ue>
P74JA Type Differential Pressure Control with Floating Action Switch
FANs 121, 125
Pressure Controls Section
P
Product Bulletin
P74JA
Issue Date
0891
© 1991 Johnson Controls, Inc.
1
Code No. LIT-121470
P74JA Type Differential Pressure Control
With Floating Action Switch
Application
The P74JA differential pressure
control is commonly used on
water systems to control a
pressure differential between
supply and return lines. The
control's SPDT floating action
contacts position a motor
actuated valve operated by a
Johnson Controls M81 or Series
M100A motor actuator. In
atypical application the valve is
located in a bypass line around
the system pump. When the
system pressure differential
increases above the control's set
point, the valve opens further.
More water is diverted through
the bypass valve, resulting in a
reduction in system pressure
differential. A reduction in the
pressure differential causes the
valve to begin closing and
results in an increase in system
pressure differential.
The P74JA is suitable for use on
other air or liquid pressure
differential applications where
SPDT non-snap acting switching
is desired.
All Series P74 differential
pressure controls are
designed for use only as
operating controls. Where an
operating control failure
would result in personal injury
and/or loss of property, it is
the responsibility of the
installer to add devices
(safety, limit controls) or
systems (alarm, supervisory
systems) that protect against,
or warn of, control failure.
Features Single-pole, double-throw,
totally enclosed non-snap
acting Pennswitch. 1/4 in. flared tube pressure
connection on the sensing
element permits easy field
installation.
Fig. 1 -- Exterior view of a P74JA
Differential
Pressure Control. Direct reading scale for quick
set point determination. Pressure differential setting
can be easily changed
without removing the cover.
General Description
The P74JA differential pressure
control incorporates two highly
sensitive pressure elements
which directly oppose each
other. A change in pressure at
either element causes
appropriate positioning of the
switch mechanism which is then
used to control supplementary
equipment such as motor
actuated valves. The switch
mechanism provides SPDT non-
snap acting (floating) action; as
such, the movable contact
(common) is made to either of
the two fixed contacts or
positioned between the two (no
contact made).
Specifications 2 P74JA Product Bulletin
Fig. 2 -- Interior view of a P74JA
Differential Pressure Control.
!
CAUTION: This control
should not be used to
control relays or starters as
short cycling or chattering
will occur.
Ordering Information
To order, specify P74JA.
Installation
Mounting
Mount the control in any position
to a flat surface or panel board.
Use two screws or bolts through
the holes in the back of the
case, or use Part Number 271-
51 universal mounting bracket.
The control should be mounted
so the connections on the
bellows are above the liquid
level of the equipment on which
it is used. This reduces the
possibility of accumulating
foreign matter inside the
bellows.
General Instructions
1.
Make sure the control is not
installed on equipment
which would require the
control to handle a load in
excess of its electrical
rating. For maximum
electrical rating and contact
action of the control, see the
label on the inside of the
control cover.
2.
Purge all tubing and lines
before connecting the
pressure control.
3.
Avoid sharp bends or kinks
in capillary tubing.
4.
Coil and secure excess
capillary length to avoid
vibration. Allow some slack
in capillary to avoid violin
string vibration which can
cause tubing to break. Do
not allow tubing to rub
against surrounding
surfaces or objects where
friction can damage
capillary.
Wiring
!
CAUTION: Disconnect the
electrical power supply
before wiring connections
are made to prevent
possible electrical shock or
damage to the equipment.
Make all wiring connections
using copper conductors only,
and in accordance with local
ordinances and the National
Electrical Code. Use Number 18
or larger wire for all connections
to the motor actuator. Number
14 wire should be used on runs
over 500 feet long.
All splices are to be made in
junction boxes utilizing approved
solderless connectors or by
soldering and then taping the
connections.
Note: Use terminal screws
furnished (8-32 x 1/4 in. binder
head). Substitution of other
screws may cause problems in
making proper connections.
Adjustment
To change the set point, turn the
adjustment wheel to the left or
right as viewed from the front of
the control. (See Fig. 2.)
Insert a screwdriver into the slot
in the wheel and rotate the
wheel until the scale pointer is
opposite the set point desired.
Rotate the wheel to the left to
increase the differential set point
or to the right to decrease the
differential set point.
Checkout Procedure
Before leaving the installation,
observe at lease three complete
operating cycles to be sure that
all components are functioning
correctly.
Repairs and Replacement
Field repairs must not be made
except for replacement of the
cover. For a replacement control
or cover contact the nearest
Johnson Controls distributor.
Fig. 3 -- Typical wiring hookup
showing connections to a motor actuator. P74JA Product Bulletin 3 4 P74JA Product Bulletin
Notes
Controls Group
507 E. Michigan Street
P.O. Box 423
Printed in U.S.A.
Milwaukee, WI 53202