Valves PART 1:
Recirculation control valve replacement cuts maintenance costs
Replacement of boiler feed pump recirculation control valves solves a maintenance problem on Baldwin Unit 3
By Mark E. Liefer, Illinois Power Co., and Herbert L. Miller and Robert E. Katz, Control Components Inc.
All three of Illinois Power`s Baldwin power plant units had the same boiler feed pump (BFP) valve design when they went into commercial operation. The original BFP recirculation valves were a top-guided plug, single-port cage design using a tapered plug and seat arrangement. This design failed in Unit 1 during the first year of operation, so a redesigned set of internals was installed in 1973. The new internals used a drilled-hole cage and were designed to resist cavitation and erosion. This same design was initially installed in the Unit 2 and Unit 3 valves.
The BFP recirculation control valves were 90-degree angle body valves with pneumatic actuators mounted below the valve bodies. The actuators were designed to allow the valves to operate in fully open or fully closed (“open/close”) positions. The plug-and-cage design valves did not provide the pressure reduction needed to allow the recirculation flow to be discharged into the deaerator storage tank. Most of the pressure drop was absorbed in two 1 1/2-inch diameter capillary tubes that carried the flow between the recirculation valves and the deaerator. These 65 feet- (20 m) long capillary tubes acted as a fixed orifice to provide the required pressure drop. Erosion of the tubes was not a problem because they were fabricated from 1 1/2-inch diameter, 304 stainless steel XXS pipe. Taking the pressure drop through capillary tubes was much quieter than taking the full pressure drop through a plug-and-cage design valve.
During initial unit startup and throughout the years, cavitation and erosion of the valve internals resulted in leaks. The valve leakage frequently increased to the point that the unit power output was reduced. The BFPs could not generate enough flow to compensate for the leakage which was returning to the deaerator. Therefore, the valve required an overhaul; the stem, plug assembly, cage and seat ring were replaced due to the heavy erosion. Several different valve stem, plug and seat materials were tried with no significant improvement in repair costs or repair frequency.
These BFP recirculation valves required replacement of internals on at least an annual basis. Replacement parts used during a valve overhaul were expensive and so were labor costs. Scaffolding had to be built to access valve internals and working overhead to repair the inverted valves was awkward and tedious. In addition to the erosion, which always was present on the replaceable parts, significant valve body erosion also became a problem. The valve bodies were welded and machined several times before the valves were replaced with a different design.
Unit 3`s BFP recirculation valves were replaced first, because its drum design boiler had more low-load operation than the once-through units. As a result, the recirculation valves were overhauled more frequently and the valve bodies were in worse condition than those on Units 1 and 2. The staff chose to use a valve specifically designed with velocity control trim that had successfully replaced four valves in the startup/bypass system on the Unit 1 boiler.
The boiler feed pump manufacturer recommended increasing pump recirculation flow and, by designing the replacement valve to specific plant conditions, the volume of BFP recirculation flow could be increased. In addition, by using feedback positioners tied into the control logic for the boiler feed pump, the flow could be modulated, resulting in a smoother transition from recirculation flow to no recirculation flow.
New modulating BFP valves
The new globe-type 4-inch by 4-inch BFP recirculation valves (Figure 1) replaced the old “open/close” angle valves and the pressure-reducing capillary tubes. The top-entry valves are arranged with their actuators on top.
These new valves are an ANSI 2500 pressure class design. The normal flow rate is 945,000 pounds per hour at a design pressure differential of 3,735 psi (3,960 to 225) and a temperature of 350 F (120 kg/s at 25.8 MPa drop, 27.3 to 1.5, 175 C).
Multi-stage pressure reduction
The new multi-stage valve trim (Figure 2) incorporates a stack of tortuous-path disks, similar to Figure 3, whose built-in, right-angle turns permit multi-stage pressure reduction over the entire valve stroke. Each disk involves multiple flow paths with 24 pressure-drop stages in each flow path. Boiler feed pump discharge pressure is continually dissipated with a controlled velocity of less than 75 feet per second (22.9 m/s) by being forced through turns etched into the disks.
In addition, each disk in the stack incorporates a pressure-equalizing ring (PER) on its inside diameter to assure that equal pressures act radially around the circumference of the plug at any position in its 6-inch (152 mm) stroke. This design keeps the plug centered at all loads and prevents plug vibration. These valve features hold noise levels to below 85 dBA at three feet (0.91 m).
Stacks of tortuous-path disks produce a linear stroke vs. valve coefficient. Therefore, for this application, the flow rate is a linear function of valve plug position and control signal.
To ensure absolutely tight shut-off, these multi-stage BFP recirculation valves are provided with pressurized seating as shown in Figure 2. When the valve is closed, upstream pressure is applied above the main plug by way of a multi-stage bleed pin which controls flow to the bonnet. This provides a seating load equal to the inlet pressure times the full area of the main plug. Shut-off provided is in accordance with MSS-SP61 (block-valve leakage requirements).
