Expo Technologies Limited
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ML384 Issue 03
–
09.01.13
Installation, Operation and Maintenance Manual for MiniPurge
£
Leakage Compensation (Model LC) and
MiniPurge
£
Continuous Flow with High Purge (Model CFHP)
conforming to NFPA 496
IMPORTANT NOTE It is essential, to ensure conformity with the standard,
that the user of the system observes the following instructions.
Please refer to the latest standard for detailed requirements and definitions.
Contents:
Section 0
Description and Principle of Operation
Section 1
Installation of the System
Section 2
Operation of the System
Section 3
Maintenance of the System
Section 4
Fault Finding
Section 5
Annex (if applicable)
Section 0
Description and Principle of Operation
All MiniPurge
£
pressurization systems 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.
Type
Leakage Compensation (LC)
and
Continuous
Flow with High Purge (CFHP)
systems comprise the
following two major parts:
- A
Control Unit (CU)
containing as a minimum, for “Y”
and “Z” Pressurization, a Leakage Compensation Valve
(LCV), Minimum Pressure and Purge Flow sensing
d
evices, and a “Pressurized”/”Alarm” indicator. The CU
supplies a ‘Pressurized’ signal showing whether the PE
pressure is satisfactory or not.
For Type “X” Pressurization, the CU has, in addition, a fully
automatic Purging controller with a Purge timer and
electrical power switch interlock.
- A
Relief Valve (RLV),
fitted to the PE, to provide a
means of limiting the maximum pressure experienced by
the PE during operation. The RLV model number has
suffixes defining the diameter of the valve aperture (in
millimeters) and material, e.g. RLV **/cs (Carbon Steel) or
/ss (Stainless Steel). All RLVs incorporate a Spark Arrestor
to prevent sparks being ejected from the PE through the
RLV aperture.
CFHP systems with a Continuous Flow of air after purging
have a calibrated Outlet Orifice which can be either within
the Relief Valve (suffix **/cf) or a separate item type SA**
or SAU**.
0.1
“Leakage Compensation” Systems, Model LC
A Leakage Compensation System, Model LC, is intended
to have minimal flow after the initial purge time. The PE is
built as leak tight as possible and the LC system merely
tops up for any enclosure leakage. The system provides an
initial high flow of purging air that leaves the PE through
the Relief Valve. After the initial purging has been
completed the Control Unit changes over to Leakage
Compensation mode and the Relief Valve closes. The only
flow thereafter is the flow through the “Leakage
Compensation Valve” (LCV) which is adjusted so that the
flow is enough to compensate for any leakage from the PE.
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 RLV. 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 and on “X”
Pressurization systems, used to operate the automatic
purge timer. Both Enclosure Pressure and Purge Flow
have to be correct before the Purge Timer can start.
0.2
“Continuous Flow after High Purge”, Model
CFHP System
The CFHP system construction is identical to a LC
model, with the addition of one or more fixed Outlet
Orifices to provide a deliberate “leak”
at a known flow
rate. The Outlet Orifice is pre-calibrated so that the
pressure drop at the desired flow rate is known. The
Minimum Pressure Sensor within the Control Unit will
be set to the same value as the pressure drop. When
the PE pressure exceeds the calibrated pressure the
Continuous Flow must be taking place.
The Leakage Compensation Valve in the CU is opened
sufficiently to provide enough air to compensate for
any accidental leakage as well as to provide the
Continuous Flow through the outlet orifice. In this way
a high flow rate is provided during the initial purge
period which is thereafter reduced to the desired
Continuous Flow rate. Even if the PE had no
accidental leakage there would still be a flow from the
outlet orifice.
There are three ways of providing the calibrated Outlet
Orifice. Please consult the system specification sheet
to determine which has been supplied. The choice:
- Type SAU** where an 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. (** denotes the metric thread size
of the SAU body)
- Type SA** where the orifice size is fixed and the way
to change the flow rate is either to change the setting
of the Minimum Pressure Sensor or to replace the SA
with one of another size. (** denotes the nominal
thread size of the SA body)
Section 13: FM ML384 Manual
