gas hazards and preventative equipment
TRANSCRIPT
Solar Cells, 19 (1986-1987) 355-365 355
GAS HAZARDS AND PREVENTATIVE EQUIPMENT*
RONALD COLLINS and EDWARD FLAHERTY
Matheson Gas Products Inc., Secaucus, NJ 07094 (U.S.A.)
(Accepted January 31, 1986)
Summary
The safety hazards gases pose to users in the photovoltaic industry are reviewed. Information describing gas handling equipment is also outlined. Basic gas handling considerations are discussed with an explanation of the salient hazardous characteristics of gases such as flammability, explosivity, toxicity, corrosivity and reactivity.
A tabulation of the specific hazards each gas possesses is presented along with key physical properties. An extensive reactivity table is presented depicting gases that can be mixed with each other safely. Emphasis is placed on clearly understanding the chemistry of the gas used and its hazardous characteristics so that a good design and appropriate equipment can be chosen to handle the gas in an environmentally safe manner while also protecting employees.
A summary of the equipment developed to minimize hazards is described. The use of equipment such as cabinets, limiting orifices, scrubbers etc. at tempts to minimize the risks to the environment and the employee associated with cylinder handling, storage, process use and exhausting and venting of process gases.
1. Introduction
The advancing technologies of photovoltaics research have demanded higher purities of chemicals to attain greater efficiencies. In many cases gaseous rather than liquid chemicals are used because of their inherent purity advantages. The use of these gaseous chemicals poses safety con- siderations that go beyond the scope of those normally used to handle liquid chemicals. These gases are generally inorganic in nature and are produced as specialty items.
*Paper presented at the SERI Photovoltaics Safety Conference, January 16-17, 1986, Lakewood, CO, U.S.A.
0379-6787/87/$3.50 © Elsevier Sequoia/Printed in The Netherlands
356
Safety in photovoltaic manufacturing operations is enhanced by clearly understanding the chemistry and the hazards associated with the use of these specialty gases as a first step. Thereafter, with this understanding appropriately engineered hardware and controls can be provided to protect employees. To be successful it is critical that procedures be established and followed and that adequate training be given to ensure that thorough understanding of these hazards exists. The approach taken is an integrated one that instills safety awareness and knowledge of hazards and that draws from a variety of disciplines, e.g. safety and health, engineering, chemistry and management. Everyone has a responsibility for safety at the facility.
2. Properties and hazards of gases
2.1. General considerations Firstly, it must be realized that gases and vapors follow a set of gas
laws and process equipment engineers must be familiar with these relation- ships when dealing with these gases. This is elementary background material which is readily found in basic tex tbooks for chemistry, physics and engineering. However, with respect to technicians or operators who may not possess formal technical training, it is necessary to provide a training course covering these basic principles.
In general, the major hazards posed by chemicals are flammability, corrosivity, toxicity and reactivity. Gases and vapors may exhibit one or more of the above characteristics. In addition, gases may present other hazards owing to their high pressure, their cryogenic nature or their ability to rapidly diffuse into the air creating a strong inhalation hazard, which may pass unnoticed because of a lack of odor.
For each of these characteristic hazards it is difficult to assign a quantitative test to determine whether the gas possesses a single character- istic that is universal. For example, the Department of Transportation (DOT) classifies gases for transportation purposes as flammable, non-flammable, oxidizer and poison. However, in doing so they set quantitative definitions which allow some gases to be misclassified. The most salient example of this is ammonia which in reality is a flammable gas, although DOT classifies it as non-flammable. The point here is not to disparage the efforts of DOT but to demonstrate that any system used to place gases into discrete sets of classes will fail. For the most part the hazardous characteristics of each gas are as unique as the molecular formula and each must be judged on its own. This can be illustrated by considering ammonia, phosphine and arsine, which are derived from elements in the same chemical family in the periodic table and do have some predictably similar chemical properties. However, their toxicity, reactivity and corrosivity are significantly different. Therefore whatever the source of classification, (DOT, NFPA and local or regional fire code) it should not be relied upon totally when considering the hazard- ous nature of gases.
357
2.2. Flammabi l i ty Flammability of a given substance is a common hazard which requires
a fuel and oxidizer in the proper concentration range and an ignition source for flame propagation. The ignition source cannot be absolutely removed since low energy sources, e.g. simple friction, can still ignite mixtures. To protect against flammability hazards the ignition sources have to be removed and the fuel/oxidizer ratio has to be limited to concentration levels either below the lower flammability limit (LFL) or above the upper flammability limit (UFL). This is true provided the system is at standard temperature and pressure (STP) conditions in air. Data at conditions other than STP are not readily available. The flammability limits provided in Table 1 are for standard conditions in air.
