Control terminal for hazardous locations

A control terminal for use in hazardous environments, or in environments which are deleterious to electronic devices is disclosed. The terminal is housed in an enclosure which is purged with clean dry air or inert gas and is maintained at a slight positive internal pressure to prevent entry of the outside atmosphere. All electrical connections and purge controls which are active prior to completion of the purge are housed in internal and external explosion proof boxes, except for the explosion-proof flow switch which controls the purging air flow. After purge, the slightly positive internal enclosure pressure is controlled by a gas demand switch which operates a valve to admit air whenever internal pressure drops below the gas demand switch set-point. A low pressure cut out switch opens and interrupts terminal power whenever internal pressure drops to the low pressure cut out switch set point which is barely above one atmosphere, absolute. Overtemperature protection is provided by temperature sensors which activate purge air flow to purge warm air from the enclosure or to interrupt terminal power when the purge air cooling rate is less than the terminal operation heating rate.

BACKGROUND OF THE INVENTION 
This invention relates to improvements in control terminals which permit 
their safe use in hazardous locations. In particular, these improvements 
permit operation of electronic data terminals in environments containing 
explosive concentrations of inflammable gases, dusts, or vapors. 
Computer control of manufacturing processes can provide increased precision 
by enabling real-time process adjustment by employing electronic analysis 
of sampling data and control feedback signals. This requires placement of 
input/output data terminals and monitors at critical points in the process 
stream where sampling data are collected and process adjustments made. 
Many manufacturing processes, however, involve inflammable volatile 
solvents or gases, vapors, or dusts which present a danger of fire or 
explosion. In such environments, electronic data terminals must provide 
absolute certainty that they cannot cause a fire or explosion by arcing in 
the presence of the hazardous atmosphere. This assurance is critical in 
petrochemical plants, munitions plants, welding gas plants, and paint 
factories as well as in grain elevators and other operations which produce 
inflammable dust. 
One common approach to avoidance of the explosion hazard in such facilities 
has been to locate all controls remote from the hazardous environment. 
This configuration requires communication between the remote data 
terminals and the process adjustment points which adds oportunities for 
delay and error. 
In order to provide this safety while retaining the localized control it is 
desirable to provide atmospherically isolated regions for connection of 
primary power sources, electronics for control of purge timing and for 
purge maintenance, and electronics for terminal operation. 
An object of this invention, therefore, is to provide a control terminal 
which is safe for use in explosive or inflammable environments by 
preventing the explosive atmosphere from contacting any potential arc 
source. 
Besides the explosion hazard, many facilities have environments which can 
damage or destroy electronic equipment due to the effects of ambient 
atmosphere. 
A further object of this invention is to provide a control terminal which 
is shielded from the deleterious effects of dirt, corrosive vapors, 
humidity, excessive ambient temperatures, and combinations of these 
hazards. 
The terminals of the present invention provide the necessary safety and 
atmospheric shielding, and, since they can be placed in the hazardous 
environment at the process control points, eliminate the delay and error 
potential associated with remote location. Using these terminals, the 
operator at each location can monitor the process and immediately make 
adjustments as required. 
The objects of the invention and the means for accomplishing said objects 
will be readily understood by reference to the drawing and the 
descriptions thereof which will enable one skilled in the art to make and 
use the invention.

