LATEST SOOT CLEANING TECHNOLOGY

Introduction

In this article, the type of soot blower/installation procedures/precautions while installation and operation philosophy are elaborated to help operators, as soot blower is a very important auxiliary of the boiler and helps to get maximum efficiency of the boiler. Steam is a blowing media for soot blowers. Steam required for soot blowing is tapped from the superheater header of the boiler. Operation and control of the soot blower is very crucial since it saves fuel required for the boiler.

The soot blowers are used to keep the boiler surfaces free from deposits of dust & grit which would otherwise reduce the rate of heat transmission in the boiler, reducing the boiler efficiency, and reducing availability. This loosely adhering layer is a definite impediment to the transfer of heat & should be removed after it has built up sufficiently to result in a significant increase in flue gas temperature. Regular soot blowing is essential for boilers, superheaters & economizers heating surfaces if the high thermal efficiency is to be maintained.

The Soot Blowers consist of:

  • Element rotating mechanism (head): Manual or motorized with loose valve or built-in valve
  • Element (blowing tube): Stainless steel or carbon steel depending upon the flue gas temperature

The usual form of an element is a dead-end tube projecting through the boiler setting & extending into the tube bank. A number of small lateral venturi nozzles of special alloy are provided on the tube & the external head is arranged so that high-pressure steam can be admitted to the tube (element) & at the same time the element can be rotated around its axis, to ensure maximum coverage of heating surface. The element can be rotated through any desired blowing angle.

Type of Soot Blowers

1 Rotary Soot Blowers

a) Manual

b) Motorized

2 Retractable Soot Blowers

a) Short Retractable (Motorized)

b) Long Retractable (Motorized)

c) Wall Blowers (Motorized)

Aspects of Soot Blower Selection

  1. High-efficiency cleaning
  2. Minimum steam consumption
  3. Simple and robust construction
  4. Minimum maintenance
  5. Selection of proper location and type of soot blower

Long Retractable Soot Blower Installation

Long Retractable Soot Blower Installation

The Long Retractable Soot Blower is generally used in radiation zones or where flue gas temperature is more than 700ºC.

The element has two venturi-type nozzles at a distance of 50mm and at opposite directions and two nozzles carry out the helical movement. The lance tube is driven by the carriage and steam is fed into the lance tube through the soot blower valve.

When the soot blower drive motor is switched on, the carriage moves forward and the lance rotates into the furnace, steam is blown and coils are cleaned. After reaching the element at the front end position, the limit switch is activated and carriage reverses back and the soot blower stops when the rear end limit switch is activated.

Installation aspects of Long Retractable Soot Blowers (LRSB)

We have noticed that there are problems faced while commissioning Long Retractable Soot Blowers. Procedure for installation of soot blower:

  1. We have observed that the soot blower consignment is of low weight. Normally soot blowers for one boiler will weigh approx. two to three tons. In such cases when the transporter is paid on a weight basis, they try to load some other material on the soot blowers; damaging the soot blowers. You are requested to instruct the transporter in writing that he should not load any other material along with soot blowers. Once the I beam of long retractable soot blowers is bent, the problems start before we erect the same. If soot blowers, particularly long retractables, are damaged, it’s better to replace them.
  1. While lifting the soot blower, it is wrong to tie the rope along with the S.S. element; which causes the bending of the element & feed pipe. Two hooks are provided on the I-Beam at both ends. Tie rope at these points for lifting.
  1. It is recommended that wherever possible, soot blowers should be stored under cover. If this is not possible, due to site conditions, it is recommended that the soot blowers should be stored on a dry firm base. The total assembly should be covered with waterproof material.
  1. While locating the soot blower, please mark the position on the boiler wall/casing/panel and see that there should be a minimum 150 mm gap between the soot blower element and superheater coils. Previously in some cases, it was found that due to the misalignment of SH coils, an element was rubbing with coils.
  1. After the soot blower is erected, check that the soot blower element is free from all sides. There is a possibility that the element is jammed due to the refractory inside.
  1. Prevent all flue gas leakages from the mounting box. Leakages increase the temperature of soot blower parts.
  1. After doing the electrical connection, operate the Limit Switch (boiler side) manually and confirm that the soot blower retracts. In most of the cases, Limit Switches get damaged in the first trial. If the soot blower does not retract, please check the electrical wiring.
  1. Before the boiler is commissioned, one should operate soot blowers without steam when the boiler is in cold condition so as to check the working of the soot blowers. Normally, a soot blower is given less importance and is erected in a hurry which does create problems.
  1. After a few operations, tighten the chain. The chain tightness required is mentioned at the trolley end. A sketch shows the names of the parts.
  1. Normally it was found that the steam line is not flushed before putting soot blowers into operation. This causes damage to the seal due to foreign particles. Hence please flush the steam line and then reconnect to the soot blowers.
  1. Steam should be dry. Hence drain the soot blower line. Water will damage the gland packing.
  1. If the steam supply pressure is more than 15 kgs/cm2 then a pressure-reducing device must be installed. In some cases, we give an orifice plate, which should be put between the soot blower inlet flanges.
  1. The soot blower is to be operated once a shift. If you need more soot blowing you can do it twice or thrice in a shift. Do not operate continuously. Give some gaps so that the element becomes cold.
  1. In lot of bagasse or coal, dust gets deposited on the soot blower. Hence, cleaning by air blowing on a soot blower is required.
  1. For soot blowers, high-temperature grease should be used.

