PIPING VENTS AND DRAINS AN OVERVIEW

Wednesday, November 23, 2011

VENTS AND DRAINS

• Vents are needed to let gas (usually air) in and out of systems.
• When a line or vessel cools, the pressure drops and creates a partial
vacuum which can cause siphoning or prevent draining.
• When pressure rises in storage tanks due to an increase in
temperature, it is necessary to release excess pressure.
• Air must also be released from tanks to allow filling and admitted to
permit draining or pumping out liquids.
• Unless air is removed from fuel lines to burners, flame fading can
result.
• In steam lines, air reduces heating efficiency.

HYDROSTATIC TESTING

• After piping is erected, it is often necessary to subject the system to a
hydrostatic test to see if there is any leakage.
• In compliance with the applicable code, this consists of filling the lines
with water or other liquid, closing the line, applying test pressure and
observing how well pressure is maintained for a specified time, while
searching for leaks.
• As the test pressure is greater than the operating pressure of the
system, it is necessary to protect equipment and instruments by
closing all relevant valves.
• Vessels and equipment usually are supplied with a certificate of code
compliance.
• After testing, the valved drains are opened and the vent plugs
temporarily removed to allow air into the piping for complete draining.

• Positions of the required vent and drain points are established on the
piping drawings.
• PIDS shows only process vents such as vacuum breakers and process
drains.

VENTING GASES

• Quick opening vents of ample size are needed for gases.
• Safety and safety-relief valves are the usual means.
• Gases which offer no serious hazard after some dilution with air ma be
vented to atmosphere by means ensuring that no direct inhalation can
occur.
• If a combustible gas is toxic or has a bad odor, it may be piped to an
incinerator or flarestack, and destroyed by burning.
DRAINING COMPRESSED AIR LINES
• Air has a moisture content which partially carried thru the compressing
and cooling stages.
• It is this moisture that tends to separate, together with any oil, which
may have been picked up by the air in passing thru the compressor.
• If air for distribution has not been dried, distribution lines should be
sloped toward points of use and drains : Lines carrying dried air need
not be sloped.
• If the compressed air supply is not dried, provide:-
1. Traps at all drains from equipment forming or collecting liquid –
such as intercooler, aftercooler, separator, receiver.
2. Driplegs with traps on distribution headers (at low points before
rises) and traps or manual drains at the ends of distribution
headers.)