Enovation Controls Murphy DF Series, Murphy DF755, Murphy DF757 Installation And Operation Manual

DF Series Hydrostatic Head Level

Switches

Installation and Operations Manual

00-02-0831
2013-03-21
Section 15

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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. The latest version of this manual can be found at www.fwmurphy.com.

Please read the following information before installing.

BEFORE BEGINNING INSTALLATION OF THIS MURPHY
PRODUCT:

Read and follow all installation instructions.

Please contact Enovation Controls immediately if you have any

questions.

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Table of Contents

Table of Contents ...................................................................................................................... iii

Introduction ................................................................................................................................. 1

Basic Operation ............................................................................................................1

Typical Applications .................................................................................................................. 3

Water Flood Control Systems ......................................................................................3

Low Pressure Gas Blanket on Tank .............................................................................3

Typical Wiring Diagrams ..............................................................................................4

Installation of a Volume Air Cell ............................................................................................... 6

Overview ......................................................................................................................6

Specifications ............................................................................................................................. 8

Service Parts ................................................................................................................9

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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 pre-determined 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 ¼ 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 snap­switch (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).

Section 15 00-02-0831
2013-03-21 - 1 -

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