Desulfurization pump slurry circulation pump wear problems

Desulfurization pump slurry circulation pump is a large-scale equipment in the desulfurization system.

It usually uses a centrifugal type slurry pump. It is the pump with the highest flow rate and the most demanding severe operating conditions in the desulfurization process. Corrosion and wear often lead to its failure.

(1) Strong corrosive

In a typical limestone (lime) -gypsum desulfurization process, the pH of the slurry at the bottom of the tower is 5-6. After adding the desulfurizing agent, the pH can reach 6-8.5 (the pH of the circulating pump slurry is related to the operating conditions of the desulfurizing agent ). On the tower and at the point where the desulfurizing agent is added, Cl- can be enriched above 80,000 mg / L, which will produce strong Cl-activity at low pH.

(2) Strong abrasion

The slurry at the bottom of the desulfurization tower contains a large number of solid particles, mainly fly ash and desulfurization medium particles. The particle size is generally 0 to 400 microns, more than 90% is 20 to 60 microns, and the concentration is 5% to 28% (mass ratio). These solid particles are highly resistant to abrasion.

(3) Cavitation

In the desulfurization system, the slurry pumped by the circulating slurry pump usually contains a certain amount of gas. In fact, the slurry delivered by the centrifugal circulation slurry pump is a gas-solid-liquid multiphase flow. The effect of solid phase pump performance is continuous and uniform, and the effect of gas on pump performance is more complex and difficult to predict than the effect of solid phase.

Our desulfurization slurry pump impeller and casing are subject to medium corrosion and solid corrosion during long-term operation. The service life of the impeller is about one year.

Flue gas desulphurisation system

In a flue gas desulphurisation system (FGD), sulphur compounds are removed from the exhaust emissions of fossil-fuelled power stations. This is done by means of an industrial process through the addition of absorbents. This can remove up to 95 % of the sulphur dioxide from the flue gas, since the current threshold value for SO2 in the EU is 200 mg/Nm3 (Nm3 = normal cubic metre).

The wet process has become the main method of flue gas desulphurisation in large, fossil-fuelled power plants. In this method, the flue gases are steam-saturated with the absorbent in aqueous solution. Substances such as ammonia or sodium sulphite are used as absorbents; however the use of lime or limestone slurry (wet limestone scrubbing) is also widespread. The uncleaned flue gas is sprayed in a scrubber tower (absorber tower) with a mixture of water and limestone (scrubbing slurry), whereby most of the sulphur dioxide is bonded by chemical reaction.

After a number of chemical reactions, gypsum is finally produced in a suspension. After dewatering, this leaves gypsum with up to 10 % residual moisture, which provides a valuable product for the construction material industry.

The pumps used in the individual process stages are absorber circulating pumps (scrubber pumps, which - because of the high solids content and the aggressiveness of the fluid handled - are designed as non-clogging impeller pumps with special lining) and FGD auxiliary pumps (for lime
and gypsum slurries in duplex materials).

FLUE GAS DESULFURIZATION

Over the past twenty years, scrubber pumps have continued to grow in size and capacity, with individual units handling flows in excess of 5,000 gal/min. Pump capacities to 10,000 gal/min or greater, at heads ranging from 13-20 meters are necessary in order to operate with a minimum number of pump units. Pumps must accommodate lower levels of operation due to unit outages or off-peak electrical demand. Today's pump net positive suction head (NPSH) is met by the normally high (10-15 meters) suction head. FGD pumps must be durable, able to withstand constant operation at specified conditions, able to accommodate a range of pipe loads and system transients and be virtually maintenance-free for at least 24,000 hours of continuous operation. FGD units use several types of pumps, three of which will be discussed below.

Feed Pumps

Concentrated slurry feed is usually handled by rubber-lined centrifugal pumps. Positive displacement pumps with variable-speed drives are also applicable. Cast-iron, corrosion-resistant alloy, and rubber-lined pumps are common in limestone systems. Some utilities prefer to use rubber-lined pumps from a single manufacturer for uniformity throughout the plant.

Although centrifugal pumps are widely used, the screw pump that handles limestone slurry feed is a special type of rotary positive displacement pump in which the flow through the pumping elements is truly axial. Thus, the screw pump with its unique axial flow pattern and low internal velocities offers a number of advantages in those few applications where centrifugal pumps cannot be used.

