WO1998057122A1

WO1998057122A1 - Monitoring kiln linings

Info

Publication number: WO1998057122A1
Application number: PCT/GB1998/001660
Authority: WO
Inventors: Mark Francis Lucien Harper; Martin Thompson
Original assignee: Thermoteknix Systems Ltd
Priority date: 1997-06-12
Filing date: 1998-06-11
Publication date: 1998-12-17
Also published as: GB2326235A; GB9712125D0; GB2326235B; AU7782398A

Abstract

A method and apparatus are disclosed for measuring the thickness of the layer of refractory brick (15) on the inside of the steel wall (5) of a cement kiln and of any layer of clinker (19) adhering to the inner face of the brick layer. In the method: a known impulsive force is applied to a point on the outside of the kiln wall; the vibration response of that point to the imposed force is monitored, so that the mechanical point impedance at that point may be calculated; at the same time, the vibration response a short distance from the forcing point is also monitored, so that the velocity of flexural waves in the kiln wall can be deduced; and from the combination of mechanical impedance and wave velocity, and given a knowledge of the thickness of the steel wall, the densities and elastic properties of the steel, the brick and the clinker, the thickness of the layers of brick and clinker behind the steel are estimated.

Description

Monitoring ki ln l inings

This invention relates to the monitoring of the thickness of refractory linings of cement kilns while the kiln is in use.

Modern cement kilns operate a contmuous process in which the ingredients, mainly limestone and clay, are poured into the top of a kiln in powder form. They aie initially heated to 100°C to get πd of watci vapour; then they are heated to 1200°C to reduce the calcium in the limestone. They are then further heated to 1500°C to reduce all the oxides in the ingredients. At this temperature the ingredients melt to produce a liquid-phase mixture of aluminium, silicon and calcium. The mixture is then allowed to cool progressively to produce cement crystals in the form ot lumps of solid clinker which are then decanted, cooled and milled and mixed with gypsum (calcium sulphate) to produce a usable cement mixture. The kiln itself is typically a steel cylinder, around 5m in diametei and tens of metres long, inclined at a small angle to the horizontal, which rotates slowly during operation. The cylinder is lined internally with refiactory brick, w ich protects the steel from the high temperatures mside the kiln. The raw materials for the cement are poured into the upper end, and high temperatures are produced for example by burning natural gas. This causes the mgredients to react within the kiln, and cement is collected from the lower end. Over some portions of the inner brick surface of the kiln wall cement tends to react with the brick, and to build up a layer of solid deposit or 'clinker', which provides further protection for the kiln wall from the high temperatures. However, over other portions of the wall no clinker layer may develop, and the brick layer may be eroded by the hostile conditions within the kiln.

It is desirable for the kiln operators to know accurately the thickness of the layers of brick and clinker over the internal surface of the kiln. If the layers become too thin, for example, then the steel wall may be exposed to very high temperatures, and could be damaged or even ruptured. At present the only readily-available method of monitoring the state of the brick/clinker lining is to monitor the temperature of the external surface of the kiln wall using pyrometry: an area of raised temperature may indicate that the lining is thin. However this method does not yield the separate thicknesses of brick and clinker layers, and is in any case subject to considerable uncertainty because there are other causes of temperature variation besides thinning of the lining. Hence this method is not highly regarded in the industry.

The invention seeks to allow the said thicknesses to be known by proposing a method and apparatus for monitoring the thicknesses of both brick and clinker layers from the outside of the kiln during operation while the kiln is rotating and without interfering with normal operation. What is proposed is that an impulsive force be applied to a point on the outside of the kiln wall, and the vibration response of that point to the imposed force be monitored, so that the mechanical point impedance at that point may be calculated. At the same time, the vibration response a short distance from the forcing point is also monitored so that the velocity of flexural waves in the kiln wall can be deduced. From the combination of mechanical impedance and wave velocity, and given a knowledge of the thickness of the steel wall, the densities and elastic properties of the steel, the brick and the clinker, the thickness of the layers of brick and clinker behind the steel may be estimated.

