The conventional chlorination facility for use in cooling
water treatment consists of three major parts: Chlorine Supply
System, Metering System, and Ejector System. In addition there
is ancillary equipment: Safety Equipment, Metering and Control
Instrumentation and Chlorine Residual Analyzers.
1) Gas Filter
To prevent the impurities inherent
in chlorine from reaching the chlorinator control mechanism,
filter and trap ahead of all chlorinator control apparatus
is highly desirable. This filter is a combination sedimentation
trap and filter. The filter medium is made of spun fiber glass
that is specially impregnated, contained in a removable stainless
steel cylinder which is easily removed. The filter has two
chambers. The lower portion acts as a trap for reliquefied
gas and/or foam droplets from an evaporator. It is designed
for a 560 psi working pressure and a chlorine gas flow in
excess of 32,000 lb/day. The filter must always be installed
in the gas phase. It is not possible to filter out chlorine
impurities in the liquid phase, because the impurities are
in solution. This filter will remove particulate matter of
17 microns or larger. Note that the internals (pad) of gas
filter must be periodically changed with new filter medium
and is recommended to replace every 6 months.
2) External Pressure-Reducing Valve
If the chlorine supply line is
longer than ten or fifteen feet and is subject to much variation
in temperature due to poor insulation from ambient temperature
changes, an external chlorine pressure-reducing valve should
be installed close to the containers cylinders to avoid reliquefaction.
If the vapor pressure in the chlorine containers/cylinders
is 100 psi, the gas between the containers/cylinders and the
chlorinator will reliquefy if the temperature drops below
80oF. If a reducing valve is utilized to reduce
the pressure from 100 to 40 psi, the temperature would have
to drop to below 32oF before reliquefaction will
occur.
3) Gages
The pressure gage in the line of
manifold header pipe is always useful to determine whether
or not the system is "live" regardless the gage
on the chlorinator, since this gage would be failed. If the
system utilizes an external pressure-reducing valve, there
should be one gage upstream from this unit and one downstream.
All gages that measure liquid or vapor chlorine under pressure,
are a source of chlorine leaks. These leaks occur when the
protective diaphragm develops a pinhole from a flaw or rupture
from metal fatigue. The chlorine attacks the brass burden
tube, which will spring a leak from corrosion. Brass corrodes
quickly from the inherent moisture in packaged chlorine. Unless
these gages can be isolated safety, in the event of a leak
they represents a serious risk for a chlorine leak.
This type of gage is vulnerable
to leaks because of the silver or hastalloy diaphragm that
protects the brass Borden tube. The Borden tube flexes to
indicate pressure changes. Constant flexing of the diaphragm
causes metal fatigue that results in diaphragm failure. When
this occurs, the brass Borden tube corrodes quickly from the
inherent moisture in the chlorine. This results in a severe
leak. These gages should be replaced about once every five
years. To protect personnel as well as to provide the best
possible method of minimizing the danger of a leak from a
diaphragm failure, chlorine pressure gages and pressure switches
should be installed with a header valve.
4) Alarms
Pressure switches that sound an
alarm are helpful, and should be considered on most installations.
Two kinds of pressure alarm devices are available. One is
an adjustable mercoid type switch that has adjustable contacts
which close for either rising or falling pressure-the two
functions are now not available in the same housing. The other
is a combination indicating gage and pressure switch. The
US. Gage model 3050 is adjustable to sound an alarm at both
high and low pressure. Its contacts are adjustable over the
entire range of the gage.
High pressure alarms are no need
for gas withdrawal systems and a low pressure alarm is highly
desirable. It warns the operator of imminent exhaustion of
the chlorine supply. In some cases it can be used in lieu
of weighing devices. For example, consider that five-ton containers
are in a chlorine system with a usage rate of 4000 lb/day.
If the low pressure alarm is set at 30 psi on the vapor supply
line, there would be approximately 30 to 40 lb of chlorine
in each cylinder in the piping system, or a total of approximately
150 to 200 lb of chlorine remaining in system at 30 psi. At
4000 lb/day, the usage is 167 lb/hour, when the alarm sounds,
the operator has about one hour to prepare for putting another
bank of containers in service.