Fresh Water Pumps

In a limestone FGD system, the most likely points at which fresh water would enter are the ball mills, pump seals, and the mist eliminator wash system. A fresh water plump for this service can be a standard centrifugal pump. The important items to be specified are properties of the service water, available net positive suction head (NPSH), materials of construction, type of drive, and type and size of motor.

Recycle Slurry Pumps

The typical slurry pump has many features that set it apart from the typical centrifugal pump used for clear liquids. Wall thicknesses of wetted-end parts (casing, impeller) are greater that in conventional centrifugal pumps. The cutwater, or volute tongue (the point on the casing at which the discharge nozzle diverges from the casing), is less pronounced in order to minimize the effects of abrasion. The radial and the axial-thrust bearing on the slurry pump are heavier, too, than those on standard centrifugal pumps.

Materials of Construction

Pumps are made in two types of materials: all-metal pumps and rubber-lined cast-iron pumps. European and American engineers have tended to use different materials of construction. American power plants have generally installed rubber-lined cast-iron pumps while European plants have generally installed all-metal pumps. Since the pump parts in contact with the slurry are subjected to abrasive-corrosive action, the "wetted" parts must be constructed of a corrosion-resistant material that is either harder than the slurry solids or resilient. As David Root observed in "New Technology Throws Down Challenge to Pump Industry," International Power Generation, March 1996, pp. 59-60, the hard iron pump is a single casing, back pull-out design that increases ease of maintenance. It is constructed of duplex stainless steel for highly abrasive media. The cast-iron pump casing is split vertically with through-bolting used to connect the halves as well as the piping to the suction and discharge nozzles. In the rubber-lined cast-iron pumps each casing half is lined with easily replaceable, bolted-in segmented rubber liners. A good design provides for 100 percent reinforcement of all liners. This approach precludes collapse of the liners if a vacuum is inadvertently created. Rubber has advantages for modern scrubber pumps because it offers excellent abrasion resistance, has natural corrosion resistance and effectively dampens hydraulic noise. Rubber liners are less expensive and weigh less than metal liners or cases, so they reduce the cost and allow for easier handling during maintenance.

Most impellers today are metal. The advantage of metal is based on its durability and the fact that it allows for optimized pump efficiency through proper geometry. The maximum diameter impeller is employed regardless of the flow and head. This tactic is used to keep rotational and eye speeds at a minimum so wear life is enhanced and damage due to cavitation is minimized. Manufacturers of rubber-lined impellers claim impeller performance is maintained over 97 percent of the useful water life of the component while metal impellers slowly degrade over their working life.

The selection of impeller metallurgy normally includes either duplex stainless steel (CD4MCU) or a high-chrome martensitic white iron. The latter is typically 27-28 percent nominal chrome content. Certain formulations with carbon content under two percent enhance resistance to slurries having a pH level exceeding three, with chloride content of 75,000 ppm and higher. While duplex stainless steel can be used over the widest range of pH and chloride levels, maximum hardness with heat treatment may be 325-340 Brinell, while the available white irons may range from 450 BHN to 600 BHN, based on carbon content and heat treatment.

Generally, the advances made in elastomer compounds and metal alloys have resulted in typical wear life of five years for casing liners and three years for impellers.

Pump Accessories

Seals

Today's pumps are sealed with slurry mechanical seals, that operate without the need for flush water. It is now reasonable to expect seal wear-life of three years or more. The metallurgy of the components of the seal or shaft sleeve is important because the low pH conditions and presence of chlorides is conducive to corrosion problems.

Roller Bearings

The mechanical end of the pump features a heavy-duty shaft and bearing assembly in a cartridge that can be easily removed for maintenance in a non-contaminating atmosphere. A cylindrical roller bearing is employed to absorb radial loads imposed hydraulically and by the weight of the impeller. Double-tapered roller bearings accommodate hydraulic thrust loads. The location of these bearings provides the most efficient design for bearing size and life, while minimizing the displacement of the shaft caused by thermal gradients. A B-10 bearing life of 100,000 hours is normally specified for recycle pumps.

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