AlllDD TTS3Efl

NAT'L INST. OF STAND & TECH

nil nil I II I III! II I II nil II AlllOb MQ73M^

NBS

UBLICATIONS

JBSIR 81-2415

Furnace Pressure Probe

Investigation

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Washington, DC 20234

November 1 981

100 J.s. DEPARTMENT OF COMMERCE

.U56

81-2415

1981

c. 2

NATIONAL BUREAU OF STANDARDS
i
RATIONAL BUSBAO

or btandaxss

ltbrajit

NBSIR 81-2415

FURNACE PRESSURE PROBE

INVESTIGATION

JAN 8 1982

Quoo

m19^1

James S. Steel

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire. Research

Washington, DC 20234

November 1 981

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

LIST OF TABLES iv

LIST OF FIGURES iv

Abstract 1

1. INTRODUCTION 1

2. DESCRIPTION OF PROBES TESTED 4

3. DEMONSTRATION OF VELOCITY SENSITIVITY 6

4. RESULTS OF THE FIRST DEMONSTRATION ' . 8

5. DEMONSTRATION OF PRESSURE PROBE PERFORMANCE

IN A FURNACE ENVIRONMENT 10

6. RESULTS OF THE SECOND DEMONSTRATION 12

7. CONCLUSIONS 13

8. RECOMMENDATIONS 14

9. REFERENCES 15

iii
LIST OF FIGURES

Page

Figure I. Probes 17

Figure 2A. ISO Sketch 18

Figure 2B. ISO Drawing 19

Figure 3. Photograph of Apparatus 20

Figure 4. Probe and Fan Angles 21

Figure 5. "Circuit” Diagram of Pressure Switch 22

Figure 6. Record from a Furnace Experiment 23

Figure 7. Probe Response Relative to Group Average 24

LIST OF TABLES

Page

Table 1. Pressure Developed by Probes 16
FURNACE PRESSURE PROBE INVESTIGATION

James S. Steel

Abstract

Several pressure probes of types used in

fire endurance furnaces were tested at room

temperature and compared in a furnace. None

of the probes were capable of reliable static

pressure measurement in a furnace environment.

It is suggested that the total pressure normal

to the specimen surface be measured instead of

static pressure and that the most accurate

probe is a small flush hole in the specimen.

1. INTRODUCTION

Many probes have been designed to measure the static pressure

in fire endurance furnaces. The original object of this investigation

was to determine which probe design was most accurate, i.e., which

probe generates the correct value of static pressure in a furnace

environment. All the probes tested represent designs which are

actually in use.

1
Furnace pressure is known to be a significant parameter in the

fire endurance rating obtained in tests of doors, walls, and floor

assemblies since one criterion of integrity in such tests is the

passage of flames or gases hot enough to ignite cotton [1,2,3].^ The

specification of a positive furnace pressure is consistent with

observations and measurements of pressures developed within burning

rooms and compartments [4]

ISO Standard 834 "Fire Resistance Tests - Elements of Building

Construction” [2] states that the furnace static pressure shall be

measured and controlled so that a positive pressure is maintained

over the upper two-thirds of the door. This report discusses the

problem of measuring the pressure to which a fire test specimen is

exposed and summarizes the results of experiments conducted to

compare pressure probes which are sometimes used to measure small

pressure differences in a furnace environment.

McCaffrey and Rockett [5] used tubes flush with a wall for

pressure measurements in their investigations of enclosure fires.

Owen and Parkhurst [6] give a comprehensive description of the

problems of measuring static and velocity pressures. They state

”. . . static tubes are much more sensitive to angular deviations

than pitot tubes.” Our data strongly confirm this statement.

^Numbers in brackets refer to literature references listed at the end of

this paper.

2
In fluid mechanics, distinction is made between total pressure,

static pressure, and velocity pressure. Historically, static pressure

has been the parameter selected for measurement in fire endurance

furnaces. This was a questionable selection since specimen penetration is a function of the sum of the static pressure and the component

of velocity pressure normal to the specimen surface rather than

merely the static component. This choice of measurement parameter

led to the development of several "static" pressure probes, some of

which were tested here. All of these probes can be expected to

develop the correct static pressure when the velocity component of

pressure is neglibible. When the velocity component is significant,

then probe responses may differ.

