NUREG/CR-3242

SLA-9929- M S

RI

The Los Alamos National Laboratory/

New Mexico State University

Filter Plugging Test Facility

Description and Preliminary Test Results

D. L. Fenton*

J. J. Dallman*

P. R. Smith*

R. A. Martin

W. S. Gregory

Manuscript submitted: October 1983

Date published: October 1983

Prepared for

Division of Risk Analysis

Office of Nuclear Regulatory Research

US Nuclear Regulatory Commission

Washington, DC 20555

NRC FIN No. A7029

*Mechanical Engineering Department, New Mexico State University, Las Cruces, NM 88003.

Los LosAlamos National Laboratory /J aU 'i_©__Los Alamos, New Mexico 87545
f
THE LOS ALAMOS NATIONAL LABORATORY/NEW MEXICO STATE UNIVERSITY

FILTER PLUGGING TEST:FACILITY

Description and Preliminary Test Results

by

0. L. Fenton, J. J. Dallman, P. R. Smith, . R. A. Martin,. and W. S. Gregory

ABSTRACT

A facility to test the plugging effects of

combustion products on high-efficiency particulate air

filters has been constructed. This facility can supply

experimental data to support pressure-drop models .of

filter plugging under fire accident conditions, which

are needed for use in an existing fire accident

analysis computer code. The test facility includes--a

specially designed null-balance filter-weighing

system. The resolution of this system is approximately

2 to 3. .g out of 14 kg for a commercial 0.6- by 0.6-m

filter. Using this system, commercial filters can be

tested to provide data with which to correlate pressure

drop and smoke aerosol mass accumulation and flow.

rate. Some recently accomplished tests and future test

plansý are discussed.

I. INTRODUCTION

A. Background

Most nuclear facilities depend on ventilation systems to bring air into,

through, and out of them. Because the air passes through contaminated areas,

1
most ventilation systems contain air cleaning systems of which the highefficiency particulate air (HEPA) filter is an important component. A standard

HEPA filter consists of pleated filter material enclosed by a plywood or metal

frame. The filter material (which is made up of a thin mat of fine, intertwined

glass fibers) is folded back and forth around thin sheets of asbestos or alumi1

num,-which are called separators. The nominal airflow capacity of. these filters

is 0.47 m3 /s.

An air cleaning system-may consist either of a series arrangement of these

filters or of banks containing several hundred filters. HEPA filters typically

exhibit cleaning efficiencies of 99.97% or better, and operate very effectively

in most noncorrosive atmospheres at normal airflow conditions. However, there

is concern about the degradation of performance or destruction of these units

when they are subjected to accident conditions. The areas of concern are

(1) the structural integrity of the HEPA filter media, (2) the influence of

design and construction on structural integrity, (3) the filtration effectiveness, and (4) the plugging or clogging of the media by combustion products.1

This report deals with area (4).

HEPA filters may be exposed to large quantities of smoke (solid and liquid

aerosol) under fire accident conditions. If the filters become sufficiently

plugged, upstream safety zones that previously were maintained at negative gauge

pressures (slight'vacuums) could assume positive gauge pressures and leak radioactive aerosols. 2

B. Purpose

The Los Alamos National Laboratory filter plugging program is part of a

more comprehensive accident analysis effort sponsored by the US Nuclear

Regulatory Commission. 3 The filter plugging experiment program was begun in

1981, which is when we realized that there were no known data for simultaneous

measurements of pressure drop, flow rate, and combustion product mass accumulation on HEPA filters for realistic fire conditions. These data are needed to

calculate the empirical coefficients used in expressions relating filter pressure drop to volumetric flow rate in the Los Alamos FIRAC accident analysis computer code. 4 An example of a typical semi-empirical expression of this kind is

Ap= Po(l+ rmp) , (1)

2
where

Ap =K + K (.2)

Equation (1) assumes that a linear relationship exists between Ap/Q and mp. In

Eqs. (1) and (2), ApO represents the clean filter pressure drop resulting

from both laminar friction and turbulent dissipation in pascals, and

a filter plugging.coefficient dependent on filter and material

properties• (kg- 1),

m= accumulated mass of material on filter.(kg),

Q = volumetric flow rate (m3 /s), which = VA,

Qo= clean filter volumetric flow rate, which = V0A,

V = velocity (m/s),

A = filter cross-sectional flow area (m2

KL, KT = dimensionless laminar and turbulent coefficients, respectively,

= air dynamic viscosity (Pa's,) and

p = air density (kg/mr3).

