Thermal Flow Sensors: Installation Guidance
The probe is made of special steel in one piece, using a robust electronic and mechanical construction. By means of this, absolute tightness and a high pressure resistance is obtained. The chemical stability of the sensor material must be verified individually for every application Assembly is independent of flow direction. A basic requirement is that the sensor tip must be completely surrounded by the monitored medium, whether the medium flows or is at rest. With smaller cross sections care must be taken that the sensor tip does not narrow the pipe profile considerably.
In order to avoid function failures due to flow turbulence, no parts which influence flow cross-section or flow direction should be installed immediately before or behind a sensor. Recommended values for inlet and outlet are 4 to 8 times the interior pipe diameter.
All flow monitors feature an array of LEDs giving a visual indication of the flow rate. If the red LED illuminates, the flow falls short of the preset limit and the switching output is not enabled. The yellow LED indicates that the limit has been reached and the output enabled. In addition to the yellow LED a further 4 green LEDs may illuminate representing a relative measure for how much the limit has been exceeded.
Flow monitoring sensors are available with a M12 plug connector or fixed cable. The connection cable from the sensor 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.
Sensors with short thread-pieces of the STK... type are particularly suited for fitting into T-pieces. Sensor length is designed in such a way that the sensor tip is completely immersed in the medium without touching the opposite side.
Sensors with long thread-pieces of the ST... type are suitable for larger pipe diameters or for use with longer assembly thread-pieces.
Sensor 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.
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 be-low 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 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 to ANSI B 1.20.1
This thread 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 to ANSI B 1.20.3
This thread 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.
The sensitivity of thermodynamic flow monitors depends on the thermal characteristics of a medium. The detection range of a standard sensor for oil, for example, is three times as great than for water and for air is approx. 30 times greater than for water due to the reduced heat conductivity. Unless stated otherwise, the technical sensor data are specified for water.
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.
Flow monitors 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 sensors
Flow monitor sensors are available in screw lengths of 25 mm to 300 mm. Sensors for use in explosive environments are made of two components if longer than 110 mm and joined corrosion-proof through laser welding. The sensor 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 large section pipes
mounting of the sensor with a standard flange
use of extra long welding sleeves if the piping is surrounded by a supplementary insulation.
For these cases, special shapes can be supplied up to a maximum length of 300 mm. Immersion depth "L" is determined by the distance between the sealing face and the sensor tip. Stainless steel 1.4571 (AISI-316 Ti) is used for this special model.
Standard lengths which can be supplied are: L = 80 and 120 mm; in the Ex-area 80, 110 and 140 mm.
Approval for safety applications
Sensors for personal security must have a qualification approval according to EN 954-1 and must be labelled accordingly. Sensors that are not labelled must not be used for applications of this kind.