Coal, Nuclear

Sticking with Air

Issue 6 and Volume 109.

A decision to replace more than 300 aging damper actuators forced Entergy to make a choice between pneumatic and electric actuator designs.

At Entergy’s Independence Plant in Newark, Ark., two Combustion Engineering coal-fired boilers can each generate more than 800 MW of power. With dual furnaces, each boiler has eight corner windbox damper units, each using 19 dampers. That’s 152 windbox dampers on each boiler, for a total of 304. These dampers route air flow through separate dedicated compartments to ensure proper firing of pulverized coal.

The actuator for each multi-bladed damper must accurately and reliably position its damper to a particular open setting, as dictated by a centralized distributed control system (Figure 1). For some years, the boilers performed quite successfully with Hagan pneumatic-operated, linear piston-type actuators. Last year, however, Entergy decided the time had come to replace the aging devices. With more than 300 actuators involved, the project represented a major investment that warranted serious analysis of replacements as to cost and performance.


Figure 1. A distributed control system in a centralized control room orchestrates all boiler control functions at Entergy�s Independence Plant in Newark, Ark., including the positioning of 152 windbox dampers on each boiler.
Click here to enlarge image

After a review of available designs – both electric and pneumatic – Entergy decided to stick with pneumatics. The reasons for this choice, which some might think at odds with the trend toward electric actuators, are covered in this article.

Independence

Through five subsidiaries, Entergy Corp. serves the electric power needs of nearly three million customers in Arkansas, Louisiana, Mississippi and Texas. Entergy owns and operates power plants with a total generating capacity of about 30,000 MW. The Independence Plant is one of three Entergy power plants using pulverized coal as an economical and reliable source of fuel.


Figure 2. Simplified plan view of dual furnaces in Combustion Engineering coal-fired boiler at the Independence power plant. Note eight windbox damper units-four per furnace-feeding combustion air for corner-firing of furnaces.
Click here to enlarge image

The dual furnaces at Independence feature windbox damper units firing from all four corners – referred to as “corner-firing” (Figure 2). With two furnaces, that adds up to eight windbox damper units. Extending 40 feet up, each corner unit has 19 compartments, one above the other, as shown in Figure 3. Each compartment receives combustion air through an actuator-positioned, windbox damper. At the other end of the compartment, air discharges through a nozzle on the burner front. The nozzle is aimed toward the center of the furnace, where a fireball exists during proper burning of the pulverized coal.

Click here to enlarge image

For the 19 compartments of each corner unit, dampers are assigned to handle air flow for either auxiliary air (AA), fuel-air (FA) or overfire air (OA). Starting at the furnace floor, first there’s an AA compartment. The next compartment up is FA and then another AA above that. These two functions alternate up to the top, where the last two compartments are OAs. This results in eight FA compartments, nine AA compartments, and two OA compartments.

The actuators for the AA dampers modulate to control the windbox-to-furnace differential pressure. The actuators receive a control signal from a differential pressure controller. The AA dampers also provide combustion air to the furnace.

The FA dampers, through their actuators, follow a preset curve based on speed of the coal feeder. As the coal feeder speeds up, the dampers open proportionally to provide the needed combustion air. The FA dampers also influence the ignition point of the coal. Note the separate passage for pulverized coal in the center of the FA nozzle (Figure 3.)

Actuators for the OA dampers modulate their damper positions in relation to boiler load and assist in controlling NOx emissions. If, for any reason, too much air enters the furnace from the lower part of the 40-foot high fireball, extra NOx will be generated. By cutting back on air going into the furnace at the bottom, the OA dampers will reduce the level of NOx, as measured in the stack. When the OA dampers open, the AA dampers below close to maintain windbox/furnace differential, resulting in more air flow entering above the fireball.


Figure 4. Close-up of new pneumatic damper actuator – one of 304 mounted on the outside walls of the windboxes.
Click here to enlarge image

Any damper actuator failure or malfunction can upset the balance of combustion, in a manner depending on the assigned function. If an FA damper is not positioned correctly, for example, the coal ignition point may not be at the proper distance from the burner tip in front of the bucket. Typically, it should be about 2 to 3 feet into the furnace. If the damper doesn’t open enough, then the ignition point comes back toward the burner tip, causing it to overheat and possibly resulting in serious distortion of the metal.

Realize that these are the ill effects of just one FA damper not being positioned correctly. The windbox damper unit on a furnace corner has eight FA dampers. This places heavy emphasis on the reliability of the actuator for every FA damper.

Selecting the Right Damper Actuator

In selecting new actuators, Entergy evaluated several factors: initial cost, installation cost, and reliable, long-term performance. The LP Type pneumatic actuators ultimately purchased from ABB Instrumentation (Figure 4) had been designed purposely for the exact same mounting as the Hagan drives being replaced.

In reviewing initial costs of available actuators, Entergy checked out electric and pneumatic models and discovered that the pneumatic models were 30 to 50 percent less expensive than electric models. Only minor differences in installation cost existed between the electric and pneumatic models, so this factor did not play a significant role in actuator selection.

With respect to reliable performance, the LP unit possesses excellent temperature ratings. Even though mounted on the outside walls of the windboxes, the actuators can be exposed to temperatures greater than 200 F. Most electronics, when used on pneumatic actuators, are only good to 175 to 180 F. The actuator cylinders on the ABB LP actuators have piston seals made of Viton, good for up to 300 F.

The LP unit features a built-in damper position feedback that sends a 4 to 20 mA signal to the distributed control system. A high temperature excursion could cause this feedback to fail, but the distributed control system will generate an alarm identifying that failure. What’s more, the actuator itself will still function to position the damper as called for by the pneumatic 3 to 15 psi control signal.

As to the LP’s long-term ability to provide sensitive and accurate damper positioning, Independence had the history of good performance with the similarly designed Hagan units. With a 2-1/2 inch diameter cylinder and a 5-inch stroke, the LP units could use the same 80-psi air supply as the original Hagan units to develop a 200-lb thrust for double-acting damper positioning. Further, the LP came with an updated pilot valve in its positioner to help assure accurate and sensitive damper positioning. p

Author: Steven Coker is a senior engineer at Entergy’s Independence Plant. He has more than 26 years’ experience in instrumentation and controls design, construction and maintenance. Coker is a senior member of ISA and a graduate of Louisiana Tech University.