits and valves The benefits of being able to control steam pressure with control valves upstream of the secondary superheater is achieved by using control valve designs that: n Provide block valve shut off performance n Can control the flow with high pressure drop These two changes result in fewer valves and increased reliability using proven experience in the applications. The use of fewer valves with higher capacity such as illustrated in Figure 6 and 7 have many benefits. The control system is greatly simplified just because there are fewer loops. The smaller number of valves also improves the control function because of fewer introductions or shutdowns of extra equipment into the system as load is changed. The number of potential leakage paths is also reduced in proportion to the valve quantity change. Using reliable designs also reduces the maintenance cost and time. There are likely many other benefits in reducing the number of loops that could range from more space on the control panel to just a cleaner plant with more space for maintenance. To achieve the control system benefits it is essential that the proper valve design is selected so that one is assured of good control and tight shut off when needed. A design that meets these criteria is illustrated in Figure 8. The generic description of the valve design is a pressurized seat with characterized tortuous path trims and cage design. Figure 8 shows the valve in three different positions; fully closed, internal pilot valve open and modulating. The flow direction is over the plug, which is also referred to as flow to close. 6 Upgrade of Coal Fired Plant Startup Valves CCI. All rights reserved.

7 When the stem is pulled up, View B, it opens the pilot valve inside the plug so that the high pressure above the plug can be relieved. The actuator can then lift the plug off the main seat and start the flow control function by modulating the plug. A spring, not shown, inside of the plug helps to keep the stem and plug separated during the modulating mode. A differential area between the maximum diameter of the plug, X dimension, and the top part of the plug, Y dimension, also provides a significant separation force between the stem and the plug. In View C, the plug is now being easily modulated by the actuator as pressure forces on the top and bottom of the plug are essentially balanced. Figure 7, Showing a reduction of the number of valves on a B&W unit. When the valve is closed, View A Figure 8, the inlet fluid leaks by the piston ring balance seal and pressurizes the volume above the plug. This pressure times the plug area results in a very large force holding the plug against the seat ring and assuring a tight shutoff. Frequently this force is a ton of load for every inch (25 mm) of seat ring circumference. The wedging angles of the seat amplify this force and a block valve seal is achieved. The actuator force adds to this pressure load and also causes a very tight seal between the pilot seat and the stem. Similar forces per unit length of seal circumference can be achieved on the pilot seat. The second feature of the Pressurized Seat control Valve is the trim design that is used. It consists of a characterized trim that is made up of multi-path multi-stage components that are selected to control the fluid velocity as the pressure is let down. Typical multi-path multi-stage disk and trim for these valves is shown in Figure 9. The right angles in the flow path cause the resistance to the flow and lower the fluid velocity. With lower velocities the fluid energy is reduced, as the square of the velocity, and this allows good flow control for the high pressure drop. A number of these disks are brazed together to form the valve trim and since each disk can be difference the design can be customized to fit the plant conditions of pressure drop versus flow. Capacity of the trim is achieved simply by adding enough disks to the stack to satisfy the flow needed CCI. All rights reserved. 890 Upgrade of Coal Fired Plant Startup Valves 7

8 point where boiler pressure is held constant while heat is added to the water. As the water is heated the density is changed in the fluid through the bypass valve. In order to maintain constant pressure the valve must move to adjust for this small flow rate change that is altered only by the density change. With a single stage valve trim the pressure swings can be extreme with changes as much as plus and minus 200 psi (1.4 MPa). To avoid this with a single stage valve the operator usually puts the valve in manual control. However with the unique characterization permissible with the multi-path multi-stage designs the valves continue the heat up transient in automatic control with small perturbations in the boiler pressure. Actual results for such a unique characterization of a disk stack are shown in Figure 10 in comparison to a linear characterization. The pressure swings are reduced by a factor of 8 to 10 with the characterization. The longer valve stroke resulting from adding of disks to achieve capacity has an additional benefit of providing better control because less fluid change results from the minimum change in plug position in comparison to a single stage cage design. The minimum change in plug position is driven by the actuator resolution and a longer stroke enhances the control function. Disk Stack Flow Linear Stack All Disks Have the Same Number of Passages and Turns, the Same Flow Area Flow is Directly Proportional to the Valve s s Stroke at Constant Differential Pressure Characterized Stack All Disks are Not the Same Provides Precise Variable Flow Versus Pressure Drop Over the Full Range of the Valve % Flow Modified Linear Linear 2 Turns 8 Turns 18 Turns Modified Equa % Stroke Figure 8, Pressurized Seat Design. With the unlimited ability to characterize the flow versus valve stroke a unique design to fit the application requirements can be achieved. The equal percentage form is the ideal characterization for the pressure control valve between the superheaters. A unique characterization is needed for the superheater bypass valves because at some point in the heat up the valves are stroked to a Figure 9, Typical Multi-path Multi-stage Disks and Trim. The trim characterization can include a single stage cage design with large flow windows on top of the disks. The cage would be used for the portion of operation when pressure drops are minimal and good control is easily achieved with a single stage design. 8 Upgrade of Coal Fired Plant Startup Valves CCI. All rights reserved.

