EGE-Elektronik Spezial-Sensoren GmbH - Special-Sensors for Automation

Flow Sensors - Technique and application

Probes

The temperature sensitive measuring elements are fitted in the tip of the probe. The probe tip and the adjoining thread/mounting part are made in one piece of stainless steel in many probes. This guarantees absolute tightness and high compressive strength. Special materials are used in corrosive, and particularly in oxidizing media, since stainless steel shows only limited resistance to corrosion in this application. In standard applications, probes can be mounted independently of the direction of flow of the medium.
In any case, it is important to make sure that the pin of probe is completely surrounded by the medium to be monitored. Please note that for smaller cross-sections the sensor tip narrows the tube's cross-section. This results in a higher flow rate. In order to avoid malfunctions caused by unstable flow patterns no fittings that could affect the flow cross-section or the flow direction should be placed directly in front of and behind the sensor. The point of reference for the input/outlet section is approximately 4 to 8 times the tube diameter.


Assembly

Probes with short thread-pieces of the STK... type are particulary suited for fitting into T-pieces. Sensor length is designed in such a way that the probe tip is completely immersed in the medium without touching the opposite side.
Probes with long thread-pieces of the ST... type are suitable for larger pipe diameters or for use with longer assembly thread-pieces.
Probes threads are G-pipe threads to DIN ISO 228 and also comply with the BSP standard. A flat gasket centered by a step on the sensor ensures a good seal. A good seal can also be ensured using Teflon tape. For pressure above 30 bar or very high screw-down torques, a flat gasket may be damaged, especially if it is made of plastic. In this case, a recess must be incorporated into the fitting which will keep the gasket in the right position in the case of high loads. PTFE gaskets must always be used with this technique. For high pressure applications, metal gaskets must be used.
The standard material for gaskets is AFM 30/34. Special gaskets made of other materials such as moving iron, copper or PTFE are also available on request.


EX probes

Probes for gas and dust explosion hazardous environments are design approved to ATEX 100a and operated with an approved switching device of series SZA..., SEA... or SS 400.



A rising pipe should be used in case of open systems or in the presence of air pockets. Deposits and air pockets do not impair sensor function in the case of lateral assembly, providing the sensor is completely immersed in the medium. Assembly from below assures flow monitoring function even if there are air pockets in the pipe. However, the monitored medium level must not fall below the upper edge of the measuring tip. Assembly from above is only applicable if there is no air in the pipe.



NPT threads

NPT threads can be provided as an alternative for all types which have a G1/2 or a G3/4 thread. NPT threads are conical and must be screwed into an equally conical counter-part. Two types of NPT threads must be distinguished. NPT thread according to ANSI B 1.20.1 does not ensure a good seal by itself and requires the use of a sealing medium, e.g. Teflon tape. It is not possible to use flat gaskets with this type of thread. NPT thread according to ANSI B 1.20.3 does ensure a good seal by itself and requires no further sealing medium. When this type of thread is used, special attention must be paid to the kind of metal used for both parts of the thread, so as to avoid metal seizing when the parts are screwed tight.


Flange types

Standardised pipe connections are required particularly in the chemical, pharmaceutical and foodstuff industries. Sensors for use in these areas are supplied with flange connections per DIN or ASME. Sensor and flange form a corrosion-proof connection using laser or inert gas shielded arc welding.


Food-approved screw connections

For hygienic reasons the food and pharmaceutical industries place special demands on the mechanical and electronic characteristics of sensors.
Probes with food-approved connections, e. g. Triclamp or dairy pipe connections (DIN 11851) comply with the 3-A sanitary standard 28-03. Due to the temperature changes involved, the usual cleaning cycles CIP and SIP place a particular demand on sensor electronics. Therefore, special protective measures are taken. Sensor materials for these applications is mainly the special steel AISI 316 L. Customer-specific connections, e. g. GEA-Varivent or APV flanges are available, as are other special metallic materials.


Extra long probes

Flow probes are available in screw lengths of 25 mm to 300 mm. Probes for use in explosive environments are made of two components if longer than 110 mm and joined corrosion-proof through laser welding. The probe length should be selected such that the measuring tip is within an area of stable flow characteristics.

Main applications are:

- detection of small flow velocities in pipes with large cross section

- mounting of the sensor with a standard flange

- use of extra long welding sleeves if the piping is surrounded by a supplementary insulation.


