Salt is everywhere at Frisia Zout in the Dutch city of Harlingen because salt is precisely what Frisia produces. Since the company's plant is located only a few meters away from the sea, salt is even in the air. This poses a significant problem for the switch boxes that control the company's outdoor valves because the salty air causes exposed metal parts to corrode.
Noncontact Position Detection
Traditional switch boxes are based on mechanical feedback systems that depend on rotary or lifting movements, e.g., conventional actuating switching elements that work via switching cams. But new noncontact solutions, like the F31K2 valve position sensor from Pepperl+Fuchs, use two inductive elements to separate mechanical movement and position.
With the inductive sensors, the position of valve actuators can be detected touch-free using simple metal targets. When a metallic surface (target) enters the inductive sensor's magnetic field, the evaluation electronics detect the metal and trigger an output. Since no physical contact is necessary, the sensor can be hermetically sealed from the actuator.
Combined in a single module and enclosed in rugged, translucent plastic, the sensor and its components are protected from moisture, mechanical vibration, impact, and UV light. Sealed LED windows are not necessary for the pilot lights that indicate the power supply, sensor, and valve conditions because of this dual mechanical protection.
“A lot of our equipment is made of polyester or stainless steel to ensure that it can withstand salt-induced corrosion,” said Rudolf Bergsma, head of the plant's electrotechnical division. The switch boxes mounted on the plant's pneumatic valve actuators were housed in highly corrosive aluminum boxes that needed frequent replacing. “When I saw the F31K2 sensor's plastic cover and read about its double-housing design, I realized that this could be just what we were looking for.” Bergsma then contacted Pepperl+Fuchs (headquartered in Mannheim, Germany) and Dutch valve supplier Bray Controls to organize onsite testing.
According to Bray Controls’ Jos De Jong, the sensor “is compact and gives a very clear indication of the valve's position. With the inductive sensors, there is no need for mechanical contacts — what is not there cannot corrode. For additional protection, the switches are fully encapsulated in a box within a box. At a very reasonable price, the sensor looked like the perfect solution to me.”
The F31K2 sensor is part of an open solutions concept for valve position feedback. Based on simple designs that can be used in a wide variety of applications, the concept's basic elements are an actuator (puck) that contains the targets and a dual sensor. These can be installed without mounting brackets because of the mechanical standardization of drives according to VDI/VDE 3845 and NAMUR mounting holes. The sensors cover all drive sizes, including those with drive shaft diameters of up to 90 mm, with just two sizes of puck. The dual sensor attaches directly to the drive housing. Then, the puck only needs to be pushed onto the drive shaft and fixed in place according to the valve's current position.
In addition to all AC and DC output circuits, the sensor supports NAMUR and AS-Interface options. Versions suitable for use in both safe and hazardous areas are available and these come with the corresponding approvals. Cable and plug accessories for all commonly used industrial plug connections complete the product range.
Models can be linked using terminal compartment connection technology. A large terminal compartment and pluggable terminal block mean rigid or large-diameter cables can be used easily in environments where mounting work could prove difficult.
“The sensors have worked perfectly — we haven't had any problems with them at all. The onsite operating personnel are extremely satisfied with them, not least because of the high visibility of the signals given off by the beacon. I am certain that, over time, we shall replace all of our switch boxes with F31K2 sensors,” according to Bergsma.
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