In one aspect therefore the invention provides a method and apparatus for monitoring the thicknesses of both brick and clinker layers within the metal outer wall of a cement kiln in which method:

a) a known impulsive force is applied to a point on the outside of the kiln wall;

b) the vibration response of that point to the imposed force is monitored, so that the mechanical point impedance at that point may be calculated;

c) at the same time, the vibration response a short distance from the forcing point is also monitored, so that the velocity of flexural waves in the kiln wall can be deduced:

and

d) from the combination of mechanical impedance and wave velocity, and given a knowledge of the thickness of the steel wall, the densities and elastic properties of the steel, the brick and the clinker, the thickness of the layers of brick and clinker behind the steel may be estimated by a procedure which will be explained below. In a second aspect the invention provides apparatus for monitoring the thicknesses of both brick and clinker layers within the metal outer wall of a cement kiln, which apparatus comprises:

a) means for applying a known impulsive force to a point on the outside of the kiln wall;

b) means for monitoring the vibration response of that point to the imposed force, so that the mechanical point impedance at that point may be calculated;

c) means for monitoring at the same time the vibration response a short distance from the forcing point, so that the velocity of flexural waves in the kiln wall can be deduced;

whereby from the combination of mechanical impedance and wave velocity, and given a knowledge of the thickness of the steel wall, the densities and elastic properties of the steel, the brick and the clinker, the thickness of the layers of brick and clinker behind the steel may be estimated.

The known impulsive force (a) should have a bandwidth of at least 5kHz and should impart momentum of order 1 kg m s^"1 or greater. This may be achieved for example by striking the wall of the kiln with a small steel hammer, in which case the applied force will rise rapidly from zero to a maximum value of order 1 ,000 Newtons and then decline back to zero at a comparable rate over a time interval of order two milliseconds.

Because the kiln wall is moving, and will normally be very hot on the outside (temperatures of 600°C being common), it is not convenient to use conventional contacting vibration sensors such as piezoelectric accelerometers to detect vibrations (b) of the kiln wall in response to the applied force (a). Instead, non-contacting means such as induction probes or capacitance probes may be used. These probes are devices which produce an electrical signal which varies when the probe is held close to a metal surface and the distance of the metal surface from the probe varies by a small amount. Suitable induction probes and capacitance probes may be obtained commercially, for example from Briiel & Kjaer of Na_τum. Denmark. Vibrations (b) of the kiln wall in response to the applied force (a) may thus be detected because they will produce changes in the signal from the probe in proportion to the vibrational displacements of the kiln wall. The distance of the probe from the mean position of the wall should be of order 1mm while the vibrational displacements of the wall will be of order 1 micrometre. It is important that the distance of the probe from the mean position of the wall be kept constant, because the sensitivity of the signal from the probe to vibrational displacements of the kiln wall will vary with that distance. Alternative non-contacting means of detecting vibrations of the kiln wall include laser velocimetry and microwave displacement detectors. Having obtained measurements of applied force and resulting displacement these may now be used to calculate mechanical impedance. The value of the mechanical impedance is only of interest over the range of frequencies within which the kiln wall will respond to forcing very much like a homogeneous flat plate because further calculations will be much simplified by limiting the frequency range to this range. This frequency range will be limited at the low frequency end by the cylindrical nature of the kiln, and at the high frequency end by the finite travel time of vibrations through the thickness of the layers of the kiln wall: this travel time must be smaller than the period of the highest frequency in the range. Typically the range of frequencies from 1 kHz up to 5 kHz will be usable. In order to calculate the mechanical impedance Z(ω) of the kiln wall as a function of angular frequency ω from the applied force as a function of time f{t) and the resulting displacement at the forcing point χ(t) the following procedure may be used. First the applied force as a function of frequency is obtained via the Fourier transform of the applied force :

Next the velocity at the forcing point as a function of frequency is obtained via the Fourier transform of the differentiated displacement:

