This article was originally published on July 19, 2021, and updated on October 28, 2024.
If you work in a refinery, petrochemical plant, or power generation facility, you likely face the challenge of managing high temperatures and pressures associated with steam applications. Under these harsh conditions, pressure gauges and other instruments can become damaged, which can lead to operational failures. A steam siphon is one solution that can provide a simple yet effective way to protect your instruments from these elevated process temperatures.
As a product leader in process gauges at Ashcroft, one of the most common questions I’ve heard (and one that our customer service team often receives) is, “What is a steam siphon and how can it protect my pressure gauge?” In this article, you will get the answer as well as details on the two most common types of steam siphons and how they work. You will also get my guidance on selecting the right siphon for your application.
From this point on, when I mention the term siphon, I am specifically referring to a steam siphon, as other siphons can also dissipate heat for high process temperatures not related to steam.
What is a Steam Siphon?
Steam siphons are crucial accessories designed to dissipate heat in steam-related applications. While this article focuses on using siphons with pressure gauges, it's important to remember that any steam present in a process line could damage any instrument if proper precautions are not taken.
Refineries, petrochemical plants and industrial factories use steam as a pressurized fuel for energy consumption. With steam also comes very high temperature and associated process pressure. When there is a need to measure steam pressure in the process line, the pressure gauge must be protected from these elevated temperature and pressure surges. Without protection, the gauge could be rendered damaged and inoperable.
This is where a siphon becomes essential. Siphons shield gauges from high process temperatures, particularly steam, and prevent damage when exposed to steam surges.
The two most common types of siphons are:
- Pigtail siphons. These instruments are specifically made for vertical piping configurations and are identifiable by their distinctive looped shape resembling a pig's tail. This design is effective in creating a natural trap for liquid accumulation, which acts as a barrier to protect your pressure gauges from the high temperatures of steam. By allowing steam to cool and condense into liquid form within the loop, pigtail siphons ensure that only the cooler liquid reaches the sensitive instruments, thereby preventing potential damage.
Figure 1. Ashcroft® 1100 Pigtail Siphon
- Coil siphons. This style of siphon is meant for both vertical and horizontal piping applications. They are designed with a loop with a small dip that traps water when placed horizontally. The water forms a barrier to stop the steam's high heat from damaging your pressure gauges. By letting only cooler liquid reach the gauges, coil siphons help keep pressure measurements accurate and reliable. Their ability to fit different pipe setups makes them useful in many industrial environments, ensuring pressure gauges stay protected.
Figure 2. Ashcroft® 1098 Coil Siphon
Steam Siphons Material and Maximum Allowable Working Pressure (MAWP)
Siphons are designed to be low-cost and are typically available in carbon steel, stainless steel or brass. Siphon materials have corresponding temperature and pressure ratings, which is important when choosing the right one for your application.
The MAWP rating for siphons directly corresponds to the material of the instrument and the wall thickness of the siphon. Choose carefully based on your process. While the siphon may be compatible with your MAWP and corresponding steam temperature, you will also want to consider its dissipation capability. In other words, you need to determine whether the siphon can effectively reduce the process temperature to sufficiently protect the instrument.
How Steam Siphons Protect Your Pressure Gauges
Now that you know more about siphons in general, let’s talk about how they work and how they can protect your pressure gauges.
Pigtail and coil siphons are looped or coiled pipes that are placed directly on piping before a pressure gauge. Pressure gauges generally have a standard operating temperature rating of 200 °F (93 °C), possibly as high as 250 °F (121 °C), and even low-temperature steam services can reach a temperature that will damage a gauge.
Before installation, the siphon loop should be filled with water. When the steam hits the water, it’s cooled down before it can reach the gauge and cause damage. While in service, the siphon allows the steam to change from a gas to a liquid by dissipating the heat energy of steam which then condenses, providing a barrier of liquid in the loop of the siphon.
There is a chance that air can get trapped inside the siphon but it’s no need for concern. Air is compressible so you will still get an accurate reading from your gauge.
The most important thing is keeping your instrumentation safe from extreme temperatures, which makes a pressure gauge siphon a must-have for steam applications.
Ready to learn more?
I hope this article cleared up any confusion you may have had about how siphons work. As you can see, siphons are simple devices but make a big difference in the performance of your gauge. The Ashcroft 1098 and 1100 siphons are each an excellent and cost-effective choice for protecting pressure gauges in your power generation, oil and gas, refinery, chemical and petrochemical, and water and wastewater applications.
You can also read these related articles:
- How Does Temperature Affect Pressure Gauge Performance?
- How Does Media Temperature Affect Pressure Transducer Performance?
- When to Use a Welded Instrument Assembly
- Why Did My Pressure Equipment Fail? 6 Instrument Killers
- What’s the Right Pressure Instrument Mounting Assembly for Me?
- Do Measurement Instrument Accessories Affect Accuracy?
Contact us today to talk to one of our industry experts and get all of your questions answered. Let us be your one-stop-shop for pressure and temperature instruments!