Choking in DP measuring lines of ball tube mill - A Case study

Alok Kumar GuptaEngineer / C&I Paras TPS Site.

Project : Paras TPS Rating : 1×250 MW No. of Tube Mills : 3Type of Tube Mill : BBD 4772

Introduction For a Ball tube mill a direct relation exists between the primary air flow entering the mill and the pulverized fuel extracted from the mill. This relation (air-coal ratio) shall be maintained to get good balance in the analog loop load control.The air coal ratio is widely dependent on the tube mill load content. For this reason it is necessary to maintain constant level of pulverized coal in the mill.The level is measured by deep tube system which is designed to give a reliable signal independent of tube mill load/output.

PrincipleIn this method, a flow controller maintains air flow at very low speed in the measuring pipes. The pressure in pipe is dependent on specific weight of the liquid ‘w’ and the distance ‘h’ between the injection point and free atmosphereThen the pressure p = w × hIt is possible to graduate manometer to read level ‘h’ directly. If the liquid surface is not at atmospheric pressure but in pressurized vessel it is possible to measure the level using differential pressure measurement.

Working of DP PanelThis system uses measuring probes placed above (top) and in the pulverized coal (bottom) of the tube mill. 2 nos. pressure taps are also provided on the line which is connected to the upper tap of the mill for other uses which require measurement of pressure inside the tube mill. Two air circuits are used, one for the measurement and second for purging to avoid blockages. The measuring taps and pipes should be without blockage otherwise it will result in false reading. Measuring air circuit introduces instrument air continuously in the measuring pipes at low speed for measurement of pressure in the tube mill. A buffer tank stabilizes the pressure of measuring air which is controlled by a pressure reducing valve to get constant differential pressure (120 mmwc) in the pressure reducing orifices. This system maintains continuous constant air flow and velocity in the measuring pipes independent of the pressure existing in the tube mill. The correct functioning of the system is ensured by a differential pressure switch. Each of the six lines going to the mill is connected to a Normally open and Normally closed 2/2 pneumatic valve. Each pair of Normally open and Normally closed 2/2 pneumatic valves are connected in parallel to 3/2 solenoid

operated poppet valves. This arrangement disconnects low pressure measuring air circuit during high pressure purging of air lines going to the tube mill.

Problem StatementThe trouble started in November 2010 end with the choking of bottom side DP line of both sides. Due to this, it was difficult to appropriately control the mill level/parameters.

Preliminary stepsFollowing steps were taken for clearing the blockage of DP line since the emergence of problem:

Firstly the impulse line was cut open just before entering the mill. This particular line was found clear of blockage by blowing air from panel through the line . Air at the pressure of 5 Kg/cm2 was blown through the cut line but to no effect.

The next step was to go inside the mill and check the taps/nozzle. The screws provided for poking had been welded (SS). So the only option was to cut open the bracket/guard of nozzle. Using armor/ wire the line was poked but it was ineffective. Again through the nozzle air of 7 kg/cm2 pressure was blown but that didn’t help either.

Further Actions taken As the blowing of air upto a pressure of 7 kg/cm2 was ineffective so it was

decided to employ a mechanized hydraulic pressure pump (which is often used for impulse line hydro test). A collection of fittings were arranged to connect the fitting of hydraulic pump as it was of non standard size and finally it was connected to the nozzle. High pressure air along with mild sulphuric acid was pumped through the nozzle. Subsequently the pressure was raised to 70 kg/cm2.

ResultAs a result of pumping high pressure air with dilute sulphuric acid the blockage was finally cleared.

ConclusionThe blockage inside DP line could not be cleared using conventional blockage-clearing techniques viz. air blowing , poking etc. Hence a new methodology was adopted after exhausting all the available methods. This new methodology involved the use of mechanized hydraulic pressure pump for blowing air mixed with mild sulphuric acid through the DP lines. The use of sulphuric acid was particularly helpful as it involves oxidation and sulphonation which disintegrates the coal lumps.

Photographs

1. Delta-p Panel

2. Cut DP line just outside mill

3. The wonder machine- manual hydraulic pump