What Is the Maximum Allowable 316 Stainless Steel Sheath Wall Thickness for a Low-Pressure Steam Heating Application at 130 Degrees Celsius with Condensate Return?

Process engineers constructing electric immersion heaters for low-pressure steam systems such as steam jackets, humidification chambers and sterilisation equipment face a unique set of constraints with the existence of condensate return. Unlike pure water or dry steam applications condensate return systems have a mixture of liquid water, dissolved oxygen and typically trace amounts of carbonic acid from dissolved carbon dioxide. The environment is somewhat corrosive to 316 stainless steel, especially at the water-steam interface where oxygen concentration gradients produce localised corrosion. The corrosion allowance is determined by the wall thickness of the 316 sheath and more importantly the surface temperature compared to the saturation temperature of the steam. If the sheath surface temperature is too low, it encourages formation of condensate coating and corrosion, but if it is too high, it increases scaling and thermal stress. This article defines the maximum wall thickness permitted for 316 sheaths in condensate return steam service based on corrosion rates and optimisation of heat transmission.

Corrosion Mechanisms in the Return of Steam Condensate
The saturation pressure of low pressure steam at 130°C is about 2.7 bar absolute. When steam meets a surface below saturation temperature, it produces condensate in the form of a liquid coating. This condensate scavenges oxygen and carbon dioxide from the steam and from air in-leakage, and forms a moderately acid solution, with a pH of 4.5 to 6.0. 316 stainless steel shows in this environment corrosion rates in general of 0.02–0.08 mm/year depending on oxygen content and the rate of condensate regeneration. More importantly, localised attack-pitting and crevice corrosion-occurs in the waterline where the sheath penetrates the condensate layer. Formation of oxygen concentration cells can result in corrosion rates near the waterline that are two to five times higher than the completely immersed rate. For a five-year design life heater, a corrosion allowance of 0.2–0.4 mm is required for general corrosion + 0.3–0.6 mm is necessary for waterline attack. A 1.0 mm sheath would not have sufficient margin and would perforate in two to three years. The 1.6 mm sheath gives an adequate margin for a five year service life. However, thicker walls can lead to a higher sheath surface temperature at a given watt density, which may affect the condensation behaviour.

Influence of wall thickness on condensate film formation and corrosion
For a heater working in steam at 130°C, the sheath surface temperature should be higher than the saturation temperature to avoid formation of condensate film. Dry steam conditions with no liquid water on the sheath are obtained when the surface temperature is 5-10°C above saturation. 316 corrosion rate in dry steam is modest - < 0.01 mm per year. The corrosion rates above are for a continuous condensate film at or below saturation surface temperature. The sheath surface temperature is calculated as the bulk steam temperature plus the temperature rise due to the heat flux through the sheath wall. Thicker wall adds conductive resistance and so increases the surface temperature for a given watt density. A thick walled sheath can actually prevent corrosion by keeping the surface dry. A narrow walled sheath can run cooler and encourage condensate attack. Consider a heater at 8 W/cm² in 130°C steam. The outer surface temperature of a 1.0 mm wall 316 sheath is about 133°C, only 3°C above saturation. Marginal circumstances may produce intermittent condensate. The outside surface temperature of the 1.6mm wall sheath is about 138°C - 8°C above saturation, therefore there is a safe margin for dry operation. It is not the addition of material that minimises corrosion, but the change in surface state from wet to dry that the thick wall provides. This is an important lesson to steam system design: larger walls can be good for corrosion resistance even in condensing situations.

Recommended Wall Thickness for Steam Service Condensate Return
The following table lists acceptable sheath wall thicknesses for electric immersion heaters in low pressure steam service with condensate return. Values are calculated for continuous operation, steam quality typical for industrial boiler systems and a projected service life of 5 years.

Steam Temperature Operating Watt Density Recommended Minimum 316 Wall Thickness Recommended Maximum 316 Wall Thickness Sheath Surface Temperature at Maximum Thickness Expected Corrosion Rate Dominant Failure Mode 110°C (1.4 bar) 6 – 10 W/cm² 1.2 mm 2.0 mm 115 – 122°C 0.05 – 0.10 mm/year Condensate film corrosion 120°C (2.0 bar) 6 – 10 W/cm² 1.2 mm 2.0 mm 125 – 132°C 0.03 – 0.08 mm/year Transitional wet/dry 130°C (2.7 bar) 6 – 10 W/cm² 1.4 mm 2.5 mm 135 – 145°C Below 0.02 mm/year (dry) Thermal cy
Any temperature, intermittent condensing 4 – 8 W/cm² 1.6 mm 2.5 mm Variable 0.08 – 0.15 mm/year (wet cycles) Waterline pitting 
For steam temperatures exceeding 140°C, the sheath surface temperature may surpass 160°C at thick walls, which can produce localised boiling and scale development if dissolved particles are present in the water. In these instances it is desirable that a low-end wall thickness within the specified range is obtained. For steam temperatures below 120°C it is difficult to establish dry sheath conditions with any practical wall thickness since the saturation temperature is low and the temperature rise due to heat flux is limited. In low-temperature steam service, engineers should accept that there will be a film of condensate and therefore specify wall thickness on the basis of corrosion allowance rather than dry operation.

Design Modifications to Attain Dry Sheath Operation with Thin Walls
If a thin-walled 316 sheath below 1.2 mm is required in steam service (usually for fast thermal response in low pressure systems), three design changes can result in dry operation despite the lower temperature rise. One is to enhance the watt density. An increase in watt density from 5 W/cm² to 10 W/cm² may raise the sheath surface temperature by around 15–20°C and may change the operation from wet to dry. The trade-off is higher risk of local overheating if flow of steam is stopped. The second adjustment is to run the heater in an intermittent mode at a greater peak temperature. A heater that cycles between 130°C and 180°C will spend most of its time above the saturation temperature, with just short periods of condensate contact during startup. If the heater is in dry condition in minutes, corrosion damage during startup is limited. The final change is the installation of a condensate drain to prevent liquid water from forming around the heater. In the absence of any standing condensate, the corrosion rate is greatly reduced even if the surface is moist with a thin film. For most industrial steam systems a wall thickness of 1.6 mm is a good compromise between corrosion allowance, dry operation margin and mechanical strength. The engineers have to provide the needed margin on surface temperature, often 5–10°C above saturation, and ask the manufacturer to determine the wall thickness and watt density combination which will provide this margin. A supplier unable to do this estimate may not appreciate the special needs of steam condensate service.