Instrumentation - Dew Point
June 4, 2006

Introduction Acid Dew Point Air Dew Point Principle of Operation Instruments

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Introduction

There are two important dew points that are important in the operation of an acid plant.  The most important is the acid dew point which has an impact on the life of the process equipment.  The second is the dew point of instrument air.

Acid Dew Point

The acid dewpoint is important to know because excessive acid condensation will lead to increased corrosion of process equipment.  The areas in the plant where this is critical are points in the process where the gas has been cooled close to or below its dewpoint.  These areas are:

• Exit Cold Heat Exchanger to Absorber Tower (Economizers/SO3 Coolers in parallel)

• Exit Cold Reheat Exchangers to Absorber Tower (Economizers/SO3 Coolers in parallel)

The dewpoint of the gas is approximately 140°C to 180°C.   Knowing the exact dewpoint of the gas can enable the acid plant to recover the maximum amount of heat in heat recovery equipment (ie. economizers and boilers) by cooling the gas closer to its dewpoint temperature.

Air Dew Point

A dewpoint monitor is sometimes specified for an instrument air system where the air quality is important for the proper operation of a plant.  Instances where this is important are plant locations where the ambient temperatures are very low (-40°C/°F).  If the instrument air is not dried to the required dewpoint, water will condense and freeze in instrument air lines resulting in instrument failures, safety hazards, and possibly a plant shutdown.

Principle of Operation

Condensed Liquid Film - A probe is inserted into the gas stream and the detector is gradually cooled by increasing the flow of air to the probe.  As the detector temperature falls, a point is reached where a thin film of acid begins to condense onto its surface.  Two electrodes are fused into the detector surface, and as the acid condenses, a current begins to flow between them.  The magnitude of the current depends on the thickness of the acid film.  The air flow to the probe is adjusted until a steady current is obtained indicating that the thickness of the film is constant.  At that point the rate of condensation of acid is equal to the rate of evaporation, and the temperature at which this occurs is the acid dew point temperature.  The precise temperature is measured by the probe thermocouple which indicates the temperature of the detector surface.

Instruments

The following instruments are available for measuring acid dew points.

Manufacturer	Type	Model	Mounting	Remarks
Land Combustion	Constant current through condensed liquid film	410 A	3" 150 lb flange with 1 1/2" swagelok fitting and PTFE compression ring	Instrument Air: 6 CFM @ 60-100 psi Electrical: < 20 W Accuracy: +/- 4°F (2°C) over full ambient range
Land Combustion	Constant current through condensed liquid film	200A	3" 150 lb flange with 1 1/2" swagelok fitting and PTFE compression ring	Identical to Model 410 A except portable