Referring to the FIGURE, the invention features consist of the gas purged 
enclosure 1, an external explosion-proof junction box 2a, a seal 
junction 3, between the junction box 2a, and the local electrical conduit 
4, a purge gas inlet line 5, and a pendant intrinsically safe keyboard 6. 
Examination of the enclosure 1 as illustrated in the FIGURE shows both the 
external and internal explosion-proof junction boxes 2a and 2b, 
respectively, the explosion-proof purge gas flow switch 7, the CRT and 
data processing system 8, the mechanically operated low pressure cut-out 
switch 9, the mechanically operated gas demand switch 10, the enclosure 
pressure check valves 11, the solenoid air valve 12, the first 
overtemperature purge control switch 13, and the second overtemperature 
terminal power cut-off switch 14. 
To accomplish the object of the invention of providing a control terminal 
which is safe to operate in a hazardous environment, the terminal 
incorporates electrical protection circuits to ensure safe startup, 
operation, and power-down. All components, except the explosion-proof 
purge gas flow switch 7, which are electrically active prior to purge are 
contained within the two explosion-proof boxes 2a and 2b of the FIGURE. 
The external explosion-proof junction box 2a, is used for connections to 
the local electrical power installation, which is in a safe area of the 
facility, by means of electrical lines in conduit 4. The connection is 
made through a seal 3 which provides a gas tight seal to prevent flow 
of the hazardous atmosphere, which may be in the conduit, into the 
explosion-proof box 2a. The primary electrical power terminations are made 
within the external explosion-proof box 2a which is connected to the 
internal explosion-proof box 2b through an explosion-proof union. Within 
the internal explosion-proof box 2b are the control electronics, control 
relays, the purge timer, purge indicator LEDs, and the electrical 
termination points for the various valves and switches needed for 
start-up, normal operation, and power-down. 
Note that purge status indication is carried to the front panel of the 
enclosure by fiber optics bundles. 
DETAILED DESCRIPTION OF THE INVENTION 
Referring again to the FIGURE, operation of the control terminal, which is 
the subject of this inention, is described thus. 
Upon application of AC power and a source of clean dry air or inert gas, 
the terminal enclosure 1 pressurizes to 3 to 6 psi, preferably 5 psi 
gauge, through the normally open solenoid gas or air valve 12. As the 
enclosure pressure rises, the mechanically operated low-pressure cut-out 
switch 9, closes at a pressure of between 0.1 and 0.5 inches, preferably 
0.4 inches of water column, gauge, then the gas demand switch 10, also 
mechanically operated, closes at a pressure of 3 to 5 inches, preferably 4 
inches of water column. Note that, although the low pressure cut-out 
switch 9 and gas demand switch 10 are closed mechanically due to the 
response of their diaphragms to enclosure pressure, they are not 
electrically activated until the purge timer in the internal explosion 
proof box 2b has timed out and the relays within the explosion proof box 
2b have closed. At 5 psi, enclosure pressure purge check valves 11 open 
and exhaust air from the enclosure 1. These valves are equipped with flame 
and spark suppressors in case ignition should occur within the chamber 
during purge. The explosion-proof flow switch 7, monitoring the flow of 
air through the enclosure, closes when there is sufficient flow to 
exchange four times the volume of air in the enclosure in a time period 
between 15 minutes and 35 minutes, preferably 33 minutes, to meet NFPA 
type X purging specifications. Closure of the flow switch 7 starts the 33 
minute purge cycle timer located in the explosion-proof box 2b. 
At the end of the 33 minute purge cycle, the timer causes power to be 
applied to the terminal 8, all signal lines to be connected to the 
terminal through the signal relays, in the explosion-proof box 2b, and 
enclosure pressure control to be handed-off through control relays in the 
explosion-proof box 2b to the normal operation circuits. Because the 
enclosure pressure is well above the closure point of the gas demand 
switch 10, it remains closed and the solenoid air valve 12 is immediately 
energized, or closed, cutting off the air supply. 
The unit is now in normal operation and will continue operational until 
power is removed or the air supply is interrupted resulting in a drop in 
pressure which cuts off power through the low pressure cut-out switch 9 at 
a pressure of 0.4 inches of water column, gauge. At the conclusion of the 
purge, the enclosure is pressurized to 5 psi gauge. Slowly, the air leaks 
out of the enclosure. When the pressure falls below 4 inches of water 
column, the gas demand switch opens causing the solenoid air valve 12 to 
open and allow air flow into the enclosure. Once the enclosure pressure is 
back above 4 inches of water column, the gas demand switch 10 again 
closes, energizing the solenoid air valve 12 and again closing the air 
supply. The gas demand cycle repeats continuously, maintaining the 
enclosure pressure, until power is removed, or the air supply is 
interrupted. 
Shut down of the terminal occurs whenever the enclosure pressure drops 
below 0.4 inches of water column. This may be caused by interruption of 
air supply or by enclosure leaks of sufficient size to keep enclosure 
pressure below 0.4 inches. When the air supply is interrupted, eventually 
pressure falls below 4 inches, causing gas demand switch 10 to open and, 
in turn, the solenoid the air valve 12 to open. Since the air supply has 
been interrupted, no air flows into the enclosure. The enclosure pressure 
continues to drop until finally, the low-pressure cut-out switch 9 opens 
at 0.4 inches of water column which immediately removes terminal power 
and, all signals and transfers enclosure pressure control back to the 
start-up purge circuits wholly contained within the explosion-proof box 
2b. 
Two stages of over-temperature protection are provided. When enclosure 
temperature reaches 54 degrees Celsius (130.degree. F.), an 
overtemperature purge control switch 13 causes the solenoid air valve 12 
to open, allowing the enclosure 1 to pressurize, the check valves 11 to 
open, and the enclosure to be purged to warm air. This flow of air 
continues until enclosure temperature falls to 43 degrees Celsius 
(110.degree. F.), where first control switch 13 cease purging and normal 
operation resumes. If the flow of air is insufficient to cool the terminal 
and temperature continues to rise, a second overtemperature terminal power 
cut-off switch 14 opens at 60 degrees Celsius (140.degree. F.) cutting 
terminal power, and thus removing the major source of heat in the 
enclosure and protecting the terminal electronics from over-temperature 
operation. Terminal power is restored when the enclosure temperature falls 
below 49 degrees Celsius (120.degree. F.) closing the second cut-off 
switch 14. 
While I have shown and described a particular embodiment of my invention, 
various modifications in construction, components, operating parameters, 
and application can be made without departing from my invention in its 
broad functional aspects. For instance, it is clear that a control 
terminal and/or a computer, as described herein, can be used in any 
environment, not only in environments presenting a fire or explosion 
hazard. Consequently, this invention could be employed to protect 
sensitive electronic control terminals and computers from non-explosive 
environments which may cause deterioration of the terminals due to 
humidity, corrosion, or contamination. Other such variations will be 
obvious to those skilled in the art. The appended claims, thus, address 
these anticipated modifications as lie within the scope of our invention.