Installation and operation of a control panel of soot blower

Description

Control Panel controls the sequential operation of motorized soot blowers automatically. Each soot blower motor is run by its switchgear. Indicating lamps are provided on the panel, facilitating knowledge of the operation condition of the blower motor. This helps the operator to monitor and control the operation effectively from a remote place. Remote push buttons are provided near the blower to operate the soot blower locally. A panel door interlock facility is provided for safety purposes. All overloads have a single phasing-preventing feature.

H.R.C. fuses are provided for the safety of the motor. It enables the three modes of operation, namely

  1. Manual mode 
  2. Automatic mode 
  3. Semi-Automatic mode

Modes Of Operation

1. Manual Mode

In this mode, auto/manual selector switches of all soot blowers are kept in manual position. Manual push button which is provided on a panel or locally mounted push buttons are used for operation. Indicating lamps are provided on the panel, indicating blower operation. The valve status indicator can monitor valve operation. The valve indicator will switch off and on in every revolution. The illuminated indicator indicates that the steam valve on the soot blower is closed and the illuminated indicator also indicates that the valve is open. In this mode, the soot blower operates till the push button is pressed. Always keep the steam valve on the soot blower in a closed position. In auto mode & semi-auto mode, a panel can be operated.

  • Single cycle mode 
  • Recycle mode

2. Automatic Mode

In this mode, all auto/manual selector switches of all soot blowers are kept in auto position and the auto on push button is actuated. Soot blowers will start operating serially in sequence. The soot blower motor stops after the completion of a cycle. Auto cycles interrupt if the stop button PBST is pressed.

3. Semi-Automatic Mode

This mode enables the operation of even a single soot blower in automatic mode. Those soot blowers, that are not working or have to be operated in manual mode for some reason, are kept in manual and others are kept in auto mode. W4hen the auto-on button is pressed, the blower, which is kept in auto, works serially in sequence. In the case of a single blower operating in auto mode, the blower will operate and will stop after laps of set delay. (Set on Timer T-1)

In the case of recycle mode all soot blowers work serially in sequence as mentioned above. But once again first soot blower, which is serially in sequence, operates after laps of time set on the ‘recycle interval’ timer T-5

Erection

  1. Erection of the soot blower is a very important factor for trouble-free running of the equipment. The seal box has to be fabricated by the customer and considering the expansion of the membrane panel, slope to the soot blower has to be provided. If the expansion is downwards, then the rear-end of the soot blower will be down than the front end; so that in the hot conditions, the soot blower will be in a perfectly horizontal position.
  1. While commissioning the soot blower, move the carriage by 500mm by hand crank. After the soot blower starts, it should go forward. Manually press the limit switch at the front end and ensure that the soot blower carriage reverses. If not, then check the electrical wiring and correct the same.
  1. All steam lines and drain lines should be taken cleared off all the grit and cut by blowing steam or air until they are clean.
  2. While connecting the steam line, no force should be exerted on the soot blower flange to watch the holes. In the case of boilers having steel casing, care should be taken so that if the location of the soot blower head changes, piping should also be free to move with the head, otherwise the head will not be horizontal while operating.
  3. The erection of the soot blower is a very important point. But it is being neglected by most of the erectors, assuming a small auxiliary plant compared to the boiler. Wrong erection of the soot blower results in damage to elements and soot blower parts.

Frequency of Operation

The interval between soot-blowing periods should be determined by observation under service conditions. As a guideline, the boiler should be cleaned.

a) Shortly after start-up, full W.P. was obtained

b) Approximately every eight-hour service

c) Observe the flue gas temperature at the boiler outlet. If the temperature is increased by 10ºC, then soot blowing is required

The boiler should be on at 75% load when soot blowing, to ensure that increased gas velocity through the boiler helps to carry away the loosened deposits originating from the soot blowing.

a) Do not start two soot blowers at a time

b) Never run a soot blower without steam when the boiler is running

c) The normal sequence of soot blowing is to follow the gas flow through the boiler. The first soot blower to be operated should be nearest the burner

Warming up

To avoid damage to the boiler and to the blowers necessitates the use of dry steam. Hence, it is important to use a certain procedure for warming up.

a) ‘Crack’ the steam supply valve to warm up the steam pipes and allow drainage of condensate through the drain valves

b) After approximately 5 minutes, the drain valve has to be closed, allowing the steam valve to be opened fully

Scavenge Air / Scaling Air

Each blower is provided with air scaled mounting box with an air inlet of 1″ BSP. The inlet pressure required should be 50 to 76 mm of water gauge more than the boiler furnace pressure. In positive pressure installations, it is necessary to fit separate scaling and scavenging air-piping systems from the F.D. fan ducting.

Conclusion

Installation and operation of soot blowers are very important because they help keep the heating surface clean without fouling and get the maximum possible heat transfer for maximum efficiency.

Author

Dilip Ambekar

Trustwell Group of Companies

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