Here I enclosed some of the meaning of the "Minimum" in Piping Knowledge..
The Minimum is mostly tell about the standard requirement when do the design or the field execution.
Hopefully this information can help you to do the right of your jobs on piping world.
There are :
MINIMUM LENGTH SPOOLPIECE
This means the total length of the assembled piping components is as short as possible -
observing certain rules (those written and unwritten). Observing the rules means complying with
them and not violating them. You have failed when you violate them. You, the Designer must
incorporate a mixture of common sense and technology in your work. Normally, the minimum
length spoolpiece is determined by finding the sum of the physical dimensions of the components
involved AND the dimensions required for "sound" construction, fabrication, assembly. Also,
metallurgy (distance between welds), maintenance needs, and pipe stress requirements are
factors in determination.
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MINIMUM DISTANCE BETWEEN WELDS
This is subject to interpretation by various "experts" - but the following is what will work. The weld
that is referred to here is usually a circumfrential butt weld or fillet weld. In practice we will use it
for ANY weld. When a metal is subjected to heat as in welding, at a distance from the heated
"point" what is called the HEAT-AFFECTED ZONE or HAZ is produced. The HAZ is a weakened
place in the metal, obviously since it is weakened, it's reliability is reduced. This is not a problem
because all welded piping has two HAZ at every weld (one on each side of the weld). THERE IS
A DEFINITE PROBLEM WHEN TWO HAZ OCCUPY THE SAME LOCATION. Two HAZ occupy
the same location when two welds are positioned so that their HAZ overlap. THE WEAKNESS
PRODUCED BY THE OVERLAPPING HAZ IS UNDESIRABLE AND SHOULD BE AVOIDED!
The overlapping HAZ can be avoided by observing a prescribed minimum distance between the
welds. When this is done, we successfully avoid the HAZ problem. So what is the minimum
distance between welds? This has never been written in any Fluor Daniel document. Some
projects have adopted a dimension, but not Fluor Daniel. In the Piping training courses we
instruct the students to use 3" (76mm) as the Fluor minimum between welds. They also observe
this 3" with threaded piping - with the theory that it may be seal-welded (back-welded). The 3"
was adopted with the theory that the HAZ was produced 1" (25mm) from the weld. So, if you
have 3" between welds, the HAZ will never overlap. Everybody (including myself) that has been
to a fabrication shop or to a jobsite has seen welds closer together than 3" - and nothing has
exploded!!! Yes it happens, and often - but good design logic and planning will keep us away (in
most situations) from that arrangement. If you get into the habit of designing to bare minimums
you are setting yourself up for trouble.
A lesser "minimum" is possible, but I tell my students to stay away from it unless all reasonable
possibilities have been explored. The chart below contains the absolute "minimums", if you
believe you need this - rethink!!
Absolute Minimum Between Welds
Pipe Size Inches Millimeters
1/8" 3/16 6
1/4" 1/4 7
3/8" 5/16 9
1/2" 7/16 11
3/4" 1/2 14
1" 11/16 7
1 1/4" 13/16 22
1 1/2" 15/16 25
2" 1 3/16 31
2 1/2" 1 9/16 37
3" 1 3/4 45
3 1/2" 2 51
4" 2 1/4 58
5" 2 3/4 71
6" & Larger 3 76
MINIMUM DISTANCE FROM THE VALVE TO THE HEADER
On the P&ID this means that the Piping Designer must place the valve as close to the header as
he can - this effort is the responsibility of Piping and NO OTHER DISIPLINE! Observing the
rules, here are some components that could dictate that distance as determined by the Piping
Designer:
- The branch connection length (sockolet, tee, stub-on).
- The minimum distance between welds or the rise to a standard elevation
established by the Project (maybe all lines will rise to a common BOP).
- The center-to-end of the elbow.
- The center-to end of the elbow.
- The minimum distance between welds.
- The length of a flange.
- The thickness of the gasket.
You have satisfied the Process requirements shown on the P&ID with the above.
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MINIMUM DISTANCE FROM THE NOZZLE TO THE VALVE
On the P&ID this means that the Piping Designer must place the valve as close to the equipment
nozzle as he can - this effort is the responsibility of Piping and NO OTHER DISCIPLINE!
Observing the rules, here are some components that could dictate that distance as determined by
the Piping Designer:
- If the nozzle is flanged and the valve is flanged and is a different size; the thickness of the
gasket, the length of the flange, the length of a reducer, the length of the flange, the thickness of
the gasket.
- If the nozzle and valve are flanged and are the same size; the thickness of the gasket. If the
valve needs to be moved back to allow knuckle clearance at the handwheel; the thickness of the
gasket, the length of the flange, the length of a spoolpiece, the length of the flange, the thickness
of the gasket.
You have satisfied the Process requirements shown on the P&ID with the above.
NOTE! NOTE! NOTE!
DO NOT EVER BELIEVE THAT YOU MUST ALWAYS HAVE A PIECE OF PIPE BETWEEN AN
EQUIPMENT NOZZLE AND A VALVE IF THE TWO ARE THE SAME SIZE AND RATING AND
NO OTHER INTERFERENCES EXIST! BOLT THEM TOGETHER IF THERE ARE NO
INSTRUMENT OR BLEEDS BETWEEN THEM. USE YOUR COMMON SENSE.
MINIMUM LENGTH OF PUMP (OR COMPRESSOR) SUCTION LINE
This is not specified on the P&ID but is "the law". To keep the horsepower of the pump
(compressor) down and thereby making the pump
operate with a smaller and less expensive motor, suction lines are
kept as short and direct as possible. In Piping we do our absolute
best to keep this piping short. Several factors prevent us from using
solely component dimensions; equipment plot arrangement separates the
pump from it's suction source, headroom and walkway requirements
"push" the piping away from a direct route, stress & force caused by thermal expansion of the
piping will result in the need of bend leg or loops. When all of this is considered, the piping is not so minimum, but is 100% in compliance with making the lines as direct as possible.
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MINIMUM WALKWAY WIDTH OR HEADROOM HEIGHT
These are numbers provided in Piping's layout/design specification 50001. The numbers are 2'-6"
& 7'-0" respectively and are not to be less, they can be more. If you cannot minimally achieve
these numbers, discuss the problem with your supervisor. It is doubtful someone will go to the
field and measure your installed design unless it is obvious that there is an unworkable situation -
be conscientious and keep your design to the specified requirements.
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MINIMUM SLOPE
This note could be found on the P&ID, but is wron if you ever find it. ALL slope amounts are to be
identified by Process engineering. If you see this situation - inform your supervisor.
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MINIMUM DETAIL
- This usually refers to a Piping Plan Drawing and it means that the Project does not require a full
detail drawing - follow your Project instructions to find what is required.
Sources : Friends of Fluor Daniel Inc
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