It should be pointed out that flammability data are obtained empiri- cally. The Bureau of Mines bulletins completely describe the test pro- cedures [1]. Clearly, as background gases are changed, e.g. from nitrogen to helium or argon, flammability limits may go up or down depending on the heat capacity values of the background gas used. It is also generally accepted that as the temperature or the pressure is raised the flammability range increases, i.e. the LFL gets marginally lower and the UFL gets higher. Obviously as the oxidizer is changed from air to chlorine or some other substance the limits drastically change.
So far we have briefly talked about flame propagation when fuel oxidizer and ignition sources are present. Some chemicals spontaneously propagate flames when exposed to air wi thout ignition sources at a specific temperature. This temperature is called the autoignition temperature. Again this is an empirical determination which depends on many variables including pressure, rate of heating and geometry of test apparatus. In general, materials that have an autoignition temperature at or below room temperature in air are classified as pyrophoric materials. One prime example of this is silane.
Finally, no discussion on flammability would be complete without a brief explanation of explosions. Explosions are characterized on the basis of flame speed and their directional nature. An explosion can be viewed as a rapid equilibration of a high pressure gas with the environment. Uniform reactions are those in which the reaction occurs more or less uniformly throughout the mass of material. Propagating reactions are those in which the reaction initiates at a specific point in the material and propagates from that point as a reaction front through the unreacted material. Detonations are the most severe explosions, which equilibrate very rapidly and have directional nature.
It needs to be emphasized that explosions do not necessarily have to be rapid oxidations. Many compounds that are endothermic in nature, i.e. possess a positive enthalpy of formation, can potentially undergo rapid explosive decompositions. Acetylene and germane are examples of this. The enthalpy of formation for some gases are listed in Table 1. It should be noted that for gases with positive enthalpies of formation there is a potential
TA
BL
E 1
Ph
ysi
cal
pro
pe
rtie
s
Che
mic
al
form
ula
Sill
4 si
2H
6
Si3
HB
S
iCI4
Si
F4
SiH
2CI2
S
iHC
13
GeH
4
HF
B
F3
B
CI3
H
C1
B2H
6 A
sH3
P
H3
H2S
e H
~ N
F3
C
BrF
3
CH
F3
C2F
6 C
F4
C
C14
C
12
Mol
ecul
ar
wei
ght
32
.11
8
62
.22
1
02
.19
1
73
.88
1
04
.08
1
01
.00
79
1
35
.45
30
7
6.6
3
20
.00
6
67
.80
5
11
7.1
7
36
.46
1
27
.66
8
77
.94
6
33
.99
8
80
.47
6
34
.07
6
71
.00
1
48
.91
7
0.0
14
1
38
.01
2
88
.00
48
1
53
.82
32
7
0.9
06
~k
Hf °
(kca
l m
o1
-1)
Fla
mm
ab
ilit
y li
mit
s (v
ol.
%)
+8.2
+
23.0
4
+22.0
1
--156.5
08
--384.6
6
--75.0
2
--I 2
2.6
+
21
.7
--6
4.7
89
--
27
1.0
82
--
96
.28
--
22
.02
+
12
.29
+
15
.88
+
3.2
0
+8
.05
--
4.2
3
--2
8.4
3
--1
50
.72
--
16
4.5
--
31
0.0
--
22
1.0
--
24
.6(g
as)
0
Au
toig
niti
on
Vap
or
Boi
ling
po
int
tem
pera
ture
pr
essu
re
(°C
) (°
C)
at
20
°C
(l
bf
in -2
)
Py
rop
ho
ric
<
:25
--
--
11
1.5
P
yro
ph
ori
c
<:2
5
47
--
14
.3
Py
rop
ho
ric
<
:25
4
.3
+ 5
3.0
5
No
n-f
lam
ma
ble
--
3
.76
+
57
.6
No
n-f
lam
ma
ble
--
--
--
90
4
.1-9
8.0
2
1.2
2
1.3
+
8.2
P
yro
ph
ori
c
<:2
5
7.7
+
31
.8
Un
kn
ow
n
--
--
--8
8.4
N
on
-fla
mm
ab
le
--
15
+
19
.5
No
n-f
lam
ma
ble
--
--
--
99
.8
No
n-f
lam
ma
ble
--
1
9.1
+
12
.4
No
n-f
lam
ma
ble
--
6
27
--
85
0
.9-9
8
38
-52
--
--
92
.8
0.8
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8
--
21
9
--6
2.5
0
~ -
~9
8
<:2
5
60
7
--8
7.7
U
nk
no
wn
--
1
39
--
42
.0
4-4
4
26
0
26
6
--6
0.3
O
xid
ize
r --
--
--
12
9
No
n-f
lam
ma
ble
--
2
07
--
57
.8
No
n-f
lam
ma
ble
--
6
47
--
82
.0
No
n-f
lam
ma
ble
--
--
--
78
.7
No
n-f
lam
ma
ble
--
--
--
12
8.0
N
on
-fla
mm
ab
le
--
1.9
+
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Ox
idiz
er
--
94
--
34
.1
C~
359
that, under certain temperature, pressure or catalytic conditions, these materials can decompose rapidly or slowly.
