Here some information that needed to the piping designer and engineer for the vent , drain and bleed that required when do the design of the piping process and maintenance..
This is probably one of the MOST MISUNDERSTOOD terms regarding Piping Design, with low
point being equally misunderstood.
In terms of venting the high point of piping, THE HIGH POINT IS NOT THE SINGLE HIGHEST
POINT IN THE ENTIRE SYSTEM!!!!! Instead, it is correctly, ANY place in a piping system in
which air or vapor can collect - INCLUDING THE SMALL VOLUME THAT WOULD BE CREATED
IN AN ECCENTRIC, BOTTOM FLAT SWAGE. In almost all cases, a piping system has
SEVERAL high points - THERE IS ALMOST NEVER ONLY ONE HIGH POINT. I have seen
piping system with 10 or more high points in them. If the line was to be hydrostatically-tested,
EVERY ONE OF THOSE HIGH POINTS WOULD HAVE A VENT!!!!!
The exact same philosophy regarding high points pertains 100% to low points - low points are
different in that liquid is "pooling" and collecting. A piping system can, like the above, have
MANY LOW POINTS.
HYDROTEST OR HYDROSTATIC TEST
This test is done by filling a piping system with water and increasing the pressure to a specified
amount, maintaining that pressure for a
specified amount of time, examining for leaks or loss of pressure, then draining the system.
High point vents and low point drains are required for this type of test. These vent and drains are
used one time only at the time of hydrostatic test - to allow the filling of the line with water and
removing the water from the line.
The construction of the vent or drain is governed by your Project instructions, these vents and
drains are usually unvalved threaded connections using a screwed cap or screwed plug. On
some Projects unbolting of flanged connections in a high point or low point to permit the gas or
liquid to escape is permitted in the Piping Field Pressure Test Specification - READ THE
SPECIFICATION AND FIND OUT WHAT IS ALLOWED!!!
The specified pressures and duration are covered in the Piping Field Pressure Test Specification
for the Project.
PNEUMATIC OR AIR TEST
This type of test involves using air pressure maintained for a specified amount of time to check
the integrity of the system. High point vents and low point drains are not required because water
is not involved.
PROCESS VENTS AND DRAINS
These are required by the Process Engineering group and are necessary for the correct
functioning of the process. Process vents and drains appear on the P&ID and are usually valved.
Bleeds are used to depressurize and sometimes drain a volume of pipe for maintenance
purposes. Because they are used again and again, they are valved.
If a bleed is used only to depressurize, it can be placed in any safe orientation.
If a bleed is also used to drain liquid, then it needs to be located at the lowest point of the volume
of pipe being drained/depressurized.
Examples of bleed locations include;
1. Between a control valve and the upstream block valve.
2. Between a relief valve and the upstream block valve and between the relief valve and the
downstream block valve.
3. At the lower connection of a level gauge.
4. At the lower connection of a level switch/alarm.
ITEM PURPOSE CONSTRUCTION ISO DRAWING P&ID
Process Process Valved with YES YES YES
Vent/Drain requirements. blind flange.
cap or plug.
Hydrostatic Pressure test Unvalved with YES NO NO
Vent/Drain blind flange,
cap or plug.
Bleed Depressurizing Valved with YES NO YES
and/or draining blind flange,
for maintenance cap or plug.