∞ . (ω) = j- x(t) e^ιwt dt (2) o

Finally the mechanical impedance is obtained from the ratio of these two calculated quantities:

E(ω)

Likewise the vibration response y(t) to the forcing f {t) at a short distance d_i from the forcing point is also monitored (c). The velocity of flexural waves on a flat plate varies with angular frequency ω and the following procedure may be used to calculate velocity over the aforesaid frequency range from y(t) and d_l . Firstly the Fourier transform of the response y(t) is obtained: F(ω) = J» e^/ωr dt (4)

Next the phase Φ[F(ω)] of Y(ω) is obtained as follows:

Finally the velocity as a function of frequency is obtained as follows:

ω

V(ω) = (6) φ[F(ω)]

In order to further determine the thicknesses of brick and clinker layers (d) the following procedure is used. The bending stiffness B and mass per unit area m of the kiln wall are first found as follows. The mechanical impedance is related to these quantities as follows:

Z(ω) = 8 ^B m (7)

The velocity of flexural waves is related to the same quantities as follows:

By combining these two equations the bending stiffness B and mass per unit area m of the kiln wall may be separately calculated: Z(ω) V (ωj

B(ω) (9) ω

Z(ω) ω m(ω) = (10)

8 V² (ω)

These quantities are functions of angular frequency ω but over the frequency range of interest they should not vary substantially. The effect of measurement noise on the estimated quantities may be reduced by averagmg results for B and m over the aforesaid frequency range.

We now use these quantities to determine the thickness t_b of the brick layer and the thickness t_c of the clinker layer by findmg values of these thicknesses which simultaneously satisfy the following equations foxB and m :

1 (/. + t 7 (E t + E, t. )

B = B_{s +} B_{bc +} -E_st_sE_bct_br - ^K _c' '! ~ ^{>κ bc)} (1 1 )

/7I = m_Λ + 72_j,. ( 12)

m which t^ , B^ and m_h( are respectively the thickness, bendmg stiffness and mass per umt area of the brick and clmker layers in combmation, and in which B_s is the bendmg stiffness of the steel wall which may be calculated from its known thickness t, and elastic properties. E_s is the Young^'s modulus of the steel, v_s is the Poisson^'s ratio of the steel, m is the mass per umt area of the steel wall and v_bc is the Poisson^'s ratio for the brick and clinker or a representative value if they do not have the same value (the precise value used is not critical). These quantities aie related to the desired thicknesses t_b , t_c by the following equations: ',+', = ' (14)

[t_b+t7² (E_ct_e+E„t_b)

B b,e = B b. +B c +-E c t cEj b b. (15)

(E_ct_c+E_bt_b)² - (v R, ', + _tE₆'.

in which p_b and p_c are the densities of the brick and clinker respectively while B_b and B_c are the bending stiffnesses of the separate brick and clinker layers and are given by the further equations

B_b = ^Eb (16) ^b 12(l-v_fc-)

B = — ^e- — ^c— (17)

' 12(l-v;)

in which E_b is the Young's modulus of the brick. E_r is the Young's modulus of the clinker. v_b is the Poisson^*s ratio of the brick, and v_c is the Poisson's ratio of the clinker.

These formulae are not used to obtain the desired values of t_b and t_c directly; instead, trial values of t_b and t_c may be inserted until values which result in the correct values of B and m are found. This may be done by straightforward trial and error or by using a standard iterative scheme such as the Newton- Raphson method. A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 shows a section through the wall of a cement kiln with transducers for making the necessary measurements in which the sensors are induction probes .

Figure 2 shows a section through the wall of a cement kiln with transducers for making the necessary measurements in which the sensors are laser Doppler velocimeters.