2.3. Corrosivity The corrosivity characteristic is one of the hazards that is difficult to
classify. Attempts to classify materials as corrosive use measures such as pH, ability to attack flesh and ability to attack steel coupons. This classification appears to also cover oxidizers and potentially some toxic properties. An a t tempt has been made in this paper to classify gases as corrosive and this is shown in Table 2.
2.4. Toxicity Clearly the most important route of entry to the body for gases is
inhalation. One of the accepted guidelines for measuring toxicity is the threshold limit value (TLV) published by the American Conference of Governmental Industrial Hygienists (ACGIH). However, many newer chemicals do not have TLVs assigned to them. This situation is likely to persist in the future and may be exaggerated when industry starts using new materials that are somewhat exotic in nature. Many of the TLVs are not readily achieved owing to reactivity of the gas with air or moisture. The individual is not exposed to the specific gas itself bu t to byproducts of either an oxidation or hydrolysis process. In some cases a hydrolysis or oxidation product or the gas itself is so overwhelmingly irritating to the nostrils that the sense of smell provides a fast warning. This is especially true for acid gases such as hydrogen chloride, chlorine etc.
Clarity of definition is lacking. Some gases can be considered toxic owing to their corrosive nature in damaging human tissue, e.g. hydrogen chloride, boron trichloride and chlorine. In addition, they can be toxic in the sense that small amounts can assimilate into the body and interfere drastically with metabolic pathways, e.g. arsine and phosphine. TLVs of various gases are shown in Table 2. Two substances can have the same TLV but one can be odorless and tasteless and the other not. The risk of being injured by the materials with the good warning properties is less than the risk of being injured by the odorless and tasteless material. Further detailed toxicity information is available from a number of sources, some of which are listed in ref. 2.
2.5. Reactivity The reactivity hazard clearly requires the expertise of a chemist to
decide what combination of gases can or cannot be mixed together safely. A "classification" of gases according to reactivity class is shown in Fig. 1. The matrix in Fig. 1 gives a summary of gases that can or cannot be mixed. This guideline is provided in the sense that if a combinat ion is forbidden the two gases should not be mixed until the reactive properties of both of these chemical species have been further researched. The restriction matrix does not assume specific conditions. It is based on common chemical sense that
360
TABLE 2
Hazards
Chemical A CGIH a Odor Hazards b formula t lv- twa
(ppm)
I m p r o p e r react ion wi th air or mois ture
Sill4 5 Pungent R, F Si2H 6 NE Pungent R, F Si3H s NE Pungent R, F SIC14 NE Repulsive C, R, T
acid SiF4 NE Repulsive C, R, T
acid SiH2CI2 NE Repulsive C, R, T
acid SiHC13 NE Repulsive C, R, T
acid GeH4 0.2 Pungent T, F HF 3.0 Repulsive C, R, T
acid BF3 1 ceiling Repulsive C, R, T
acid BC13 NE Repulsive C, R, T
acid HCI 5 ceiling Repulsive C, R, T
acid B2H6 0.1 Sickly sweet T, R, F AsH 3 0.05 Garlic T, F PH3 0.3 Putrid T, F H2Se 0.05 Obnoxious T, C, R, F H2S 10 Rotten eggs T, C, R, F NF3 10 Moldy T, C CBrF3 1,000 Ether-like T CHF3 NE Ether-like A C2H6 NE Ether-like A CF4 NE Ether-like A CC14 10 Ether-like T C12 1 Repulsive T, C, R
acid
Air -+ SiO2 + H2 Air -+ SiO2 + H2 Air -+ SiO2 + H2 Moisture -+ HC1
Moisture --> HF
Air/moisture -~ SiO2, H2, HC1
Air/moisture -~ SiO2 • H2, HC1
No reaction at STP Moisture -+ fumes
Moisture -+ HF
Moisture -+ HC1
Moisture -+ fumes
Air/moisture --> boric acid No reaction at STP Air/moisture -+ phosphoric acids Moisture --> fumes slightly Moisture -+ fumes slightly No reaction at STP No reaction at STP No reaction at STP No reaction at STP No reaction at STP No reaction at STP Moisture -+ fumes
aNE, not established b T, toxic; C, corrosive; R, reactive; F, flammable; A, asphyxiant.