Other more complicated theoretical models of filter plugging are available

in the literature, such as the dendrite and increasing fiber models. 6 The

models quantitatively predict filter plugging as idealized cases in which the

loading particles either form fiber-like dendrite chains or increase the .fiber

diameter, respectively. (In. actuality, both effects probably are occurring in

the case of smoke.) These models are extensions of existing pressure drop theories for clean filters. A computer code that simulates smoke movement through a

facility must have the capability to alter the flow dynamics in accordance with

material accumulation on the filter. The FIRAC computer code is capable of predicting temperature and smoke distributions throughout a nuclear facility ventilation system. The data developed in this program will be transformed into filter plugging relationships for the FIRAC code, which, automatically will adjust

3
the flow conditions in accordance with the mass of material on the filter. This

simulation capability is essential if an analyst is to predict flow reversals

an-d the effect "of the changing ventilation conditions on the character of the

fire. That is, reduced airflow may create greater smoke production and flow

reversals that will send backflow.to less. contaminated zones-wilthin the plant.

Therefore, the purpose of the Los Alamos filter plugging program is to supply

experimental data to support the generalized pressure drop model:

AP/Apo f(mp (QIQo . (3)

0p .0

This expression relates the filter plugging variables

1. pressure drop,

2. volumetric flow rate, and

3. mass accumulation

in a systematic way. The fQrm of'the polynomial F in Eq. (3) and some empirical

coefficients will depend on other important filter plugging variables, including

4. aerosol type/chemi stry,

5. aerosol concentration,.

6. aerosol size distribution,

7. water vapor,

8. air temperature, and.

9. other things (such as electrostatic effects, flow conditions, and

filter media parameters).

Some theoretical aspects of filter plugging are discussed in Refs. 7--9.

C. Scope

The current filter .plugging test series is part of a Los Alamos fire test

plan to support the development and verification of the FIRAC computer code.

Ultimately, we wish to mode'l and predict •transient combustion product generation

and transport in nuclear fuel cycle facilities using FIRAC. To do this, we need

to develop experimental 'fire simulation and measurement capabilities in the

following areas.

1. Fire simulation and classification

.2. Compartment fire behavior

4
3. Combustion product behavior during transport. (deposition and modification by aerosol dynamic changes)

4. Filter plugging behavior

5. FIRAC code verification

The testing in these areas should include release rate oven, compartment fire,

filter plugging, and ventilation system facility experiments.

To supply the. needed filter plugging data, Los Alamos and New Mexico State

University (NMSU) have constructed a unique filter plugging test facility. Its

purpose is to supply experimental data for 0.61- by 0.61-m HEPA filters under

conditions simulating those postulated as credible for fires in nuclear facilities. Industrial fires, such as these are expected to differ from other kinds of

fires in the types of materials involved and the ventilation conditions (availability of oxygen). A typical fuel mixture may be composed of the materials

listed in Table I. These materials are likely to burn under both oxygen-rich

.and oxygen-starved conditions (over- and under-ventilated conditions) to produce

particulate material, water vapor, and gaseous combustion products. Thus, some

unique capabilities were required of the new facility.

First, we needed the capability to determine the accumulated mass gain of a

clean 14-kg HEPA filter because of smoke and moisture clogging. Previous tests

using polystyrene latex spheres led us to expect plugging (arbitrarily defined

to be a 50% reduction in flow rate from the design value) to occur from an

accumulation of under 500 g of dry solid material. To resolve 2- to 3-g smoke

accumulations out of 14 kg, we designed and constructed a special null-balance

filter-weighing apparatus. Second, we needed the capability to burn mixtures of

TABLE I

TYPICAL FUEL MIXTURE COMPOSITION

Component Composition (%)

1. Polymethylmethacrylate.. 45

2. Cellulosic 26

3. Elastomer 18

4. Polyvinyl Chloride 8

5. Hydraulic Fluids 2

6. Polystyrene 1

5
materials selected from Table I that have variable burning efficiencies. To do

this, Battelle Pacific Northwest Laboratory (PNL) designed and constructed a

special combustor. for Los Alamos.' Lastly, we needed the capability of adding

varying amounts of heat and moisture to the airflow. Heat was supplied by a

commercial air heater, and water was sprayed into the air through a commercially

available nozzle.

