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Comprehensive monitoring of containment shells

Safe operation of magnetically coupled centrifugal pumps
Comprehensive monitoring of containment shells

With the “…-safe” monitoring device series – comprised of Mag-safe, Shell-safe and Double-safe – almost all critical conditions and safety levels of a containment shell inside a magnetic coupling pump can now be safely monitored and detected.

The magnetic field lines in magnetic coupling pumps cut through the metallic containment shell, which is heated due to the eddy current losses produced by the electrical conductivity of the material. The highest temperature at the containment shell occurs in the area around the magnetic coupling. A temperature probe is normally used to monitor the temperature rise at the containment shell. This standard Pt100 sensor has a flat thermowell bottom and is provided with an integrated spring which ensures sufficient and permanent contact with the containment shell surface. The temperature probes work reliably in a liquid-filled pump and prevent the flashpoint of the pumped liquid from being exceeded in the containment shell area. However, the Pt100 is not suitable as dry running protection. The temperature rise caused by the eddy current losses of the metallic containment shell occurs in the centre of the magnets whereas the Pt100 sensor is located outside the magnetic area. Although the temperatures at the centre of the magnets increase dramatically within a few seconds in case of dry running, the Pt100 sensor shows only a small reaction after a few minutes.

Besides dry running protection, the Mag-safe (Figure 1) is also used to monitor the containment shell surface temperature according to the Explosion Proof Directive as well as the antifriction bearings. If these bearings wear out, the rub ring of the outer magnets first of all contacts the bearing bracket. If this is not detected, the outer magnets cut off the Mag-safe wire and the pump is shut down before the containment shell has a chance to be damaged.
Ceramic shell monitoring
When it comes to monitoring non-metallic containment shells, particularly ceramic shells, no satisfactory solution exists as yet for end users. Temperature monitoring with the current monitoring options does not make sense. Due to the high specific electrical resistance of ceramic, no eddy currents are produced and there is thus no temperature rise. It is still not possible to monitor for ceramic containment shell failures or the resulting widespread destruction of other components due to loose ceramic parts.
The Shell-safe was developed as a containment shell monitor in order to meet the increasing safety requirements of end users (Figure 2). It consists of a very thin film with an embedded nickel wire, which has a defined resistance of 100 ohms at 20 °C. This film is affixed to the ceramic containment shell and covers its total surface area. A contact is established to a transmitter placed in the connection head via a bushing on the containment shell flange and a downstream plug connector. The transmitter controls the circuit of the nickel wire, as well as the insulation of this wire and of the connections to the surrounding casing parts and containment shell, by measuring the resistance. An alarm is activated in case of wire damage or a short-circuit.
The flange, the containment shell and an intermediate gasket are glued together to form one component. Power is supplied via an Ex-proof cable entry. An insulation monitor connects the transmitter to the exterior of the pump. The Shell-safe is intrinsically safe electrical equipment according to EN 60079-11 and therefore subject to the regulations of the Explosion Proof Directive. A conformity assessment procedure has been performed by a notified body. A type examination certificate is available. As intrinsically safe equipment with an Ex ib marking, the Shell-safe can be used in hazardous areas and is regarded as a category 2 device in zone 1.
Double shell monitoring
Ever higher safety requirements play an increasingly important role today when handling highly hazardous and toxic liquids. The trend is to provide a second barrier for such liquids in order to avoid uncontrolled product leakage in case of containment shell damage. Apart from the well-known secondary seals (dry-running mechanical seals), which form a secondary containment together with the pump’s bearing bracket, Dickow has also developed a patented double containment shell called Double-safe, which is shown in Figure 3. There is hence no more need for dry-running secondary mechanical seals or the associated monitoring and shut-off devices. Standard components without any additional connections or design measures can be used to hold the mechanical seal.
For the first time, the Double-safe can be defined as a genuine second barrier because both the inner and the outer shell are rated for the maximum permissible operating conditions. If ever one shell becomes defective owing to unforeseen operating conditions, complete containment by the second shell is guaranteed. The inner and outer shells are locked together by friction. Both shells have an identical wall thickness. There is no air between the inner and outer shell in the pipe section. The interlayer material has very high thermal conductivity, high specific electrical resistance and good chemical corrosion resistance. Any eddy current losses which are produced in the metallic outer shell are safely guided through the interlayer to the inner shell without additional heating or air insulation. The eddy current losses of both shells are then cooled by the internal circulation flow, so that no external liquid is required to cool the outer shell.
An insulated thermocouple wire interwoven into the interlayer material serves to monitor the interspace and is fed to a plug connector at the containment shell flange. The downstream plug connector establishes contact with a transmitter placed in the connection head. The transmitter controls the circuit of the nickel wire, as well as the insulation of this wire and of the connections to the surrounding casing parts, by measuring the resistance. An alarm is activated in case of wire damage or a short-circuit. An insulation monitor connects the transmitter to the exterior of the pump.
In the event of an inner or outer shell failure, this monitor detects wire break due to cutting of the inner or outer magnets or to corrosive attack by the pumped liquid. No other connections are required and expensive monitoring equipment for the interspace is unnecessary. The interlayer material cannot leak in case of damage to the containment shell. There is no health risk and no obligation to test for compatibility with the pumped liquid.
The Double-safe is simple electrical equipment according to EN 60079-11 and as such not subject to the regulations of the Explosion Proof Directive. No conformity assessment procedure has to be performed by a notified body and the Double-safe has no Ex marking. In combination with an intrinsically safe circuit, it can also be used in hazardous areas. The head transmitter and cable entry require a separate assessment procedure.
www.cpp-net.com search: cpp0217dickow

Jürgen Konrad
Head of Technical Department,
Dickow Pumpen
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