The user also has the option to select a window in the housing of the electronics. Without a microprocessor, the flow switch could be subject to more noise, have drift issues and need more frequent calibration to maintain the set point. The microprocessor based electronics monitor for any open circuits or flow signal that goes out of range. For pump applications where a low flow alarm is desirable, the current will fall to less than or equal to 3.6 mA during the fault. The live signal allows the operator to monitor conditions to which the sensor is exposed and possibly develop custom diagnostics for the fluid system.Īlong with the trending capabilities of using the current output, this output will also go low or high when a fault condition occurs according to NAMUR NE 43. If there is turbulence in the line, possibly being caused by a closed valve with the pump still running, the sensor will see this turbulence as a higher flow rate than what is actually occurring. The current will be repeatable for a given low flow set point. For example, in a low flow condition the current may be 8 mA and at normal flows 12 mA (output varies for each application). It is not a linear 4-20 mA output, similar to a flow meter, but the current will act as a live signal that varies with heat transfer. Along with the ease of installation come many diagnostic features incorporated in the microprocessor based electronics.Ī useful diagnostic feature in the electronics is the current output. Wiring is simplified with the terminals easily accessible without removal of the bezel or any circuit boards. The electronics for the thermal dispersion switch can be integral to the probe assembly or remotely located, enclosed in an explosion proof or other suitable housing. With pressure ratings up to 600 psig (41 bar) and standard 316 stainless steel material of construction it is suitable for most pump applications. The lack of pins at the end of the probe eliminates plugging in viscous applications while the thin wall allows increased sensitivity with the process liquid. It has a very high pressure rating and is available in a variety of construction materials.Ī unique design that may be preferred for liquid applications is the spherical tip probe. Twin tip probes can be beneficial as multiple manufacturers have similar designs. The twin tip is essentially two tubes welded to the end of the probe that are in the process liquid. The standard probe design offered by thermal dispersion switch manufacturers is a twin tip construction to house the sensors. Dual ultrasonic gap switches have pump control modes where the unit performs auto-fill or auto-empty as needed. Common applications for these switches are sumps or wet wells. They are unable to detect decreasing flow rates, and their "gap" at the sensor creates opportunity for plugging or fouling with some liquids. It is inherent in these technologies that the presence or absence of liquid at the sensor location is all that is detected, not actual fluid flow. Vibrating forks and ultrasonic gap switches are other technologies employed for pump protection. In terms of the installation, the pipeline must be horizontal. A mechanical flow switch may be desirable if there is limited on-site power. Viscous liquids or those that may accumulate deposits on the operating mechanism can decrease reliability of the switch. Moving parts are subject to wear, possible jamming and increased maintenance over time. When the vane moves a predetermined distance, a magnetic sleeve or other device rises to draw the magnet in to actuate the switch. Typically, a vane or paddle is in the flow stream and swings in the direction of the flow. They use a mechanical operator, driven by liquid flow, to actuate a relay. Prior to the availability of electronic controls, they were generally the only method employed. Mechanical flow switches are an alternative. Flow meters typically are more costly than flow switches. Flow meters can be used, but a continuous flow measurement may not be needed for the application. There are many technologies that can measure liquid flow and function as part of a pump protection scheme. Monitoring liquid flow rate and triggering an alarm or pump shutdown to prevent damage has substantial benefit, even in a system of comparatively modest size.
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