FW MURPHY DF755 Hydrostatic Head Level Switches User Manual
- June 13, 2024
- FW MURPHY
Table of Contents
Hydrostatic Head level Switches DF Series
Installation and Operations Manual
Please read the following instructions and warnings before installation.
Visually inspect the product for any damage that may have occurred during
shipping.
Introduction
The DF series are diaphragm-operated hydrostatic head pressure level switches.
A pressure-sensitive diaphragm operates a snap-switch that can be wired
directly to electric pilot circuits to control pumps at predetermined levels.
A typical application is starting and stopping electric driven pump(s) to
maintain tank levels. This series of level switches can also be used with
engine driven pumps.
The nitrile sensing diaphragm is impervious to most liquids and is sensitive
enough to control levels with 1/4 in. (6 mm) repeatability. See the next
section, Basic Operation, for limits of switch trip point adjustability. The
case is aluminum with a glass-filled nylon bottom plate.
This is a highly reliable level switch and can be worked into almost any new
or existing system without major modification or special tools.
The DF755 and DF757 are suitable for atmospheric tanks in non-hazardous areas.
The SPDT snap-switch for the DF755 is preset for a 4 in. (102 mm) differential
in liquid level. The DF757 trip point is adjustable over a 108 in. (2743 mm)
range.
Basic Operation
As the liquid level rises, hydrostatic head pressure is applied to the diaphragm (A), as shown below. The diaphragm moves upward pressing the actuator arm (B) to activate the snaps itch (C).
Model DF755 is factory set and operates at approximately 2 in. (51 mm) and 6 in. (152 mm) above the level at which the diaphragm is mounted. The trip point(s) for Model DF757 are adjustable between 2 in. (51 mm) and 110 in. (2794 mm) for high and low (make/break) operation by knobs (D).
Set Point Adjustment for the DF757
- Locate the threaded adjustment shaft and adjustment knobs (see drawing).
- To increase the low-level set point, rotate the lower knob counterclockwise.
NOTE: If the adjustment shaft turns when rotating the adjustment knobs, firmly grasp the adjustment shaft with a pair of needle nose pliers and then rotate the knob.
In order to consistently bring you the highest quality, full-featured
products, we reserve the right to change our specifications and designs at any
time.
FW MURPHY product names and the FW MURPHY logo are proprietary trademarks.
This document, including textual matter and illustrations, is copyright
protected with all rights reserved. (c) 2021 FW MURPHY. A copy of our typical
warranty may be viewed or printed by going to
www.fwmurphy.com/warranty.
Typical Applications
This section covers some general situations where the rugged DF Series
switches provide a simple-to-install solution.
Applications include:
| Water Flood Systems | Diesel Day Tanks |
|---|---|
| Crude Oil Tanks | Sumps |
| Saltwater Disposal Systems | Cooling Towers |
Water Flood Control Systems
The diagram below displays eight DF Series switches installed on raw water and
clear water tanks. When raw water rises to a predetermined level, DF#1 stops
the supply pump. As tank level falls below the set level, DF#2 starts the
supply pump. If the tank level continues to fall, DF#3 initiates a shutdown of
the supply pump. DF#4 stops the transfer pump before the raw water tank is
completely pumped out.
When clear water reaches the predetermined level, DF#5 stops the transfer
pump. As tank level falls to the set low level, DF#6 starts the transfer pump.
If the tank level continues to fall due to the failure of the filters section,
DF#7 initiates shutdown of the transfer pump. (A DF755 located at this level
will also operate backwash equipment.) DF#8 stops the injection pump before
tank pumps completely out. An OPL Series Pressure Gage stops the injection
pump when pressure reaches an established high or low pressure.
Typical Tank Mounting
The following graphics show different methods of mounting DF755 switches in
tank applications.
Typical Wiring Diagrams
Starts at Low Level, Stops at High
Start motor when predetermined low level is reached and stop when high level
is reached. Keeps tank level within selected limits. Motor starter is equipped
with Hands Off Auto (HOA).
Single Magneto Shutdown
Wiring of magneto to Normally Open (NO) switch terminal shuts engine down at a
predetermined high level (Shown at right). Wire to Normally Closed (NC)
terminal to shut down on low level.
