What Is a Waste Heat Boiler?

Waste heat boiler refers to a boiler that uses waste heat from industrial production processes to produce steam. It belongs to a high-temperature, high-pressure heat exchanger. The waste heat boiler was used to generate some low-pressure steam earlier, and the heat recovered was limited, but it was only used as general auxiliary equipment for production. With the development of production technology, the parameters of waste heat boilers have gradually increased. Waste heat boilers have been transformed from process boilers that produce low-pressure steam to power boilers that produce high-pressure steam. New results of waste heat research are constantly emerging. New results of research are constantly emerging. New results of research in the areas of waste heat boiler design, manufacturing, use, and safety management are constantly emerging.

Compared with ordinary boilers, the main features of waste heat boilers are:

High temperature and pressure, such as in the production of synthetic ammonia

1. The impact of water quality treatment on the service life of waste heat boilers

(1) Water treatment outside waste heat boiler

Water is one of the important factors affecting the service life and safe and normal operation of the waste heat boiler. The quality of the water in the adding furnace often directly determines whether the waste heat boiler can operate normally. Therefore, before the water is added to the waste heat boiler, it must undergo a series of beautification measures such as softening, desalination, and thermal deaeration to achieve the purpose of improving water quality.

(2) Water treatment in waste heat boiler

To fully extend the service life of the waste heat boiler and maximize its thermal power efficiency to ensure the safe and normal production of the boiler, it is not enough to rely on water treatment outside the furnace. At the same time, the quality of water treatment outside the furnace should be emphasized. Control the quality indicators of the water in the furnace, and pay attention to strengthening the water treatment in the boiler. Such as: before the newly installed boiler is put into operation, the boiler should be cooked, and the boiler should be pickled after being put into operation. After the boiler is put into operation, the boiler should be continuously discharged while the boiler is continuously discharged, and the boiler should be regularly filled with chemicals. .

2. The impact of water quality transportation on the service life of waste heat boilers

Water is the lifeblood of the normal production of waste heat boilers. If the boiler lacks water and causes a dry pot phenomenon, it will inevitably disrupt normal production and cause serious consequences. Therefore, while paying attention to the water treatment in the boiler, we must strengthen the operation of the multi-stage centrifugal water pump, which is the equipment that transports water, so that it can run continuously and stably, so as to provide a water source guarantee for the efficient production of waste heat boilers. In addition, when the characteristics of the multi-stage centrifugal water pump are constant, more attention should be paid to the recirculation pipeline at the outlet of the feed pump, the check valve installed on the outlet pipe of the feed pump, and the bypass valve on the check valve on the outlet pipe of the pump. Such as the working characteristics and working conditions of pipes and valves.

(1) Influence of the recirculation pipe at the exit of the multi-stage centrifugal pump

In order to feed the feed water to the boiler and make the feed water regulating valve have good regulating characteristics, the pressure of the boiler feed pump outlet must overcome the working pressure of the drum, in addition to the flow resistance of the feed pipe and the feed pump outlet to the drum. The static pressure difference caused by the height difference must be maintained at a certain pressure difference before and after the feed water regulating valve. The outlet pressure of the feed pump is usually 30% ~ 40% higher than the working pressure of the drum.

(2) Influence of check valve on outlet pipe of multi-stage centrifugal pump

In order for the standby pump to be ready to start at any time and to supply water immediately after startup, the population and outlet valves of the standby pump must be in the fully open position. In order to prevent the high-pressure water in the outlet pipe of the feed pump from returning to the treatment tank through the standby feed pump, causing the standby feed pump to reverse and the outlet mother pipe pressure to decrease, and water cannot be normally supplied to the boiler, the outlet of each feed pump should be installed Check valve.

There are many other factors that affect the service life of waste heat boilers and normal safe production. Only by regulating the various indicators that affect its production within a certain range can its benign effects be fully exerted and the service life extended. The design, manufacture, installation, operation and maintenance of boilers involve a variety of disciplines such as fluid mechanics, heat transfer, metallurgy, boiler principles, physics, chemistry, automatic control and measurement, etc., and effectively improve the theoretical and practical level of boiler operators. It is of great significance to save fuel, protect the environment and produce safely. ^[1]

(1) The quality of soft water for boilers The quality of soft water for boilers is poor, which leads to fouling of heat exchange tubes and affects steam production. Soft water has a high salt content, and scales are formed on the outer tube wall of the heat exchange tube, which greatly hinders the heat transfer process.

(2) Failures caused by the welding quality of pipe and center tube are mostly concentrated in some fillet welds, and weld beads that cannot be inspected by X-ray inspection after welding are mostly slag inclusions, pores, unfused, unmelted Defects such as penetration, and its strength are difficult to adapt to the thermal expansion and contraction of the equipment during production and operation, resulting in leakage.

(3) The problems in design and installation cannot ignore the changes in thermal expansion and contraction during equipment operation.

