Pumps and Systems, May 2009
Find out why level controls can fail in municipal wastewater treatment plants and similar systems and what technologies are available to combat failure.
Leaving the smallest parts of a system unnoticed can be a costly mistake. Peter Jankowski, previously a plumber on call, remembers one fateful Christmas Eve when he received an emergency call from a nearby farm where the sanitation system had apparently failed. When he and his partner arrived at the scene, they already had a good idea of the problem. "Most of those failures relate to overflowing cesspools," he says. "But usually it is not the pumps that are failing; it is the level-control switches."
In applications like domestic cesspools, but also in municipal wastewater treatment plants and similar systems, level controls are used to keep the wastewater level within a certain defined range. The level controller sends a signal to the control box to start or stop the pump when the level reaches the upper or lower limits of the tank.
A wide range of different products and technologies are deployed in trying to master this task, each with its advantages and disadvantages. How do you select the right one? To make the correct choice, it is important to understand why these level controls are so prone to error. As simple as their task may seem, these little devices have to battle a wide range of problems, depending on the fluid they have to handle.
Unlike clean water, wastewater contains a large amount of suspended solids. The tanks are designed so that the solids sink to the bottom, where they are collected in a holding pit. One problem that often causes the failure of controls is that some solids, like toilet paper, get caught in them. Another factor is turbulence and wave formation, which may shake the device and prevent it from switching correctly.
The effect of capillary action is often underestimated. Sulfur dioxide is a gas that municipal wastewater emits. This gas can get sucked in through pores in the cable into the level-control switch. It is absorbed on metal surfaces and tends to form sulfuric acid in the presence of moisture, causing the internal metal parts of a device such as microswitches to corrode. In fact, this is a common cause of failing control switches and usually remains undetected because the damage is not visible from the outside.
Level sensors can be grouped into two categories: continuous level monitoring or point level detection. Point level detection is usually adequate for pump control. The main types of level control include ultrasound, pneumatic and float switch.
Ultrasound Level Control
"Technologically, the best solution is probably ultrasound," says Jankowski, "but its price is usually disproportionally higher than that of the alternatives." The sensors emit high-frequency sound waves that are reflected back and detected by the emitting transducer. They are not in physical contact with the medium, so they are not affected by viscous, bulky or aggressive liquids, such as oils, slurries, grease or solvents.
Apart from their price and comparatively greater difficulty of installation, ultrasonic level sensors have other crucial drawbacks. Sound waves are sensitive to variations in the air through which they are traveling. Moisture level and temperature influence the speed of sound. Foam, steam, turbulence or vapors can prevent proper reflection of the sound waves or even absorb them.
Pneumatic Level Control
Pneumatic level detection uses a hydrostatic measuring method. It is a relatively inexpensive means of level detection. The actual sensor consists of a cast iron plunger installed at a specific distance above the bottom of a vessel, such as a tank, and attached to a plastic tube leading to a transducer. The compression of a column of air against a diaphragm is then used to actuate a switch.
Although they are relatively simple and inexpensive to install, pressure-actuated level controls cannot be used with process liquids that may crystallize in the riser or corrode the transducer. A common misconception is that these pressure controls may be used for hazardous, potentially explosive atmospheres, since no electronics are inserted in the medium. If the plunger is not constantly below the surface of the process liquid, however, explosive vapors may rise through the plastic tube to the transducer, where the conditions for spark formation may exist.
Float Switches
After he had cleaned the inspection point well enough to climb in the pit, Jankowski advised the residents not to use the restrooms until they were done replacing the level control switch. "I was just attaching the new switch when I heard a rush and right then, I knew I was in trouble," Jankowski remembers, laughing. "That was the day I decided to build a level control that was better than anything else on the market so that I would not have to climb into these cesspools quite so often."
Having worked in the pump industry for 45 years, Jankowski believed only a float switch would meet his requirements for quality, functionality and cost efficiency. The bulb shape of the float switch has no pores or edges where solids can become trapped and prevent movement. The diameter only needs to be small, unlike wide-angle switches. Being centrally weighted, the switching point is always at the same angle, regardless of turbulence. The weight is suspended on its own cable, which means it can be simply pulled out of the tank or cesspool, instead of having to be demounted. This makes maintenance much easier. Foam or rising bubbles do not cause hysteresis as they would with floating switches.
However, sometimes the problem is not the actual switch, but the cable. Exposed to the medium, it may swell, become stiff or even dissolve completely. A thermoplastic elastomere (TPK), on the other hand, withstands most fluids, chemicals and gases, such as sewage and fecal water, liquid manure, emulsions, diesel, fruit acid and alcohol. The cable remains flexible, even at low or high temperatures. A level controller fit with a Teflon cable and special housing may be used in solvents, one of the most aggressive mediums.
Float switches are generally suitable for water, and some can be used with drinking water (ACS-certified), aggressive chemicals and in Divisions 1 and 2 hazardous areas (equivalent to the European potentially explosive atmosphere ("Ex") zones zero, one and two) such as gas tanks. For small tanks with limited space, miniature level controllers are available. Quality float switches are environmentally friendly, complying with the restrictions on the use of certain hazardous substances directive (RoHS).
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