ABTCo, located in Alpena, Michigan, is a decorative wood paneling producer using three boilers to generate 7 MW of electricity – half the power needed in the plant plus steam for pulp processing. As in many boiler operations, the main enemy is corrosion, which can destroy system components and lead to boiler failure. Although internal boiler surfaces are protected by a film of magnetic oxide, which forms naturally at high temperatures, feed water containing dissolved oxygen or with a high acidity can break down the film and corrode the underlying steel.
Corrosion can also develop in the boiler feed water system and condensate steam components, if dissolved oxygen is present. Bicarbonate alkalinity in the feed water can rapidly decompose to form carbon dioxide. This can lead to the formation of carbonic acid, which can cause severe corrosion of iron and copper alloys in the steam distribution system, especially if traces of dissolved oxygen are present. To prevent this, ABTCo dispenses chemical into its boiler feed water: sodium phosphate to control pH, sodium sulfate to eliminate dissolved oxygen and neutralizing amines to control the formation of carbon dioxide. “The process is designed to keep our boilers operating n top condition,” said Bill Hay, ABTCo power plant superintendent.
“So the injection system we use is extremely important. If it fails to perform properly, we can end up with a severe corrosion problem.” Until recently, injection was accomplished using four reciprocating plunger pumps. Operation of these pumps centers around a packed plunger liquid end, which induces fluid motion by a sealed plunger that draws in and expels process fluid, displacing a set amount of fluid during each pump stroke. Check valves on the suction and discharge of the pump operate 180 degrees out of phase to permit filling the liquid end during the suction stroke and to prevent backflow during the discharge stroke.
However, plunger pumps are not without problems. Plunger pumps use precision packing, which requires regular grease lubrication, to effect a tight seal between the plunger and the plunger bore. A small amount of controlled leakage of process fluid is common, in order to cool and lubricate the packing. In addition, a certain amount of wear is also unavoidable as the plunger reciprocates. This causes the packing to wear and increases the leakage of process fluid past the plunger.
The problem with the leakage goes beyond the wastes chemical, the problem with leakage goes beyond the wasted chemical and associated expense. “It’s becoming a compliance problem as well. More and more the EPA and other federal and local government agencies are cracking down on chemical spills in the workplace, both from an environmental and safety standpoint. It’s best to avoid leaks if at all possible and a leak-free metering pump is a good place to start.”
ABTCo first tried to solve the problem with maintenance procedures. The high cost of replacement parts, plus a long lead time, ultimately led to adoption of another solution: installation of Cheminjector-D hydraulic diaphragm metering pumps, made by Hydroflo. Unlike reciprocating packed plunger pumps, these pumps feature a diaphragm head that utilizes a Teflon disc diaphragm to prevent fluid from contacting the plunger. Hydraulic oil is displaced by the plunger’s movement to flex, forcing liquid on the other side on the diaphragm to move. Fluid flow through the diaphragm head is controlled by ball or disc check valves. Fluid is drawn into the diaphragm head during the pump’s suction stroke. When the stroke reversed, discharge occurs. Normal hydraulic oil losses are conually replenished by mechanically opening the displacement chamber to atmospheric pressure for a short period at the end of each suction stroke and the beginning of each discharge stroke.
ABTCo installed six Cheminjector-D pumps in its power plant operation, each capable of pumping 5.2 gallons of chemical per hour at 1,100 psig discharge pressure. “There used to be a puddle of chemical on the floor below the pumps. Not anymore,” hay said. “The power plant is a lot cleaner 9and safer) than it was in the past. The packing on the reciprocating pumps had to be adjusted on a regular basis. We used to need spare parts all the time, just to keep things going. The diaphragm pumps, however, have no packing to adjust, so maintenance is easier and less time consuming.”