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Technical Note

Structural design of a probe system for a

high-temperature pressurized fluidized-bed combustor

G J Thompson, N N Clark and J E Smith*

Center for Industrial Research Applications, West Virginia University, Morgantown, West Virginia, USA

Abstract: A lack of information about the operating characteristics of a jetting pressurized fluidized-bed combustor (PFBC) can be attributed to the environment (1227 K and 2.93 MPa) in the bed. To gain information about a PFBC’s operation, a pressure and temperature probe has been designed to measure the small pressure fluctuations and bulk fluid temperature in the feed jet area.

This research concentrates on the structural design and analysis of a multi-tier dual-static-pressure and temperature probe system. Each tier contains two branches placed one above the other in the axial direction; they are arranged to measure the differential pressure and temperature at a given radial and axial location in the feed jet. Each branch is a structural tube member, of 6.35 mm outside diameter, with a smaller tube, of 1.07 mm inside diameter, placed inside for the pressure measurement. A thermocouple is placed on the upper branch of each tier for the temperature measurement. Inconel 600 alloy is selected for the structural and pressure measurement tubing and thermocouple sheathing material because of cost, availability and manufacturing concerns. This paper presents the results of the structural design of the probe system to be fitted in the bed.

Keywords: computer-aided design, analysis, flow measurement, modelling, pressure measurement, vibration

NOTATION

inside diameter (m) d_i

d_o outside diameter (m)

1 INTRODUCTION

A complete design for a jetting pressurized fluidized- bed combustor (PFBC) pressure and temperature measurement system requires both fluid dynamic and structural analyses of an intrusive probe within the bed.

The general concern for most fluidized-bed probe measurement systems is the sizing and spacing of the probe tubes so that the intrusion effects are minimized while the information gained about the bed is maximized, as discussed by Atkinson and Clark [1] and Rowe and

Masson [2]. A wide range of probe types has been discussed in the literature, including optical probes by Oki et al. [3, 4] and Ishida and Tanaka [5], pressure probes by Sitnai [6] and Geldart and Xie [7], a novel pneumatic probe by Flemmer [8] and Flemmer et al. [9]

and capacitance probes by Werther [10]. In room-tem- perature laboratory-scale beds, probes may be delicate but, in hot pilot-scale beds, the probe must be robust to withstand the operation.

To gain useful information about the jet a multi-tier dual-static-pressure probe layout was adopted for the design. The stresses imparted on the probe result from the jet momentum and solid particles (coal) impacting the surface of the probe exposed to the jet. Further information on the probe response and fluid dynamic aspects for this design has been detailed by Clark and Karamavruc [11]. Driving design parameters include cost, ease of installation and manufacturing concerns.

2 BED CONFIGURATION

The research bed for this work is a vertical pressure vessel, nominally 0.254 m diameter by 3.048 m high, containing combustion gases and solid particles from

The MS was recei6ed on 7 January 1999 and was accepted for publication on25January 1999.

*Corresponding author: Centre for Industrial Research Applications, 339Engineering Sciences Building, West Virginia Uni6ersity, Morgan- town, WV26506-6106, USA.

A00399 © IMechE 1999 Proc Instn Mech Engrs Vol 213 Part A

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the coal combustion process. The bed operates at a nominal 2.93 MPa and 1227 K for up to 3 weeks at a time. The feed area for the reactants starts at the lowest portion of the bed with a 0.076 m diameter;

the area becomes progressively larger in diameter in the axial direction. A vacant port is located near the feed jet at the base of the bed; a simplified cross-sec- tional view of the bed and port area is shown in Fig.

1. It is estimated by Smith et al. [12] that the jet penetration length will end in this region.

3 STRUCTURAL DESIGN

The structural design of the probe for the pressure and temperature measurements in the bed depends on the

availability and mechanical limitations of materials suitable for the bed environment. The probe must be stiff enough to withstand the forces from the fluid motion and thermal growth but small enough to mini- mize its intrusion into the flow field. Multiple probes in the jet area of the bed enable the maximum amount of information to be retrieved, a desirable feature since the unit operation time available for the testing is normally limited and adjustments to the probe position would prove costly.