This report describes the construction and preliminary check-out of the

filter plugging facility. Full-size HEPA filters initially operating at their

3 design pressure drop and flow rate .of approximately 2.54 cm-ww.g. and 0.47 m /s,

respectively, were selected for testing to ensure realistic results and to eliminate the uncertainty of scaling. (HEPA filters are operated under these same

conditions in nuclear facilities.) However, a special problem resulted from

this experimental approach. Because. the mass of a cleanHEPA filter is on the

order of 14 kg, how could the accumulated particulate mass (on the order of

100 g) be measured reliably? Several design options, were considered, but we

finally decided on a null technique. This technique incorporates a carefully

designed balance and counterweights so that the initial weight of the. HEPA filter can be cancelled or nulled. . During tests when the HEPA filter is being

loaded with particles, the imbalance is measured directly by means of a sensitive force transducer. We achieved the desired mass resolution (2 to 3 g) with

the balance.

Real-time measurement of HEPA filter mass gains was determined to be too

difficult to accomplish at the present time. This would require the.HEPA filter

to be suspended permanently on the force transducer. in such a way as to

subtract-mechanically or electrically--the aerodynamic forces applied by the

flow through the filter. Instead, the flow is diverted from the filter (or

stopped), and it is unclamped from its loading position for the mass-gain

measurement.

The design details of the equipment and some preliminary test results are

described in Sec. II. These results must be considered tentative because they

involve the initial operation of all the equipment. For these tests, we used a

commercial high-volume particle generator to create a stearic acid aerosol.

Stearic acid produces a submicron-size aerosol and is. safe for human contact.

6
II. HEPA FILTER PARTICULATE LOADING EQUIPMENT

The- HEPA filter- being tested was attached to the mass balance and the surrounding ductwork. The ducts-upstream from the test filter bring air and the

test aerosol to the filter; downstream ducts conduct air from the filter and

provide the means for measuring both the pressure drop across the filter and the

actual volumetric airflow rate through the filter. A special branch is located

upstream from the filter to divert the test aerosol from the test filter during

weighing. The equipment and instrumentation are as follows.

1. 3465A Digital Multimeter

Hewlett Packard, Colorado Springs, Colorado

2. 6271A DC Power Supply

Harrison Laboratories, Berkeley Heights, New Jersey

3. No. TD-6-1K Force Transducer

Schaevitz Engineering, Camden, New Jersey

4. 700-1A Soni-Mist Nozzle

Heat Systems Ultrasonics, Plain View, New York

5.. Model 258 Smoke Generator

Hiac/Royco Instruments, Menlo Park, California

6. Model 351 Toggle Clamps

De-Sta-Co, Detroit, Michigan

7. Two Manometers (one inclined and.*one vertical)

Dwyer Instruments, Michigan City, Indiana

8. Pneumatic Cylinders

Miller Fluid Power, Bensenville, Illinois

9. Psychrometer -

Dwyer Instruments, Michigan City, Indiana

10.. Axial Flow Fan

Joy Manufacturing, Chicago, Illinois

A. Air-handling Equipment, Smoke Generator, and Water Spray Nozzle

The air-handling equipment is composed of a fan housed within a box and

both wood and metal ducts. The maximum capacity of airflow through the system

is approximately 34 m 3/min. The wooden ducts are made of 1.9-cm-thick plywood

and are reinforced with 5.1- by 5.1- by..63-cm angle iron; they were constructed

at the NMSU Mechanical Engineering Machine Shop. The metal ducts are made of

7
heavy-gauge galvanized sheet metal and are reinforced with angle iron; they were

fabricated by a local refrigeration contractor. The sheet-metal ducts constitute the three main sections of the air ductwork: (1) the diverging transition

section; (2) the by-pass and by-pass exit ducts; and (3) the converging, transition, and exit ducts. The center of all the ductwork is 1.08 m above the

ground, which allows the assembly process to proceed more quickly than if the

ducts were on the ground., The ductwork for the most part measures 0.61 m by

0.61 m. Figures 1 and 2 show the major components of the system. The ductwork

and the test filter instrumentation are shown in Figs. 3 and 4, respectively.

