March 2010, Vol. 22, No.3

Small Communities

A Primer on Liquid-Level Control Floats

Kelly Barger

Many small communities utilize distributed wastewater systems that rely on septic-tank effluent pumping (STEP) systems. These systems commonly use high-head turbine pumps to move liquids, and they typically use liquid-level control floats to turn the pumps on and off.

Liquid-level control floats are a point-level liquid measurement device — the ideal and simplest way to monitor and control liquid levels in situations where strict process control is unnecessary. Liquid-level control floats are passive devices that contact the liquid being handled. They react only when physically moved by the liquid, and the float’s rising and falling triggers an internal switch to toggle pumps on or off.

Understanding how these devices work and the right practices for choosing and installing them ensure that liquid-level floats are a simple and effective solution for monitoring and controlling liquid level in distributed wastewater systems.

Power Versus Signal Floats

Different types of floats are used, depending upon the particular application. Floats can be either “power” floats or “signal” floats. With power floats, the operating current for the pump passes from the control panel through the float to the pump. In signal floats, a much smaller current flows from the control panel through the float and back to a relay on the control panel. When engaged, the relay operates a motor contactor, which sends the operating current to the pump.

 

Wide Versus Narrow Angle

Liquid-level control floats are designed to operate over a specific angle of movement. The most common float types include wide- or narrow-angle mercury or mechanical floats. Narrow-angle floats commonly operate across a 10-degree angle, and wide-angle floats operate across a 45-degree angle. The operating ranges of the floats are selected depending on the desired pumping operation or on–off cycle

An example of the operation of a narrow-angle float is a 10-degree-angle pump-down float that actuates when the float rises to an angle of 5 degrees above horizontal and shuts off when the float drops to an angle of 5 degrees below horizontal. This configuration would lead to more frequent pump operation.

A wide-angle float, however, typically is used to minimize pump operation. For example, a 45-degree float might be used to turn off a pump at the float’s lowest level to protect the pump from running dry and then allow the pump chamber to fill to a higher level. This scenario prevents the pump from continually cycling off and on.

A float’s baffle controls the angle at which it triggers the switch. In mercury floats, the baffles are machined metal. The machined metal baffle is designed to enable the mercury to flow inside the float from side to side.

Mechanical floats use a plastic molded baffle. A mechanical float has a ball bearing that drops from one side of the float to the other. The side with the switch has a lever. When the ball bearing hits the lever, it pushes a button on the switch. If the baffle is not designed correctly, the float will not operate at the design angle; therefore, the pump will not cycle correctly.

 

Vertical-Float Switch

A vertical-float switch is yet another type of liquid-level control float. These floats are commonly used in small-diameter pits or pipes where a mercury or mechanical float does not have enough room to float and angle up or down. These vertical floats are completely submersible.

A wide variety of materials is used to construct vertical-float switches to enable them to withstand harsh environments and high temperatures. Stainless steel, polypropylene, high-impact polystyrene, and acrylonitrilebutadiene styrene (ABS) plastic are all commonly used.

 

Cord Considerations

Liquid-level control floats are constructed to operate as either normally open or normally closed (pump-down or pump-up). In the normally open, or pump-down, application, the liquid level in the float chamber drops when the pump is activated. The float triggers the pump to turn on when the float moves above the horizontal. The pump then shuts off when the float reaches its negative design angle.

In the normally closed, or pump-up, application, the opposite occurs — the liquid level in the float chamber rises. When the float reaches its negative design angle, the pump is activated. When the float reaches its positive angle, the pump turns off.

In either application, the float cord is tied to a rigid surface — its tether point. The tether point is left adjustable and not tightened down until the pump-on and pump-off levels are determined. The time between the pump-on and pump-off levels can be lengthened by increasing the length of the float cord and shortened by shortening the float cord.

In STEP applications, the float should be set to achieve a 4-in. pump-down to avoid excessive stirring of the tank contents. Float-switch cords are PVC-jacketed and come in a many lengths, such as 15 m, 23 m, 30 m (50 ft, 75 ft, 100 ft), to meet the needs of the particular installation.

Most liquid-level control floats are wired with 16-2 AWG SJOW cord. To avoid excessive power drop, the cord length on power floats should not exceed 30 m (100 ft) in most cases. If necessary to locate floats farther away from the control panel, a panel should be selected that channels a low-voltage signal through the float to a relay, which enables energizing of motor contactors.

Troubleshooting

Common problems observed with float installations include improper float selection, improper connection to control panels, excessive cord length, use of splice boxes between pump chamber and control panel, and insufficient clear space around the float. If the pump operating range is too large or too small, the wrong float angle may have been installed.

Mercury and mechanical floats often are used interchangeably for the same applications. However, mercury floats are commonly rated at 115V/13 amps or 230V/6.5 amps. In a 230-V pump system with the control panel operating the floats as “powered,” the operating amperage of the pump easily can exceed the amperage limit of the mercury floats.

Liquid-level controls generally consist of one to five floats, each performing a specific function, such as pump on and off; timer on and off; redundant off; and high or low level alarm. If the floats are not connected to the proper terminals in the panel, the floats’ functions will not occur at the proper liquid level or in the sequence desired.

One of the most common maintenance issues has been water entering splice boxes and shorting out floats. As all splice boxes eventually leak, running the float leads uninterrupted from the pump chamber to the control panel will eliminate this problem.

If floats are mounted without sufficient clear space in which to rise and fall, “hanging” can occur, resulting in improper pump operation. Excessive cord length between the tether point and the float body can result in excessive drawdown. Likewise, insufficient length can result in failure of the float to rise or fall as designed, especially after lengthy exposure to wastewater, which results in loss of flexibility of the cord. Leaving approximately four fingers' width of cord length between the float and the cord collar will ensure good operation.

With the proper specification for the particular application, including material selection, pump-motor horsepower, desired pump operation, installation location, and tank or pit size, a liquid-level control float is a simple solution for monitoring and controlling liquid levels in distributed wastewater systems.

 

Kelly Barger is the owner of Tennessee Pump and Controls LLC (Knoxville, Tenn.), which specializes in onsite sewer applications.

 

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