It is important to keep in mind that while the probes were

designed to measure static pressure, the actual parameter to which

the specimen responds is the sum of the pressures perpendicular to

its surface. This pressure is easy to measure; the appropriate

probe is simply a small hole in the specimen surface that can be

connected to a transducer.

The pressure can be expected to vary over the surface of a

specimen due to motion of gas within the furnace.

3
2. DESCRIPTION OF PROBES TESTED

The eight probes tested in the first experiment were selected

as being representative of probes actually used in fire endurance

testing. Drawing of the probes are given as figure 1.

ISO Standard 3008 [3] includes a sketch (see figure 2k) of a

probe for monitoring furnace pressure. A scale drawing of the probe

design is given as figure 2B. Dimensions not given in the sketch

were arbitrarily selected; most notable of these are the bend radius

and the length of vertical tubing after the bend. In any event,

construction of this probe would be complicated by the necessity of

bending large thick walled steel tubing on a small radius. The

probe actually constructed (probe 1 in figure 1) was much easier to

build, and probably responds to air currents in much the same way as

would a standard ISO probe. In fact, since the bend radius and

vertical height were not specified in the ISO standard, an argument

could be made that probe 1 is essentially an ISO probe.

Probe 2 is a tee made from 19 mm outside diameter tubing. It

is identical to probe 1 except no baffle is included.

Probe 3 is a 25 mm diameter stainless steel disc, 6 mm thick.

The sensing hole is 3 mm in diameter and is drilled through the

center of the disc. The probe is referred to as a "lollipop."

4
Probe 4 is identical to probe 3, except the edges of the disc

are rounded. This probe is a classic static pressure probe in fluid

mechanics and must be aligned so that the sides of the disc are

parallel to the flow. This probe is called a "round-edged lollipop.

Probes 5 and 6 are plain open tubes, of 4.75 and 9.50 mm

diameter, respectively.

Probe 7 is a closed-end tube, 9.5 mm diameter, with 4 cross

drilled holes, each 1 mm diameter. This probe resembles the static

pressure measuring component of probes used in many commercial

versions of combination pitot and static tubes. This probe is

called a "perforated tube."

Probe 8 was constructed from a cast iron pipe cap turned to a

spherical shape. Five sensing holes, each 1 mm in diameter, were

drilled in the sphere in mutually orthogonal directions. The original probe was used in the first demonstration. A second similar

probe constructed of stainless steel was used in the second demonstr

ation furnace (furnace test) because the epoxy used in mounting the

first probe made it unsuitable for high temperature use. This probe

is called the "tea ball."

5
3. DEMONSTRATION OF VELOCITY SENSITIVITY

There are three types of errors which can be attributed to

pressure probes. These are: (a) inaccurate transmission of static

pressure, (b) pressure head due to height and density difference

(the chimney effect), and (c) dynamic response of the probe.

Errors in pressure transmission can be caused by factors such

as clogged or restricted probe orifices, foreign substances in

pressure lines, and leaks. These reflect poor experimental technique,

not faults in probe design.

The chimney effect is directly proportional to the height of

the "chimney” formed by the vertical extension of probe tubing

inside the furnace. In all of the probes tested here, this height

was zero and the chimney effect (pressure head) was not investigated. The chimney effect can be significant in some cases, e.g., a

pressure probe mounted vertically in a floor assembly.

The ISO standard 3008 specifies a probe with a non-zero chimney

height (figures 2 A and B) . This height is not given but could

typically be 8 cm or less. Calculation shows the chimney effect

error to be about - 1.2 Pa for a furnace temperature of 700°C and a

8 cm chimney.

6
The major potential source of error is velocity sensitivity.

While the absolute sensitivity of a probe to velocity will vary with

gas density, which is inversely proportional to gas temperature, the

relative sensitivity can be expected to remain unaltered.

The experimental method chosen was to use a stream of room

temperature air directed toward the probe at various angles. The

pressure developed by the probe in moving air was measured. The

number of readings required was considerably reduced by symmetry

considerations.

By this method, the additional uncertainties of operating in a

furnace environment were avoided. The experiment as expected yielded

a consistent and valid comparison of behavior of several probe types

in substantially identical environments.

