Handling Liquefied Compressed Gas

Safetygram 30

Handling liquefied compressed gas

A liquefied compressed gas can be defined as a gas, which when compressed in a container, becomes a liquid at ordi- nary temperatures and at pressures ranging from 25 to 2500 psig. Liquefied gases have boiling points that range from –130°F to 30°F (–90°C to –1°C). At 70°F (21.1°C) the cylinder contains both liquid and gas. Cylinder pressure, or the “vapor pressure” of the gas, is directly affected by ambient temperature. Increases or decreases in the temperature will cause the vapor pressure to increase or decrease, respectively.

Liquefied gases are packaged under their own vapor pressure and are shipped under rules that limit the maximum amount that can be put into a container to allow for liquid expansion with rising temperatures. (The various transpor- tation regulatory agencies have established very detailed requirements for the filling limits of liquefied compressed gases to prevent the possibility of container overpressurization. Consult the regulations for your region for more information.)

Typical liquefied compressed gases are listed in Table 1.

Containers Liquefied compressed gases come in a variety of containers.

Because the product exists in both the liquid and gas phases in the container, many containers are equipped to access both phases. This is accomplished by the use of full-length eductor tubes (dip tubes) and gooseneck eductor tubes (see Fig. 1).

Cylinders with a full-length eductor tube, or what is sometimes called a full- length dip tube, have a tube that runs from the inlet of the cylinder valve to the bottom of the cylinder. When a cylinder with this valve configuration is in the upright position, the inlet of the tube is immersed in liquid, and the liquid phase will be removed.

Some cylinders are equipped with two valves: one having a full-length educ- tor tube for liquid withdrawal and the other a valve without an eductor tube for gas withdrawal or inert gas padding (see Fig. 2 and section on liquid-phase withdrawal).

Another type of valve configuration is called the gooseneck eductor tube. The gooseneck goes only a short distance into the cylinder and then bends to the cylinder side opposite the valve outlet. In the upright position, the gooseneck is above the liquid level and provides gas. To remove the liquid, the cylinder is placed on its side with the valve outlet facing up. This puts the gooseneck into the liquid. Figure 1: Figure 2:

Gas Withdrawal Valve Gas

Liquid Gas Withdrawal Valve Liquid Liquid

(left) Cylinder with gooseneck dip tube. (right) Some cylinders are equipped with two valves, one Cylinder with full-length dip tube. with and one without an eductor tube.

In larger horizontal containers, such can be withdrawn from the rear of as high pressure “Y” cylinders, low the trailer with all dip tubes oriented pressure ton containers (multi unit upward (to the vapor phase). Liquid tank car tank), and tube trailers (see can be withdrawn from the front of Figs. 3, 4, and 5), dip tubes are required the trailer where the dip tubes will to access both the liquid and gas be oriented downward into the liquid phases. phase.

“Y” cylinders use what is referred to as The most common type of liquefied a “C” configuration. This configuration gas container uses a standard cylin- is very similar to the gooseneck, but der valve. In the upright position the the inlet to the gooseneck is oriented liquid level is well below the valve in the same direction as the valve inlet, preventing liquid from be- outlet. The product flows into the ing removed. If liquefied gas is to be dip tube and out the valve outlet in withdrawn, the bottom of the cylinder a flow path shaped like the letter “C.” must be elevated above the valve to This means when the valve outlet is allow the liquid phase to be in contact pointed up, the gas phase is accessed, with the valve inlet. Special inversion and when the valve outlet is pointed racks are usually used to provide a down, the liquid phase is accessed. safe method for cylinder inversion.

Ton containers have two valves. The container is oriented so the valves are one above the other. The valves are connected to dip tubes that run to the cylinder sides. The top valve will have the dip tube in the vapor, and the bottom valve will have the dip tube in the liquid.

On tube trailers with liquefied com- pressed gases, gooseneck dip tubes are used, but the dip tube orientation is determined by the end of the tube trailer that is being accessed. Typically all valve outlets will be aimed down- ward on a tube trailer. Normally, gas

