Introduction to Level Controls
 

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 60 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 from 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 316SS 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 12A AC, 10A 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:

  1. 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-pressurized vessel.
  2. To monitor a safe level or detect 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
      A. General Purpose
      B. Weatherproof
      C. Explosion-proof
  7. Media compatibility with wetted parts
  1. Method of attachment
       A. External pipe mounted
          1. threaded
          2. socket weld
          3. flanged
       B. Side mounted
          1. threaded
          2. flanged
       C. 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.