Most of the devices featured
in this technical guide use the magnetic attraction or repulsion
system of operation. This rugged electro-mechanical design
has proven itself for more than 50 years and millions of
heavy duty applications. The electro-mechanical system consists
of an attractor of magnetic material, usually 430 stainless
steel, or a magnet inside the control and a movable magnet
outside the device. As the attractor moves in or out of
the magnetic field the magnet moves toward or away form
the switch assembly to either make or break one or more
electrical contacts. This switch action performs the task
of operating equipment, alarms, shutdowns, and/or reports
such functions to a computer or central location. Controls
featuring this magnet operated system can be used at temperatures
up to 800°F (425°C) and pressures up to 1750 psig
(121 bar). Liquid level controls are available to handle
fluids from water to more aggressive liquids requiring all
316 SS construction. Dry bulk controls can handle materials
from carbon black to soybean flour. Electrical contacts
of SPST, SPDT, or DPDT configuration are provided to accommodate
applications from 12 amperes AC, 10 amperes DC, to milliampere
requirements of high technology low current, low voltage
intrinsically safe or "dry" circuits. General
purpose, weatherproof or explosion-proof enclosures protect
the electrical switch assemblies from the environment.
Dwyer® Dry Bulk or Liquid Level Controls
are designed to perform two functions:
| • |
To
maintain a level. This task is usually completed by
using a control with a wide deadband, or two fixed
deadband controls and a holding relay. Thus when the
level of a liquid or dry bulk material reaches a pre-determined
high (or low) point a pump is shut off or turned on.
This cycle is repeated continuously such as in a sump,
tank, lubrication equipment, bin or other pressurized
or non-pressured vessel.
|
| • |
To monitor
a safe level or de4tect a not-to-exceed level - that
is to act as an alarm, or shut down, or both. This
can be either high or low functions or both. This
is accomplished by using a control with a fixed deadband.
There are a great variety of controls suitable for
this purpose.
|
There are basically 8 determining factors
when selecting a control for a specific function.
| 1 |
Pressure |
| 2 |
Temperature |
| 3 |
Specific gravity or bulk density |
| 4 |
Fixed or adjustable deadband |
| 5 |
Electrical requirement |
| 6 |
Enclosure type |
| |
• |
General Purpose |
| |
• |
Weatherproof |
| |
• |
Explosion-proof |
| 7 |
Media compatibility with wetted parts
|
| 8 |
Method of attachment |
| |
• |
External pipe mounted |
| |
|
1 |
threaded |
| |
|
2 |
socket weld |
| |
|
3 |
flanged |
| |
• |
Side mounted |
| |
|
1 |
threaded |
| |
|
2 |
flanged |
| |
• |
Top mounted |
| |
|
1 |
flanged |
| |
|
2 |
threaded |
If the control is to be externally mounted,
can the chamber be a welded type such as for use with clean
non-corrosive liquids or should it be flanged so that the
unit can be inspected and/or easily maintained? Welded chamber
benefits are that they are usually less expensive than flanged
controls and can be used with higher pressure and temperature.
The disadvantage is that the unit cannot be disassembled
for cleaning or inspection. Flanged units can be disassembled
but are more expensive and have lower pressure and temperature
capability.
Controls for top mounted service are
generally capable of performing a greater variety of functions
such as operating 1, 2 or 3 pumps, providing high and low
alarm or combining functions such as operating a pump and
providing both high and low alarms in one unit.
Consult the factor or your nearest
sales office if you cannot determine the best control for
your application based on the information in the technical
guide.
