Volume XXXIV: ASME Pressure Vessels

ASME PRESSURE VESSELS

The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.

DEAERATORS

What is a deaerator? What does it do? How does it work?

A deaerator is a device that is widely used for the removal of oxygen and other dissolved gases from the feedwater to steam-generating boilers.

• Deaeration is the process of removal of oxygen, Carbon dioxide and other non-condensable gases from boiler feed water thereby reducing the risk of corrosion in the pressure parts of the boiler.

  • Deaeration of two types namely Mechanical Deaeration and Chemical Deaeration
  • Mechanical deaeration works on the principle of Henry’s law of physics
  • In particular, dissolved oxygen in boiler feedwater will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). • Water also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion.

Why Deaerate Boiler Feedwater?

  • Water is heated during deaeration to near the temperature of the boiler water, thus minimizing the risk of thermal shock.
  • The deaerating process removes non-condensable gases (oxygen and carbon-dioxide) which tend to act as insulators inhibiting the transfer of heat within the boiler.
  • Removal of corrosive oxygen and carbon-dioxide controls corrosion within the boiler and piping, extending the life expectancy of the system and reducing maintenance cost.
  • Higher temperature feed water reduces the drop in boiler operating pressure which can occur when cold water is added.
  • Mechanical deaeration can reduce the amount of chemical consumables used for water conditioning in turn cost saving.

How are Deaerators Designed?

Henry’s law of partial pressure

  • At a constant temperature, the amount of given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
  • The law further states that the solubility of gas in a particular temperature is proportional to the pressure of that gas above the liquid.
  • An everyday example of Henry's law is given by carbonated soft drinks. Before the bottle or can is opened, the gas above the drink is almost pure carbon dioxide at a pressure slightly higher than atmospheric pressure. The drink itself contains dissolved carbon dioxide. When the bottle or can is opened, some of this gas escapes and giving the hiss sound.
  • Because the pressure above the liquid is now lower, some of the dissolved carbon dioxide comes out of solution as bubbles. If a glass of the drink is left in the open, the concentration of carbon dioxide in solution will come into equilibrium with the carbon dioxide in the air, and the drink will go "flat".
  • The easiest way to deaerate the boiler feed water by forcing the steam into the feed water, this action is called SCRUBBING.
  • The feed water entering the deaerator is heated to the saturation temperature corresponding to the steam pressure maintained in the deaerator. This will reduce the solubility of dissolved gas in the water to zero.

TYPES OF DEAERATORS

There are three types of deaerators. They are:

  1. Spray Type
  2. Spray with Tray Type
  3. Vacuum Type

SPRAY TYPE DEAERATOR

spray-type-deaerator

SPRAY WITH TRAY TYPE DEAERATOR

spray-tray-deaerator

 

DEAERATION PROCESS DESCRIPTION

Deaeration is based on two scientific principles.

  • The first principle can be described by Henry's Law. Henry's Law asserts that gas solubility in a solution decreases as the gas partial pressure above the solution decreases.
  • The second scientific principle that governs deaeration is the relationship between gas solubility and temperature. Easily explained, gas solubility in a solution decreases to almost zero as the temperature of the solution rises and approaches saturation temperature.

A deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide, and other non-condensable gases from boiler feed water. The feed water is sprayed in thin films into a steam atmosphere allowing it to become quickly heated to saturation temperature.

Spraying feed water in thin films increases the surface area of the liquid in contact with the steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations. This process reduces the solubility of all dissolved gases and removes it from the feed water. The liberated gases are then vented from the deaerator.

TYPICAL PLANT DEAERATOR

  • It is of spray and trays type deaerator consist of a feed water storage tank and vapor tank with vent condenser.
  • Water is sprayed from the top of vapor tank through ten spray nozzles on set of multi-level perforated trays below it for easy and complete scrubbing with steam .
  • Steam is fed in storage tank through distribution header below the water level. Partial scrubbing of steam with water takes place by raising the water temperature (Since solubility of gases like oxygen carbon dioxide decreases with increase in water temperature).

The complete removal of gases is taking place in vapor tank with incoming water spray. Water sprayed from the top (fixed in two headers with five nozzles in each to spray the CEP water into fine particles covering entire cross section of the tank) and passed through perforated SS trays at different levels to provide enough scrubbing of water with upcoming steam from storage tank through interconnection pipe.

Typical Deaerator Technical Data

  • Integral, thermal-mechanical deaerator of spray and tray type (multi-level trays)
  • Water is sprayed through 10 nozzles arranged in two rows with 5 nozzles.
  • Heating and scrubbing action removes all the dissolved gases.
  • With mechanical deaeration DO level : 20 to 30ppb
  • With chemical deaeration DO level :5 to 10ppb
  • Chemical used : Cortrol-Carbo hydrazide

This is presented to you as a service from BOARDMAN INC. located in Oklahoma City, Oklahoma.

Since 1910, Boardman has been a respected custom fabricator. We take pride in our ability to take the most stringent specifications and requirements to provide a high quality solution to our customers.With more than 75 years of ASME Section VIII, Division I engineering experience, we have the unique ability to provide custom solutions to our customers.

Fabricated Projects Include:

  • Trayed Towers & Columns
  • ASME Pressure Vessels
  • Molecular Sieves
  • Rotary Dryers & Kilns
  • API Tanks
  • Acid settlers
  • Stacks, Scrubbers
  • Thermal Oxidizers
  • Accumulators, Condensers
  • Crystallizers
  • Ducting
  • Bins
  • Large Diameter Piping

The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4” thick.

BOARDMAN INC. is available for shop tours and Pressure Vessel and Static Equipment Fabrication Classes.

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