Any device which infers the water level or reacts to it, typically for the purpose of monitoring would be considered a remote (Indirect) Level Gauge. Where the boiler gauge glass is not visible to the operator, or as a part of a different operational philosophy, the code allows for (1) of the gauge glasses to be replaced by (2) remote indicators. Even though these indirect indicators do not allow the operator to view the boiler level directly, they do offer other capabilities such as control contacts and analogue signals that can be used for alarm points and trips. It is important however to understand the merits of each technology as much of the level monitoring takes place from a convenient remote location.
The Electro Eye-Hye System:
Ever popular for its simplicity and its dependability is the Electro Eye Hye System. The Electro Eye Hye unit, comprises three main parts:-
The Electolev which is a water column mounted vertically on the Boiler Drum.
The Indicator/s which can be located near the Boiler Drum, on the Firing Floor and in the Control Room some distance away.
The Control Unit which houses the motherboard and Relays
The Electrolev consists of an array of conductivity probes which detect the presence of water. As the water level in the drum changes a corresponding change is detected by the probes in the Electrolev. As each probe is submerged, it completes a circuit thus providing an incremental representation of the level. Mounting the probes horizontally in this way allows for a greater number of probes to be installed thus maximising the resolution of sensing.
Clark Reliance offer a variety of different indicators for the Electro Eye Hye System. There are small Bi-Colour models for the Control Room and much larger ones for the Firing Floor. Current indicator models all make use of LED technology and thus provide low energy consumption and a high MTBF. They can be panel mounted or wall mounted and can also be weatherproofed for outdoor installation or the occasional hosing down of the works area.
The Control Unit houses the detection circuitry, this being one individual circuit at each probe level. As detection circuits sense the presence of water, they operate control contacts within their detector modules providing both indication and control functions. These devices thus eliminate the need for separate devices for indication and control. Advanced systems may incorporate self-checking features that will alert the operator of system failures and a 4-20mA output signal for connection to the customer’s DCS/PLC. Some versions of the Electro Eye Hye will continue to operate even in the event of a major circuit failure, providing a “Fault Tolerant” system.
The Differential Pressure Transmitter:
Differential Pressure Transmitters are the most popular device for level monitoring. The device compares the head pressure generated by the water in the drum to a datum level. This reference is usually derived from a chamber attached to the drum and filled with condensate from the sub cooling of steam.
Generally there are several factors to consider, when installing such a device, which affect the water level and can infer inaccuracies: Water inside the drum is generally quite turbulent, the drum may not be level. The boiler water is continuously subject to steam out flows and make-up water inflows. The DP cell requires calibration and one needs to cater for the variation in the densities of steam and water with saturation temperature or pressure. One also has to consider the density of saturated steam above the water and the density of saturated water in the drum. This translates to quite a complicated setup process.
For critical applications such as boiler drum monitoring, consideration of system redundancy, reliability and the failure state of control outputs should be considered. It is important to understand that some technologies are susceptible to “single component failure” such as the DP Cell (not sold by Clark Reliance) and monitoring devices which include magnetic floats.