18 August 1998
under local control of the mixed air controller or for override to allow for 100 percent outside air supply or
minimum outside air supply.
d. Transducers are used for changing pneumatic signals to electric signals and vice versa or for
changing a current signal to a voltage signal. Transducers are used in conjunction with sensors and
controllers. Electric to pneumatic transducers can be used to convert field equipment panel electrical
output signals to pneumatic signal inputs to a local pneumatic control loop or to a pneumatic actuator.
Pneumatic to electric transducers can be used to convert local loop pneumatic signals to electric signals
inputs to the field equipment panel.
e. A local loop pneumatic controller must be retrofitted with a CPA port for supervisory control. The
operation of a local loop 3 to 13 psi air signal on a CPA changes the setpoint plus or minus 10 percent
(the percentage will vary depending on the manufacturer of the controller). The UMCS will drive a
transducer to change the setpoint from the high to the low (or low to high) setting.
f. The designer will determine the failure mode of operation for each CPA point. In order to fail to
high, low, or local loop control, main air is fed through a pressure reducing station to produce fixed
pressure input to the three-way EP valve. A similar arrangement will use a Form C relay in lieu of a 3-
way solenoid. If the required failure mode is to remain in the last command state, the 3-way EP valve or
Form C relay is eliminated and the transducer (on UMCS failure) remains at the last command position.
g. The local electric controllers will have the same functions as the pneumatic controllers described
5. TIME CLOCKS. The implementation of UMCS time dependent control programs requires
elimination of existing time clocks. The existing time clock start/stop contacts are replaced with start/stop
contacts operated from the field equipment panel.
6. SINGLE LOOP DIGITAL CONTROLLERS. Existing single loop digital controllers may have to be
replaced with controllers having an EIA 485 serial interface with adjustable data transmission rates up to
19.2 Kbps in order to interface with the UMCS. The controller will provide the UMCS with process values,
setpoints, alarms and controller status (local-off-auto) and will allow the UMCS to perform remote
controller setpoint adjustment.
INSTRUMENT AIR SUPPLY.
a. Existing instrument air supplies must be checked for water and oil contamination. If
contamination is present the affected pneumatic lines must be replaced and tested prior to UMCS
operation. All other devices in the local control loop that have been contaminated must be replaced.
c. The instrumentation air supply must have an air drier. Filtration must be provided before and
after the air dryer.
d. Air filters must be installed with bypass and isolation valves to permit filter
replacement without instrument air supply disruption.
e. Pressure switches must be installed for all major supply air branches to detect loss of air supply.
f. Air drying and filtration at buildings must be provided when instrument air enters a building from
an outdoor distribution system.
g. The designer must evaluate the cost effectiveness of replacing a damaged instrumentation air
system versus replacement of pneumatic control devices with electric devices (e.g. actuators, controllers).