Referring to Figure 1 , a light hard conical tip 3 is fixed to a force transducer 1 so that an impulsive force may be applied to the steel wall 5 of the kiln by striking the tip 3 against the wall 5 and the applied force may be known by measuring the output signal of the force transducer 1. The tip 3 must be hard enough to give a very short sharp tap to the wall 5 and thus maximise the bandwidth of the force pulse. A bandwidth of several kilohertz will normally be desirable in order to cover the frequency range within which the kiln wall 5,15,19 responds to the applied force essentially as a flat plate. This frequency range will normally include the range 1kHz to 5kHz. The transducer 1 and tip 3 are driven against the wall 5 at a forcing point 17 by activating an electromagnet whose coil 11 and armature 13 are indicated in Figure 1. The electromagnet 11 ,13 is rigidly fixed to a rigid frame 15 to which two induction probes 7,9 are also rigidly fixed. The response of the kiln wall 5 to the forcing is monitored by the induction probe 7 which is held close to the forcing point 17 and at a known constant small distance from the wall 5 by the frame 15. The displacement of the moving wall 5 due to the force apphed by the tip 3 may be known by measuring the output signal of the induction probe 7. The response of the wall 5 is also monitored by a second mduction probe 9 which is held at a distance from the forcing point 17 and at the same constant small distance from the wall 5 as the probe 7 by the frame 15.

The output signals from the force transducer 1 and the mduction probe 7 are used to calculate the point impedance of the kiln wall at the forcmg pomt 17, as already explained.

The said point impedance taken over the said frequency range w ill depend on the product of the total mass per unit area of the kiln wall and the bending stiffness of kiln wall taking into account the steel 5, the brick layer 15, and the clmker 19 attached to the brick. The output signals from the force transducer 1 and the induction probe 9 are used to calculate the velocity of bendmg waves near the forcing pomt 17. This velocity taken over the said frequency range will depend on the ratio of the two properties of the kiln wall already referred to - that is to say, the ratio of the bendmg stiffness of the kiln wall to the total mass per unit area of the kiln wall. Thus, having calculated the pomt impedance at the forcmg pomt 17, and the wave velocity near the forcing point 17 the bendmg stiffness of the kiln wall and the total mass per umt area of the kiln wall may be separately deduced. If the density and elastic properties of the steel wall 5, the brick layer 15 and the clmker 19, and the thickness of the steel wall 5 are known then the thicknesses of the brick layer 15 and the clinker layer 19 may be finally deduced, as already explained. In an alternative embodiment of the invention the induction probes 7.9 are replaced by capacitance probes which operate by measuring the capacitance between a metal plate at the front of the probe. They also need to be held at a known constant small distance from the wall 5 by the frame 15, and so the alternative embodiment is identical to the first described embodiment except that the induction probes are replaced on the frame 15 by capacitance probes.

In a third embodiment of the invention illustrated in Figure 2 the induction probes 7,9 are replaced by laser Doppler velocity measurement devices 21.23 which detect the velocity of the vibrating kiln surface 25 by means of laser beams 31 ,33 reflected from the surface 25. Because the surface 25 typically is rough because of oxidation of the steel and is moving it will be necessary to spread the laser beams by means of beam-spreading lens systems 27,29 so that they have a width that is much greater than the width of a typical irregularity of the surface. By this means the beam will cover very many irregularities on the surface, and so the spurious velocity signal which results from a single irregularity of the surface passing through the laser beam as the kiln rotates will be much reduced by the effect of averaging. The frame 15 is not required in this embodiment since the distance of a laser velocimeter from its target is not as critical as is the distance of a displacement measuring device such as an induction probe or a capacitative probe.

To illustrate the orders of magnitude of the quantities involved, some illustrative figures are given below.

Take the steel wall of the kiln to be 25 mm thick with a Young's modulus of 212 GPa, a Poisson's Ratio of 0.291 , and a density of 7900 kg/m³. Take the brick lining of the kiln to be 200 mm thick with a Young^'s modulus of 160 GPa, a PoissonMs Ratio of 0.20, and a density of 2500 kg/πr .

Take the clinker layer attached to the brick to be 100 mm thick with a Young's modulus of 100 GPa, a Poisson^'s Ratio of 0.20, and a density of 2000 kg/m³.