The fan or air blower is an axial vane fan made by Joy Manufacturing. It

is rated at 68 m3 /min and is powered by a 5.7-kW Dyna Corporation motor

operating at 1720 rpm. The fan is mounted in a wooden box with an adjustable

inlet that allows the airflow rate to be varied. The air is cleaned initially

by a furnace filter at the inlet, which extends the operating life of -the prefilter. To reduce fan losses, a bleed-off branch downstream from the box has a

damper and returns air to the fan box.

The fan box exit is 0.3- by 0.3-m square and includes a vibration isolation

device that connects the fan, box to t.he duct. The vibrations of the fan motor

are damped by the ,isolation device, which is 15.24 cm long and made of a

Adjustable cone

By-pass

Vibration isolator PrefNlter . exhaust duct

B1 eed-off By- pass

0.15 m 0.29 m

F 8i.3 x1•.U

Wind tunnel system-upstream section (top view).

8
HEPA filter

Support frame

Flexible ducts

Flow straighteners

Khaust duct

Pitot tube

. 0.273 m

.Static pressure ports -i I I L- Static pressure ports ..... ~ ~~~0.61 m m , . m O . j .1

0914 0 m 0 914 m 0 914 m

0.29 m'

Lp 7.87 m •

Fig. 2.

Test facility duct work.

Fig. 3.

Test filter instrumentation.

9
flexible, strong fabric that commonly

is used by the ventilation industry.

The vibration isolator is connected to

a 0.3- by 0.3- by 0.61-m sheet-metal

duct. The airflow is made to expand

or diverge smoothly to a 0.61, by

:•-v -0.61-m duct size. This is to accommodate the HEPA filter size used in

testing. The expansion is accom-

-plished by a 1.2-m-long sheet-metal

transition duct in which the wall

angle from the horizontal centerline

is not ,greater than 7

The prefilter -collects particles

greater than 1 or 2 pm, lint, and dust

concentrations greater than 22.9 mg/m 3

as established in Ref. 1. In the nuclear power industry, the main purpose

of the prefilter is to prolong the

life of the" HEPA .filter, whereas at

the NMSU facility the :purpose is to

-create a clean airs.tream into which

g..., . •the generated particles can be inTestfileriinstrumentation.. jected (Fig. 1). The prefilter also

:,,acts as a flow straightener, destroying the large f:lo:6wfeatures created by the fan and transition duct.

The generation duct is a 0.61- by 0.61- by 0.61-m .section with two ports

.(for particle and water spray injectionss); Fig. 5 shows the arrangement of these

injection ports. The smoke or aerosol is produced by a Royco.Model 258 smoke

generator ,(shown schematically in Fig. 5). The smoke generator is essentially a

commercial pressure cooker with six special nozzles placed -.in the lid. Thejet

.nozzles are immersed in a liquid mixture of alcohol and stearic -acid. Each nozzle produces 85 L/min. The generator has an output mass concentration of 5 mg/L

with a median particle diameter of 0.3 pm. (These generator performance specifications are for the .smoke material, dioctyl phthalate or DOP, 10 which was not

used in the filter tests discussed here.)

10
Ultrasonic

I spray nozzle

II

J: __ Z a

II "I;:" I " IL [)_L__.] "• •wa~ater

-]J_

Smoke generator.

Compressed

air

Fig. 5.

Generation duct.

A significant portion of the first experimental HEPA filter loading. tests

involved water mist injection to determine the importance of this variable. 1

Water vapor concentrations were selected to bracket those expected by combustion. The water mist was produced by a Soni-Mist Ultrasonic Spray. Nozzle

(Model 700-IA), which is capable of producing 0.21 m3 /min at 414 kPa. The exact

size of the water spray droplets is unknown, but they are believed to be ini-

-tially in the size range of 5--15 im.

The diverting or by-pass duct allows the operator to pass the flow from the

test branch to a similar branch by means of sliding sheet-metal panels as shown

in Fig. 1. Each panel slides in across the duct to block the flow, but only one

panel is used at. a time (depending on which test condition exists). Either the

filter is being loaded with particles (the by-pass branch is blocked), or the

filter is being weighed (the test branch is blocked). This procedure can be

used to ensure consistent flow conditions (constant air velocity and constant

particle and water droplet concentrations).

