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API RP 752 is a recommended practice to provide
guidance for managing the risk from explosions, fires and
toxic material releases to on-site personnel located in new
and existing buildings intended for occupancy. Moreover,
this RP was developed for use at refineries, petrochemical
and chemical operations, NGLs extraction plants, natural
gas liquefaction plants, and other onshore facilities covered
by OSHA 29 CFR 1910.119.
Shortly thereafter, blast rated modules (BRMs) were
introduced to the market and, to a large degree, have
replaced wood framed trailers in blast prone areas. As
BRMs became more acceptable, larger buildings that
required a blast resistant rating and larger open spaces
(such as control rooms, laboratories, tool cribs, lunchrooms,
administrative buildings, etc.), were built modularly and
scalable through fastening BRMs together. This
customisable concept has mostly replaced site built steel
reinforced concrete structures.
BRMs are now commonplace in chemical plants and
refineries. From the outside, many BRM’s from various
suppliers look identical. However, it is what is on the inside
that counts.
Although there are exceptions, the general blast rating
standard established in the chemical and refining industry is
a peak free-field pressure of 8 psi, 200 ms, medium damage
(MD) rating. Often, when a BRM is requested, the typical
considerations are blast ratings and price.
When evaluating BRMs in relation to site safety or risk
management implications, there are multiple factors that
should be considered.
Weight of BRM
Steel is typically the most expensive item in a BRM. It can
be assumed that the heavier the BRM, the more steel was
used, and therefore the more it costs. As a means of
lowering costs, weight/steel is often engineered out of
BRM designs.
Many BRMs are engineered to be unanchored and able
to be placed on the ground or a concrete pad, designed to
slide in the event of a blast. In an event, inhabitants in the
BRM would likely be subjected to kinetic energy. The
velocity of the building during a blast can cause furniture,
computer monitors, filing cabinets, microwaves, etc., to
move very quickly inside the building, which
may cause injury.
The study of injury biomechanics evaluates
how injuries are caused and how they can be
prevented. By considering the human body as a
structure from the engineering perspective,
one can analyse an event to determine how
injury occurred and how the injury pattern
might be altered or eliminated under a
different set of circumstances.
Newton’s first law of motion states that a
body at rest will stay at rest until a net external
force acts upon it. Assuming the same
dimensions, a heavier object will move less
than a lighter object when subjected to the
same force.
Therefore, as a baseline consideration,
weight can be advantageous. Considering an unanchored or
slide design BRM, its weight is proportional to the
movement that occurs in the blast. Increased weight results
in less movement and therefore lowers ones exposure to
injury.
Wall deflection
One of the ‘fatal four’ items is ‘struck by objects’. The
damage level of the BRM indicates the degree of deflection
of the exterior wall in the event of a blast. Low damage
denotes little to no deflection by the exterior wall, and
high damage indicates significant wall deflection. The low
damaged building would likely be able to be reused, while
the high damage building would likely be unusable.
The significance of the wall deflection emerges as a
consideration when evaluating the biodynamic effects to
occupants who are in the building at the time of an event.
Simply stated, for any item that is either mounted or
attached to the wall, it is capable of becoming a projectile
if the interior wall is penetrated by a millisecond thrust
from the exterior wall. Also, non-structural items, such as
lighting and power panels that are wall mounted, can
become compromised during a blast and may cause fire or
electrocutions.
The construction methods of suppliers vary with regard
to wall deflection. Some BRM suppliers accommodate for
these potential risks by allowing an adequate space
between the exterior wall and the interior wall. On a 12 ft
wide module (exterior dimensions), the inside space
becomes quite valuable. Some suppliers may decide to
forego space for an added layer of safety.
Mechanical integrity
Although OSHA has not explicitly ruled on ‘blast
resistance’, some BRM manufacturers have interpreted that
Mechanical Integrity requirements (part of OSHA’s Process
Safety Management (PSM) rule), 29 C.F.R. § 1910.119(j), apply
to BRMs and have self-imposed this standard on their
company and processes.
OSHA’s PSM Rule already broadly requires that
adequately protective, quality materials with integrity are
used in hazardous applications, particularly where the use,
storage, manufacturing, or handling of highly hazardous
Figure 1.
A 12 ft. x 40 ft. blast resistant building.