i t is poss ib le for two species to reac t . T h e y m a y n o t r eac t a t STP b u t a t o t h e r more severe cond i t i ons . The genera l t y p e s o f r e s t r i c t i ons are ac id /base c o m b i n a t i o n s , Lewis ac id /base c o m b i n a t i o n s , f u e l / o x i d i z e r c o m b i n a t i o n s , o r g a n o m e t a U i c / f l u o r o c a r b o n c o m b i n a t i o n s , a d d i t i o n to d o u b l e - b o n d com-
b ina t i ons e tc . Pressur ized gas in a f ixed vo lume is a p o t e n t i a l ene rgy source . I f a
" s t a n d a r d " size large c y l i n d e r o f 43 1 is p ressu r i zed to 2500 l b f in -2 t h e n the
No
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362
potential energy represented by the gas in the cylinder is equivalent to two sticks of dynamite.
When liquefied gases under pressure are released they rapidly vaporize and cool to temperatures which can cause frostbite. In addition to this frost- bite hazard, the pressure of the liquefied gas in the source container can drop to vacuum conditions. This must be carefully noted since it can be mistakenly assumed that there is no pressure in a system or no material in a cylinder.
3. Engineering controls
In addition to the chemical hazards that can be addressed with regard to the use of compressed gases there are a number of very real physical hazards which must be considered. These are cylinder handling, cylinder storage, pressure, process gas considerations and exhaust/vent gas treatment.
3.1. Cylinder handling The greatest physical hazard is present upon actual receipt of the
process gas cylinders. The cylinder should be off-loaded by means of a lift gate on the truck or onto a loading dock. It should never be dropped from the truck to the ground.
Once off-loaded, the cylinders should be secured on a properly sized handtruck for transport to the cylinder storage area. For small cylinders, a cart similar to a grocery cart should be fitted with a properly sized rack to ensure safe transport.
3.2. Cylinder storage Cylinder storage areas should be located away from any high traffic
activity. They should be secure areas to preclude unauthorized access to the cylinders themselves.
The ideal location is a locked indoor area which is well ventilated. A fenced outdoor area can also be used but in hot climates the cylinders may be overheated, subjecting them to excess pressure. In cold climates the cylinders would have to be moved to an indoor storage area prior to use so that proper process inlet temperatures are maintained. With gases such as dichlorosilane this step is critical since the cylinder pressure is so low.
The cylinders should be restrained. Gases should be separated by type, e.g. flammables should be stored with flammables. Oxidizers should never be stored with flammables or other reactive gases. If the same storage area must be used the oxidizers must be located at least twenty feet away from the flammables. Local fire codes may require additional precautions. It is advisable to check these codes. Full cylinders should be stored in a different area from empty cylinders. In addition, especially in the case of corrosives, careful inventory control should be maintained to limit cylinder storage time to six months.
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3.3. Pressure A general hazard associated with compressed gases is the high cylinder
pressures. Pressurized gases when released will expand rapidly whether the release is purposeful, as in opening a cylinder valve, or accidental as in a leaking cylinder valve or in a release from a pressure relief device. Such expansion can be controlled by a pressure regulator but if accidental and uncontrolled can result in the release of t remendous amounts of energy. It has been said that cylinders with sheared valves can penetrate masonry walls under the force of escaping gas. At present, most cylinder valves when sheared at the cylinder result in a limiting orifice of approximately 0.3 in (0.75 cm) to limit the rate of release of gas and reduce cylinder velocity. This may prevent large heavy cylinders from leaving the ground but smaller and/or lighter cylinders are more likely to take off like rockets.
In a chemical reaction system, improperly controlled pressure within the system can cause vessels to burst, create leakage in unsuitable equipment or produce runaway reactions. In addition extremely rapid diffusion can intensify the other hazards of some compressed gases. Asphyxiant gases can rapidly displace oxygen from the surrounding air, giving no warning of potential hazard. Flammable gases can rap id ly come into contact with sources of ignition and toxic gases can spread to workers at considerable distances from the cylinder itself.
