About a year ago, a vintage 1980 pneumatic damper actuator on a steam boiler at Equistar Chemical’;s Corpus Christi, Texas, plant was replaced with a state-of-the-art Contrac electric drive from ABB Inc., Warminster, Pa. Electric drives, also known as actuators, must position combustion air dampers on steam boilers precisely and reliably. As boiler load varies, there is a damper position that optimizes the amount of excess air in the fuel/air mixture. Too little results in undesirable stack emissions; too much reduces boiler efficiency and wastes fuel. The new drive provides precise damper control on this boiler, resulting in fuel savings estimated between $200,000 and $350,000 annually.
Such savings are due primarily to the fact that the electric drive enables boiler operation with just the right amount of excess air at lower firing rates. We installed another electric drive on a second boiler in June 2004, after the first performed reliably. We expect this will provide additional fuel savings of the same magnitude as the first drive installation.
Scope out the problem
Constructed in 1980, the Corpus Christi plant covers 2,050 acres. There are more than 400 employees and resident contractors on site. The largest part of the plant is an olefins facility that manufactures products such as ethylene, propylene, benzene and fuel products. Another unit manufactures butadiene (BD). These intermediate products are typically used to make chemicals and plastics for consumer products ranging from food packaging and containers to rubber tires and antifreeze.
To serve the extensive energy needs of the entire site, there are four Clarke-Chapman Type 40SAB18W packaged boilers located side-by-side (Figure 1). Each is rated to produce about 224,000 lb/hr of superheated steam at 900 psig and 900°F. Each of the four boilers can feed steam into a common, high-pressure (HP) header. Steam from this header is typically used to operate steam turbines for electric power generation, process gas compression or centrifugal pump operation. Process heat exchangers represent another major use.
Each boiler is rated to produce about 224,000 lb/hr of superheated
steam at 900 psig and 900°F. This photo shows the air intake
side of the four boilers. The two in the foreground were upgraded with
electric damper drives.
The four boilers have excess capacity that allows the Corpus Christi plant some flexibility for a variety of operating conditions. Boilers are started up or shut down as required by the plant load.
Identical in rating, the boilers have been altered. For example, one was modified to burn fuel oil and two have undergone a major revamp to use multiple burners with high turndown (10:1).
The two boilers with high-turndown burners were selected for the electric damper drive upgrades. Following an engineering analysis, we reasoned that more precise damper control could readily achieve the desired operation with less excess air at low firing rates, thereby increasing boiler efficiency. We had anticipated a 4% to 6% increase in boiler efficiency -- the resultant savings surpassed these expectations.
The drive is in control
All four boilers are individually monitored and controlled by a distributed control system (DCS) located in a centralized control room. The schematic diagram (Figure 2) shows how the correct amount of excess air is determined and combustion-air intake is controlled.
This schematic shows the major coponents of Equistar's damper drive control system.
Intake air flow upstream of the induced-draft fan is measured by a Venturi primary element that creates a pressure drop proportional to air flow rate. This value is transmitted to the DCS as one input to determine control action.
An oxygen analyzer in the boiler stack monitors excess air, sending a second input signal to the DCS. Boiler load is a third input since the proper amount of excess air varies with the load. Based on these inputs, the DCS calculates the target amount of excess air and sends a 4-20 mA signal to the Contrac electric actuator via its Power Electronics Unit (Figure 3). The latter develops a proper control response to energize the drive’;s bidirectional electric motor. Through precision gearing, the motor turns the drive’;s lever arm to accurately position the air damper.