Siemens 125-5033 - Manual (Page 23)

Applications
Siemens Building Technologies, Inc. 15
The fume hood operator may silence the horn by pressing the Horn Silence button on the
ODP without affecting the Emergency Purge sequence.
In addition to the ODP indicators, the emergency alarm point will toggle to ON and will remain
ON until the sequence is canceled. If the emergency alarm is alarmable in the field panel, the
condition can be indicated at selected terminals if the APOGEE Automation System is set up
for it.
Emergency Purge Strategies
The use of the Emergency Purge button is determined by the work practices established by
the laboratory safety officer. As a general rule, there are two groups that should be aware of
the use of the Emergency Purge button: the laboratory worker and the laboratory safety
officer.
Laboratory worker – In most cases, the laboratory worker uses the Emergency Purge
button to increase the fume hood airflow to minimize a dangerous situation.
The Emergency Purge button should not be used as a way to work around the established
face velocity and minimum flow setpoints in an effort to make the fume hood "even safer".
Laboratory safety officer – For the safety department personnel to use the Emergency
Purge feature effectively, they must select the purge mode that best suits their needs. The
selection may be based on knowledge of industrial hygiene, the chemicals used in the
laboratory, the organization's emergency procedures, and the capacity of the central exhaust
system.
If the maximum purge mode is selected, the emergency timer and the emergency setpoint to
their maximum values.
If the controlled purge mode is selected, the emergency timer is set to 0. Select the value for
the emergency setpoint to indicate how much you want the emergency airflow to increase
over the normal airflow. For example, if your emergency procedures call for closing the fume
hood and starting the emergency purge, decide how much airflow you want compared to the
minimum airflow. If you want twice as much airflow, set the emergency setpoint to 200%; if
you want a 50% increase, set the emergency setpoint to 150%. If you do not intend to have
the laboratory worker close the fume hood, then select the face velocity you want the fume
hood to control to in an emergency. Compare that value to the normal face velocity setpoint
and calculate the emergency setpoint in percent.
NOTE: During a controlled purge, the increased airflow called for in the controlled purge
may exceed the range of the flow sensor. If so, the system cannot accurately
regulate the airflow and the controller will open the exhaust flow control device
100%.
If a sequence initially employing maximum purge with a switch to controlled purge is desired,
set the emergency setpoint as described for the controlled purge. Select a value for the
emergency timer to tell the system how long to leave the exhaust flow control device open
before switching to controlled purge. Base this on the types of accidents anticipated, the
reason you require a maximum purge, and the reason you want to switch it down to a
controlled purge.