Then the mechanical impedanceZ and flexural wave velocity v will have the following values:

for the steel wall alone:

1

Z = 62,000 Ns/m v = ω x 860 m/s

for the steel and brick in combination:

Z = 2,800.000 Ns/m v = ω ^: x 3.000 m/s

and for the steel, brick and clinker in combination:

Z = 5,700.000 Ns/m v = ω ^: x 3.870 m/s

Thus it is clear that the addition of a brick layer may be expected to make a very considerable difference to the measurable properties of the kiln wall. The further addition of a clinker layer may make a further, clear, although less dramatic difference.

Claims

1. Apparatus for monitoring the thicknesses of both brick and clinker layers within the metal outer wall of a cement kiln, which apparatus comprises: means for applying a known impulsive force to a point on the outside of the kiln wall; means for monitoring the vibration response of that point to the imposed force, so that the mechanical point impedance at that point may be calculated; and means for monitoring at the same time the vibration response a short distance from the forcing point, so that the velocity of flexural waves in the kiln wall can be deduced.

2. Apparatus as claimed in Claim 1, wherein the means for applying the known impulsive force is a hammer with a hard conical tip.

3. Apparatus as claimed in Claim 2, wherein the hammer is fixed to a force transducer, and when used the applied force is known by measuring the output of the transducer .

4. Apparatus as claimed in any of the preceding Claims, wherein the means for monitoring the vibration response is a non-contacting induction or capacitance probe .

5. Apparatus as claimed in any of the preceding Claims, wherein the distance of the probe from the mean position of the wall is of order 1mm.

6. Apparatus as claimed in any of the preceding Claims which also includes computational means whereby from the combination of mechanical impedance and wave velocity, and given a knowledge of the thickness of the steel wall, the densities and elastic properties of the steel, the brick and the clinker, the thickness of the layers of brick and clinker behind the steel may be estimated.

7. Apparatus for monitoring the thicknesses of both brick and clinker layers within the metal outer wall of a cement kiln as claimed in any of the preceding Claims and substantially as hereinbefore described.

8. A method of monitoring the thicknesses of both brick and clinker layers within the metal outer wall of a cement kiln, in which method, using an apparatus as defined in any of the preceding Claims: a known impulsive force is applied to a point on the outside of the kiln wall; the vibration response of that point to the imposed force is monitored, so that the mechanical point impedance at that point may be calculated; at the same time, the vibration response a short distance from the forcing point is also monitored, so that the velocity of flexural waves in the kiln wall can be deduced; and from the combination of mechanical impedance and wave velocity, and given a knowledge of the thickness of the steel wall, the densities and elastic properties of the steel, the brick and the clinker, the thickness of the layers of brick and clinker behind the steel are estimated.

9. A method as claimed in Claim 8, in which the known impulsive force has a bandwidth of at least 5kHz, and imparts momentum of order 1 kg m s- or greater.

10. A method as claimed in either of Claims 8 and 9, in which, from the combination of mechanical impedance and wave velocity, and given a knowledge of the thickness of the steel wall, the densities and elastic properties of the steel, the brick and the clinker, the thickness of the layers of brick and clmker behind the steel are estimated in the manner described hereinbefore, in which:

1) the applied force as a function of frequency is obtained via the Fourier transform of the applied force;

2) the velocity at the forcing point as a function of frequency is obtained via the Fourier transform of the differentiated displacement; and

3) the mechanical impedance is obtained from the ratio of these two calculated quantities and then :

4) the Fourier transform of the vibration response is obtained;

5) the phase of this transform is obtained; and

6) the velocity as a function of frequency is obtained; thereafter, in order to further determine the thicknesses of brick and clmker layers, the bendmg stiffness (9) and mass per unit area (10) of the kiln wall are found from the mechanical impedance (7) and the velocity of flexural waves ( 8) , and finally, these quantities are used to determine the thickness of the brick layer and the thickness of the clinker layer.

11. A method of monitoring the thicknesses of both brick and clinker layers within the metal outer wall of a cement kiln as claimed in any of Claims 8 to 10 and substantially as described hereinbefore.