CHEMICAL LABORATORY SAFETY COMPRESSED GASES: SAFE HANDLING PROCEDURES

Objectives

Compressed Gases: Safe Handling Procedures: American Chemical Society chemical safety video courses

Introduction

Compressed gases are common constituents of numerous laboratory operations.

As diverse as these gases and their applications are, they all have something in common: a potential for explosive release with extreme danger to life and property.

OSHA regulations (the "Laboratory Standard") require your laboratory's administration to establish, communicate, and train employees in the use of safe practices for working with hazardous materials, such as compressed gases.

Everyone in the laboratory who deals with compressed gases must know how to handle and use them safely.

High-Pressure Hazards

All compressed gases represent kinetic energy stored as high pressure.

High-Pressure Emergency Response

There is very little you can do to mitigate a catastrophic release of a compressed gas from a gas cylinder in the laboratory.   Exit as quickly as you can and follow your facility's emergency response plan.

Gas Hazard Labeling and Information

Before handling or using any compressed gas, find out as much as possible about its uses, properties, and hazards.

The Material Safety Data Sheet (MSDS) provided by the gas vendor is your best source of safety information about the gas.   The new ANSI MSDS standard covers 16 areas of safety and health, including toxicology, Threshold Limit Values (TLVs) or Permissible Exposure Limits (PELs), signs and symptoms of exposure, emergency procedures, and disposal methods.   The American Conference of Governmental Industrial Hygienists publishes annual compilations of current TLVs and Short-Term Exposure Limits (STELs) for several hundred chemicals, which are useful for planning or training purposes.   Material Safety Data Sheets are particularly valuable for training new lab personnel or familiarizing medical personnel with the properties and hazards of a material.

Receiving and Inspecting Cylinders

Safety in handling compressed gases starts as soon as you receive the gas at your facility.

• Immediately upon delivery, chain the gas cylinders in three-point contact (on a cylinder cart or in contact with a wall or other cylinders on the other two sides).
• Remove the shipping cap and inspect and test the cylinder valve for leaks with a polymer-soap solution. A leak will be indicated by a "fish-egg" pattern of bubbles around the packing nut, valve stem, or at the cylinder valve base.   If you find a leak, tag the leaking valve clearly and call the gas vendor promptly for assistance.

The Matheson Gas Company recommends the following inspection checklist for received cylinders:

• Is the cylinder leak tight, giving off no odors and issuing no visible fumes or hissing sounds?
• Is the product identified by name?
• Do all forms of identification on the cylinder agree?
• Are the cylinder and valve free from such damage as dents, gouges, burn marks or corrosion?

If the answer to any of these questions is NO, do not use the cylinder and contact the supplier for instructions.

Also check the hydrostatic test dates stamped on the cylinder neck.   According to the DOT cylinder manufacturing standard, all reusable compressed gas cylinders must be hydrostatically pressure-tested by a gas supplier at 5- or 10-year intervals to verify their integrity at operational pressures.    Generally, inert gas cylinders are tested at 10-year intervals and other gas cylinders at 5-year intervals. If the cylinder does not carry a current test date, return it to the gas vendor.

Moving Compressed Gas Cylinders

Before moving a cylinder, screw the protective valve cap firmly over the cylinder neck to protect the cylinder valve, which is the weakest part of the cylinder.   Keep the protective valve cap and gas-tight valve outlet caps (or plugs if the gas cylinder is designed to accept them) on at all times except when the cylinder is connected for use.

Never move a cylinder with the regulator attached.

Do not hoist the cylinder by hooking through the cylinder cap slot.

Use a gas cylinder cart rather than a two-wheel dolly for moving gas cylinders.

Do not slide or lift a large gas cylinder or roll it on its side.

Use a two-person carry cradle to manually lift and carry a standard cylinder over railroad tracks, soft ground, or other rough terrain for field laboratory uses.

Do not transport gas cylinders on public roads in a passenger car or truck unless you have all required DOT permits.

Confined-Space Hazards

Be very cautious when moving gas cylinders in any confined space. Inert, heavier-than-air gases have killed workers by asphyxiation in test chambers, pump pits, dry ice storage chests, and even in open-top tanks and block-wall enclosures.

When transporting a cylinder in an elevator, secure it in a transport dolly bearing the sign "DO NOT ENTER ELEVATOR WHEN COMPRESSED GAS IS IN TRANSIT".   Then, send it up unescorted and walk up the stairs to meet it.    To ensure that no one gets on the elevator until the cylinder is off-loaded at its destination, inform all laboratory personnel, including non-technical personnel, service personnel, office staff, and visitors, about the hazard and about the "cylinders-ride-alone" policy.

Do not enter any space that may enclose a toxic or oxygen-deficient atmosphere.   If you encounter such a situation, follow your facility's emergency response plan.

Storing Compressed Gases

Chain cylinders that are not required for immediate laboratory use in three-point contact in a ventilated, dry area (preferably outside) away from heat or ignition sources.   The storage temperature must never exceed 125^oF (52^oC).

To meet building and fire codes, separate cylinders of oxidizing and flammable and toxic gases by at least 20 ft (6.1 m) or a 1-hour rated fire wall at least 5 ft (1.5 m) high, and secure them at all times.

In addition to other safety features, building codes also require inside cylinder storage rooms to have at least one outside wall.   If the gases to be stored are toxic, the enclosure must be fitted with a gas detection system with a hazard-level alarm and a ventilation system that exchanges air at specific rates based on the toxicity and on the total cylinder volume.

