Civil Engineering Reference
In-Depth Information
Almost all gas feeder control schemes are based on the use of variable vacuum or
the use of an orifice proportioning positioner. In most cases, the manual dosage ad-
justment is retained for operator use.
Gas feeders can be started and stopped simply by starting or stopping water flow
through the injector. Usually this is all that is necessary to start or stop a typical gas
feed system such as a chlorinator or sulfonator.
Automatic Control for Chemical Feed Systems
Various automatic control schemes are possible for chemical feed systems beyond the
automatic flow proportioning discussed for each type of feeder. For example, automatic
pH control is possible using a pH sensor, controller, and pH adjustment chemical feeder
for sodium hydroxide with an automatic stroke positioner.
Automatic chlorine residual control is possible using a chlorine residual analyzer,
controller, and chlorinator with an automatic orifice positioner. These are ''feed back''
systems where the final control parameter, such as pH, is controlled by a previous feed
of chemical. ''Feed forward'' systems are also possible where a parameter concentra-
tion prior to chemical feed is determined and related to the chemical feed for automatic
control.
Another method that is gaining wide acceptance is the use of an ORP probe, to
control chlorine feed. These probes, which measure the change in redox potential,
have been used as a surrogate for direct chlorine residual measurement with success.
The ORP probe measures the rate of oxidative disinfection. The output from the ORP
probe can be converted into a 4- to 20-millivolt signal that can then be used to control
the chlorine feed. The higher the signal, the higher the chlorine dose. Typically, the
signal used to control the chlorine feed rate is established by trial and error using a
residual analyzer. The probe is typically located near the beginning of the basin, and
in some cases a second probe is located at the end of the basin. In order to calculate
the
Ct
value, a chlorine residual analyzer may need to be installed as well. Some ORP
probes are available with a residual analyzer as part of the unit.
Various types of ''feed forward'' or ''feed back'' systems or combinations can be
devised in theory. The problem is that there are substantial delay times in such systems
that most analog controllers are not designed to handle. The delays result from the
time required to change a chemical feeder setting and get the change to the injection
point, as well as process delays to the sample point, delay in the sample lines, and
delay in the analyzer. The total delay from the time a feed rate is changed until it is
read out by an analyzer can be 5 minutes or longer. The control results can be unstable
and can lead to wide, cyclic variations of the controlled variable. There are ways of
overcoming these problems with proper design and equipment selection, but proper
design of such systems is a very specialized skill.
Automatically controlled systems must be arranged so that auxiliary systems such
as mixers, dilution water, and slakers are started and stopped as needed. Such auto-
mated systems can be designed and applied; however, their complexity and mainte-
nance requirements are such that they should not be used unless their benefits clearly
outweigh their operational disadvantages.
Buildings
When designing enclosures for chemical facilities, care must be taken to review the
applicable building and fire codes. Many chemicals are hazardous and are not com-