2 Table 1: Some Common Liquefied Gases* How to withdraw product Gas Vapor Pressure @ 70°F (21.1°C) safely Ammonia 114 psig (7.76 atm) Product withdrawal should be care- Carbon Dioxide 830 psig (56.5 atm) fully supervised by qualified people Chlorine 86 psig (5.85 atm) with the proper equipment. Personnel Hydrogen Chloride 613 psig (41.7 atm) should be aware of the associated Hydrogen Sulfide 247 psig (16.8 atm) hazards of the product and equipment Methyl Chloride 59 psig (4.01 atm) and thoroughly understand applicable Monomethylamine 44 psig (2.99 atm) safety regulations and emergency Nitrous Oxide 745 psig (50.7 atm) procedures. Sulfur Dioxide 34 psig (2.31 atm) There are two different methods of Sulfur Hexafluoride 298 psig (20.3 atm) product withdrawal from a liquefied Tungsten Hexafluoride 2.5 psig (0.17 atm) compressed gas container: as a vapor * Liquid petroleum gases, such as propane and butane, have not been included as they are too (gas) or as a liquid (liquefied gas). numerous to mention. Vapor-phase withdrawal Figure 3 Figure 4 Liquefied compressed gases in a cyl- inder or any container exist in liquid Pressure Relief Device and gaseous form at a pressure equal

Valve to the vapor pressure of the particu- lar gas (see Table 1 for specific vapor pressures). The cylinder pressure will Dip Tubes “Y” cylinders use a C configuration dip tube. remain constant at the vapor pres- sure of the material as long as there is Valve any liquid remaining in the cylinder.

Pressure Relief Device When the contents of the cylinder are withdrawn to the point that no liquid remains, the pressure in the cylinder will begin to diminish as the remain- Ton containers have two valves, each connected to a dip tube. ing vapor is used.

The first step for removing vapor is to Figure 5 orient the package to access the vapor phase of the product. Cylinder End Plug

5 10 /8" Radius

1 22 /2" Length Approx. 20" Radius Approx.

22" O. D. Tube Dip Tube 1/2" Approx.

Tube trailer front dip tube configuration.

3 When vapor is removed from the cyl- inder, the temperature and pressure Special warnings regarding vapor-phase equilibrium is disturbed, and both will decrease. Liquid will vaporize to withdrawal replace the gas that was removed, ab- CAUTION! EXCESSIVE TEMPERATURE CONSIDERATIONS: Any enhanced sorbing the heat of vaporization from vapor withdrawal method listed here that involves heating should be the remaining liquid and the contain- controlled to prevent exposure of the container to temperatures exceed- er. This heat can usually be recovered ing 125°F (52°C). from the ambient air surrounding the cylinder. If the withdrawal rate of the DANGER: Never heat an aluminum cylinder with electrical resistance gas is such that the energy required to elements. Only cylinders made of steel should be electrically heated. vaporize the liquid cannot be recov- Aluminum cylinders can be severely damaged by excessive temperature ered from the surrounding air, the exposure. liquid phase will begin to cool. CAUTION! ELECTRICAL REQUIREMENTS: All electrical systems for indi- The phase equilibrium is a func- rect heating and monitoring for a flammable gas supply system should tion of the system temperature. As be designed to comply with the applicable national or local electrical the temperature of the liquid phase code requirements. Typical electrical code requirements include classifi- increases, so will the vapor pressure; cation for instruments and/or limiting the surface temperature of heated the converse is also true. If the liquid resistance elements to a specified fraction of the autoignition tempera- cannot recover enough heat from its ture for the flammable gas. surroundings to keep up with the AIR PRODUCTS’ POSITION ON CODES: Various national and/or local demand for gas, the liquid will cool. codes prohibit the general direct heating of containers. However, these This is called “sub-cooling” or “auto- codes are interpreted as only pertaining to heating systems that apply refrigeration.” It is common for vapor heat energy directly onto the containers. withdrawal to cool the cylinder to the point where moisture condenses on This interpretation is based on the examples of prohibited heating meth- the external cylinder, valve, and pip- ods cited in various code references: radiant flame, steam impingement ing surfaces. If the surfaces are chilled on the container, immersion in a heated water bath or electrical resis- below water’s freezing point, the con- tance heater elements in direct contact with the container. densed moisture can solidify into ice. Nonheated, flow-enhancing options should be evaluated as a first If the rate of withdrawal of vapor is preference. If none are suitable, a properly engineered and approved excessive, serious safety problems INDIRECT heating system is acceptable for liquefied compressed gas can arise. Sub-cooling can cause the containers. Direct heating methods such as flames, steam impingement, vapor pressure to collapse to the point electrical resistance elements, water bath immersion, hot plates, and where the cylinder pressure is below ovens should not be used. that of the process. This pressure inversion can cause backflow of the Water-bath immersion is not recommended: The direct immersion of a process materials into the cylinder, or cylinder into a water bath is NOT RECOMMENDED as a heating method, “suckback.” It is also possible to cool a since repeated or prolonged exposure to heated and agitated water can cylinder enough to actually embrittle degrade the cylinder’s external surface and can eventually compromise the metal, potentially leading to a the cylinder’s mechanical integrity. Use of acidic or alkaline water or the cylinder failure. Ice formation on the use of conditioning salts and other materials has caused cylinder failures cylinder and especially on the valve when used in water baths. and piping, coupled with the decrease in flow as the cylinder pressure drops, is sometimes misinterpreted as block- age in the valve. This can lead to users applying excessive heat to the valve, possibly creating leakage at the outlet

