User manual HONEYWELL PRESSURE SWITCHES DATASHEET

[. . . ] Pressure switches Mechanical pressure switches 21 General information about explosion protection Basic principle The basic principle of explosion protection is that: a) combustible materials (gas, vapour, mist or dust) in dangerous quantities b) air (or oxygen) c) ignition sources must not occur in the same place. The permanent or temporary occurrence of explosive mixtures as per a) and b) is often unavoidable, therefore when operating electrical installations care must be taken to ensure that no ignition sources can occur. With this in mind, the CENELEC technical committee has adopted the following European standards which are recognized in all EU member states. · General requirements EN 50 014 · Pressure resistant encapsulation "d" EN 50 018 · Oil encapsulation "o" EN 50 015 · Increased safety "e" EN 50 019 · Overpressure encapsulation "p" EN 50 016 · Intrinsic safety "i" EN 50 020 · Sand encapsulation "q" EN 50 017 · Cast encapsulation "m" EN 50 028 The guidelines relevant to FEMA products ­ besides the "General Requirements EN 50 014" ­ are "Pressure resistant encapsulation d" and "Intrinsic safety i". [. . . ] Pressure limiters (DB) Pressure limiters are devices which switch off the heating system on exceeding and / or falling below a predefined pressure limit and lock it to prevent automatic restarting. Pressure limiters "of special construction" (SDB) Pressure limiters "of special construction" perform the same tasks as pressure limiters. In addition they must satisfy the extended safety requirements of section 3. 3 (of "Pressure 100/1"). Pressure switches Pressure switches "of special construction" 45 Safe condition According to DIN VDE 0660, Part 209, the safe condition of the system is reached if a cut-off command is present at the output contact which means that in the safe condition, the microswitch in the pressure limiter is actuated (opened) and the control circuit is interrupted. Series connected switching devices must react in the same way. The operating mode of the safety pressure limitation thus corresponds to the closed circuit principle. Additional requirements for pressure limiters "of special construction" 8 7 6 3 2 1 5 Section 3. 3 of VdTÜV Memorandum "Pressure 100/1": Pressure limiters "of special construction" must, in the event of a breakage in the mechanical part of the measuring element, lead to cut-off and interlock of the heating. This requirement is also fulfilled if the mechanical part of the measuring element is calculated for vibrating stress or has withstood a test with 2 million operating cycles and the pressurized parts of the measuring element are made of corrosion-resistant materials. (Abbreviated excerpt from VdTÜV Memorandum "Pressure 100/1"). Therefore there are two possible ways of meeting the requirements for pressure limiters "of special construction": a) By a self-monitoring pressure sensor which is designed so that a breakage in the mechanical part of the measuring element leads to cut-off to the safe side (see Fig. 1) b) By certification of endurance testing with 2 million operating cycles during the component test (see Fig. 2) a) Self-monitoring pressure sensor with safety diaphragm (for maximum pressure monitoring only) Fig. 1 is a cross-sectional diagram of a pressure sensor which fulfils the "special construction" requirements. The measuring chamber is bordered by the housing (1), base (2) and measuring bellows (3). All parts are made of stainless steel and are welded together without filler metals. When the pressure rises the measuring bellows (3) moves upwards, supported by the back pressure spring (5). The setpoint spring installed in the switching device acts as a counterforce. A transfer bolt (6) which transfers the pressure-dependent movements of the measuring bellows (3) to the switching device located above is placed on the inside of the base. A plastic diaphragm (7), which is not in contact with the medium and in normal operation follows the movements of the measuring bellows but itself has no influence on the position of the bellows, is clamped in the upper part of the transfer bolt. On breakage of the measuring bellows (3), the medium can escape into the interior of the bellows. The medium pressure is now on the underside of the diaphragm (PL). An additional force is generated because of the far larger effective area of the diaphragm compared with the bellows, and this pushes the transfer bolt (6) upwards. This results in cut-off to the safe side. The cut-off condition thus achieved is normally interlocked electrically or mechanically, so that the system also remains cut off when the pressure drops again. [. . . ] (with external interlock in the control cabinet) Suggested connection for the external interlock, see page 31. or DWR. . . -205 Pressure limiter for safety monitoring: Pressure switches Pressure switches "of special construction" 51 DWR series P r e s s u r e m o n i t o r s f o r s t e a m a n d h o t w a t e r, fuel gases and liquid fuels DWR 625 Technical data Pressure connection External thread G 1/2 (pressure gauge connection) to DIN 16 288 and internal thread G 1/4 to ISO 228 Part 1 (for gas applications internal thread permissible only up to 4 bar). Switching device Rugged housing (200) made of seawaterresistant diecast aluminium. Materials Pressure bellows: Material no. [. . . ]