Designing a Shorter Vertical Leg for Sanitary Steam Traps

Sep 01, 2006
Volume 19, Issue 9


Steam traps are part of a steam-in-place system. The current design allots 18 in. of vertical leg for condensate backup. A design with a sensitive bellows has been proven in laboratory tests to need only 6 in. of vertical leg during the 15 min. of 121°C sterilization. Loads of 1 to 27 lb/h are covered by the capability of the new trap, equivalent to required steam for vessels 20 to 40,000 L.

Biopharmaceutical facilities include hundreds of sanitary steam traps. Steam traps require an 18 in. vertical leg for condensate backup during steam-in-place sterilization (SIP). This adds 18 in. to the elevation of equipment, portable vessels, utility panels, and clean room ceilings. The vertical leg is required by the original equipment manufacturer to guarantee that the condensate backup does not reach the temperature instrument that is monitoring a SIP.

Steam traps are sized for the high condensate load during initial heatup of the vessel. However, the condensate load during SIP maintenance is 50 times lower than the heatup load. Jacobs asked if Spence Engineering Co., the manufacturer of Nicholson Steam Trap, could guarantee their steam traps with 6 in. condensate backup at low loads during SIP maintenance. This article reports on laboratory tests proving a modified bellows in a standard steam trap can hold to the 6 in. maximum.


A diagram of a thermostatic sanitary steam trap for vertical installations is shown in Figure 1. The working element in this trap is an alcohol-filled bellows. When this bellows is in contact with steam, the alcohol expands, which forces the plug into the seat and stops the steam flow. When cooler condensate hits the trap, the alcohol condenses, which contracts the bellows, pulls the plug off of the seat, and allows the condensate to be purged from the trap.

Figure 1. Sanitary Thermostatic Steam Trap
Several additional features make this a sanitary steam trap. The bellows is located above the plug, which allows a small layer of condensate to remain at the bottom of the trap when the trap is hot. This water layer acts as a seal to prevent small leakages of steam. When the SIP cycle is finished, all condensate drains from the trap as the trap cools. The trap is angled so any condensate left in the trap after the SIP cycle will drain freely. The material is 316L stainless steel to minimize any rouging of the system. The internal surface finish is also less than 20 micro-inch roughness average to prevent any particulate adhesion.

The trap operates on the difference in temperature between the steam and the condensate. This difference is referred to as the sub-cool of the steam trap. As condensate backs up above the trap, its temperature drops. A trap with a high sub-cool requires the condensate to back up a greater height to allow it to cool. The piping immediately above a steam trap is uninsulated so the condensate can cool quickly. Steam traps located in a cramped, hot mechanical space where heat cannot be dissipated will not operate properly. This is a bad facility design and should be avoided.

The range of temperature in SIP is 259 to 280 °F, corresponding to 20 to 35 psig. The trap has a passive way to determine when the condensate is subcooled. Steam pressure squeezes the bellows, which is counteracted by the vapor pressure of the bellow's fill to compensate for the condensate temperature. Proper design of a steam trap can reduce the sub-cool, making it more sensitive to condensate backup. This sensitivity is controlled by the mechanical properties of the bellows and the bellows fill.

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