Steam and Boiler Pressure Gauges: Siphons, Ranges and Materials

A steam or boiler pressure gauge must be protected from the heat of the steam, usually with a pigtail siphon, and fitted with a range well above the working pressure, commonly around 1.5 to 2 times the safety valve set pressure. Stainless steel wetted parts, a robust case and a gauge isolation or test cock are standard. This guide explains how steam gauges differ from general purpose gauges, why the siphon matters, how to choose the range and materials, and how to install, check and maintain them. Boilers are regulated equipment in Australia, so always confirm the requirements for your own boiler with your inspector.

Why steam needs special treatment

Steam is hot, and the heat is the problem. At 7 bar gauge pressure, saturated steam is at roughly 170 degrees Celsius. The gauges in our range have a stated working temperature of up to 60 degrees Celsius, and the datasheets recommend a pigtail for higher temperatures. If hot steam reaches the Bourdon tube directly, it can overheat the tube and the movement, shift the calibration, damage the seals and cook any liquid fill in the case.

The solution is a siphon, usually in the shape of a coil called a pigtail. Steam condenses in the coil, and the water that remains acts as a cushion. The gauge sees the pressure, but the temperature at the tube is close to ambient.

How a pigtail siphon works

1. Steam enters the pigtail from the boiler or pipe.
2. As the steam passes through the coiled tube it loses heat and condenses.
3. The condensate fills the coil and stays there, so hot steam never reaches the gauge.
4. Pressure is transmitted through the water to the Bourdon tube, so the reading is correct.

Always fill the siphon with water before the first start-up if it is possible to do so. This prevents steam reaching the gauge during the first few minutes. Check that the pigtail is not blocked by scale or corrosion, because a blocked siphon causes a gauge to read zero or to stick.

A straight tube, known as a U-bend or a loop, can serve the same purpose, but the pigtail is the most common form in industry.

Choosing the range

Boiler codes typically require the gauge range to be larger than the safety valve set pressure, and many specify a range of about 1.5 to 2 times the maximum allowable working pressure. For a boiler with a safety valve set at 10 bar, a 0 to 16 bar gauge puts the set pressure at 63% of the scale and the normal working pressure lower. Check the code and your boiler’s documentation, including AS 1228 for boilers, for the exact rule that applies. Our pressure gauge range selection guide sets out the general method for other applications.

The dial size matters too. A boiler gauge is often read from a distance by operators, so the 100 mm and 150 to 160 mm dials are common. The AL-F-2 has a 100 mm dial and the AL-H-2 has a 150 to 160 mm dial. Mark the normal working pressure and the maximum allowable pressure on the dial or with a marker, as your site procedure requires.

Materials for steam duty

• Stainless steel case and wetted parts: resist corrosion from condensate and humid plant rooms. The AD-D-2, AL-F-2 and AL-H-2 have a 304 stainless steel case and a 316 stainless steel tube with a 316L connection.
• Safety glass window: a laminated or shatterproof window is safer in a plant room. These gauges use a shatterproof safety glass window, with a polycarbonate lens as an option on the AL-F-2.
• Brass and copper alloy: acceptable for low pressure steam where the siphon keeps the temperature down, though stainless steel gives a longer life.

Dry or liquid filled on steam?

Most steam gauges are supplied dry, or with a fill chosen for the temperature. The pigtail keeps the gauge cool, so a liquid fill can be used where there is vibration, for example near a steam engine or a pump. Make sure the fill liquid is rated for the temperature at the gauge. The stainless steel gauges in our range are supplied dry, with liquid filling available on request, and our team can advise whether it suits your steam application. For more on the pros and cons, read liquid filled vs dry pressure gauges.