5) Automatic Switchover Modules
Unattended chlorination stations
must have special provisions to preclude the possibility of
chlorine supply failure. Traditionally chlorine supply systems
have been provided with scales for determining the amount
of chlorine remaining in the connected cylinders/containers.
If this is the sole means of accounting for the chlorine supply,
it requires frequent checking and operator judgment to determine
when the containers/cylinders will become empty. This diminishes
facility reliability. The operator may find that he is faced
with repeated interruption in chlorination.
To solve the problem of chlorine
supply reliability a great many stations have adopted the
automatic switchover concept in lieu of scales. There are
two types: the pressure system and vacuum system.
(1) The pressure switchover system
This is designed to provide chlorine
gas feed or liquid vaporizing equipment with an uninterrupted
supply of gas or liquid. Low pressure resulting from depletion
of an on-line source initiates switchover by the system to
a new, full source of gas or liquid.
Under normal operation of the pressure
switchover system, liquid or gas flows from the source in
service and an illuminated amber light on the graphic display
indicates which line valve is open. An illuminated white light
for the stand-by source indicates its availability. When the
on-line source is depleted, the amber light goes out and a
red light illuminates to indicate depletion of the on-line
source. The open line valve closes and the closed line valve
opens. On completion of the two valve operations, the amber
light on the stand-by source has become the on-line source.
When the exhausted source is replenished,
its corresponding red light will go out and its white light
will illuminate to indicate that it is now on stand-by and
ready for service. As a safety feature, the indicator/controller,
upon receipt of a contact closure from any external alarm
device such as a gas fire or smoke detector will automatically
close both line valves, thereby stopping the flow of the liquid/gas
from the source.
(2) The vacuum switchover system
This is recommended when 100% gas
source stand-by is required. The switchover module is connected
to the outlets of two (2) vacuum regulators. All vacuum regulators
used with an automatic switchover system are supplied with
a gas flow indicator.
The operation of vacuum operated
automatic switchover module is as follows:
At start-up, both gas source valves
are opened. The system withdraws gas from the first source
until the source is empty.. (Withdrawal from the second source
is prevented by the switchover module) At the time, the vapor
pressure in the initial gas source approaches zero (0) psig,
the vacuum level increases. This increased vacuum level, due
to no gas feed, causes the switchover module to change position,
seating off the empty source and bringing the full source
on-line. This source will continue to feed until depleted.
While one source is feeding, the empty source is required,
of the switchover module as long as sources are operational.
The vacuum type automatic switchover modules are available
for system capacities up to 4000 PPD (75 Kg/h).
6) Vacuum Regulator
This is used to reduce chlorine
gas under pressure to a vacuum and is integrated into any
gas feeding system. There are four basic functions of the
vacuum regulator component in a gas feeder. For more further,
refer to Technical Data (Bulletin A2.41009) of Capital Controls
Company.
- Reduction of gas pressure to
a vacuum
- Safety shut-off of gas pressure
upon loss of vacuum
- Vacuum regulator pressure relief
- Isolation of vacuum regulator
via vacuum sealing valve.
7) Chlorinators
Refer to Series 2000, 4000 and
4800 catalogs of Capital Controls Company.
8) Ejector Systems
The ejector system is critical
to the operation of an all vacuum gas feed system. No part
of the system can operate if the ejector is improperly sized
or malfunctioning. Proper selection of the type and application
of the ejector is essential. The ejector system is comprised
of; ejector, ejector operating water supply, vacuum line from
the ejector to the gas feeder and ejector discharge system.
For more, refer to Technical Data (Bulletin A2.41002) of Capital
Controls Company.
9) Weighing Devices
This is a device to monitor chlorine
gas consumption. Also, by knowing the exact amount remaining,
the operator may leave the plant and know that it will not
run empty during personnel absence. It is possible to indicate
the tank content remotely on DCS panel, or to alarm locally
or remotely. The devices using a hydrostatic load cell with
remote dial read-out are popular for installation using cylinders
or ton containers.
To be continued.
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