Posted: Posted on January 22, 2018 Safe Inspection of Pressure Vessels

When is Disassembly Required?

Owners and operators of high pressure gas distribution equipment, such as tube trailers and multiple-element gas containers (MEGCs), are required to recertify the equipment’s DOT and UN pressure vessels every 5 to 10 years.  Depending upon the situation, disassembly of the equipment may or may not be required.  If the equipment has large tubes (≥ 18” OD) and has not been disassembled in the last 10 years, disassembly will be required to examine the neck surfaces of each tube.  With small tubes (9-5/8” OD), disassembly may not be required and requalification can be performed at the owner or operators site or a retest facility.  When disassembly is required, the equipment must be delivered to a requalification facility for removal of the valves and pressure relief devices from the tubes and visual examination of the interior and exterior of the vessels.  Disassembly may also be desired so shotblasting and re-coating of the pressure vessels and other structural components can be performed.

When you can opt to have the unit requalified at your facility by a company that has been certified by a US DOT special permit to perform requalification using acoustic emission testing (AET), the unit does not need to be depressurized nor does it need to be drained and replaced with an inert gas such as nitrogen.  FIBA has extensive experience in AET and on-site testing of vessels.  We would be glad to help you determine if recertification by AET is an option for your equipment.

Owner/Operator Responsibilities

When on-site AET is not an option for the equipment, you will need to transport the equipment to a recertification facility.  This appears straightforward for equipment owners and operators until you consider what state the vessels were in before they are brought to the recertification site.  Undoubtedly, the vessels have been filled with some type of gas throughout the previous retest cycle.  On the hazard scale, the gas may be a relatively benign product, such as compressed air, or a very problematic gas, such as silane (SiH₄) or boron trifluoride (BF₃).  As the owner or operator of this compressed gas equipment you must consider:

1. What must be done to the equipment to deliver it to the recertification facility in a safe and responsible manner?
2. What will happen to the equipment when it arrives at the requalification facility?

As the gas manufacturer/supplier, you have the resources and safety protocols readily available to remove the contents of the vessels.  The recertification facility will likely not have such resources.  Thus, it is extremely important that you drain and purge the equipment’s pressure vessels prior to shipping, using whatever methods are best for the contents.  Once drained and purged, the vessels can be refilled using a low pressure, inert gas (usually nitrogen) or, in the case of some specialty gas products, a vacuum can be introduced to the vessels.  Upon successful draining and purging, the equipment can now be declared empty (as defined by 49 CFR 173.29) and it can be shipped with product placards and labels either removed or temporarily covered.  Owners and operators should notate on their bill of lading that the equipment has been drained and purged and also indicate the previous gas service.

Handling of Equipment at the Requalification Facility

The vessel has arrived at the recertification facility.  Now what?  At FIBA, it is our policy to ensure that any compressed gas distribution equipment arriving at our facility has been drained and purged to remove the hazards associated with the prior gas service.  We have developed a procedure that all our facilities follow (FIBA procedure P-70) where we perform a check of all incoming vessels prior to being allowed onto our premises.

The transport driver stops outside the gate of our facility and informs us that they are dropping the unit off for retest.  An employee trained to perform the P-70 procedure meets the driver and inspects the bill of lading.  Our employee is looking for the drained and purged statement and the previous gas service.  When this is known, a series of tests can be performed to ensure that each vessel has, indeed, been drained and purged.

The employee dons the proper personal protective equipment for the potential hazard and then determines the correct testing device to use in order to detect the presence of the last known gas.  This can vary from a simple combustibility detector, to a more elaborate 4-gas analyzer, to using specialty gas detection tubes and a metered gas extraction device.

The first test is to determine the pressure state of the vessels.  FIBA requires that there be no more than

29 psig of residual pressure is inside the vessel.  This is done using a pressure gauge.  Once we have determined that the pressure has been verified to be within limits, a check of the gas residue can be performed.  For a flammable or combustible gas, we determine the level of explosivity.  The level has to be below the lower explosivity limit (LEL) of the gas.  Ideally, we want this level to be zero.  In the real world, some gases can permeate the steel of the vessel and you will get a low-level detection of the gas, even if the unit has been drained and purged.  For non-flammable, but potentially harmful gases, we want a reading that is zero, but below the threshold limit value (TLV) for the gas that is being tested – again keeping in mind that the unit may exhibit some trace of the gas even after draining and purging.  Once the vessel has passed these checks, the driver is allowed through the front gate to park and unhook.  If we run into a unit that fails these tests (otherwise known as a “hot” unit), senior management or sales is immediately notified of the problem so the customer can be notified.  Together, a corrective action can be determined.

Why Do We Check for Hazmat?

Why the double check at the gate to inspect for hazardous material residue?  One word:  Humans.  We all deal with the human condition daily.  We make mistakes.  We miss steps.  We ship units that have not had all of the protective measures taken.  We don’t mean to miss something, but our good intentions do not help a technician tasked with disassembling a unit that is expected to be empty and ends up being injured by a vessel that is “hot”.  We may double check a hundred units at the gate and find nothing.  It’s the one we do find that has saved our employees from potential harm.  We think you’ll agree that, in this area of our business, just one oversight is one too many.