Immersion depth "L" is determined by the distance between the sealing face and the sensor tip. Standard lengths which can be supplied are: L = 80 and 120mm; in the Ex-area 80, 110 and 140 mm.


Inline

Inline sensors are inserted directly into the line of a pipe. This design does not feature any measuring pins protruding into the flow. EGE inline sensors SD of series 500 are suitable for flow volumes from 0.5 ml/min to 6 l/min. These sensors excel through smooth measuring pipes, low pressure loss and fast response to flow changes. A multitude of connection options are available.


Chemical stability of probe housings

The chemical stability of the materials used must be verified individually for every application. Basically, no problems occur if the probe and the piping are made of the same material. It is always advantageous if the sensor housing is made of a more noble material than the piping.
The screwed cable gland on the rear side of the ST... sensors is designed in nickelplated brass. Order material PVDF for screwed cable glands in applications that are cleaned with alkaline cleaning agents as is the case, for example, in the food industry.

Stainless Steel belongs to the group of chromium-nickel alloys containing further components such as molybdenum or titanium. The proportions of the different alloy components is critical to the resistance to corrosion in the medium. For this reason, there exists a large number of materials identified by numbers to the DIN EN ISO 7153-1:2000 standard. Due to its good corrosive resistance in many areas of application, AISI-316 Ti (VA4) stainless steel is a frequently used material. It may be used in installations used to obtain water, in air conditioning systems, in food processing industries such as dairy products, meat products, beverages, wine production or in kitchen installations. Stainless steels have a restricted stability in chlorinated or poorly oxygenated atmospheres. Special alloys must be used for such applications.


Special materials

Hastelloy B2 (2.4617) belongs to the group of highly corrosion-resistant nickel-molybdenum alloys.
This material has excellent characteristics in reducing media, e.g. in hydrochloric acid of any concentration and for a large range of temperatures. It can also be used in hydrochloric, sulphuric, acetic and phosphoric acid media. Good resistance against corrosion such as pitting, crevice corrosion, chlorine induced stress, corrosion cracking, hair-line corrosion, abrasion and corrosion within the heat influence zone allows for a large range of applications. In the presence of oxidising components such as iron or copper salts, the use of this material is not recommended.

Hastelloy C-22 (2.4602) belongs to the group of high corrosion-resistance nickel-chromium-molybdenum-tungsten alloys. The material is characterised through high resistance against crevice corrosion, pitting and stress corrosion cracking in oxidising and reducing media. It also displays good behavior in the presence of a large number of corrosive media, including strong oxidants such asiron (III) chloride and copper (II) chloride, hot media, e.g. sulphuric acid, nitric acid, phosphoric acid, chlorine (dry), formic acid and acetic acid. Furthermore, it has satisfactory characteristics in humid chlorine gas, as well as in sodium hypochlorite and chlorine dioxide solutions.

Titanium (3.7035) is a light metal with mechanical strength values equivalent to those of high quality steel. The good chemical resistance of this metal is due to the fact that an oxide film is formed on its surface, as is also the case with stainless steels. If this protective layer undergoes mechanical damages in an oxygenated enviroment, it is immediately renewed (titanium will resist even aqua regia). Titanium is not stable in environments containing no oxygen or in reducing enviroments. It is particularly suitable for applications in chloride-containing media. Experience in the chemical industry and in paper bleaching factories has shown that titanium is the only material allowing undisturbed production. The excellent characteristics of titanium also give optimum results in sea water cooling sytems and sea water de-salinising plants.
The material is particularly suited for the application of coating with other metals and metal ceramics. These supplementary coatings noticeably increase its chemical stability and thus the lifetime of sensor housings.




High temperature

High temperature sensors are manufactured from temperature resistant components and feature FEP cables.
The functional range of these special probes of series 400 is specified as +10...+120 °C. Temporarily 135 °C is permissible for max. 10 min. High temperature sensors of series 500 can be used for media temperatures of up to 160 °C / 320 °F


Connection

Flow monitoring probes are available with a M12 plug connector or fixed cable. Special models for the hazardous areas have a terminal compartment.
The connection cable from the probe to the amplifier may be up to 100 m long. For distances above 30 m a shielded cable is preferred. In all cases the chosen wire strength must be checked against the requirements.


Approval for safety applications

Sensors for personal security must have a qualification approval according to EN 61508 and must be labeled accordingly. Sensors that are not labeled must not be used for applications of this kind.


« back to function        forward to amplifiers and compact devices »

« back to overview

powered by webEdition CMS