Common materials (steel and stainless steel) can be eliminated from the design analysis since these materials will not withstand the stresses imparted by the jet owing to their low yield points at high temperatures.

All design work assumes that the probe must have a maximum stress less than the 1000 h rupture stress for

Fig. 1 Simplified bed cross-section

Proc Instn Mech Engrs Vol 213 Part A A00399 © IMechE 1999

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Table 1 Inconel 600 alloy material property [13]

Value Units

Property

12.4 MPa

1000 h rupture strength

MPa 35

Yield point

MPa

Tensile strength 70

GPa Modulus of elasticity 144

GPa 54

Shear modulus of elasticity

—

Poisson’s ratio 0.338

MPa 50

Rotating beam fatigue strength

1.0×10⁸ cycles

since the bed, and hence the jet area, increases in size in the axial direction, assuming constant mass flux through the jet area. A temperature loading can also be applied to examine the thermal growth of the structure. For this analysis, an assumed constant temperature of 1255 K is used for the temperature loading;

however, Haji-Sulaiman et al. [15] found there can be as much as a 100 K temperature difference in the jet area.

Three different restraint models have been assumed for the analysis. For each restraint set, a linear statics analysis incorporated the pressure loading together with the 1255 K temperature loading to examine the thermal expansion. A normal mode dynamics simula- tion was performed to determine the structure’s natural frequency. The first model uses fixed translational restraints on the outermost portion of the disc as shown in Fig. 3a. This model is not a realistic boundary condition since it results in excessive stresses for the 1250 K temperature loading, but it serves as a refer- ence for the other two models, which allow thermal growth. The second restraint set, shown in Fig. 3b, assumes no axial (with respect to the access port) movement at the top half of the outer periphery of the disc but provides for the thermal growth. The last restraint set assumes no axial movement, but the X and Z restraints have been moved to the outer peripheries.

Table 2 summarizes each analysis. The linear static solutions for analysis sets 1, 4 and 6 are similar. Each set has the same 8.76 MPa maximum stress at the branch – stem interface on the upper two branches. A maximum deflection of 0.025 mm is given in analysis set 1 because of the bending. A maximum deflection of 1255 K. This requirement imparts a safety factor in the

design since the bed operates at 1227 K for 500 h.

3.1 Material selection

The Inconel and Incoloy alloys from Inco Alloys have been selected as probe materials because these nickel – chromium alloys maintain their strength in high-temperature environments. Specifically, the Inconel 600 alloy has been selected for its strength, availability of bar and tube stock, and cost; Table 1 is a summary of the material properties [13]. Although other materials, both metals and ceramics, that have better physical properties than Inconel 600 alloy could be obtained, the resulting cost increase to connect the probe to the transducer made these materials unacceptable.

3.2 Probe analysis

The geometric design of the multi-tube probe com- prises three sets of thick-walled tubing joined to form a single rigid structural member. The ends of each set of tubes are arranged to record the pressure information while maintaining a sufficient amount of space from the rest of the probe assembly; the design is illustrated in Fig. 2. The members extend from the access port and only reach to the middle of the bed centre-line at the furthermost branch location. The design consists of three sets of probes, a disc and a web section joined together.

A finite element analysis (FEA) using the I-DEAS [14] linear static solution module has been performed to analyse this design. The mapped meshing option in I-DEAS results in the use of eight-node (linear) brick elements; it also decreases the number of nodes and elements used to represent the geometry and increases the accuracy of the solution.