(4) The problems in the use process did not operate strictly according to the process indicators, and abnormal conditions such as over-temperature, boiler water shortage, and dry pot caused malfunctions. The internal heat-resistant lining of the waste heat boiler was not ovened as required. After the boiler was replaced, a large number of cracks were found in the internal heat-resistant lining. ^[1]

The first thing to do is to start with the boiler production quality. Be sure to choose a manufacturer qualified to make Class III containers. During the manufacturing process, no defects that violate the welding requirements are allowed. It is necessary to strictly control the proportion of the heat-resistant lining casting material, to make special molds, to densely cast, and to fully maintain it. The components of the boiler shall be assembled according to the design requirements, and shall be completed under the supervision of a technician. For example, the assembly of heat-resistant bushings, liners and protective sleeves of each row of pipes must be strict, careful, standardized and firm. In particular, the assembly of the gas outlet center control valve must be standardized. Measure the size and adjust the valve head stroke to ensure that the valve is live and reliable. Maintenance of cracks and leaks in the tube and the central tube. The treatment method for cracks and leaks of tube sheet welds must first pass the color inspection, determine the cracks, and then grind the grooves, and perform repair welding according to the welding process requirements. For the cracks in the welds of the tube and the tube sheet, try to avoid blockage of the tube. Use the method of re-welding along the weld in the tube to maintain the heat exchange area as much as possible.

Operators should have the ability to operate proficiently to avoid accidents such as over-temperature, boiler shortage, and dry pot during normal operation of the equipment. At the same time, the heating and cooling rates are strictly controlled during the heating and cooling processes. Scale is easily formed during the operation of the boiler. For systems with dosing equipment, pay attention to the proportion of the medicine. After dosing, the sewage should be discharged in strict accordance with the requirements to avoid the formation of a new scale layer. For systems without dosing equipment, regular furnace cleaning is necessary. ^[1]

(I) Hydrostatic test

After the boiler is installed and repaired, a hydrostatic test must be performed. The method is as follows;

1. Check whether there are tools and sundries in the upper and lower stoves, and then install the manhole. Seal the safety valve with a blind flange and remove it after the hydraulic test.

2. Check the pressure gauge and catheter to confirm good performance. Inject fresh water, the test water temperature is preferably about 20 ° C.

3. Open the upper air bleed valve to exhaust the air, and close the air exhaust valve after emptying. Increase the pressure gradually, about 0.35MMPa per minute.

4. After reaching the test pressure of 3.0Mpa, keep it for 30 minutes, check all parts, especially whether there are leaks in the water pipes connected to the upper and lower furnaces.

5. If there is no problem, gradually open the lower drain valve, and pay attention to avoid vacuum in the furnace.

6. There is waste heat in the boiler, no hydraulic test is allowed.

(2) Preparations before starting

1. Check whether all parts of the furnace tube are connected correctly and whether the tightness is appropriate. Make sure that there are no people or debris left in the furnace. Then cover the manhole cover and check whether the gasket is damaged.

2. Check whether the water level meter is functioning normally. Open the main steam valve a little and then close it.

3. Check the installation of water supply valve, blowdown valve, water level valve and pipeline valve. Operate it by hand once and put it in the normal position.

4. Open the upper furnace air release valve.

5. Feed the water with the feed pump and check the drain valve for leaks. When the water level reaches the upper limit of the water level, stop the feed pump and check for leaks in the joint seals of all components. ^[1]

(Three) heating up and boosting

1. Check the instruments, meters, soot blowers, manholes, etc. on the furnace body for looseness.

2. Check the performance of the pressure gauge and confirm whether the gauge valve is open.

3. Check the water level to make it slightly higher than normal.

4. Open the steam shut-off valve a little and then close it gently, otherwise it won't open after rising.

5. Burn the incinerator, gradually increase the temperature and increase the pressure. When the temperature and pressure increase, in order to prevent the steam valve from getting stuck, gently turn the valve handle. When the steam pressure reaches 0.1-0.2Mpa, close the vent valve.

6. Gradually increase the pressure, not exceeding 0.2-0.3Mpa per hour. Further inspection, tighten the manhole, handhole cover, furnace body valve and flange. When the pressure reaches 1.0Mpa, do it again.

7. When the water level exceeds the upper limit, gradually discharge the sewage to bring the water level back to normal.

8. Discharge part of the low-pressure steam to warm the steam pipe system.

(D) steam supply

1. Open the drain valve of the steam pipe system to supply steam and fully discharge the residual water.

2. Gradually open the main stop steam valve to supply steam to the air heater and condenser. At this time, check the piping system for noise and avoid water hammer. If normal, fully open the steam stop valve for normal operation.

3. When steam enters the condenser after the pressure control valve is opened, supply steam to each gas consumption unit.

(V) Operation in operation

1. Monitor the water level meter to keep the water level at 100 mm on the center of the upper furnace tube. The water level displayed by the water level meter should be within a small range above and below; when using the automatic water level control device, do not rely too much, you must carefully observe The water level of the water level gauge is used to discriminate between true and false water levels, and the water level gauge is often kept clean.

2. Make sure that the pressure gauge valve is kept open. When the sensitivity decreases, it must be checked. Pay attention to antifreeze when the furnace is shut down in winter.

3. Keep the heating surface clean, and decide whether to use soot blower according to the amount of steam generated, exhaust temperature, and exhaust pressure loss to ensure thermal efficiency (usually at least once a day). ^[3]