Steam Service · Pressure Gauge Protection · Piping Accessories · High Temperature
7 Proven Reasons Every Steam Pressure Gauge Needs a Siphon Tube : Plus Critical Installation Rules
A pressure gauge connected directly to a steam line will fail within weeks : possibly dangerously. The siphon tube is the single most important, least expensive and most overlooked protection device in steam instrumentation. This guide explains exactly why it is essential, how it works, the 3 types, material selection and the 7 installation rules every technician must know.
What Is a Siphon Tube and Why Is It Critical for Steam Pressure Gauges?
A siphon tube (also called a siphon pipe or coil siphon) is a small section of coiled or U-shaped tubing installed between a steam line and a pressure gauge. Its sole function is to trap and maintain a column of condensed water between the hot steam and the gauge internals. This water column protects the gauge from three things simultaneously: damaging steam temperature, steam condensate pulsation, and direct steam contact with the Bourdon tube.
A standard Bourdon tube pressure gauge is rated for a maximum process temperature of typically 60 to 80°C at the gauge socket. Steam at even moderate pressures is at temperatures well above this. At 6 bar gauge, steam temperature is 159°C. At 10 bar gauge it is 184°C. Direct connection of a gauge to steam at these temperatures causes the Bourdon tube to anneal (soften and lose its spring characteristics), giving permanently wrong readings before it eventually fails. The siphon tube prevents this by ensuring only cool condensate contacts the gauge.
The siphon is also used in some high-pressure liquid and slurry applications where pulsation protection is needed : but steam service is where it is mandatory without exception. When specifying pressure transmitters for steam service, a remote seal or impulse line fills a similar protective role.
Without a Siphon Tube vs With a Siphon Tube
Steam at 160-250°C enters the gauge socket directly. The Bourdon tube metal heats up and anneals within weeks. Zero shifts. Span changes. Gauge reads wrong : but looks fine from the outside. Eventually the Bourdon tube cracks or ruptures, releasing live steam. Gauge lifespan: weeks to months.
- Bourdon tube anneals from heat
- Condensate pulsation hammers internals
- Gauge oil (if liquid filled) breaks down
- Catastrophic failure risk from steam release
- Calibration drift from first week of service
Steam enters the siphon and condenses in the coil. A column of cool water fills the siphon and the gauge socket. Only water at near-ambient temperature contacts the Bourdon tube. Steam pressure is transmitted accurately through the water column without heat. Gauge lifespan: years of reliable service.
- Bourdon tube stays at safe temperature
- Water column cushions pulsation
- Gauge reads accurately and reliably
- No steam contact with gauge internals
- Correct reading from first day of service
7 Proven Reasons a Siphon Tube Is Essential on Every Steam Pressure Gauge
Steam at 6 bar is 159°C. The Bourdon tube alloy (typically phosphor bronze or stainless steel) loses its elastic modulus at elevated temperatures. Once annealed, it cannot recover : the gauge is permanently inaccurate. The siphon's condensate column keeps the gauge socket below 50°C regardless of steam temperature.
A gauge exposed to steam heat drifts in zero and span within the first week. Operators trust the reading without knowing it is wrong. In a steam distribution system where pressure is the primary safety indicator, a drifted gauge is a hidden hazard. The siphon maintains a stable temperature at the gauge, preserving calibration for years.
Steam systems experience regular pulsation from steam traps, condensate slugs, and valve operation. These pulses travel at the speed of sound in water and hammer the gauge internals at high frequency. The water column in the siphon absorbs these pulsations before they reach the Bourdon tube, eliminating pointer flutter and mechanical fatigue.
Glycerine-filled or silicone-oil-filled gauges are widely used in steam systems to damp pulsation and protect from vibration. Without a siphon, the fill fluid heats up, expands, forces fluid out of the gauge, and eventually degrades. The siphon keeps the fill fluid at safe operating temperature.
Steam condenses on cold surfaces. In a new installation, the siphon fills with condensate within minutes of steam being admitted. Once filled, it acts as a permanent barrier. No steam molecules ever reach the gauge socket after this initial fill : only the pressure of the steam, transmitted hydrostatically through the cool water column.
