The document discusses several issues related to pipe stress analysis in industrial systems. It describes cases where pipe stress caused unexpected problems, such as pumps misaligning and failing due to lack of space for thermal expansion in a large diameter pipe. It emphasizes that rules of thumb are not sufficient for determining when pipe stress analysis is needed, and that transient conditions and unusual factors must be considered. It recommends using experienced pipe stress engineers to properly analyze critical piping systems.

1. installed right in the field. One “famous”
case in the business was when all these
high temperature lines were analyzed,
constructed, and put into service. Pipe
supports failed, pumps were misaligned,
and overall some interesting challenges
occurred. Well the solution to fix all the
problems was simple; the “locking pins”
that should have been removed after
construction, were left in. Whoops!!
Then there are other cases where things
are not so easy to see. In some systems,
under transient conditions, hot streams
can meet colder steams causing low cycle
thermal fatigue. In another case, a
compressor train would work fine for
months; however, when a “blue norther”or
heavy rain would occur, heavy vibration
was experienced that would sometimes
shut the plant down. The problem was the
temperature changes in the structure and
piping would cause the drive turbine to
move enough to cause vibration.
One lack of understanding in the business
is that basically only the normal design
conditions should be evaluated. Any weird
or unusually stuff will just have to be taken
care during operation and startup. Not
always true! One example is acoustical
analysis of piping systems. For example,
it is known in industry that reciprocating
pumps and compressor can cause
vibration in piping systems. Ignorance is
not an excuse when these systems are
installed and fail. The design never met
code if it was installed without an
acoustical analysis. Another good
example is pressure relief systems. In
many cases, when a relief valve blows
down, the transient pressure-momentum
conditions cause a dynamic impulse that
KnightHawk Project Update
When designing or maintaining
Petrochemical, Pulp & Paper,
and Power Equipment, pipe
stress is most always a consideration.
However, to a great extent it is not known
what the range of problems associated
with pipe stress issues really are. It is also
not well understood what should be
included in a pipe stress analysis. Many
issues and seminars are available to
address pipe stress, but unfortunately
individuals who have never worked in a
production facility conduct these analyses
in many cases. Seeing and experiencing
the real world makes a difference.
Many entities have rules of thumb
governing when pipe stress should be
performed on a particular project. They
might say, “An evaluation below 250 ?F or
below 3 inch nominal diameter is not
required to perform a pipe flexibility
analysis for non lethal services”. While
these statements are meaningful and have
good intentions, they cause problems. A
good example is as follows. Several years
ago I worked on a problem concerning a
maintenance issue with some water
circulation pumps. Very frequently the
pumps would misalign and vibration would
start, sometimes the pumps would wreck.
The temperature was only 180 ?F and
guess what the pressure was only 100 psi.
Pipe stress should not be a problem right,
well wrong. It was a pipe stress problem
and it was simple, the pipe was 48 inches
in diameter and it was close coupled to the
pumps. There was no room for any
thermal expansion, and the large pipes
would simply push the pumps around,
causing failure and downtime.
A good analysis is fine, but what if it is not
Cliff’s Notes:
BORSIG Technologies, Inc., KnightHawk Engineering Division is pleased to add Paul
Parker to the staff. Paul has a BS in mechanical engineering and 30 years of experi-
ence. He has worked in production most of his career including Borden and BASF. Paul
is a highly qualified individual that will serve KnightHawk well. Welcome Paul!
KnightHawk has performed many pipe stress analyses in industry. We have worked in
Chemical, Off shore, Nuclear & Fossil Power, Pulp & Paper, and Refining. We routinely
perform design of critical pipe systems including failure analysis and troubleshooting.
Summer is near and school is fixing to wrap up this month. Time to start planning a va-
cation. We were considering going to California, but we didn’t want to take out a sec-
ond mortgage to pay for the gas, besides the power might
not be on at the new Disney Theme Park. Anyway, we wish
all well and appreciate your business.
Cliff Knight
cknight@borsigtech.com
Issue 01.04April/May, 2001
PHONE: (281) 282-9200 •FAX: (281) 282-9333
WEBSITE: www.knighthawk.com
Pipe Stress Loads on Rotating and Static Equipment
The Nozzles Don’t Always Break, But Is It Costing You Money?
• Diesel Engine Bearing Failure Analysis –
Large Container Ship
• Piping Analysis at Nuclear Power Plant –
FEA, Structural Dynamics
• CFD of Gas Turbine Inlet – OEM
• Critical Pipe Stress Analysis in Oxygen
Service - Petrochemical
• Centrifugal Compressor Failure Analysis
at Refinery
• High Pressure Plunger Pump Design and
Analysis – Industrial
• Compressor Skid Pipe Stress Analysis –
Off Shore
• Reciprocating Compressor Analysis – Off
Shore
• Transfer Line Exchanger design Audit
• Pelletizing Die Design – CFD and FEA
• Liquid Ring Compressor Analysis –
Acoustical, FEA, CFD, Rotordynamics,
Field Data Acquisition.
• Heat Exchanger Research and Develop-
ment Project
• Pressure Vessel Analysis – FEA
• Refractory line Piping Design and
Analysis – FEA
• Pellet Quality Problem – Polymers Plant
can break supports or damage equipment.
These loads must be calculated and
accounted for in the mechanical design.
The load cases that must be considered
include but are not limited to the following:
1. Thermal
2. Pressure
3. Wind Load
4. Occasional loading
5. Earthquake
6. Miscellaneous conditions such as
acoustical, flow induced vibration,
water hammer or steam hammer,
etc.
The major problem with piping connected to
rotating equipment, is not that the pipe will
break, but that the equipment nozzles will
be overloaded. Again, the “compressor
nozzle” most likely will not break, but
sufficient deflection can be introduced to
cause misalignment or “rubbing”.
Well we can’t cover everything about piping
in one article, but it is recommended that a
qualified pipe stress engineer should review
every critical piping system in an industrial
environment.