# What are Water trap and Pigtail siphons?

**URL:** <https://forumautomation.com/t/what-are-water-trap-and-pigtail-siphons/2889>\
**Category:** Flow Measurement\
**Tags:** interviewquestions\
**Created:** [December 29, 2017, 6:40pm UTC](https://forumautomation.com/t/what-are-water-trap-and-pigtail-siphons/2889 "2017-12-29T18:40:50Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![sivaranjith](https://forumautomation.com/letter_avatar_proxy/v4/letter/s/ba9def/32.png) [@sivaranjith](https://forumautomation.com/u/sivaranjith)\
**Post date:** [December 29, 2017, 6:40pm UTC](https://forumautomation.com/t/what-are-water-trap-and-pigtail-siphons/2889/1 "2017-12-29T18:40:51Z")

</div>

## Why trap and Siphons are needed?

Many industrial processes utilize high-pressure steam for direct heating, performing mechanical work, combustion control, and as a chemical reactant. Measuring the pressure of steam is important both for its end-point use and its generation (in a boiler). One problem with doing this is the relatively high temperature of steam at the pressures common in the industry, which can cause damage to the sensing element of a pressure instrument if directly connected.

So that to create a barrier techniques such as pigtails are used.

## Water trap and Pigtail siphons:

 ![pigtail siphon](https://cdn.forumautomation.com/original/2X/8/8bf730c7abe5e86d265ff0add986c0a75cac9222.png)

To avoid the direct contact of hot steam through the steam pipe a pigtail shaped curve is made at the [impulse line](https://forumautomation.com/t/what-is-filled-impulse-line/2853) to the [pressure gauge](https://forumautomation.com/t/types-of-pressure-gauge-based-on-the-working-principle/2709). where condensed steam (water) will accumulate and act as a liquid barrier to prevent hot steam from reaching the pressure instrument.

The same effect is created when we made a water trap in the line. Water trap is a low spot in the impulse line.

When siphons are installed, it should be filled with water or any other suitable separating liquid.

## Applications:

- Used to protect the pressure gauge from the effect of hot pressure media, temperature above 65°C

- Used to reduce rapid pressure rise, which can affect the gauge.

- Used at the inlet and outlet steam flow measurement of boiler