The experimental apparatus was quite simple, as can be seen in

figure 3. A ducted fan was used to blow air toward the various

probes. There were chalk marks on the building floor to aid in

placing the fan. The probes were mounted on a vertical plywood

board, 1.2 meters square. The pressure developed by the probe was

measured using a pressure transducer and carrier demodulator, with a

resolution of 0.25 Pa. Air velocity near the probe location was

measured using a thermocouple anenomometer

7
Fan angle is the angle between the fan axis and the probe axis;

i.e., the fan blows perpendicular to the wall at a fan angle of 0°,

and parallel to the wall at a fan angle of 90°.

Probe angle is the angle between a line connecting opposite

openings in the probe and the horizontal plane. The concept of

probe angle is meaningless for plain tubes and holes, however, for

information purposes these data are presented in the 0° probe angle

column in table 1. Figure 4 illustrates the definitions of fan and

probe angles.

Probe distance is the distance from the opening(s) in the probe

and the wall surface. In the case of the ISO probe, the distance is

specified as 25 mm. For the tea ball probe, this distance is taken

from the center of the probe.

4. RESULTS OF THE FIRST DEMONSTRATION

Results of pressure measurements listed in table 1 show values

ranging from -5.0 to +4.2 Pa. Pressures fluctuated by about +1*5

Pa, and the recorded values represent estimated averages of pressure

readings

The repeatability of test results can be checked at a fan angle

of 0°. Since the fan was blowing parallel to the probe axis, the

8
pressure should not change when the probe is rotated. These pressures

were constant to within + 0.25 Pa, which was the resolution of the

pressure sensing apparatus.

At any particular fan angle, pressure differences between

probes indicate that at least one of the probes is sensitive to

fluid velocity. When the fan angle was 45° and the probe distance

was 50 mm, two probes read -5.0 Pa and one probe read +1.7 Pa.

When fan angle and probe distance are constant, variation of

the pressure with probe angle is a strong indicator of velocity

sensitivity. The data at a fan angle of 45° show considerable

variation of pressure for probes 1, 2, 3, and 4 and some variation

for probe 8.

Probe 4, the round -edged lollipop, is expected to develop the

correct value of static pressure when the air flow is parallel to

its sides. This was true when the probe angle was 90°. Since the

correct value of static pressure is known, we can determine the

performance of the other probes and probe positions by comparison.

The results at a fan angle of 0° and a probe distance of 25 mm

are not especially enlightening. In this instance, no effect of

probe angle was expected and there was little difference in performance

among probes

9
At a fan angle of 45° and a probe distance of 25 mm, variations

in probe performance become quite marked. The correct value of

static pressure was +1.2 Pa, as measured by probe 4 with a probe

angle of 90°. All the other probes, except probe 1, indicated

negative pressures under this condition. But even the probes 1 and 4

do not show this correct value under all conditions. Both of these

probes generated negative pressures as the probe angle was changed.

Similar but less conspicuous results were obtained with other fan

angles and probe distances.

From these results, it is apparent that none of the probes

tested can be depended upon to generate the correct value of static

pressure when exposed to an air flow from an unknown direction.

5. DEMONSTRATION OF PRESSURE PROBE PERFORMANCE

IN A FURNACE ENVIRONMENT

A second demonstration of probe performance was conducted in

the NBS fire endurance pilot furnace. Five probes were compared in

a furnace environment during the fire endurance test of a floor/

ceiling specimen.

The probes tested were: the plain hole (3.2 mm dia) , the round

edge lollipop, the tea ball, the perforated tube, and the ISO probe.

A second tea ball was fabricated of stainless steel for this experiment since the epoxy used in fabrication of the first probe was

10
unsuitable for furnace use. The ISO probe was modified as described

above for ease of construction.

The pressure in the pilot furnace was expected to fluctuate

during the test, and it did. Since our purpose was to compare

probes, not to measure furnace pressure, the actual value of furnace

pressure was of secondary importance. The value sought was the

difference in the pressures developed by the various pressure probes

in the furnace environment. The apparatus was arranged to produce

these results. This was done by connecting one pressure probe to

the positive port of a differential pressure transducer and a second

pressure probe to the negative port. The transducer output is then

proportional to the difference in the pressures developed by the two

probes and the effect on the output of furnace static pressure

fluctuations is minimized.

