# Steam ships fresh water loss



## kewl dude (Jun 1, 2008)

The design of most marine steam plants included using steam to help free corrosive oxygen and other gases from within boiler feed water. A portion of this steam must always be vented to the atmosphere in order for the system to function. I do not know if anyone ever made a study? I doubt it was a lot of water? Regardless it was not something that could be shut off. 

I have edited the below removing links to images.

http://www.gewater.com/handbook/boiler_water_systems/ch_10_boiler.jsp

Quote

The dissolved gases normally present in water cause many corrosion problems. For instance, oxygen in water produces pitting that is particularly severe because of its localized nature. Carbon dioxide corrosion is frequently encountered in condensate systems and less commonly in water distribution systems. Water containing ammonia, particularly in the presence of oxygen, readily attacks copper and copper-bearing alloys. The resulting corrosion leads to deposits on boiler heat transfer surfaces and reduces efficiency and reliability. In order to meet industrial standards for both oxygen content and the allowable metal oxide levels in feedwater, nearly complete oxygen removal is required. This can be accomplished only by efficient mechanical deaeration supplemented by an effective and properly controlled chemical oxygen scavenger.

Several principles apply to the mechanical deaeration of feedwater:

The solubility of any gas in a liquid is directly proportional to the partial pressure of the gas at the liquid surface

The solubility of a gas in a liquid decreases with increasing liquid temperature.

Efficiency of removal is increased when the liquid and gas are thoroughly mixed

The solubility of a gas in a liquid is expressed by Henry's Law:

Ctotal = kP

where:
Ctotal = total concentration of the gas in solution
P = partial pressure of the gas above solution
k = a proportionality constant known as Henry's Law Constant

For example, 8 ppm of oxygen can be dissolved in water when the partial pressure of oxygen is 0.2 atmosphere; only 4 ppm of oxygen can be dissolved in water if the partial pressure of oxygen is reduced to 0.1 atmosphere.

As is evident from Henry's Law, a dissolved gas can be removed from water by a reduction of the partial pressure of that gas in the atmosphere contacting the liquid. This can be accomplished in either of two ways:

1. a vacuum is applied to the system and the unwanted gas is vented

2. a new gas is introduced into the system while the unwanted gas is vented

Vacuum deaeration has been used successfully in water distribution systems. However, pressure deaeration (with steam as the purge gas) is normally used to prepare boiler feedwater. Steam is chosen as the purge gas for several reasons:

it is readily available
it heats the water and reduces the solubility of oxygen
it does not contaminate the water
only a small quantity of steam must be vented, because most of the steam used to scrub the water is condensed and becomes a part of the deaerated water

In order to deaerate the boiler feedwater, water is sprayed into a steam atmosphere. This heats the water to within a few degrees of the temperature of the saturated steam. Because the solubility of oxygen in water is very low under these conditions, 97 to 98% of the oxygen in the incoming water is released to the steam and is purged from the system by venting. Although the remaining oxygen is not soluble under equilibrium conditions, it is not readily released to the steam. Therefore, water leaving the heating section of the deaerator must be scrubbed vigorously with steam to maximize removal.

Equipment

The purpose of a deaerator is to reduce dissolved gases, particularly oxygen, to a low level and improve a plant's thermal efficiency by raising the water temperature. In addition, deaerators provide feedwater storage and proper suction conditions for boiler feedwater pumps.

Pressure deaerators, or deaerating heaters, can be classified under two major categories: tray-type and spray-type (see Figure 10-3). Tray-type deaerators are also referred to as "spray-tray" type, because the water is initially introduced by spray valves or nozzles. The spray type is also referred to as the "spray-scrubber" type because a separate scrubbing section is used to provide additional steam-water contact after spraying.

The tray-type deaerating heater consists of a shell, spray nozzles to distribute and spray the water, a direct-contact vent condenser, tray stacks, and protective interchamber walls. Although the shell is constructed of low carbon steel, more corrosion-resistant stainless steels are used for the spray nozzles, vent condenser, trays, and interchamber walls.

Incoming water is sprayed into a steam atmosphere, where it is heated to within a few degrees of the saturation temperature of the steam. Most of the noncondensable gases (principally oxygen and free carbon dioxide) are released to the steam as the water is sprayed into the unit. Seals prevent the recontamination of tray stack water by gases from the spray section.

In the tray section, water cascades from tray to tray, breaking into fine droplets or films which intimately contact the incoming steam. The steam heats the water to the saturation temperature of the steam and removes all but the very last traces of oxygen. Deaerated water falls to the storage space below, where a steam blanket protects it from recontamination.

The steam, entering the deaerators through ports in the tray compartment, flows down through the tray stack parallel to the water flow. A very small amount of steam condenses in this section as the water temperature rises to the saturation temperature of the steam. The remainder of the steam scrubs the cascading water.

Upon leaving the tray compartment, the steam flows upward between the shell and the interchamber walls to the spray section. Most of the steam is condensed and becomes a part of the deaerated water. A small portion of the steam, which contains the noncondensable gases released from the water, is vented to the atmosphere. It is essential that sufficient venting is provided at all times or deaeration will be incomplete.

As mentioned, most tray and spray-type deaerators use spring-loaded spray nozzles, which evenly distribute the inlet water. Newer spray valves are designed to provide a uniform spray pattern under varying load conditions for efficient steam-water contact. The valve is designed to provide atomization of the inlet water into small droplets to improve heat transfer and to provide efficient scrubbing of the inlet water oxygen.

Steam flow through the tray stack may be cross-flow, counter-current, or co-current to the water. 

The spray-type deaerating heater consists of a shell, spring-loaded inlet spray valves, a direct-contact vent condenser, and a steam scrubber for final deaeration. The inlet spray valves and direct contact vent condenser section are stainless steel; the shell and steam scrubber may be low carbon steel.

The incoming water is sprayed into a steam atmosphere and heated to within a few degrees of the saturation temperature of the steam. Most of the noncondensable gases are released to the steam, and the heated water falls to a water seal and drains to the lowest section of the steam scrubber.

The water is scrubbed by a large volume of steam and heated to the saturation temperature prevailing at that point. The intimate steam-water contact achieved in the scrubber efficiently strips the water of dissolved gases. As the steam-water mixture rises in the scrubber, the deaerated water is a few degrees above the saturation temperature, due to a slight pressure loss. A small amount of flashing results, which aids in the release of dissolved gases. The deaerated water overflows from the steam scrubber to the storage section below.

Steam enters the deaerator through a chest on the side and flows to the steam scrubber. Because the volume of steam is large compared to the volume of water, thorough scrubbing is achieved. The steam, after flowing through the scrubber, passes up into the spray heater section to heat the incoming water. Most of the steam condenses in the spray section to become a part of the deaerated water. A small portion of the steam is vented to the atmosphere to remove the noncondensable gases.

In the jet-atomizing segment of spray-type deaerators, the incoming water is sprayed into a steam atmosphere. Here, the water is heated sufficiently to release the majority of the noncondensable gases. The water is then delivered into a high-velocity steam jet. It impinges on a baffle and is atomized into fine droplets. The high-velocity steam heats the water to its saturation temperature and scrubs all but the last traces of oxygen from the fine water droplets.

Unquote

Combined deareating direct contact feed water heater and storage tanks were installed high in the fidley sometimes inside the smoke stack. This provides the maximum suction head for the boiler feed water pumps, usually located beneath the boilers, in the lower engine room. The DC heater vent stack was usually lead up the inside or outside at the rear of the smoke stack. Depending upon atmospheric conditions one usually could easily see this feather of 'steam'.

Greg Hayden
Vista - San Diego area - California


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