The loading on the structure is assumed to consist of a pressure loading on the two sets of upper branch faces exposed to the jet. The pressure is assumed to be equal to the drag pressure where the velocity is assumed to be equal to the reactants (fluid and coal) momentum average exit velocity of the jet at the feed

tubes. The velocity of the flow is lower at the probe Fig. 2 Solid model shaded images of the probe design

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Fig. 3 (a) Restraint set 1, fixed translation on the outer disc; (b) restraint set 2, fixed Y translation on the outer disc and fixed X and Z translations at the branch – web interface; (c) restraint set 3, fixed translation on the outer disc with rotational restraint about the Y axis via X and Z restraints

2.72 mm is estimated for analysis sets 4 and 6 and is due to the thermal expansion of the material at the 1255 K temperature loading. The displacements in analysis sets 4 and 6 were checked for interference between the stem and branch; no such interference was found. The results from analysis set 2 illustrate the effects when a model is over-restrained. The results from this model cannot be used since the material exceeded its yield point (and ultimate strength), but

they demonstrate that this probe could be used if it were designed to expand in the refractory lined access port.

The natural frequency, approximately 380 Hz, for each of the three restraint sets does not appear to be dependent on the restraints that were specified. The first mode results in the probe structure rotating about the Z axis. The largest stress occurs at the base of the stem at the plate as shown in Table 2. The natural frequency

Table 2 Finite element analysis solution summary

Analysis set Result summary

The maximum deflection, 0.025 mm, occurs on the upper branch in the Z direction. The maximum 1 Restraint set 1: pressure loading

von Mises stress, 8.76 MPa, occurs between the branch and stem

2 Restraint set 1: pressure and The results for this case illustrate the need for thermal growth. The stress results are invalid since the thermal loading stress is above the yield; non-linear effects are not taken into account since a linear static analysis was

performed

The first mode is 382 Hz. The probe will rotate about the Z axis 3 Restraint set 1: normal mode

dynamics

Restraint set 2: pressure and The maximum deflection, 2.72 mm, occurs on the upper branch in the Z direction. The maximum von 4

thermal loading Mises stress, 8.76 MPa, occurs between the branch and stem The first mode is 380 Hz. The probe will rotate about the Z axis 5 Restraint set 2: normal mode

dynamics

The maximum deflection, 2.69 mm, occurs on the upper branch in the Z direction. The maximum von 6 Restraint set 3: pressure and

thermal loading Mises stress, 8.76 MPa, occurs between the branch and stem The first mode is 380 Hz. The probe will rotate about the Z axis 7 Restraint set 3: normal mode

dynamics

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is high enough for this design to be used in the bed.

The effects of the element type (linear or parabolic) on the results were examined using analysis set 4 boundary conditions. This study shows that the total displacement for the linear element representation (2.69 mm) is within 1 per cent of the parabolic representation (2.72 mm), but the von Mises stress in the linear element discretization (8.76 MPa) underpredicts the stress compared with the parabolic elements (9.93 MPa) by 13.4 per cent. The linear element solution takes approximately 1126 s of central processing unit time whereas the parabolic element solution requires over 21 950 s, over a nineteenfold increase in the solution time. Although a linear element representation underpredicts the stress by approximately 13 per cent compared with a parabolic element representation, the resulting stress (9.93 MPa) from a parabolic element analysis is still less than the 1000 h rupture strength, and the linear model was deemed acceptable for the remainder of the analysis. Since the calculated stress from either the linear or the parabolic element representation is significantly less than the fatigue strength (50 MPa at 1250 K and 1.0 x 10⁸cycles or about 25 Hz at 1000 h), fatigue should not pose a problem for this probe design.

4 CONCLUSIONS

The design and analysis of a high temperature pressure probe have been reviewed for application into an exper- imental PFBC. Each branch of the probe consists of individual structural tubing, 6.35 mm d_oby 3.05 mm d_i, with the pressure measurement tubing, 1.59 mm d_o by 1.07 mm d_i, placed inside. The Inconel 600 alloy has been selected as the probe material for its good mechanical properties at the bed temperature, low cost and availability. The probe consists of three tiers, each tier consisting of a set of the structural – pressure measurement tubing. A web section is used to stiffen the tube assembly so that the maximum stress is modelled to be less than the 1000 h rupture limit. Thermocouples are placed on the top branch of each tier for temperature measurement. The ceramic plug encases the probe assembly so that it can be placed in the access port. The pressure measurement tubing is connected to a differential pressure transducer, which is in turn connected to a data acquisition system to record the pressure reading.

REFERENCES

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