When a signal to open the valve is received, the actuator lifts the stem, opens the pilot plug and allows the main plug to be hydraulically balanced with downstream pressure. There is a step on the main plug`s outside diameter that creates a differential area.
When upstream pressure acts on the main plug differential area, it provides an axial biasing force that causes it to remain on the main seat. As the stem continues to move in the opening direction, the pilot-plug shoulder engages the main-plug retaining ring to lift it off the main seat. This axial biasing force causes the pilot-plug shoulder and the main plug to remain in contact under all operating conditions. When a signal to close the valve is received, the actuator moves the stem in the closing direction. The biasing force on the plug causes it to move with the stem until the main seat is contacted. The stem continues downward to seat the pilot plug, restore upstream pressure above the entire main plug diameter, and assures a tight, leakproof seal. The new pneumatic-actuator control systems (Figure 4) operate on 70- to 100-psig (0.5 to 0.7 MPa) supply air and produce input/output signals of 4 to 20 milliamps. Upon power failure, the recirculation valves open fully in 10 seconds.
The original system design had flow transmitters on the discharge line of each boiler feed pump. The recirculation lines did not have a flow transmitter. The control system was set up to close the recirculation valve at 35 percent of rated flow, 800,000 pounds per hour (100 kg/s) during a pump startup. It would open the valve at 33 percent of rated flow, 750,000 lb/hr (95 kg/s) when the pump was being taken out of service. The flow through the recirculation valve was 750,000 pounds per hour (95 kg/s) at these conditions. The opening or closing of the recirculation valve had a significant effect on the feedwater flow going to the boiler. Operators had to anticipate this change and often put the feed pump controls in manual during this transition. The pump manufacturer recommended changing minimum flow through the boiler feed pump to 45 percent of rated flow 1,000,000 pounds per hour (126 kg/s). The new valves were designed to pass this flow in the fully open position. The valves have a linear flow characterization curve that allows the flow through the valve to be varied by changing the position of the stem and plug assembly. By using this ability to modulate flow, the energy loss in the boiler feed pump could be minimized. The modulating valves would look at the combination of pump recirculation flow going to the deaerator and pump flow which was going to the boiler. The valves would open or close to maintain a total flow of 1,000,000 pounds per hour (126 kg/s).
The new design boiler feed pump recirculation valves were installed during a four-week Unit 3 scheduled outage in the Spring of 1992. At that time, the valves were set up to operate in the “open/close” mode. During the scheduled outage in the Spring of 1994, a new control system was installed on Unit 3. The control system was set up to operate the valves in the modulating mode. This was accomplished by adding flow orifices and flow transmitters to the recirculation lines. The signal from these new transmitters was tied into the control system. Position feedback from the valves is used to position the recirculation valve stem and plug assembly so that the combination of recirculation flow and flow to the boiler equals 1,000,000 pounds per hour (126 kg/s). This has proved quite successful and has made the transition from recirculation to no flow nearly undetectable. The valves have provided reliable, leak-free service since their installation. During the Spring of 1995 scheduled outage on Unit 1, the recirculation valves on both turbine-driven boiler feed pumps were replaced with identical new design valves set up for modulating control. The Unit 2 boiler feed pump recirculation valves will be replaced during its Spring 1996 scheduled outage. END
Mark E. Liefer is an engineering coordinator for Illinois Power. Graduating in 1976 with a bachelor`s of science in mechanical engineering from University of Missouri-Rolla, Liefer began working for Illinois Power`s Baldwin Station in May of that year. He has held several positions with the company and currently supervises performance and project engineers at Baldwin Station.
Herbert Miller is vice president of operations at Control Components Inc. Miller holds a bachelor`s of science in mechanical engineering from Ohio Northern University and a master`s of science in mechanical engineering from Northwestern University.
Robert E. Katz is a product standardization manager at Control Components Inc. Katz holds a bachelor`s of science in mechanical engineering from Carnegie-Mellon University and a master`s of business administration from Pepperdine University.
Illinois Power`s Baldwin Power Station
Baldwin Power Station
Illinois Power`s Baldwin Power Station is a mine-mouth coal-fired plant with three 600-MW units. Units 1 and 2 are once-through, cyclone-fired units and Unit 3 is a drum design, pulverized coal-fired unit. All three Baldwin units are 2,400 psig (16.6 MPa), 1,000 F/1,000 F (538 C/540 C) with two steam turbine driven BFPs per unit. The six pumps are identical with a rated capacity of 2,270,000 pounds per hour (286 kg/s) at 3,580 psig (24.7 MPa) at a design speed of 5,850 rpm. Baldwin Unit 1 began commercial operation in 1970, Unit 2 in 1973 and Unit 3 in 1975.