Starts at High Level, Stops at Low
Start motor when predetermined high level is reached and stop when low level
is reached. Motor starter equipped with HOA.
Dual Magneto Shutdown
Shutdown dual magneto engines using an FW Murphy MS2120 Magnetic Switch.
Diagram at right shows hookup for low-level shutdown. Wiring changes and
mounting locations are necessary for high-level shutdowns.
Specifications
Snap-switch Ratings SPDT
5 A @ 125, 250 or 480 VAC
1/2 A @ 125 VDC, 1/4 A @ 250 VDC
Case/Lid: Aluminum
Bottom Plate: Glass-filled Nylon
Process Connection: 1 NPT
Maximum Pressure Rating: 25 psi (172 kPa [1.72 Bar])
Conduit Connection (electrical): 1/2 NPT
RoHS Compliant
Installation of Volume Air Cell
This section provides installation and operation information for using a DF755 level switch with an FW Murphy MAC1 Volume Air Cell.
Overview
When attached to the DF755 level switch, the MAC1 Volume Air Cell can monitor
water levels on a sump, activate alarms or start a pump directly. The MAC1
Volume Air Cell is non-corrosive. Stainless steel 1/4-20 mounting studs are
provided with the unit.
The MACT1 Tubing Kit provides 4 ft. (1.2 m) of flexible, non-corrosive 1/4 in.
(6 mm) tubing (cut to fit). The kit also includes fittings necessary to attach
tubing
Volume Cell Operation
As liquid rises around the volume cell, it compresses the air inside the cell
and forces it up in the sensor line. As the water level continues to rise, the
air pressure increases. In time, sufficient pressure is applied to activate
the internal snap-switch, which starts the pump. As the liquid level is pumped
down, pressure decreases, the snap-switch reverses, and the pump stops. The
pump is held in a standby condition. An air purge may be required in the
sensor line. For additional information, consult FW Murphy.
Choosing a Volume Cell
The volume cell should be constructed of material which will be unaffected by
the liquid being measured. For proper pressure to level ratio, the minimum
dimensions of the volume cell should be 6 in. (152 mm) inside diameter and 3
in. (76 mm) inside depth. The sensor line can be of any diameter or material
either flexible or solid, as long as it is of sufficient length to reach from
the volume cell to the desired location for the DF755. All fittings and
connections should be airtight to avoid loss of charge. Tube lengths longer
than 4 ft. (1.2 m) should have a provision for intermittent air purging.
Installation of the Volume Cell
Install the volume cell with reference to the level at which you wish the pump
to start and stop. Secure the volume cell with a substantial bracket that will
not allow the cell to float or tilt when the water level rises. Install the
DF755 well above the highest water level and in a position that allows access
for adjustment or repairs.
NOTE: Periodically operate the pump manually until the water level
reaches a point approximately 1/2 in. (13 mm) below the bottom of the Volume
Cell. This automatically recharges the unit and compensates for normal
absorption of air into water. Small electric air pumps are available to
automatically charge the system continuously.
Service Parts
DF755
| Description | Part Number |
|---|---|
| Cover (Aluminum) | 15050081 |
| Case (Aluminum) | 15050082 |
| Cover Screws (3), #6–32 x 5/16 Round Head | 80040607 |
| SPDT Snap-Switch and Movement Assembly/Repair Kit (5 amp) | 15000122 |
| Screws (3) for Switch Assembly to Case, #6–32 x 1/4 Round Head | 80040605 |
| Diaphragm Repair Kit | 15000123 |
| Bottom Plate (1 NPT Connection) | 15050083 |
| Bracket and Movement Repair Kit | 15000313 |
| SPDT Snap-Switch and Insulator Repair Kit (5 amp) | 15000121 |
DF757
| Description | Part Number |
|---|---|
| Cover (Aluminum) | 15050081 |
| Case (Aluminum) | 15050594 |
| Cover Screw (3), #6–32 x 5/16 Round Head | 80040607 |
| SPDT Snap-Switch and Bracket Assembly/Repair Kit (5 amp) | 15000174 |
| Screws (2) for Switch Bracket and Assembly to Case (#6–32 x 1/4 Round Head) |
80040605
Spring and Piston Assembly/Repair Kit| 15000190
Diaphragm repair kit| 15000123
Bottom Plate (1 NPT Connection)| 15050083
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References
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