The by-pass duct is made of sheet metal and diverges from the test branch

at an angle of 450. The by-pass branch is 0.61- by 0.61-m square and leads to a

11
HEPA filter, which is followed by a converging transition duct and exit. A movable cone at the exit provides adjustable regulation of the flow. Figure 1'

gives the overall dimensions of. the by-pass duct.

After passing through the test filter, the airstream proceeds through;:a

0.61- by 0.61- by 0.61-m wooden duct into a converging sheet-metal transition

duct. The transition duct changes gradually from the 0.61- by 0.61-m square to

a cylindrical duct 0.273 m in diameter. There 'is a series of three 0.91-m-long

cylindrical ducts, resulting in a total length of 2.74 m. The exhaust is at the

end of the third duct, which also contains 'a pitot tube for measuring the centerline velocity pressure. An inclined Dwyer manometer (read in centimeters

water gauge) gives the velocity pressure. Flow straighteners are placed a distance of five. duct diameters upstream of the pitot tube to prevent flow swirl.

The resolution of the manometer is :about 0.005 cm w.g. Because the relation between velocity pressure and velocity is nonlinear, the velocity resolution is

approximately 0.1 m/s.

B. Test Section

The test section of the system is at the HEPA filter being tested. The

test filter is about 4.8 m, or about eight duct diameters, from the branch ductwork. The instrumentation at the test section 'includes a mass balance', a force

transducer, and a vertical manometer to measure static pressure drop across the

filter.

1.., Filter Weighing System. The balance beam'assembly is designed to null

the starting filter weight. The filter'.is attached to one end of the beam and

can be balanced on the other side by moving adjustable counterweights.

The balance is delicate and is.protected from vibration and outside forces

(including people) by a large support structure. The structure is. made of

7.62-cm-square steel tubing with a wall thickness of 0.317 cm. The tubing<is.

welded together into the frame shown in Figs. 6 and 7. The frame is 1.77 m

high, 0.9906 m wide, and 2.24 m long. It is leveled using adjustable legs. The

beam (shown in Figs. 6 and 8) is 1.27 cm thick, 5.1 cm wide, and 160.3 cm long.

It is held rigid by a cradle that balances on a knife-edge. All the components

of the balance beam are made from mild steel to facilitate the machining of the

knife-edge and cradle angles.

A knife-edge' support assembly was selected for the balance beam because of

.its inherent simplicity. The alternative support assemblies that we -considered

12
III I

0.155m

T

1.19 M

0.576 m

1 .77 m

Fig. 6.

Balance and support structure (front view). beam assembly

13
V.

Adjustable stops

Force transducer

Balance beam

Adjustable jack

Adjustable

legs

Fig. 7.

Balance beam assembly and support structure (side view).

14
1.27 cm rod Small counterweight (0.966 kg) Radii of Curvature 5.12 threads/cm

Cradle 0.254 mml

Knife-edge 0.203 mm

Fig. 8.

Balance beam assembly.

required significantly more construction effort and additional utilities (such

as clean compressed air) *for low-friction operation. Faries gives an analysis

of the contact stresses i nvolved that indicates that the maximum shearing stress

is generally the most important one because this stress is responsible for promoting flaking of metal particles from the surface. 1 1 With these considerations

and assuming a safety factor of 4, the knife-edge radius of curvature was calculated as 0.203 mm for mild steel and a length of 10 cm. To achieve the desired

stress distribution, the cradle, which fits atop the knife-edge, had to have a

radius of curvature of 0.254 mm.

The knife-edge component is a solid five-sided polygon 5.1 cm high and

10.1 cm long. (The exact dimensions are 3.8 by 1.9 by 3.8 by 3.8 by 1.9 cm.)

Figure 8 shows the knife-edge and the general location of the beam. The cradle

is 6.52 cm thick, 15.2 cm wide, and 10.1 cm deep. Both the knife and cradle

were inspected under a microscope to ensure that the design values. were

obtained.

The large counterweight mass is 11.16 kg. It can be adjusted by sliding it

along the beam and is secured with a thumbscrew. The small counterweight mass

is 0.966 kg. It is adjusted by turning the weight on a 2.27-cm threaded rod;

two 1.26-cm nuts lock the weight after the 'initial balance has been achieved.

