Apparatus and method for determining a liquid level in a steam drum

1. A multi-variable transmitter for determining a density-compensated liquid level in a vessel containing a mixture of liquid and/or vapor and communicating a signal representative of the liquid level to a remote monitoring location, comprising:

one or more pressure sensors for measuring (a) an operating pressure, and (b) a differential pressure between the mixture in the vessel and a constant reference leg;

an integral microprocessor for calculating the respective specific gravities of the liquid and vapor portions of the mixture based on the measured operating pressure, and then computing the density-compensated liquid level in the vessel based on such calculations of specific gravity and the measured differential pressure; and

a transmission module for communicating the signal representative of the computed liquid level to the remote monitoring location.

2. The multi-variable transmitter as recited in claim 1 , and further comprising a temperature sensor which produces a signal that is proportional to the temperature of the mixture, said integral microprocessor evaluating the signal to confirm whether the mixture is at or near a saturated condition.

3. The multi-variable transmitter as recited in claim 1 , and further comprising an alarm that is initiated when the computed liquid level exceeds a predetermined limit.

4. The multi-variable transmitter as recited in claim 1 , wherein said vessel includes an upper tapping point, a line extending from and in liquid communication with the vessel through the upper tapping point and terminating at a condensate pot, which condenses steam from the vapor portion of the mixture to provide the constant reference leg for measurement of the differential pressure.

5. The multi-variable transmitter as recited in claim 4 , and further comprising an additional line extending from and in liquid communication with the condensate pot and terminating at the multi-variable transmitter.

6. The multi-variable transmitter as recited in claim 5 , wherein said vessel also includes a lower tapping point, another line extending from and in liquid communication with the vessel through the lower tapping point and terminating at the multi-variable transmitter.

7. A method for determining a density-compensated liquid level in a vessel containing a mixture of liquid and/or vapor, comprising the steps of:

providing a processing apparatus, including one or more pressure sensors and an integral microprocessor;

measuring an operating pressure of the mixture using the processing apparatus;

measuring a differential pressure between the mixture of liquid and vapor in the vessel and a constant reference leg using the processing apparatus;

calculating the respective specific gravities of the liquid and vapor portions of the mixture based on the operating pressure using the microprocessor of the processing apparatus; and

computing the density-compensated liquid level in the vessel using the microprocessor of the processing apparatus based on the calculations of specific gravity and the measured differential pressure.

8. The method as recited in claim 7 , and further comprising the step of communicating a signal representative of the computed liquid level to a remote monitoring location.

9. A method for determining a density-compensated liquid level of drum contents at or near a saturated condition, comprising the steps of:

providing a processing apparatus, including one or more pressure sensors and an integral microprocessor;

measuring an operating pressure within the drum using the processing apparatus;

measuring a differential pressure between the drum contents and a constant reference leg using the processing apparatus;

confirming that the drum contents are at or near a saturated condition based on the measured operating pressure; and

computing the density-compensated liquid level in the drum using the microprocessor of the processing apparatus based on calculations of specific gravity of the respective liquid and vapor portions of the drum contents and the measured differential pressure.

10. The method as recited in claim 9 , and further comprising the step of communicating a signal representative of the computed liquid level to a remote monitoring location.