Remote gas storage rooms with piping systems to distribute hydrogen and other flammable gases into laboratories must be fitted with hard-wired gas detection and alarm devices, classified electrical wiring, and sprinklers, and must qualify as fire-rated construction under state and local building codes.

Minimizing Inventory

One way to mitigate compressed gas hazards in a facility is to minimize the total laboratory inventory of potentially hazardous gases.

You can also minimize the cylinder count in laboratories by running a gas line from the regulator of a single cylinder to individual instruments.

Cylinder Valve Pressure-Relief Devices

Except for poison gas cylinders, the valve on any cylinder larger than lecture bottle size has a built-in device to allow controlled venting of pressure: a rupture disc, a fusible plug (made from a low-melting-point material), a fusible plug/rupture disc combination, or a spring-loaded poppet valve.

Securing the Cylinder in the Laboratory

Never move a cylinder with the regulator attached.    When moving a cylinder within the lab, screw the valve cap in place to protect the cylinder valve.   Locate gas cylinders within the lab so that the valve is accessible at all times.   Never expose cylinders to temperatures higher than 125^oF (52^oC), since expansion of the gas or melting of the fusible plug may completely vent the cylinder contents through the pressure relief.

Attach compressed gas cylinders securely at all times at their points of use in the laboratory to minimize any chance of toppling, which could shear off the cylinder valve or the regulator.   Secure them with a wall-mounted cylinder clamp or chain and a chain snap, or you may use a bench clamp if the projecting lab bench top is deep enough to fully engage the clamp. check that the cylinder clamps are installed before you move the cylinder into the lab, and provide individual restraints for each cylinder.

Also securely clamp lecture bottles and small calibration gas cylinders in the laboratory since the mass of a regulator on a small cylinder may make it topple.

Using Compressed Gases

Pressure Regulators

Pressure regulators are mechanical devices that reduce the high pressure in the cylinder to a suitable delivery pressure.   The type of regulator you use depends on the kind of gas you are using.   The regulators for each kind of gas are designed with fittings that exactly match the CGA connection on the appropriate cylinder valve.

You must use a regulator that exactly matches the CGA fitting on the cylinder valve, or you are creating the potential for a serious hazard. NEVER use a cylinder adapter to attach a regulator to a noncompatible cylinder valve.

Always replace the gasket washer on flat-faced CGA fittings at every cylinder changeout.   Never use Teflon thread tape or other sealing compounds on any CGA valve fitting.

Do not "crack" open the valve on an unregulated cylinder to blow out dust because the valve might freeze in the open position.

After the cylinder is mounted securely in the laboratory, inspect all the outlets and valves and the regulator for damage or leaks.   Feel the cylinder valve CGA fitting for dent or scratch flaws across the mating surfaces with your finger or use a bright light and a magnifying glass.   Do not overtighten the CGA regulator fitting nut or use Teflon thread tape to stop a leak caused by a fitting flaw.   Contact the gas distributor for assistance with leaking cylinders.

Pressurizing and Depressurizing the System

Always attach a regulator before opening the gas cylinder valve.

To pressurize the system:

• Hand tighten the regulator on the gas fitting and snug it with a wrench. do not use excessive force, and do not use a wrench extension lever--it may distort the machine threads.   A proper connection will go together smoothly.

• Turn the pressure-setting knob counterclockwise (closed) to avoid a high-pressure gas surge on the diaphragm.   Close the small regulator outlet valve, and open the cylinder valve slowly one turn or less while facing away in case the regulator diaphragm ruptures, or the pressure gauge malfunctions, or other parts, such as fittings and low-pressure components, suddenly fail ballistically.

• Turn the pressure-setting knob (usually clockwise to increase the outlet pressure setting.

• Leak test all fittings from the cylinder to the regulator with a polymer-soap solution.

Depressurize the regulator whenever the equipment is unattended or not operating.   Never leave partly assembled apparatus attached to gas cylinders.   To depressurize the system:

• Close the main cylinder valve.

• Bleed off the regulator pressure and turn the pressure-adjusting knob counterclockwise to avoid rupturing the regulator diaphragm when the main cylinder valve is again opened. Return the regulator to the vendor if the gauges do not read zero.

• Close the small regulator outlet valve.

• Remove the regulator and cap the cylinder if it will not be used for a period or is at low pressure.

• Always replace the valve cap before removing the cylinder from the laboratory.

• Never empty cylinders to less than to atmospheres pressure (30psi) to avoid contamination from backflow.

Regulator Maintenance

Pressure-reducing regulators usually fail in one of several ways:

• The pressure increases past the setpoint (regulator "creep") because the regulator poppet valve seat is worn, obstructed by contamination, or eroded by corrosive gas service.

• Gas escapes from the regulator bonnet vent, or the outlet pressure gauge shows a pressure drop due to material fatigue of the diaphragm or the spring.

Preventive regulator maintenance will minimize occurrences of sudden regulator failure. Corrosive gas service regulators should be serviced about every six months, and toxic gas regulators should be overhauled annually.   Any time a regulator shows gauge pressure discrepancies, leaks, or other abnormal performance, remove it from service and return it to the vendor for overhauling.

Tubing and Fittings

Gas from the regulator is delivered to a laboratory instrument or a process through tubing that is compatible with the gas, pressure, and temperature.