The small mass can produce a resolution of 3.71 g per turn. Figure 8 shows the

dimensions of the counterweights. The maximum and minimum filter weights that

can be nulled are 29.3 kg and 5.86 kg, respectively.

The filter is attached to the beam by a cradle-and-bearing arrangement

(Fig. 8). High-precision, self-aligning ball bearings prevent the filter from

15
binding- or sticking. This is important for the operation of the force transducer and is discussed later. Air cylinders and adjustable stops hold the' test

filter stationary to protect the knife-edge from the destructive horizontal

forces that occur during the particle loading mode. The filter also must be

able to hang freely when a mass measurement is to be taken. Two double-acting

air cylinders fulfill both of these requirements and allow quick conversion from

one mode to the other. The air cylinders are capable of a 15.2-cm' stroke of

which 13.6 cm is used in the actual system. The filter is stopped at a height

equal to that of the duct by an iron frame or adjustable stop that is connected

to the support frame. Figures 6 and 7 show the exact positioning of the cylinders and stop.

The particle mass collected by the filter is measured by a force transducer. The transducer has a range from 0 to 1000 g and has a reported linearity

error of 0.6 over the full range. The transducer operates at 15 V, which is

supplied by a direct current power. supply. The transducer reads out on a

digital multimeter.

Two 7.62-cm-square steel tubing cross beams located underneath the test

filter support the transducer assembly. The assembly includes (1) a 0.63-cmthick plate, (2) a Micro-Control jack with a 5.1-cm throw, and (3) the force

transducer. The force transducer is bolted to the jack, which is bolted to the

plate, and the plate is bolted to the cross beams. The separation distance between the filter bottom and the transducer platform is 0.64 cm. The descent of

the filter onto the transducer is controlled to some extent by the highprecision ball bearings in the filter attachment; The bearings allow the filter

to consistently hang and swing free so that the bottom of the filter remains

parallel to the horizontal and forms a 900 angle with the transducer platform.

The calibration curve of the force transducer, is shown in Fig. 9. The

points were taken after the filter weight had been nulled. Known values of mass

were placed on top of the filter, and the transducer reading was recorded. A

third-degree polynomial curve-fit yields the following equation.

M 4.39 x 10-8 V3 - 3.24 x 10.4 V2 + 0 932 V - 13.48 ,

16
where V is the. transducer output voltage measured in millivolts and M is the

mass in grams. Note that the actual linearity of the transducer/mass balance

combination is 11% of span.

2. Flexible Ducts. The flexible ducts shown in 'Fig. 10 allow.the separation of the test filter from the duct system when making a mass measurement or

filter change. Eight quick-release. latches bolted to the filter flange of the

flexible ducts enable the separation to proceed quickly. Thetwo ducts measure

.61 by .61 by .28 m, and the flexible material is the same material used in the

vibration isolation device.

3. Test Filter Static Pressure Drop. The static pressure drop across the

filter is measured in centimeters water gauge by a Dwyer vertical manometer with

1.6

1.2

tn

o0.8

0.4

0.0

0 0.2 0.4 0.6

Test mass (kg)

0.8 1.0

Fig. 9.

Transducer calibration.

17
Flexi ble

ducts

HEPA

Ups.t-ream filter Downstream

... . L.... .

IilIter

: Fii1 ter

flanges .&

-toggle

clamps

Fig. o0t.

Testt :filIter. section.

a resolution ;of 1 Rm. The upstream and downsstream .static ports *are placed

0.45 m from their respective .fi-:lter.-,faces. '-The large -distance is a result of

the length of the flexible ducts.

III. PRELIMINARY HEPA FILTER 'PLUGGING TESTS

A preliminary series of 19 idealized p~lugging test~s was conducted during

the winter of 198•1, (1) to :test the ýnull-bal,-ance filter-weighing system and

(2) to determine the -effects of concentration and moisture addition. 1' 1 2 In

these tests, ;dry stearic acid particles with ,a mean diameter of 0.488 um were

generated using a commercial :condensa.tjion-type generator. Stearic acid was cho-

.sen ýbecause ,a fairly monodisperse, spherical., nontoxic, control aerosol could be

generated for the approximate size desired to simulate a fire. (We were only

abl.e to generate stearic acid aerosol loadings of about 60 to 120 mg/m3n

.full-scale forced-ventilation fire tests at the Lawrence Livermore National

Laboratory have-produced concentrations of .real smoke in excess of 5 .g/m 3 ., while

burning porous cri~bs of solid. fuels.) A-. spray nozzle was used to inject water

at selected flow -rates -up -to fully saturated conditions.