In the actual experiment, only one transducer was used. The

transducer was connected between successive pairs of probes by a

rotary selector valve, the fluid analog of a 2-pole, 12-position

electrical switch. This required only one transducer and one chart

recorder (2 pens) and eliminated errors caused by mismatching transducers and transducer drift. A "circuit" diagram is given as figure

5. In position 0, the transducer was open to the air, giving a

check on the transducer zero output. In position 1, the actual

11
total furnace pressure was measured using the flush hole probe. The

remaining 10 valve positions compared successive pairs of probes.

The transducer output and a signal proportional to switch

position were fed to the strip chart recorder. The valve position

was advanced every 30 seconds. An active filter with a 1-second

time constant was used to condition the transducer output before

recording. Figure 6 is a graph of transducer and switch position

output from one of the experiments. In position 1, the actual

furnace pressure is recorded and the fluctuations can be easily

seen.

A major limitation of this technique is inadequate knowledge of

the flow pattern in the furnace. The differences in probe response

can be measured, but since the cause of the variation is not known,

then the error function is not well defined.

6. RESULTS OF THE SECOND DEMONSTRATION

The data from this demonstration present some problems of

intrepretation. Several readings were taken over the course of each

test; there is no real standard of performance for comparison and no

certainty in the knowledge of air speed and direction.

12
Rather than select one probe as a standard, the data are

presented as deviations from the group average pressure. Since only

deviations were measured, this scheme has the advantage of not

requiring a knowledge of the furnace pressure. Examination of the

data in figure 6 will show the difficulty of determining the actual

pressure in position 1. This "noisy" signal is actually a filtered

representation of the furnace pressure. In subsequent positions,

this varying pressure was imposed on two probes, and the measurement

was made of the differential pressure. The reduction of the noise

level results from canceling the effects of furnace pressure variation. This noise canceling effect demonstrates that the "noise"

level is very similar in all of the probes tested. Figure 7 shows

the deviation of pressures generated by the probes from the group

average

The two probes with the greatest deviation are the flush tube

and the round-edged lollipop. Since the direction of air flow is not

known, it cannot be assumed that the round-edged lollipop probe

measures the correct value of static pressure. What has been shown

is that different probes in the same environment may generate different

pressures

7. CONCLUSIONS

In the first demonstration, the velocity sensitivities of eight

probes were measured at certain flow angles at room temperature.

13
These data show that probe design has an appreciable effect on the

pressure value obtained when air velocity is a factor. In the second

demonstration, five of these probes were compared in a furnace environment. The data show a maximum deviation between probes of about

0.7 Pa.

The demonstrations show that errors can result from probe

selection. What was not shown by experiment is which probe design

gives the most accurate results. The pressure normal to the specimen

is more representative of specimen exposure than static pressure.

The simplest and most accurate probe is a small flush hole in the

specimen

8. RECOMMENDATIONS

It is recommended that probes of the flush hole type be installed

in the specimen mounting frames for fire endurance tests of walls and

floors. For tests of windows and doors these probes should be installed

in the walls near the specimen. When the fire resistance of the

specimen would not be compromised, the probe should be a small tube

passing through the specimen, with one end flush with the fire

exposed face.

14
9 . REFERENCES

[1] ASTM E 119-79, Standard Methods of Fire Tests of Building Construction and Materials, Part 18, Annual Book of ASTM Standards, 1979.

[2] ISO 834, Fire Resistance Tests - Elements of Building Construction,

International Organization for Standards, 1975 plus Amendment 1 (1979).

[3] ISO 3008, Fire-Resistance Tests - Door and Shutter Assemblies,

International Organization for Standards, 1976.

[4] Fang, J. B., Static Pressures Produced by Room Fires, Nat. Bur. Std.

(U.S.), NBSIR 80-1984 (1980).

[5] McCaffrey, B. J. and Rockett, J. A., Static Pressure Measurements of

Enclosure Fires, J. Research of National Bureau of Standards, V. 82,

No. 2, 107-117, 1977.

[6] Owen, E. and Parkhurst, R. C. , The Measurement of Air Flow, Pergamon

Press, New York, 1977, p. 48.

