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Resolving problematic self-priming of slurry liquids at power plants ... YOKOTA Slurry/Corrosion Resistant Self-Priming Centrifugal Pump (PAT.)

Air heater circulation flush pump in operation at a thermal power plant

Pumping challenges ... Wear, corrosion, and self-priming

If asked, "What are the main concerns regarding pump operation?", most answers would be, "wear", "corrosion", and, "pump stoppage due to air mixed in with the pumped liquid". These are all troublesome problems that are also costly.

"Wear" normally refers to wear of the shaft and other parts but wear from slurry liquids especially affects the pump itself. "Slurry" refers to a liquid that has granular particles mixed and if these particles are hard, such as sand or limestone, they can readily cause pump wear.

As for troublesome "corrosion", it easily occurs when pumping liquids that contain acids, alkalines, and salts.

To deal with "pump stoppage due to air mixed in with the pumped liquid", it is common to use "self-priming pumps" with a foot valve or similar device installed on the suction side to ensure the flow of priming liquid. However, since the self-priming capability is not completely sufficient, there is the constant concern that the pump may stop if air is suctioned during pumping.
There are many cases where the use of a self-priming system has been abandoned for "forced pumping" using "submersible" (can be submerged in water) or "vertical shaft" pumps. However, these pumps present other serious concerns such as their vulnerability to corrosion and the difficult and costly procedure of removing them from out of the water for maintenance and inspection.

It seems impossible that one pump could simultaneously deal with all three problems of "wear", "corrosion", and, "pump stoppage due to air mixed in with the pumped liquid". It seems like we should just give up....However, there is a land-based pump that can operate easily under such difficult conditions and overcome all three of the problems simultaneously. It's the YOKOTA Self-Priming Centrifugal Pump (PAT.).

What's different about the YOKOTA Self-Priming Centrifugal Pump?
While these problematic conditions are present simultaneously at locations such as chemical factories, refineries, and wastewater treatment facilities, we will use the example of a coal-fired thermal power plant to explain how this pump is typically used.

Coal-fired thermal power plant (Full view)

Overview of coal-fired thermal power plant

Since coal is the source for approximately 20% of all the energy used for electric power generation in Japan, there are a large number of coal-fired thermal power plants constructed and operating in the country.
At these plants, coal is pulverized and then burnt to generate highly pressurized steam inside of boilers. This steam is used to turn turbines and generate electricity.

The burnt coal produces smoke and, since Japan has the most stringent environmental standards in the world, the coal-fired thermal power plants must perform a thorough flue gas treatment for air pollution control.
In a typical treatment process, the flue gas, which contains substances such as coal ash (fly ash), nitrogen oxides (NOx), and sulfur oxides (SOx), is sent to an electro-static precipitator that removes ash particulates and then to a denitrification unit that removes nitrogen oxides.
The high-temperature flue gas is then channeled to the air heater where heat exchange is performed with inducted fresh air so that it can be sent to the boiler. Due to this heat exchange, the heat of the flue gas can be effectively utilized.
After the heat of the flue gas has been transferred to the fresh air, it is sent to the gas-gas heater where heat exchange is performed again with the flue gas sent to the chimney and then it is channeled to the desulfurization unit.
Then, after the sulfur oxides have been removed by the desulfurization unit, the flue gas passes back through the same gas-gas heater to be exhausted from the chimney.

Flue gas treatment process at a coal-fired thermal power plant

Circulation flush pump for the air heater and gas-gas heater

The internal filter of the air heater must be cleaned periodically due to ash and rust left by the flue gas that accumulates in the heater, reducing the heat exchange efficiency.
Cleaning is normally performed by using a "circulation flush pump" that suctions water stored in a pit, pressurizes it, and then sprays it out under high pressure. Since the material accumulated on the filter is dispersed into the water, creating slurry, a huge rotor blade is installed in the pit, constantly stirring the mixture until cleaning is completed.
The internal filter of the gas-gas heater is periodically washed off using a similar process.

Air heater filter cleaning system	Air heater	Gas-gas heater

Resolving circulation flush pump concerns

The circulation flush pump for the air heater and gas-gas heater must have sufficient pumping capability to suction and pump slurry liquid. Furthermore, the pump material must be highly corrosion resistant since the liquid used to clean the gas-gas heater contains strong acids.

At a certain coal-fired thermal power plant, conventional vertical shaft pumps were used until 1986. However, they presented the following concerns.

	The pumping equipment was large and costly.
	The main part of the pump was submerged, making maintenance troublesome and costly.
	Due to slurry, pump impeller, casing, and other parts suffered considerable wear. Also, bearings would wear out, causing vibration. This entailed replacement costs.
	Pump corrosion was also severe. This entailed replacement costs.

In order to solve these problems at this plant, land-based horizontal pumps were planned for the first time to be used as circulation flush pumps in 1986 when the plant underwent an expansion.
Foot valves cannot be used due to the slurry liquid. For this reason, using land-based horizontal pumps requires them to have a strong self-priming capability. The YOKOTA Self-Priming Centrifugal Pump (PAT.) was selected since it is quite possibly the only pump capable of self-priming and high-pressure pumping of slurry liquid.

The YOKOTA Non-Water Hammer Check Valve was installed to prevent water hammer when the pump is stopped and ensure stable pumping. The Non-Water Hammer Check Valves close at exactly the moment when change in flow from normal to reverse direction occurs, that is, when the liquid in the pipes stops flowing after the pump is stopped. Due to this, water hammer due to reversed flow does not occur, thereby preventing pipe ruptures and pump damage.
YOKOTA's independently developed special stainless steel YST alloy, proven to have high corrosion and wear resistance under a wide range of conditions, is used for these pumps and valves.