4 connection, the valve packing, and ous products. Overpressure protec- Warning: Manifolding liquefied especially from a fusible metal relief tion should be provided on any compressed gas cylinders, together device if the valve is so equipped. lines, including the vaporizer without good engineering practices to circuit, in which liquid product can prevent product migration of one cyl- How can one improve the be trapped by isolation valves, inder’s contents into another cylinder, gas-phase withdrawal check valves, or other system may result in: components. rate? 1. Container rupture. Depending on the cylinder or contain- 3. Container Switching: This method 2. Major property damage. er geometry and ambient conditions, uses two or more identical con- some method might be necessary to tainers or banks of containers 3. Serious injury or death. enhance product withdrawal from that can be switched to the online 4. Noncompliance with local, liquefied compressed gas cylinders to position either manually or auto- national or international shipping maintain required flow rates. This is matically. When the primary, and fire/occupancy regulations. especially true with small cylinders active container vapor pressure and nearly empty larger containers. drops below the threshold capable CAUTION, Cylinder Heating of supplying the desired gaseous Considerations: Any equipment used Product withdrawal enhancement flow rates, the secondary, alternate to heat a cylinder of liquefied com- methods have a definite preference container is brought online in pressed gas must include redundant of selection based on inherent safety place of the previously active con- over-temperature protection, such considerations and consequences of tainer. This permits the first as a system temperature control- system failure. This Safetygram pres- container to rewarm by absorbing ler (thermostat) with a maximum ents the recommended methods in ambient heat. The switching setpoint of 125°F (52°C) along with a order of preference and their associ- sequence is repeated throughout separate, independent over- ated requirements. the high flow demand. This temperature shutdown device, such 1. Use a Larger Container: This will method may not be feasible if the as a fusible link, in the power supply increase the outer surface area ambient temperature conditions to the heater. The over-temperature of the container to allow more do not provide adequate heat to shutdown must be installed between heat transfer from the rewarm the off-line container the heat source and the cylinder. environment. within an acceptable period of time. 5. Convective Conditioning: 2. Vaporization: The user can with- Gas withdrawal can be enhanced draw liquid phase through an 4. Container Manifolding: by heating the atmosphere sur - external vaporizer, thereby Manifolding a number of liquefied, rounding the container to provide converting the liquid to gaseous compressed gas containers in par- additional thermal convective product. This is the preferred allel permits the user to achieve heating of the containers and their method for high flow require- the required gaseous flow rate by contents to increase product vapor ments. This method requires withdrawing product from all con- pressure. This approach is best liquid-phase withdrawal from the tainers simultaneously. This accomplished if the container is container using an eductor tube. method benefits from the larger held within an enclosure or The liquid is then vaporized via thermal mass of the manifolded small room and it provides a standard vaporizer, tubing coil, cylinders and product and provides gradual, controlled heating of the or other vaporization means. This additional container surface container contents. However, this method can provide the highest area for ambient heat transfer, system may not prove feasible for withdrawal rates, but may not be thus enhancing total gaseous containers located outdoors, suitable for high-purity appli- withdrawal capability. However, within a large room, or in a highly cations. It also creates the high- manifolding may not be adequate ventilated/exhausted enclosure. est release flow rate potential due if ambient temperature conditions to downstream leakage or opera- do not yield sufficient heat flow or tor error, which should be product vapor pressures within adequately addressed for hazard the containers.