15
Fan

Probe

Probe

12345

67

8

Angle

Distance

Angle

ISO

TEE

SQ

LP

RD

LP

4.75

TUBE

9.5

TUBE

PERF

TEA

BALL

VELOCITY

CO CO

CO 'b 'e "e

\D rH m 00 lO o o rH

I I CN CN 1 o in 1 1 1 1 m 1 m 1 1 1 1 1 CN CN

• • 1 • • I 1 1 1 • • 1 • • 1 1 1 1 • • 1 • • CsJ CM CO cn CM CO CO cn o o rH rH

I I I I I I I I

CM

'cr

I I

I I o

CN m 1 LO

• • 1 • • CN CM CN CN

I I

I I

I I

to m 1 m 1 1 1 1 O 1 CN m • 1 • • 1 1 1 1 • 1 rH

1

t—

1 t—1 1 O

1 rH

1 1 rH

1 m m CM

•

1 1 • 1 1 •1 1 CM 1 1 • 1 CO 1 1 cn 1 1 o 1 1 • 1 1 rH 1

1 1 + 1—1

caa

CO

ca Puc

•H

I m I I I'll CM

I I

I I o

i I in I I

I I 'S'

I

I I O 1 1 o 1 1 m 1 1 O

• 1 1 1 1 • 1 1 m o CM cn

CO

<uM

3

toto

<uu

PLi

0)P.o

I—

0)>

(1) Q

I I CM n.

I I • • • CM CM CM

I I

I I

CM O O

'3'

I I CM m CM

I I

t—I O I—

I + oI + +

I in CM

I • • • O iH O

I I I

I I I I I I I I I

O

O iH

I I I

in m CN m CM 1 1 1 m m lO 1 1 1 o o CN m • • * • • 1 1 1 • • • • • • 1 1 1 • • • • « • Cs| CN CN CN CN CM CM rH O CN rH o CM CN rH CO CO rH

I I I

o UO m m 1 1 1 m o UO CN O 1 1 1 o LO o o UO

• • • • • • 1 1 1 • • • • • 1 1 1 • • • • • • CN CO CN CN CN CM 1—

o rH <1* m o rH CN O CN CN

I I I I I I I I I I I

CM CM in o o

cn cn cn cn cn

I I

I I o in m

O CM o o .H

I +

I I t

I I t m m CM o CM

CM CM I—I 1—I O

I I I I I

t I I

I I I

o m o m o 0^ Ml- Ml- ON

I I o m o

Mf CTi o m o m o cyi Ml- Ml- tTi

I I o m o ^3- <y> o m o m o

<y> Mf mt ON

I I o m o

Mjin m m m m

CM (M CM CM CM o o oin m in

in m m m m

CM CM CM CM CM o o o

in in in m m m m inCM CM CM CM CM o o o • m m in

o o o o o o o o m m m in m m

MT MT Mj- MT Ml- m m in

-3- Mt mt o

a\

o o o o o

o> o^ o^ o^ o o o

C7^ <T\ CTi

16
Figure 1. Probes

17
K

(/i

c

0

>

UJ

c

o

u

18

FIGURE

2

—

Static

pressure

probe
GOC

*Hs

nJMPOWMP

CN

0)

GO

•H

19
Ui

20

Figure

3

.

Photograph

of

Apparatus
PLAN

ELEVATION

Figure 4. Probe and Fan Angles

21
00

00

00

UJ

00

oe

o.

22

Figure

5.

"Circuit"

Diagram

of

Pressure

Switch
Figure 6. Record from a Furnace Experiment

23
FLUSH TUBE

[

c

J L

ROUND EDGE LOLLIPOP

TEA BALL

PERFORATED TUBE

ISO

J L

-1.0 -0.5 0 +0.5 +1.0 Pa

PROBE RESPONSE RELATIVE TO GROUP AVERAGE

Figure 7. Probe Response Relative to Group Average

24
U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NBSIR 81-2415 December 1981

4. TITLE AND SUBTITLE

Furnace Pressure Probe Investigation

5. AUTHOR(S)

James S. Steel

6. PERFORMING ORGANIZATION (If joint or other than MBS. see instructions) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

10.

SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Several pressure probes of types used in fire endurance furnaces were tested at room temperature and compared in a furnace. It is suggested that total pressure

be measured instead of static pressure and that the most accurate probe is a small

flush hole in the specimen.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Fire endurance; furnaces; pressure probes; static pressure

13.

AVAILABILITY 14. NO. OF

PRINTED PAGES

Uni imited

I I For Official Distribution. Do Not Release to NTIS

I I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

28

IS. Price

Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $6.50

USCOMM-DC 6043-P80