Since this is the case, the most important safety element in any gas handling system is the pressure reducing regulator. These devices reduce the high cylinder pressure to a safe operating process pressure. Since the pressure reducing regulator is so important, a great deal of care should be taken in its selection. With the many types available it may be advisable to contact a gas supplier or an equipment expert to help in this selection. Some of the basic types of regulators which are available are general purpose brass single-stage, general purpose brass two-stage, mild corrosive gas regulator, corrosive halogen gas regulator, hi purity brass single-stage, hi purity brass two-stage, hi purity stainless-steel two-stage and hi purity stainless-steel single-stage.
It must be ensured that any equipment used in a system is compatible in terms of materials of construction and pressure rating.
3.4. Process gas considerations Before any gas system can be installed the location of the gas supply
must be set. In general, gases for the photovoltaics industry are located in the following areas: (i) outside the building in gas cabinets; (ii) in specially vented "gas alleys" between processing areas; (iii) in large centrally located gas rooms; (iv) in the processing area in gas cabinets.
Each of these locations has distinct advantages and disadvantages. The area must be well ventilated or gas cabinets with good ventilation should be used. Restraints for all cylinders must be provided. This includes restraints for extra cylinders of inert gas used for purging.
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The area or cabinets should have toxic and/or flammable gas detectors installed which are calibrated and checked on a periodic basis.
The actual gas handling and purging equipment should include some sort of flow limit device, purging valves to allow pressurizing and venting of the gas handling train and, of course, a properly selected pressure reducing regulator.
As an extra measure of safety, a flow restrictor of 0.01 in in diameter can be installed by Matheson in the outlet of some types of cylinder valves. With this restrictor, even if the cylinder valve is accidentally opened the outlet gas flow is restricted to approximately 36 1 min -1 at 1000 lbf in -2 differential pressure.
Other considerations in the purge panel specifications are as follows: (i) special cleaning; (ii) types of connections; (iii) pressure downstream of the regulator; (iv) materials of construction; (v) physical size; (vi) types of identification for process gas line, vent and purge gas lines.
3.5. Exhaust vent gas treatment Even though this is the last step in the gas process it is critical. Defini-
tive governmental safety regulations do not exist for the relatively young photovoltaics industry and therefore the industry is forced to develop safety considerations on its own. To design a scrubber or pollution abatement system a clear inventory must be made describing the type of gas emitted from a process, its concentration and flow rate. Thereafter, ventilation lines and exhaust ductwork must be sized and materials selected to handle the vent requirements.
Two important considerations must be decided upon before an actual scrubber is designed for the facility. The first concerns segregation of waste gas streams. It is easier to design scrubbers to handle a specific type of gas, for example, an acid gas scrubber to handle hydrogen chloride, boron trichloride, hydrogen fluoride and boron trifluoride. The scrubber system becomes more complicated when different types of gases are mixed, for example, pyrophorics, toxics and alkaline and acid gases. Scrubbers can be designed for these applications but they are multi-staged with each stage removing specific types of gases. The design of these multi-stage scrubbers should be left to experts in gas handling.
It is extremely desirable to abate contaminants at the local level since at this level volumes of contaminants are not diluted. This allows scrubbers to operate safer and more efficiently and to provide a high degree of specificity towards the type of gas.
After consideration of the above details, a decision is made on the type of scrubber to be used. There are three types of scrubber. These are wet tower, adsorption/chemisorption]reaction and incineration. All of these have advantages or disadvantages that must be considered for use at a particular facility. Wet towers can usually handle large volumes of material but they generate liquid wastes which must be handled thereafter. Dry adsorption cartridges keep the volume of waste down and do not require
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significant maintenance but they do have flow limitations. Incinerators may still require wet scrubbing and solids removal. In all cases the byproducts f rom these scrubber systems still have to be dealt with, although hopefully a properly designed scrubber has condensed the gaseous material to a solid or liquid, thereby significantly reducing the inhalation hazard originally posed by the gas.
4. Conclusions
The nature of manufacturing photovoltaics introduces a complex array of safety problems. These problems can be addressed by understanding the chemical, physical and toxic properties of these chemicals and by selecting the proper gas handling system from cylinder hand trucks to scrubber systems.
References
1 H. F. Coward and G. W. Jones, Limits of Flammability of Gases and Vapors, U.S. Bur. Mines Bull. 503 (USNTIS AD 701 575), 1952.
2 N. I. Sax, Dangerous properties o f industrial materials, 4th edn., Van Nostrand Reinhold, New York, 1975; W. Brauer, A. L. Mossman and D. Siegel, Effects o f exposure to toxic gases-first aid and medical treatment, 2nd edn., Matheson, Secaucus, NJ, 1977.