5 6. Radiant Conditioning: Heat lamps 8. Electrically Heated Blanket: The der. In the United States the stamping (or equal) can be used to provide container can be encased in a may read DOT3AA480, where 480 psig radiant heating of the container removable blanket that contains is the MAWP. In Europe the stamp- contents to increase container electrically heated resistance ing may read FP25BAR, where 25 BAR pressure for enhanced gas with- elements operated by a tempera- is the MAWP. Furthermore, certain drawal capability. The heat source ture controller. Blankets should cylinder relief devices may vent the must not directly heat the contain- have an integral covering perma- cylinder contents at pressures below er valve since the valve, connec- nently attached to their inner sur- the pressure rating of the cylinder. If tion, and relief device components face to prevent direct contact of you are not sure how to interpret the can be damaged by excessive the heating elements with the DOT stamping, or for guidance con- temperatures. This method is best container. The power input to the cerning padding a cylinder, contact utilized for indoor systems with blanket should be limited, based your supplier. no obstructions around the on maximum withdrawal rate containers and provides gradual requirements to restrict worst- How the inert gas pressure is added heating of the container contents. case heat input to the container depends upon the cylinder. If the The system may not be feasible during temperature controller cylinder has dual valves, the inert gas for containers located outdoors or runaway. can be added through the gas-phase in congested areas. valve. Be sure the inert gas source is Liquid-phase withdrawal regulated to not exceed the pressure 7. Temperature-Controlled Jacket: Just as in vapor-phase removal, the rating of the cylinder and is protected This method encloses the contain- first step in liquid-phase removal is to from backflow minimally by a check er in a removable, temperature- orient the package to gain access to valve. If the cylinder has one valve, controlled jacket that contains the liquid phase. The liquid is pushed the inert gas can be added while the a “closed-loop,” recirculated heat from the cylinder by the vapor pres- cylinder valve is oriented to the vapor transfer fluid connected to a sure of the product. As the liquid is phase, then the inert gas source can separate electric heater unit. This removed, it increases the volume of be disconnected before orienting to design separates the heating the vapor space of the cylinder. Some the liquid phase. Again, care must be element from the container and liquid will vaporize to fill the addi- taken not to exceed the pressure rat- also allows for heating or cool- tional space, but usually not enough ing of the cylinder. Some applications ing the containers, depending on to excessively sub-cool the cylinder. use air in place of the inert gas for the process pressure requirements. padding. For some products, unload- The electric heater should have a Sometimes the vapor pressure of the ing with air padding may be prohibit- heat output rated for the maxi- product is not high enough to push ed by regulations. NEVER USE AIR TO mum product withdrawal rate the liquid out at the required rate. PAD FLAMMABLE PRODUCTS. When requirements, without excessive When this is the case, a method called air padding is allowed and appropri- overcapacity. This design is widely padding can be used to pressurize ate, it is imperative that clean, oil-free, used for multiple container sys- the liquid. This enhances the rate at cooled, dry compressed air be intro- tems where there is sufficient which the liquid can be pushed from duced into the vapor space through space to locate the heating unit the cylinder. Padding is the addition its vapor valve to transfer the liquid. adjacent to the containers. of an inert gas to the vapor space in NEVER use a plant air system for air the cylinder to raise the cylinder pres- padding since vapors may backflow sure. When adding the inert gas to the into the plant air system. cylinder, the cylinder pressure rating must never be exceeded. This pressure rating is part of the regulatory stamp- ing on the cylinder. The maximum al- lowable working pressure (MAWP) of the cylinder is stamped into the cylin-

6 Extreme care must be taken when Important considerations handling the liquid phase of any liq- 1. NEVER allow any part of a uefied compressed gas. Unlike gas, the liquefied gas container to be liquid does not compress. Therefore, exposed to temperatures greater the liquid must always have a space than 125°F (51°C). to expand, especially as it warms. In the cylinder, this expansion space is 2. NEVER fill any cylinders without provided by the vapor space or head the owner’s written consent. space. 3. NEVER heat an aluminum cylinder The filling limits/fill density for lique- with electrical resistance heaters. fied compressed gases were described 4. ALWAYS refer to the Material earlier. These limits were set to Safety Data Sheet for specific prevent the cylinder from becoming chemical properties. liquid full at normal storage and use temperatures. If a vessel or system becomes liquid full, any increase in Manufacturers of systems temperature would cause the liquid to enhance withdrawal to try to expand with no space for the rates expansion. The liquid’s incompress- Consult the Thomas Register to locate ibility would result in a rapid increase vendors of cylinder heating sys- of hydrostatic pressure. These pres- tems or vaporizers. Or contact the sures can build very rapidly and can Air Products Gases and Equipment quickly cause overpressurization of Technical Information Center at the equipment. Overpressurization of +1-800-752-1597. a system takes place when its pres- sure rating is exceeded. This can result in a rupture of the system. Systems using liquefied gases as liquids should be adequately protected by pressure relief devices, especially where there is a chance to trap liquid between valves or in other components that can be isolated.

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