118
Nineteen tests (including dry particulate only, water only, and mixed conditions) were conducted. In these tests, the filter vo0lumetric flow rate initi3 ally was set at 28.3 m /min and then allowed to decrease'as the filter plugged.

•Filter plugging was arbitrarily defined to have occurred at a 50% reduction in

-flow from the design value. Table II is a summary of these results.

From the data in Table Ii, we can observe that increasing the stearic acid

concentration by increasing the generation rate from 0.'105 to 2.05 cm3 /min in

the absence of water spray successively reduced plugging times from 32.2 h to

4.87 h. Adding water spray at the higher particle flow rates (200 g/min

TABLE II

SUMMARY OF IDEALIZED-FILTER PLUGGING TESTS

USING STEARIC ACID AND WATER SPRAY

Tes t

17

18

19

4

7

6

5

1

10

9

8

.2

13

12

11

3

16

15

14

Stearic Acid

Volumetric Flow

Rate (cm3 /min)

0.0

0.0

0.0

0.105

0.105

0.105

0.105

0.241

0.241

0.241

0.241

0.941

0.941

0.941

0.941

2.05

2.05

2.05

2 .05

Water

Mass Flow

Rate (glmin)

100

200

400

0

100

200

400

0

100

200

400

0

100

200

400

0

100

200

400

Plugged

Mass

(kg)

3.447

7.729

6.996

0.415

6.481

8.060

6.583

0.401

6.179

6.941

6.336

0.550

3.796

5.429

5.450

0.454

4.251

5.345

6.449

Time to

Plug

(h)

27.0

9.52

5.27

32.2

32.9

5.787

4.93

20.7

31.5

4.53

4.13

13.4

6.22

6.23

3.90

4.87

5.97

6.05

3.00

19
and 400 g/min) resulted in faster plugging (about 3--6 h) with higher total mass

loadings (about 5.5--8 kg) of water and stearic acid. In addition, 'dry stearic

acid plugged the filters with significantly less mass loading (about

0.40-40.55 kg).

Figures 11 and 12 present the results of the eight filter tests conducted

at the highest and lowest stearic acid mass generation rates,, respectiively.

These curves can be fitted with polynomials of the form Ap/Ap 0 = f(m ), where f

is a polynomial and includes the linear form of Eq. (1). In this way the data

coul'd be used in a computer code to estimate pressure drops during filter

plugging under fire conditions.

Repetitive mass measurements at individual points during filter plugging

showed that the working resolution of the mass balance/transducer combination

was 2 to 3 g.

IV. FUTURE FILTER PLUGGING TESTS

A more complicated series of filter plugging tests will be conducted during

-1982. This series will use the same weighing technique but also will use a special combustor designed by Battelle Pacific Northwest Laboratory to burn two

materials, polystyrene (PS) and polyrnethylmethacrylate (PMMA), at high and low

mass-burning (or smoke-generation) rates. (See Table I.) The mass-burning

rates will be controlled by adjusting the inlet air supply rate. Each of these

conditions.will be repeated 3 times for a total of 12 tests. (The repetitions

will allow us to assess the reproducibility of our test results.) Because PS

and PMMA represent extremes of smoke-producing materials, we should be able to

derive filter plugging coefficients that bracket those expected in nuclear fuel

cycle facility fires. Additional experiments should be performed, including

using typical fuel mixtures, adding heat and moisture to the flow, and using

improved gas analysis. and particle-size measuring instrumentation. Such tests

should simulate and measure more complex and more realistic filter 'plugging

conditions.

20
S

3

S

U

0

'a

1 2 3 4 5 6 7 8

Accumulated mass, Ma (kg)

Fig. 11.

Resistance-mass loading curves.

201

E

3

S

0

a,

a,

&

161

8

4

I 3 4 5 6 7 8

Accumulated mass, Ma (kg)

Fig. 12.

Resistance-mass loading curves

21
V. SUMMARY AND CONCLUSIONS

Los Alamos, in cooperation with NMSU, is conducting an experimental study

in which HEPA filters are being subjected to simulated fire accident conditions.