Isolation, test connections and blowout protection

A boiler gauge installation normally includes:

• A siphon (pigtail): to protect the gauge from the steam temperature.
• An isolation cock or valve: so the gauge can be removed or replaced while the boiler is in service. Some installations use a three-way cock that can also vent the gauge or connect a test gauge.
• A test connection: so a calibrated test gauge can be fitted beside the working gauge to check it.
• A blowout back and solid front: in case the tube bursts. The Solfrunt 1981 has a solid front and full rear blowout protection, and a 316 stainless steel tube and socket.

Installation steps

1. Isolate and depressurise the line, and follow lockout procedures.
2. Fit the isolation valve and the pigtail to the steam connection. Use thread sealant suitable for steam.
3. Check the gauge thread. Several of our stainless steel gauges use a 1/4″ or 1/2″ NPT lower mount. See pressure gauge connection types and thread sizes.
4. Fit the gauge with a spanner on the socket flats, and orient it so the dial can be read clearly.
5. Fill the pigtail with water if possible.
6. Open the isolation valve slowly to let steam condense and the pressure build.
7. Check the gauge reads sensibly and look for leaks.

Daily and periodic checks

• Check that the pointer returns to zero when the boiler is cold and depressurised.
• Compare the working gauge against a second gauge or a test gauge, as your inspection regime requires.
• Confirm that the pigtail is clear. Open the isolation valve and see that the pointer moves freely.
• Look for leaks, corrosion and cracked glass.
• Check the red mark or label for the maximum allowable pressure is still visible.
• Calibrate at a regular interval and keep the record. See pressure gauge calibration.

Common problems

The gauge reads zero or is stuck

The pigtail or the connection is probably blocked by scale or debris. Isolate and clean the line, or replace the pigtail.

The gauge reads high or low compared with the test gauge

The tube may have been overheated or overstressed. Replace the gauge.

The dial is fogged or cloudy

Steam may be leaking past a damaged seal into the case. Replace the gauge and check the siphon.

The pointer vibrates

Check for pulsation from a feed pump, fit a snubber or use a liquid filled gauge.

Regulations and safety

Boilers and pressure vessels are regulated in each Australian state and territory. The standards that commonly apply include AS 1228 for boilers, AS 1210 for pressure vessels and AS/NZS 3788 for in-service inspection. Requirements for gauges, including range, markings, test connections and checking intervals, depend on the boiler class and the regulator. Always confirm what applies to your plant with a competent person or inspector. Never change safety valve settings or remove a gauge from a boiler in service without following the correct procedure.

Wear suitable protective equipment around steam. Scalding steam can cause severe burns, and a small leak can be hard to see.

Digital gauges around steam

Digital gauges are generally not fitted to the steam line itself. They suit testing on cooler parts of a system or as a reference test gauge when the gauge is isolated and cooled. The PDK range is ideal as a reference gauge for checking a working gauge. See the digital pressure gauge buying guide.

Reading and marking a boiler gauge

A boiler gauge is only useful if the operator knows what the reading means. Many sites mark the normal operating pressure and the maximum allowable working pressure directly on the dial or on the glass, so a glance shows whether the boiler is inside its limits. Keep the face clean and well lit, and make sure nothing blocks the view. If the gauge is mounted high, consider a larger dial or a second gauge at ground level. Record the reading at each shift as part of your log.

Frequently asked questions

Why does a steam gauge need a siphon?

To stop hot steam from reaching the gauge. The condensate in the siphon protects the Bourdon tube from heat.

What range should a boiler gauge have?

Typically 1.5 to 2 times the safety valve set pressure, but check the code that applies to your boiler.

Can I use a liquid filled gauge on steam?

Yes, with a siphon and a fill rated for the temperature at the gauge. Ask us if you are unsure.

How often should a boiler gauge be checked?

Check it every day against zero and for obvious faults, and at the intervals set by your inspection regime for comparison against a test gauge.

What is a pigtail?

A coiled tube between the steam line and the gauge, designed to hold condensate and keep the gauge cool.

Need a steam rated gauge?

Browse the pressure gauges category, or contact us with your boiler pressure, connection and fluid, and we will suggest a suitable gauge and accessories.