ASME B40.100 (Pressure Gauges and Gauge Attachments), EN 837 (European gauge standard), and most plant engineering specifications mandate siphon tubes for steam service above 60°C. Plant insurance policies often require documented compliance with these standards. Installation without a siphon may void the gauge manufacturer warranty.
A gauge without a siphon on steam service typically lasts 3 to 12 months before calibration is unacceptable. With a correctly installed and filled siphon, the same gauge will give accurate readings for 5 to 10 years. The cost of the siphon (typically $5-$30 USD) is recovered in the very first gauge replacement it prevents.
Steam Temperature vs Pressure: Why the Siphon Temperature Barrier Matters
Atmospheric steam
Typical process steam
Medium pressure steam
High pressure steam
Standard pressure gauge Bourdon tube maximum working temperature: 60-80°C. Every steam pressure above 0 bar g exceeds this without a siphon tube.
3 Types of Siphon Tubes: Coil, Pig-Tail and U-Shape
A full 360-degree coil of tubing, typically with 2-3 turns. The coil design maximises the condensate volume held in the siphon. The large surface area of the coil promotes rapid condensation of incoming steam. Most common type in process plants.
Best for: High-pressure steam, applications with frequent steam flow changes, general process use.
A partial coil or curved tube shaped like a pig's tail. Less condensate volume than a full coil but more compact. Standard in ASME B40.100 and widely used in the USA and Canada. Available in screwed and socket-weld ends.
Best for: Compact installations, low to medium pressure steam, where space is limited around the gauge connection.
A simple U-bend in the impulse line. The bottom of the U traps condensate permanently. Simplest design and least expensive. Used where a coil siphon cannot fit or where the impulse line itself forms the U-bend by routing.
Best for: Low-pressure steam, situations where the impulse line routing provides a natural U-trap, utility steam systems.
Siphon Tube Material Selection Guide
7 Installation Rules Every Technician Must Follow
Siphon Tube Installation: Do and Don't
- Fill siphon with water before commissioning
- Mount vertically, gauge connection at top
- Match siphon material to steam conditions
- Use 316 SS for any corrosive or unknown service
- Install isolation valves on both sides
- Install a snubber on pulsating lines
- Check condensate column during maintenance
- Use coil siphon as the default choice
- Connect gauge directly to steam without siphon
- Use copper siphon on steam above 180°C
- Insulate the siphon tube
- Admit steam before filling siphon with water
- Downgrade pipe class on the siphon vs process line
- Mount the siphon horizontally or inverted
- Forget to refill after maintenance isolation
- Use a siphon with a blocked or corroded coil
Quick FAQs: Siphon Tube for Pressure Gauges
- DP Transmitter Basics: Impulse Lines and Condensate Pots in Steam Service
- How to Size a Pressure Transmitter: Steam Service Over-Pressure Protection
- Process Variables: Pressure Measurement : Types and Instruments
- Hazardous Area Classification: Steam System Instrumentation Safety Requirements
- Cold Loop and Hot Loop Testing: Commissioning Steam Pressure Loops Safely
External References
- ASME B40.100: Pressure Gauges and Gauge Attachments Standard
- Spirax Sarco: Siphon Tubes for Steam Applications
- WIKA: Steam Pressure Measurement Accessories and Best Practices
What we learn today
- A siphon tube traps a column of cool condensate between a steam line and a pressure gauge, preventing steam at 100-250°C from contacting the Bourdon tube. The Bourdon tube maximum safe temperature is 60-80°C. Even 0 bar gauge steam at 100°C exceeds this. A siphon tube is therefore mandatory on every steam pressure gauge without exception.
- Three siphon types: coil siphon (most common, maximum condensate volume, best for high pressure), pig-tail siphon (compact, standard in North American specs), U-shape siphon (simplest, lowest cost, low pressure only). Use 316 stainless steel as the default material. Never use copper above 180°C or 10 bar gauge.
- Seven installation rules: fill with water before commissioning, mount vertically, match pipe class, use correct connection standard, install isolation valves on both sides, never insulate the siphon, add a snubber on pulsating lines. Never admit steam to a dry siphon : even brief direct steam contact permanently shifts gauge zero.