A siphon tube (also called a pressure gauge siphon or pigtail siphon) is a coiled or looped tube installed between a pressure gauge and the process line. It serves critical functions in protecting the gauge from damage caused by high temperatures, pressure surges, and corrosive media.
Key Reasons for Using a Siphon Tube
1. Protects Against High Temperature
Problem: Steam, hot gases, or high-temperature liquids can damage the pressure gauge’s internal mechanism (Bourdon tube).
Solution: The siphon tube cools the medium before it reaches the gauge by:
Creating a condensate trap (in steam applications).
Allowing heat dissipation through the coiled/looped design.
2. Prevents Pressure Surges & Pulsation Damage
Problem: Rapid pressure spikes (water hammer) or pulsating flows can fatigue the gauge.
Solution: The siphon tube dampens surges by:
Acting as a buffer zone that absorbs sudden pressure changes.
Reducing pulsation effects in systems like reciprocating pumps.
3. Blocks Corrosive/Dirty Media
Problem: Slurry, corrosive fluids, or particulate-laden gases can clog or corrode the gauge.
Solution: The siphon:
Traps condensate (in steam systems), preventing direct contact with hot/corrosive steam.
Allows sediments to settle instead of entering the gauge.
4. Maintains Gauge Accuracy & Longevity
By stabilizing temperature and pressure, the siphon extends the gauge’s life and ensures consistent readings.
Common Applications of Siphon Tubes
✔ Steam Lines (Boilers, HVAC, power plants)
✔ Hot Liquids (Oil, chemical processes)
✔ Pulsating Systems (Reciprocating pumps, compressors)
✔ Corrosive/Dirty Media (Slurry, gas pipelines)
Types of Siphon Tubes
Coil Siphon (Pigtail) – Spiral design for compact installations.
U-Type Siphon – Simple loop for moderate temperatures.
Syphon with Cooling Fins – Enhanced heat dissipation.
How It Works
The siphon tube fills with condensate (in steam systems) or process fluid.
This liquid column blocks direct contact between the gauge and extreme heat/pressure.
Pressure is transmitted hydraulically through the trapped liquid, protecting the gauge.
When is a Siphon Tube NOT Needed?
Low-temperature systems (<80°C / 176°F).
Non-pulsating, clean fluids (e.g., compressed air, water at ambient temp).
Key Takeaway
A siphon tube is a small but crucial safety device that prevents gauge failure in harsh conditions. It ensures accurate pressure readings while protecting equipment from thermal, mechanical, and chemical damage.