The results of using YOKOTA pump system clearly showed that the problems presented by conventional vertical shaft pumps were completely resolved. Additionally, facility, and maintenance inspection costs were all greatly reduced.


Comparisons of different pumps

Classification	Submerged pump	Vertical shaft pump	YOKOTA Self-Priming Centrifugal Pump
Structure
Corrosion	Corrosion occurs easily since the pump is submerged in water.	Corrosion occurs easily since the pump is submerged in water.	Corrosion does not readily occur.
Bearing maintenance	Bearing maintenance is difficult since the pump is submerged in water.	Bearing maintenance is difficult since the pump is submerged in water.	Maintenance can be performed easily due to stabilizing ball bearings.
Maintenance/Inspection	Pump and pump motor are submerged in water making maintenance inspection difficult. Pump and pump motor must be pulled up for disassembly.	Pump is submerged in water making maintenance inspection difficult. Motor must be transferred for disassembly.	Maintenance inspection can be performed easily since it is situated on land. It is not necessary to transfer the motor for disassembly.
Facility costs	Pump and pump motor costs are high.	Pump and pump motor costs are high.	Pump and pump motor costs are low.

Air heater and gas-gas heater circulation flush pump system

Pump: UHN type 200mm x 5.5m3/min x 75m x 1750rpm x 120kW

Due to the recognition of the positive accomplishments achieved with air heaters, gas-gas heaters, and other units, YOKOTA pumps and valves (including those used at other locations within the plant) have been supplied to a large number of electrical power plants.


Regarding the YOKOTA Self-Priming Centrifugal Pump UHN type (PAT.)

	Casing consists of a simple construction with a semi-double volute and a cavity holder. A single-suction, single-stage pump displaying outstanding pumping efficiency in a wide assortment of specifications.
	Outstanding self-priming power, with a maximum vacuum reaching 500-700mmHg.
	Low NPSH. Even if cavitation develops due to fluctuation in intake conditions, this pump can still continue pumping operation and does not require allowance for NPSH.
	Even if pumping becomes subject to air suction or mixture of air during pumping operation, it continues pumping without a problem. Even continuous suction of air-containing water (i.e., gas-liquid two-phase pumping) is possible.

Self-priming mechanism (PAT.)

1. Circulation flow During self-priming, the water discharged from passage A in the semi-double volute returns to the impeller through passage B-C, and is discharged again into passage A. 2. Bubble removal This recirculation flow mixes the water and the air in the center with intense vortex D within the impeller and discharges it into passage A. 3. Water-air separation and exhaust The water-air mixture is led from passage A to discharge nozzle B in a cyclonic state and automatically separated by centrifugal force, and then the water is recirculated through passage B-C. The separated air is held by cavity holder E, and then compressed and discharged against the back pressure.

Regarding the YOKOTA Non-Water Hammer Check Valve (PAT.)

	The valve opens and closes in response to water flow, therefore there is no delay in valve closure and water hammer does not occur.
	The single-disk, simple construction eliminates breakdowns. Maintenance costs can be reduced considerably.
	The No-Feed Detector can also be installed optionally.

Pressure change with time after pump operation is stopped
YOKOTA Non-Water Hammer Check Valve	Conventional check valve

Regarding YST alloy (YOKOTA stainless steel alloy YST series)

YOKOTA's independently developed special stainless steel alloy, YST series, with enhanced corrosion resistance as well as wear resistance, has been favorably received as a standard material suitable for pumps and valves in equipment for anti-pollution treatment, seawater treatment, and chemical processing among others.
Moreover YOKOTA's duplex stainless steel YST 130N is an excellent seawater and chemical resistance material.
Starting from the casting process, the YST series is wholly manufactured in-house by YOKOTA to ensure complete quality control of materials and the casting process.

Advantages of YST alloy

	Excellent corrosion resistance, especially against localized corrosion due to the presence of chloric ions.
	Improved wear resistance due to improved hardness, resulting in both corrosion and wear resistance.
	Excellent resistance to pitting and crevice corrosion in seawater.
	Outstanding strength (nearly twice as strong as general JIS materials).
	Easily welded by covered arc or TIG welding without preheating or residual heating.

YST series corrosion/wear resistance

YST series corrosion/wear resistance (Compared to when SUS316 has 100% wear due to 5% slurry)

YST130N crevice corrosion resistance Loss due to crevice corrosion Field test with actual liquid (seawater + hypochlorous acid (90-100ppm)) Test period: approx. 1 year and 9 months

YST130N pitting corrosion resistance Loss due to pitting corrosion 5% ferric chloride 40 deg C, 50hr

The final step in flue gas treatment at coal-fired thermal power plants is the flue gas desulfurization unit. YOKOTA YST alloy is also being used in these units.

The desulfurization unit uses limewater to eliminate sulfur oxides (SOx) from flue gas. The YOKOTA Mixed-Flow Volute Pump YM type, constructed with YST alloy, is used in large numbers in order to circulate (non-self-priming type pumps are used in this situation) the limestone slurry created in this process. YOKOTA's YST alloy has repeatedly proven its excellent durability and reliability under the severely corrosive and abrasive conditions of a flue gas desulfurization unit.

YST alloy used for YM type pump in flue gas desulfurization unit

For details of each product, please refer to
Self-Priming Centrifugal Pump UHN series
Mixed-Flow Volute Pump YM type
Non-Water Hammer Check Valve SL series
Special Stainless Steel YST series