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Installation, Operation and Maintenance Manual for Hoffman Purge/Pressurization Unit
IMPORTANT NOTE It is essential, to ensure conformity with the standard,
Please refer to the latest standard for detailed requirements and definitions.
All Hoffman Purge/Pressurization Unit pressurization units
provide:
a) a method of pressurizing a Pressurized Enclosure (PE)
while at the same time compensating for any leakage,
together with
b) a method of purging the enclosure, before power is turned
on, to remove any flammable gas that may have entered the
enclosure while it was not pressurized.
Continuous Flow Units, Model CF, allow air to flow through
the PE continuously by having a fixed outlet aperture, which
does not have any means of closure. The act of pressurizing
the enclosure forces air to "leak" through the outlet aperture.
The Control Unit admits sufficient air at the inlet to the PE to
compensate for the air leaking out of the outlet aperture as
well as from any other accidental leakage paths. The flow
rate is constant both during purging and thereafter. The
Purging Flow rate is monitored by a separate "Purge Flow
Sensor" located in the CU, which detects the differential
pressure across the purge flow orifice located directly before
the Outlet Orifice. The Purge Flow Sensor is set to operate
when the desired differential pressure is exceeded. The
output from the Flow Sensor is indicated on the CU.
Type Continuous Flow (CF) Units are comprised of the
following major parts:
The installation of the Hoffman Purge/Pressurization
Unit, the protective gas supply, any alarm device should
be in accordance with the requirements of NFPA 496.
The electrical installation associated with the
Hoffman Purge/Pressurization Unit shall conform to the
local codes and the relevant clauses of NFPA 496.
1.1 Installation of the Hoffman CF Unit
1.1.1 The Hoffman Purge/Pressurization Unit should be
installed either directly on or as close as possible to the
Pressurized Enclosure (PE). It should be installed so that the
unit indicators may be readily observed.
1.1.2 All parts of any unit carry a common serial number. If
installing more than one unit, ensure that this commonality is
maintained on each installation.
1.1.3 Any tubing, conduit and fittings used to connect to the
PE should be metallic, or, if non-metallic, conform to the
local codes for flammability ratings. No valve may be fitted in
any tube connecting the Hoffman Purge/Pressurization Unit
to the PE.
1.1.4 The user or manufacturer of the PE shall determine the
volume of the PE, the necessary purging volume, and the
time to be allowed for purging, using the chosen Hoffman
Purge/Pressurization Unit purging flow rate. It is the user's
Continuous Flow (CF) conforming to NFPA 496
that the user of the unit observes the following instructions.
Section 0 Description and Principle of Operation
Section 1 Installation of the Unit
- A Control Unit (CU) containing as a minimum, for "Y/Z"
Pressurization, a Flow Control Valve (LCV), Minimum
Pressure and Purge Flow sensing devices, and a
"Pressurized"/"Alarm" indicator. The CU supplies a
'Pressurized' signal showing whether the PE pressure is
satisfactory or not.
- A Relief Valve (RLV), fitted to the PE, to provide a means
of limiting the maximum pressure experienced by the PE
during operation. All RLVs incorporate a Spark Arrestor to
prevent sparks being ejected from the PE through the RLV
aperture. The RLV is fitted solely as a safety device and
does not open in normal operation.
-
An Outlet Orifice, which has been pre-calibrated so that
the pressure drop at the desired flow rate is known. The
Minimum Pressure Sensor within the CU will be set to the
same figure as the pressure drop. When the PE pressure
exceeds the calibrated pressure the Continuous Flow must
be taking place. The unit is provided with calibrated Outlet
Orifices. Where the Outlet Orifice disk is removable and can
be easily changed by the user to give different flow rates
according to the size of the PE and the available air supply
capacity.
responsibility to verify or enter this data on the PE and/or
unit nameplate. Ask Hoffman if in doubt.
Example calculations:
a) If the PE external dimensions give a volume of 20 cubic
feet, and it is NOT a motor, multiply the volume by four to get
the Purging Volume i.e. 80 cubic feet. Divide the Purging
Volume by the purge rate e.g. 32 cubic feet per minute, and
round up to the next even minute above, i.e. Purging time
would be 4 minutes.
b) If the PE is a motor, multiply the internal free volume by
ten to get the Purging Volume. For the example above,
Purging time would be 8 minutes.
1.1.5 If the PE contains an internal source of release of
flammable gas or vapor, the procedures for assessment of
the release as given in NFPA 496 shall be observed. The
user must verify that the specification of the Hoffman
Purge/Pressurization Unit e.g. pressures, continuous flow
(dilution) rate and type of protective gas are correct for the
specific application.
1.1.6 More than one PE can be protected by a single unit. If
PEs are connected and purged in "series" e.g. "Daisy
Chained", the Outlet Orifice must be fitted on the last
enclosure with the Purge Inlet to the first enclosure. The RLV
must be fitted to the last enclosure. The bore and length of
the tube or conduit used to interconnect the enclosures is
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