Flash drying protects standby plants
Special fast-drying technique provides effective corrosion
protection for units that will be in standby for a short time
By Richard E. Mallard,
Jacksonville Electric Authority
The Jacksonville Electric Au thority (JEA) has developed a technique for rapidly drying out its boilers as an effective corrosion prevention measure, even for units which will be out of service for a short time. The JEA has several steam generating units that are not in continual service. These units, whether on standby or in extended cold storage, must be maintained if they are to operate reliably when they are needed. JEA uses dehumidification as the primary method to reduce corrosion in these standby units. Engineers at JEA believe it is better to reduce the amount of water retained in standby boilers than to add inhibiting chemicals to retained water for corrosion protection.
Problems with short shutdowns
In the past, JEA employed the typical industry approach to dehumidification, which required four to five weeks to dry a system using two dehumidifier (DH) systems on each generating unit. Utilities consider this an acceptable process for a unit not needed for several months or a unit going into extended cold storage (even though considerable corrosion can occur in the lengthy drying period). However, the process was impractical for units that might be out of service for only a few days or weeks. Therefore, JEA developed a procedure to give as much protection as possible by drying out a complete unit in less than three days by “flashing-off” the boiler.
The JEA procedure for flashing off the boiler takes advantage of the heat energy present in the boiler water after the unit is released from service. Each generating unit has two DH units, an “outside unit” and an “inside unit.” The inside DH unit dehumidifies the boiler tubes, the turbines and steam lines, the condenser and condensate system and the feedwater system. The outside DH unit dehumidifies the boiler fire side, the forced draft fans and the induced draft fans. It is the areas serviced by the inside DH unit that are water-containing and, therefore, benefit from the accelerated method. Instead of the traditional method of cooling the boiler before draining it, all drains and vents on the boiler are opened as soon as the boiler pressure gets down to 200 psig. Because the saturated water temperature is 381 F, well above 212 F, the bulk of the water turns to steam when the opened boiler reaches atmospheric pressure. All the water is considered flashed off when nothing comes out of the drains and vents. At this time the forced air DH units are connected and the dehumidification is started. There are a total of three supply lines and two return lines for the inside unit. (This is unit specific.) The steps for putting the inside DH unit in service are as follows:
1) Open the low-point drains on condensate and feedwater systems;
2) Leave the DH returns disconnected so as not to melt the pipe (PVC);
3) Close the boiler vents and drains after venting has stopped; and
4) Connect the DH supply, with the DH running.
The water system must be drained while the water is hot to get most of the water out. For the first three days, all low-point drains must remain open to allow water to drain. When all drain lines appear dry they are closed, then checked every day until no water is found. Thereafter, the lines are checked weekly.
Figure 1 shows the operation of the inside DH unit. The normal flow for the DH air is into the condenser, then (1) up through low-pressure (LP) turbine, (2) through the crossover (3) counter-flow through the intermediate-pressure (IP) turbine (4) counter-flow through the boiler reheater, (5) counter-flow through the high-pressure (HP) turbine, (6) counter-flow through the open turbine block valves, (7) counter-flow up the main steam line, (8) counter-flow through the secondary superheater, (9) counter-flow through the primary superheater and finally (10) into the drum counter-flowing through the steam scrubber.
The extraction lines are open to allow the DH air to get to the steam side of the heaters. The drains go back to the heater drain pumps, and all drain valves and bypasses are open. The check valves normally allow enough flow to keep the heater dry; this will keep the steam side dry. There is a return line back to the DH unit from the heater drain pump suction (Figure 2). Figure 3 shows that, for the condensate system, a DH supply line connects downstream of the condensate pump`s discharge check valve. The flow is normal flow through the LP heaters, normal flow through the deaerator and tank (closed vents), normal flow through boiler feed pumps and then out the drains. This part of the system is difficult to dry, so the drains are left open to atmosphere and the DH air is not recycled.
The feedwater section is the last inside section to dehumidify. A DH supply is connected downstream of the boiler feed pump discharge check valve. Then the flow goes through the HP heater, the feedwater regulating valve and the economizer before entering the drum through the distribution pipe inside the drum. At this point DH air combines with the DH air from the superheaters. This air flows down the downcomers and generating tubes and out the blowdown valves, then returns to the DH units. The second DH unit is the outside unit that services the fans and the fire side of the boiler. Figure 4 shows this arrangement. There are two supply lines at the forced draft (FD) fans, two supply lines at the induced draft (ID) fans and two returns from gas fan hopper to DH unit. The flow goes two ways. The first path is (1) FD fans, (2) air heater, (3) windbox, (4) air register, (5) furnace, (6) back pass and (7) out at the gas fan hoppers. The second flow path is (A) into the ID fans (stack dampers are closed), (B) through air heaters and out at gas fan hoppers. Both flow paths return to the outside DH unit.
JEA uses the DH setup just described as the normal setup for each unit at its generating stations. To improve the basic procedure, the plants have installed quick connects and disconnects with valves. Operators on duty handle putting the dehumidifiers into and out of service. In effect, the operators are putting a unit that may be needed in just a few days into a very protective, cold storage environment, yet the units remain on an eight-hour call-up. The eight-hour restart allows two hours to get ready to fire, three hours firing time and one hour to attain minimum stable load. This totals only six hours, leaving two hours for any contingencies.
It is well established in industry that a large part of the maintenance budget is used to fight corrosion. Most companies are skilled at protecting units that are in service, but they are not nearly as protective when the units are shut down. The JEA has reduced the cost of corrosion-related maintenance and improved startup performance by protecting shut-down units better.
The results of this procedure are improved corrosion protection for units that are shut down, even for short periods. Boiler tube failures and other corrosion-related maintenance activities have been reduced. Use of this dry storage method has reduced iron problems during startup, and there are very few holds for water conductivity problems. This technique has also reduced the amount of water-treatment chemicals, such as hydrazine and ammonia, that are needed. This also improves startup times because when the unit is requested it is not necessary to take the time to drain the boiler of a water and chemical mixture because it is already empty. The operators simply start filling the boiler. In addition, the operators save the hours of work that would have been required for chemical residual testing during wet lay-up of the boiler. z
Richard E. Mallard is president of Mallard Consulting Co. He recently retired as director of operations for all three plant sites wholly owned by Jacksonville Electric Authority, Jacksonville, Fla. Mallard has 33 years of operating, maintenance and management experience. He is the developer of the Valve Open Start procedure, which was described in Power Engineering`s September 1995 issue.
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