BREATHE EASY!
by Dave Mooney, Pelsue Company
Since the OSHA guidelines began to be enforced, during the early 1990s, the
term 'Confined Space' has become a longer than usual 4 letter word to a lot of
people. In trying to comply with the varied interpretations of what determines
the space to be 'permit required' or 'non permit required', the necessary entry
equipment can become a large part of a company’s tool expense, or a
contractor's overall budget.
The fact of the matter is that, in 90 to 95 percent of confined spaces, the
only necessary equipment for a safe entry is a gas monitor, and a mechanical
ventilator. The assurance of good quality air is as simple as 1, 2,3.
1)Monitor the space before entry for toxins, combustibles,
and oxygen.
2)Continuous ventilation during entry.
3)Continuous gas monitoring during entry.
Gas monitors typically come in two styles, diffusion or pump models. A
diffusion style monitor is sampling the air continuously simply by having the
ambient environmental air pass over the sensors without the aid of an electric
or mechanical pump. This type of monitor is generally less expensive and
provides a longer run time on the batteries than the electric pump models. This
model is generally used in areas where there is not a constant need to sample
the air from a distance. If there is such a requirement, like perhaps the need
to sample the air from outside the manhole before opening, a hand operated
aspiration kit usually an efficient way to achieve the task.
The integrated electric pump monitors are simpler to use for pre entry
sampling, as the pump will draw the sample remotely, through a hose, up to a
length of 100 ft, depending on the model. Once the initial air quality is
established, and the entry is accomplished, the pump style instrument may still
be used to provide constant monitoring of the area throughout the length of
stay in the confined space.
Gas monitors, either diffusion or pump models, are available with numerous
configurations of sensors. The most common hazards in Utility confined areas
are carbon monoxide (C0) above 35 ppm, oxygen (O2) levels below 19.5% or above
23%, or potential explosive atmospheres where the lower explosive limit (LEL)
is above 10% of the volume in the space. LEL conditions may be caused from natural
ground gases such as methane, or gasoline leaking in through a faulty pipeline.
Instruments are available to monitor all of these conditions simultaneously,
with 3, 4, or 5 gas sensors per unit.
Mechanical ventilation comes in two styles as well, centrifugal or axial flow.
Axial style ventilators are characterized by a fan blade or 'propeller'
configuration where the ambient air is pulled in one end, drawn across the
motor, and expelled out the opposite end. Centrifugal style ventilators have a
blower wheel or 'squirrel cage' configuration where the air intake is on the
side of the unit.
Centrifugal style ventilators are usually required only under the following
circumstances.
(A)If the ventilator needs to be more than 50 ft from the area to be vented,
(more than 50ft of hose required), or:
(B)If there is a need to make more than 3 bends in the air delivery hose.
The rationale is that centrifugal ventilators have much more static pressure
than do the axial models. This does not mean that they deliver more air volume,
but simply that they are able to push the air further, and around more
restrictions such as bends in the hose.
In most Utility applications, axial ventilators are more than adequate to do
the job. The ultimate objective of a ventilator is to be able to take outside
air into a confined space through a hose, usually 8" in diameter, and to
achieve and maintain a positive pressure in the space. When people are working
in a confined space, you want to be sending fresh air in, and not sucking the
bad air out. Ideally, you would have enough airflow from the ventilator to be
able to completely exchange the air within the space 20 times per hour, or once
every three minutes.
____________________________________________________________________________________
The quick calculation, which allows you to determine the
proper ventilator for your application, is as follows.
1)Measure the entire confined area you are working in, width X length, X height
to arrive at the overall number of cubic feet of the space.
2)Divide the overall number of cubic feet by three (3) to determine the CFM
(cubic ft per minute) required to achieve the twenty air exchanges per hour as
mentioned above.
Example: A vault measuring 10 ft wide X 12 ft long X 8 ft high = total 960 cubic
feet.
960 cubic feet divided by three = 320 CFM
Therefore the ideal blower required to ventilate that space would need to have
an airflow capacity of 320 CFM after two bends in the hose.
Any ventilator being used to vent a confined space must have third party
certifications for both airflow and overall safety. The ventilator must show
the airflow capacity clearly on the faceplate. These numbers typically would
show as 'free air', 1 90 degree bend, and 2 90 degree bends. These are the
certified airflow numbers in CFM (cubic ft per minute) at the end of a standard
15 ft hose either with 1 or 2 90 degree bends in it. Free air is taken right at
the outlet of the ventilator and is not a number to be concerned with as the
unit is rarely used without a hose.
Although gas monitors and ventilators may seem like low tech, inexpensive
items, they are both potentially life saving devices. However, the products are
only as good as the people using them and the training they receive. If your
gas monitors and ventilators are maintained regularly, you will certainly be
able to breathe easy.