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Why Do I Need a Steam Siphon and How Does It Work?

pressure gauge | steam siphon

A steam siphon can be a useful device for protecting your pressure gauges or other instrumentation. The steam siphon is a simple device with the crucial benefit of protecting the instrument from elevated process temperature.

However, one of the most common questions I’ve heard during my time working in this industry (and one that our customer service team at Ashcroft hears frequently) is, “Why do I need a steam siphon and how does it work?”

In this article, I will explain what a steam siphon is, what it does and how it works, and go over the two most common steam siphon configurations. I will also provide some guidance on selecting the right siphon for your application.

Going forward, since there are other siphons that can dissipate heat for elevated process temperatures other than steam, when I use the word siphon, the reference is to a steam siphon.

This article will also focus on siphons specifically for pressure gauges, but keep in mind that when steam is present in the process line, damage could occur to any instrument if proper precautions to protect the instrument are not taken.

What is a Steam Pressure Gauge Siphon?

Refineries, petrochemical plants and industrial factories utilize 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 process line pressure, 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 comes in. Siphons protect gauges from elevated process temperature or, in this case, steam and damage when they are exposed to surges of steam.

The two most common types of siphons are:

  • Pigtail siphons, which are meant for vertical piping configurations.
  • Coil siphons, which are meant for vertical or horizontal applications and have a slight dip in the loop design to provide for the trapped water in a horizontal position.

Siphons are designed to be low-cost and are typically offered in carbon steel, stainless steel or brass. Siphon materials have corresponding temperature and pressure ratings. This is important when choosing the right siphon for your application.

Siphons are rated with a maximum allowable working pressure (MAWP). This correlates to the material of the siphon and the wall thickness of the siphon. Choose carefully based on your process.

Although the siphon may be compatible with your MAWP and corresponding steam temperature, also consider its dissipation capability. In other words, will the siphon dissipate enough process temperature to adequately protect the instrument?

How Does a Steam Pressure Gauge Siphon Work?

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.

To then properly measure steam pressure, a siphon attached directly to the piping before the gauge is necessary to protect it from excessive heat, which can cause damage to the gauge’s internal parts.

Prior to 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.

We don’t like to pressure you, but we have more information.

I hope this article cleared up any confusion you may have about how siphons work. As I stated before, 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.

If you want to learn more about pressure measuring instruments, feel free to visit our resource center and view any of our helpful product pages, white papers, webinars or guides.

You can also read these related articles:

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!New call-to-action

About Paul Francoletti, Product Marketing Leader - Process Gauges

Paul Francoletti is the Product Marketing Leader for Process Gauges at Ashcroft. He has spent the past 15 years in numerous roles at Ashcroft ranging from Technical Support, EPC Support Manager and now, Product Leader.