9 With the valve designs described above control is achieved under the most severe pressure drop conditions. This allows the boiler to be started from a cold condition under automatic control and then when the load is sufficient and the superheater is no longer bypassed, control of constant turbine temperature. Both of these benefits have significant payback in that the boiler and turbine upset transients during the load changes are avoided. The result is minimum wear and tear on all of the equipment involved. With the pressurized seat design for the valves block valve performance is achieved with the control valves and output is not loss by leakage around the turbine. Figures 11 present typical valve cross sections for the superheater bypass and pressure control valve designs. Both designs are pressurized seat designs with multi-path multi-stage trim. The valve on the right, globe configuration, also includes a cage on top of the disks and is used in the Pressure Control valve location either before or between superheaters. Its function is to control flow through the superheater for once through operation at reduced loads. The left valve in Figure 11, has only the multipath multi-stage disk sets and is used in the superheater bypass application during the start up and shut down transients. For the startup and shutdown transients the superheater bypass pressure drop is always high and the extra fluid velocity control in the valve is needed for good flow control. Figure 11, Cross Sections of the Superheater Bypass and Pressure Control Valves. A Retrofit Solution. Figure 10, Multi-path Multi-stage Valve Trim Characterization. In some cases it may be possible to retrofit an existing valve in the field to achieve better control and allow sliding pressure control for a major part of the load range. Reference 4 discusses the retrofit of 500 valves that were problems in the field. The retrofits discussed in the reference replaced the original trim of a valve in the field with the multi-path multi-stage trim type shown in Figure 9. There are other methods of retrofitting trim to bring about a better control valve application. An example is shown in Figure 12, for the BTB valves in which the single stage design is converted to a multi-stage design. A Drilled Hole cage has been added upstream and downstream of the main valve orifice. The seat ring diameter also has been increased to maintain full capacity. For the BT valve the Stem is retrofit with an equal percentage trim, Figure 13, also with an increase in seat ring 2007 CCI. All rights reserved. 890 Upgrade of Coal Fired Plant Startup Valves 9

10 diameter. These changes allow the valves to provide good control for the high pressure drop conditions. Control is achieved with out the usual noise, vibration, and erosion associated with the single stage trim designs. unit efficiency. The temperature at the top of the packing box is low enough so that a long life PTFE material may be used instead of the more friable Graphite packing. This stem seal significantly reduces the friction and improves control valve resolution. It assures a long term seal for the high duty valves and is especially beneficial in the case of a steam supply unit that is frequently required to change load. Figure 12, Retrofit of BTB Single Stage Valve Trim. Figure 14, Valve Bonnet with Packing Free Stem Seal and Steam Leak Off Connections. Figure 13, Equal % Retrofit of BT valves. Stem Gland Improvement Frequently the stem packing in the valves for these applications have a short life. The life is shortened because of the very high pressure drop and the extensive modulation needed during the transient. A solution to this is to consider the use of a packing free stem penetration with leak off connections that productively use the blow by steam. This stem packing box is illustrated in Figure 14. The steam leak off connections are directed to heaters, gland seals and the Deaerator so that there is no loss in Actuator Improvements In the past all types of actuators have been used for these control valves. The Pressure Control valve frequently uses a hydraulic actuator because of the need for a high seat load and the stiffness needed for control under the high pressure drop conditions. For the single stage, unbalanced control valves the higher strength capabilities dictated hydraulic actuators. If preferred any type of actuator can be used for these valve applications. A piston pneumatic actuator is well suited for the pressurized seat designs used on the Superheater Bypass and Pressure 10 Upgrade of Coal Fired Plant Startup Valves CCI. All rights reserved.

