Contamination of steam by oil

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Contamination of steam by oil

cause - effect - solution

Contamination of steam by oil debris is a dangerous type of contamination due to the deposition of oil on heat transfer surfaces. Leakage of oil through flanges or broken gaskets of steam-to-oil heat exchangers could cause oil contamination [1]. Another potential source of steam contamination is the use of rotating equipment like steam turbines or steam engines.

Oil leakage in the steam system causes problems that can lead to mechanical damage with all its consequences. Pollution is even more disastrous in the food and pharmaceutical industries, where clean steam is used.  It is therefore very important to detect steam pollution in good time and to take measures.

In this blog I discuss the risks of steam contamination by oil and how these can be prevented. The complete story can be read in the free white paper (see separate box on this page).

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Figure 1: schematic of steam cycle including small steam expander (SE) parallel to pressure reduction valve (PRV).

Causes of steam contamination

One of the contaminants is oil that can leak into the steam system. Oil-contaminated steam causes problems in the entire steam system and can lead to damage with disastrous consequences.

Steam pollution by oil can in principle be caused by two systems: heat exchangers and rotating flow machines.

Steam-to-oil heat exchangers
Steam can be used to pre-heat oil before combustion, for example on ships, or to transfer heat from a steam loop to an oil tank. Heat exchangers from steam-to-oil loops can leak through flanges or broken gaskets or through leakages in walls of the heat exchanger. In case the oil pressure exceeds the steam pressure, a driving force is present to move oil into the steam cycle.

Rotating equipment
Rotating equipment installed in the steam cycle could cause contamination in case the sealing between the oil bearings and the steam flow path fails or is not leak tight. Hydrodynamic oil bearings are a common and widely used shaft support for steam turbines. In case oil pressures exceed the steam cycle pressure, there’s a chance that oil leaks into the steam cycle.

Consequences of steam contamination

From all types of steam contamination as mentioned before most severe steam contamination is oil because oil deposits on tubes in the water side of the furnace (Figure 2). A thin layer of oil is sufficient to insulate the tubes and increases the material temperature to dangerous levels. Heat conduction of oil is 20 times lower than a layer of scale with the same thickness. The metal wall temperature significantly increases, far above the design temperature and reduces the strength of the material. As a consequence, mechanical failure occurs (Figure 3).

Figure 2: oil contaminated boiler tubes [2]
Figure 3: cracks in steel boiler tubes due to overheating by oil contamination [2]

Steam contamination by oil can cause the following problems in the boiler:

Increased foaming and carryover rate of boiler water
Steam will become less pure due to additional foaming and water carryover from boiler into the steam grid. Saturated steam contains more particles from the boiler water which are carried over by the liquid water into the steam pipes. The contaminated steam could damage valves, process systems and steam turbines. Water carryover increases the wetness of the steam and requires appropriate steam traps to protect the systems and valves downstream the boiler.

Failure of boiler water level control and protection devices
The boiler level is electronically controlled and activates the feedwater pumps to avoid the boiler from dry cooking and overheating as a consequence. Any damage of these crucial safety devices could lead to severe safety risks.

Deposition on walls of the furnace
Oil floats on top of the boiler water and can only be removed by using the top blow-down. When using the bottom blow-down instead, the oil spread over the walls and increases the problem: formation of a thin film on the boiler walls leading to overheated furnace walls.

How to prevent oil contamination in your steam system

The best way to prevent steam systems from contamination is to avoid installing equipment using large amounts of oil. In case you are considering to increase your plant efficiency by installing a small steam turbine you could think about equipment using no or very little oil.

There are turbines available on the market these days which are constructed such that it’s physically impossible to contaminate steam. When considering installing small steam turbines or engines always ask the supplier about the risk of steam contamination by the new equipment.

Steam contamination by oil can be observed by checking the furnace walls on flue gas side. The steel material starts scaling first at the flue gas side and could eventually lead to rupture with severe damage as a consequence (Figure 3).

The Innecs SteamExpander SE250

Figure 4: Innecs SteamExpander SE250 parallel to PRV

Innecs Power Systems developed a small steam expansion turbine, the SteamExpander, which makes use of oil mist lubricated ball bearings in an overhung rotor configuration. This construction ensures that oil can’t leak into the steam path and makes the system suitable for usage in the food- or any other industry where clean steam is required.

More information

Click here for more information about the SteamExpander.

If you would like to know more about this subject, or if there is oil contamination in your steam installation, please contact us by sending an e-mail to info@innecs.nl, or by calling us: +31 852 733 160.

 

References:

[1] Aalborg Technical Support - Oil conttamination in marine boilers

 

[2] https://officerofthewatch.com/2012/10/25/marine-boilers-oil-contamination/

 

Pieter van der Meer

Project Engineer

Tel: +31 (0)85 - 27 33 160
e-mail: p.vdmeer@innecs.nl
www.innecs.nl