A sensor worth its salt

“Zout” is Dutch for salt and since that is precisely what Frisia Zout produces, it is hardly surprising that their tanks and pipes are filled with it. However, salt is also in the air, as the plant in Harlingen is located only a few meters from the sea.

Salt causes exposed metal parts to corrode. Together with condensed water, this poses a significant problem for the switch boxes controlling the outdoor valves, all too often they have to be replaced due to corroded contacts and electrical parts. Frisia Zout was eager to try out Pepperl+Fuchs’ F31K2 dual inductive sensor for valve position feedback. The results seen during the six-month test phase were excellent and the company now plans to replace all of its switch boxes with the F31K2 sensors.

Corrosive conditions
3,000 meters below Harlingen lies the salt formation where Frisia Zout sources its raw material. Water is forced into the mineral formation at high pressure and salt-saturated brine is pumped up into the plant’s huge tanks. Here, soda and lime react with the brine, separating the impurities from the brine resulting in lime-like residual matter, which itself is a useful by-product used in the construction industry and fertilizers.

“We fitted two valve position sensors to the supply pipes about six months ago and two more to the drain pipes shortly afterwards,” recalls Rudolf Bergsma, the head of the plant’s electrotechnical division. He had previously read about the new F31K2 in a Pepperl+Fuchs newsletter and immediately recognized the opportunity to solve a recurring problem that he was facing with the switch boxes mounted on the plant’s pneumatic valve actuators. These switch boxes are housed in aluminum boxes and use mechanical contacts. However, they are not ideal for use in the climate of the North Sea which, combined with the boxes’ susceptibility to internal condensation, means that they constantly corrode inside and must be replaced frequently.

Box within a box
“A lot of our equipment is made of polyester or stainless steel to ensure that it can withstand salt-induced corrosion,” says Rudolf Bergsma. “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 the Dutch valve supplier Bray Controls and Pepperl+Fuchs to organize on-site testing.

“The sensor has a number of advantages which impressed us immediately,” says Jos De Jong from Bray Controls. “It 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.”

Touch-free 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. In contrast, the F31K2 valve position sensor separates mechanical movement and position feedback by using two inductive elements. As soon as a metallic surface (target) enters the inductive sensor’s magnetic field, it is detected by the evaluation electronics and the output is triggered. Simple metal targets are sufficient for valve actuators, whose positions can be detected touch-free with the inductive sensors. Since no physical contact is necessary, the sensor can be hermetically sealed from the actuator.

“The electronic solution combines a high degree of switching measurement accuracy and precisely defined hysteresis with intrinsic ruggedness,” states Pepperl+Fuchs sales engineer Wim Kamman. “To ensure that the sensor can connect seamlessly with existing control systems, options featuring all common connection types and interfaces are available. The beacon is visible from a long distance and gives on-site operators reliable information at a glance.”

Open solutions
The F31K2 sensor is part of Pepperl+Fuchs’ open solutions concept for valve position feedback, which is based on simple designs that can be used in a wide variety of applications. The concept’s basic elements are an actuator (puck) containing the targets and a dual sensor. These can be installed without mounting brackets thanks to both the mechanical standardization of drives pursuant to VDI/VDE 3845 and NAMUR mounting holes. The dual sensor is directly attached to the drive housing, and the puck merely needs to be pushed onto the drive shaft and fixed in place according to the valve’s current position.

Other members of the position feedback sensors family manufactured by Pepperl+Fuchs are the F25, F31, and F31K Series. The F25 Series models are well suited to manually operated valves and are used in limited spaces. Their circular sensor surface with two active zones offset by 90° enable the sensor and target element to be combined to form an extremely compact single unit suitable for being mounted to terminal switch boxes.
Connection options

The two inductive sensor elements in the F31 Series are arranged on top of each other. Along with the different-sized pucks available, this arrangement allows the same sensor design to be adapted for various shaft diameters and heights. The sensors cover all drive sizes up to those with drive shaft diameters of 90 mm with just two puck sizes, making brackets or mounting sets unnecessary. To date, no other valve position feedback sensor on the market offers such a product feature.

In addition to all AC and DC output stages, NAMUR and AS-Interface options are supported. Versions suitable for use in both safe and hazardous areas are available, and it goes without saying that these have obtained the corresponding approval. The product range is rounded off with cable and plug accessories for all commonly used industrial plug connections. Moreover, to keep the number of wires to a minimum, the F31 devices enable the solenoid valve control cable to be routed through the sensor. F31K models can also be linked using terminal compartment connection technology.

Additionally for the F31K2, the LEDs are integrated into the encapsulated sensor module. Because the housing cover is made of a robust, translucent plastic, sealed LED windows are not necessary for the pilot lights that indicate the power supply, sensor, and valve conditions. The housing materials provide high UV protection and are resistant to extreme temperatures and corrosion. Thanks to the large terminal compartment and pluggable terminal block, cables that are rigid or have large diameters do not pose any problems in environments where mounting work could prove difficult.

“The sensors have worked perfectly – we haven’t had any problems with them at all,” concludes Rudolf Bergsma. “The on-site operators 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.”