11 Figure 15, Advance Pneumatic Actuator. Control valves. Boosters and lockup valves are likely needed to meet speed and failure conditions. The piston actuator can provide the capabilities that are needed due to the longer stroke requirements on the multi-path multi-stage designs and the high thrust dictated by the application. An advanced new piston actuator, Reference 5, that emulates the features of a hydraulic actuator, is available and preferred because of the higher reliability and lower maintenance of pneumatic designs. This actuator, illustrated on Figure 15, achieves the high speed needed without boosters using a spool valve with a capacity about 50 times larger than normal positioning systems. High seating loads are achieved in the piston actuator utilizing 100 psi (0.7 MPa) air instead of the much lower air pressure used in conventional pneumatic actuators. This actuator has the capability to achieve full stroke without overshoot in less than 2 seconds and provide up to 20,000 pounds (90 kn) of thrust. The standard unit uses a magneto restrictive device to obtain an absolute position feedback from the control valve. It is completely contained inside the actuator and has no moving parts or linkages. Resolution of the actuator can be less than 0.25 percent for low friction applications and is never more than 1 percent for the higher friction applications. Calibration of the actuator system is all automatic and done by the controller within seconds. All of the features of the advanced pneumatic actuator are well suited for the Superheater Bypass and Pressure Control valve applications. Boiler Performance. Figures 16 and 17 show the before and after load transients for the retrofitted valve trim of Figures 12 and 13. With the retrofit the initial BT valve is opened at 13 percent load and with a pressure drop of 2500 psi ( KPa) instead of 18 percent load and 1500 psi (10 MPa). Also the remaining BT valves are opened at higher loads so that the pressure control to the turbine is maintained up to 70 percent load with the turbine control valve held constant. This allows significantly more load range in which the temperature to the turbine can be held constant. As noted also there are significantly fewer oscillations on the BTB valve during the transfer and operation of the first BT valve because the BT valves are opened earlier and under much higher pressure drop conditions. The smooth ramp up of the turbine throttle pressure reduces the stresses on the boiler as well as the turbine CCI. All rights reserved. 890 Upgrade of Coal Fired Plant Startup Valves 11

12 Another example showing the pressure control from the cold conditions up to full load is shown on Figure 18. As shown in the plot the boiler pressure is brought up to 3600 psi (24.8 MPa) and held while the fluid temperature is raised to more than 500 F (260 C). The transfer to the Pressure Control valve from the Superheater bypass then occurs at less than 20 percent load and the Pressure Control Valve then maintains the turbine throttle pressure all the way to full load while the boiler pressure is escalated to 3900 psi (26.9 MPa). The pressure drop across the Pressure Control Valve at full load is approximately 40 psi (275 kpa) as the bulk of the steam flow is through the cage section of the trim. Figure 16, Transients Prior to Valve Retrofits. The Figure 17 BT valves could have controlled pressure to higher loads than 70%. The top value of this pressure ramp is dependent upon the owner s use and need for the boiler-turbine set. Conclusion Upgrading the Pressure Control valves between the superheaters and the Superheater Bypass valves provides significant advantages for the once through coal fired boiler designs. The general upgrade to these valves is to change them to pressurized seat designs and to incorporate a multi-path multi-stage trim design. These changes assure tight shut off when required and provide excellent turbine pressure control for load changes up to near or full load which ever is preferred. The changes also allow pneumatic actuation of all of these valves so that electric and hydraulic actuators can be eliminated if desired. The advanced pneumatic actuation systems provide reliability, good resolution and speed as well as reduced maintenances and complexity. This upgrade has been shown to pay back quickly with the increase in megawatt output with minimum to zero leakage. In some cases this gain has been as high as 35 megawatt. The grid demands for load change are easily accommodated because the valves can now control pressure to the turbine reliably and allow the boiler attemperation to provide a constant temperature to the turbine. This in turn reduces the temperature transients on both the boiler and the turbine resulting in lower maintenance and repair costs. New life is gained with the existing boiler turbine equipment keeping the older units productive and efficient. Figure 17, Transients After Valve Retrofits. Figure 18, Example of Pressure Control up to Full Load. 12 Upgrade of Coal Fired Plant Startup Valves CCI. All rights reserved.

13 References 1. Sterud, C. G. and Miller, H. L., Replacement Pressure Control and Superheater Bypass Valves Permit 93 % Cyclic Load Cutback at PG&E Moss Landing, American Power Conference, Chicago, April Brailey Jr., Edwin J., Miller H. L. and Sterud, C. G., Control Valves Limit Turbine Temperature Swings, Power Engineering, April Miller, H. L., Heavy Duty Control Valves, 20 th Japan Electric Measuring Instruments Manufacturing Association International Exhibition, Tokyo, October 18-21, Miller, H. L., Stratton, L. R., and Hollerbach, M. A., Fluid Jet Energy Criterion Eliminates Control Valve Problems, Valve Magazine Spring 2006, Vol. 18, No. 2, Valve Manufacturers Association of America, Washington D. C., April Miller, S. F., Electronic Valve Controller Replaces Conventional Pneumatic Systems, Instrumentation, Systems, and Automation Society (ISA) Houston, October 1-7, CCI. All rights reserved. 890 Upgrade of Coal Fired Plant Startup Valves 13