The Electric Two-term Level Controllers (Level Switch) are designed for automatic control, signalling or controlling of fixed liquid level limit value in open or closed (pressure) vessels, within wide range of pressures and temperatures in explosion-risk atmosphere.
The electric controllers, Level Switches are equipped with an electric output which according to the needs can be used in signalling, interlocking, controlling or automatic control circuits. They may be commonly used on ships with unwatched marine power plant, having unlimited steaming range, in power plants, in agriculture, etc.
Operation of marine electric Level Switch
The floating level controller is actuated with liquid by means of float uplift force. Liquid level changes cause swinging of the float unit, which by magnetic coupling is transmitted to an intermediate magnet, whose lever closes or opens the microswitch contacts.
Design of marine electric Level Switch Controller
The level switch controller on ship is designed of the two basic assemblies separated by a diamagnetic baffle:
1. a relay unit which is the transducing part of the device unified for all the types. The intermediate magnet, whose magnetic pole is directed with the like magnetic pole of the float magnet transmitting electric pulses as a result of the magnet lever operation on a button switching the switch contacts, is enclosed in a body. External conductors are connected to the switch terminals through cable gland.
2. a float unit which is the sensing part of the controller. Depending on application of the controller, float unit is made different for each type and the float can be in a shape of cylinder with hemisperical bowls or shape of sphere.
The permanent magnet is mounted directly, or by means of leverage, to the float; the magnet pole is directed with the like magnetic pole of the intermediate magnet of the relay unit. The float unit magnet is protected with a housing.
Principle of level switch controller operation
The NC – NC contacts are closed at minimum level. When the level rises, the NC – NC contacts remain closed until the maximum level is reached. At the moment when the maximum level is reached, a „blink-like” switching takes place, that is, opening of the NC – NC contacts and closing the NO – NO contacts; the NO – NO contacts are closed until the float reaches minimum level.
The floating level controllers can be mounted in the following kinds of open or closed containers for:
• lubricating oil,
• fresh and sea water,
• pressure vessels (e.g. marine boilers),
• fecal matter,
• fluid wastes of various density,
• ship bilges,
• various containers for liquids of 12 pH concentration.
Maintenance of ship level switch
At least once a year the floating level controller is subject to detailed visual inspection, one thing to pay special attention to technical condition of parts exposed to liquid action (float, pins, cotter pins, magnet casing, lever, etc.).
Any possible contamination is to be removed, in particular, from the magnet casing, and from the gap between the body and float lever.
In case of strongly contaminated liquids, the above operations should be more often performed, in periods defined by the end-user, during inspection of the tank and its fittings.
Deposits formed, e.g. boiler scale, are to be removed mechanically or chemically, paying attention not to damage the controller parts.