The purpose of this study is to obtain supportive data with which to improve the

existing mathematical models of filter plugging under fire conditions or formulate new ones. Such models currently are being used in the Los Alamos nuclear

fuel cycle facility accident analysis computer code FIRAC.

In this paper we described the Los Alamos/NMSU HEPA filter plugging test

facility and presented some results of preliminary tests conducted in it. This

filter plugging facility can supply much-needed experimental data to support

filter plugging modeling and computer code development. We. also outlined some

recently accomplished idealized tests using stearic acid and some plans for

ongoing ones using nonradioactive combustible materials found in fuel cycle

facilities.

In summary, the idealized tests support the following conclusions.

(1) The balance beam/force transducer weighing assembly is anracceptable

weighing system for detecting HEPA filter mass accumulations with a

resolution of 2 to 3 g.

(2) These tests established that water vapor is an important variable in

the loading of HEPA filters with a combustion aerosol. The filter

lifetime is dependent on the amount of water vapor in the aerosol.

Filter 14, loaded at the maximum average particle generation rate of

3 2.0513 cm /min and at the maximum water vapor addition rate of

400 g/min, had the shortest lifetime-3 h.

(3) Water vapor loading creates a larger mass deposit on the filter at the

fully plugged condition than the dry particulate loading. The filters

loaded with water vapor had mass loadings greater than 3400 g, whereas

the filters loaded with the dry aerosol had mass loadings less than

550 g.

(4) Filters loaded with combined wet and dry aerosols have resistance/mass

loading curves that lie between the curves of the pure water-loaded and

pure particle-loaded filters.

22
REFERENCES

1. W. S. Gregory, R. A. Martin, P. R. Smith, and D. E. Fenton, "Response of,

HEPA Filters to Simulated Accident Conditions," Proc. of the 17th DOE

Nuclear Air Cleaning Conference, Denver, Colorado, August 2-5, 1982, DOE

Conf-820833 (February 1983), Vol. II, pp. 1051-1066.

2. C. A. Burchsted, A. B. Fuller, J. E. Kahn, "Nuclear Air Cleaning Handbook,"

Oak Ridge National Laboratory ERDA report 76-21 (March 1976).

3. "Fuel Cycle Facility Accident Analysis Handbook," Los Alamos National

Laboratory report LA-9180-M, NUREG/CR-2508, PNL-4149 in preparation.

4. R. W. Andrae, J. W. Bolstad, W. S. Gregory, F. R. Krause, R. A. Martin,

and P. K. Tang, "FIRAC Users Manual - A Computer Code for Analysis of

Fire-induced Flow and Material Transport Nuclear Facilities," Los Alamos

National Laboratory report in preparation.

5. R. A. Martin, P. K. Tang, A. P. Harper, J. D. Novat, and W. S. Gregory,

"Material Transport Analysis for Accident-induced Flow in Nuclear

Facilities," Los Alamos National Laboratory report LA-9913-MS,

NUREG/CR-3527 (September 1983).

6. W. Bergman, H. Hebard, R. Taylor, and B. Lum, "Electrostatic Filters

Generated by Electric Fields," Lawrence Livermore Laboratory paper UCRL-81926 (July 1979), submitted to Second World Filtration Congress,

London, September 18-20, 1979.

7. N. A. Fuchs, The Mechanics of Aerosols (Pergamon Press Ltd., Oxford, 1964).

8. C. N. Davies, Air Filtration (Academic Press, New York, 1973).

9. C. N. Davies, Ed., Aerosol Science (Academic Press, New York, 1966).

10. "Operation and Maintenance Manual for Royco Model 258 Smoke Generator,

Stock No. 197-006," Hiac/Royco Instruments, Menlo Park, California

(October 1981).

11. V. M. Faries, Design of Machine Elements (MacMillan Co., New York, 1965).

12. J. J. Dallman, "HEPA Filter Loading by Simulated Combustion Products,"

Masters Thesis submitted to Mechanical Engineering Department, New Mexico

State University (1982).

23
DISTRIBUTION

Copies

Nuc-lear Regulatory ,Commission, RI, Bethesda, Maryland 173

Technical Information Center, Oak Ridge, Tennessee 2

Los Alamos National Laboratory, Los Alamos, New Mexico 50

225

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