Kamis, 16 September 2010

PIPING STRESS ON THE ISOMETRIC DRAWINGS CHECKING

01 - DESIGN TEMPERATURE - Is there any difference between the current line list temperature
and the temperature used for the original analysis?

02 - DESIGN PRESSURE - Is there any difference between the current line list pressure and the
pressure used for the original analysis?

03 - SPECIAL CONDITIONS - Is there any difference between the current special conditions and
the special conditions considered in the original analysis? EXAMPLE; is there slug flow now
when there was not any considered in the original analysis? Was the original analysis done
considering slug flow but now there is no slug flow? Was the requirement for regeneration
deleted /added, etc., etc
 
CONCERN; Changed or new special conditions can result in more/less supports/anchors/guides

required. The as-built piping can look ridiculously over-supported or can fail because required
supports were never incorporated into the design.

04 - PIPE SIZE - Is there any difference in line size between current size and the size used in the
original analysis?

CONCERN; Span is reduced when the line size is reduced. The pipe size is used in pipe stress
calculations. Forces at steel pump nozzles are acceptable with a value of 200#/nominal inch -
cast iron nozzles @ 50#/nominal inch. Support/guide/anchor sizes are dependent on pipe size.
Loads are determined using pipe size. Changes to pipe size will change nearly all stress
calculations.

05 - PIPE WALL THICKNESS - Is there any difference in the schedule or wall thickness of the
pipe between the current wall thickness and the thickness used in the original analysis?

CONCERN; A change in wall thickness affects several things: the moment of inertia or I
changes, affecting the amount of bend leg required for flexibility, load calculations are affected
since a thicker or thinner pipe wall will increase or decrease the weight (loads that are
submitted to equipment vendors, such as for their supporting our pipe at air coolers could
be severely increased - invalidating the number to which the vendor designed - this
invariably leads to the vendor backcharging us for recalculation), , span is also affected - a
24" sch 20 pipe will span 6'-0" further than a 24" sch 10 pipe.

06 - MATERIAL OF CONSTRUCTION - Is there any difference in the metallurgy between the
current material and the material used in the original analysis?

CONCERN; It may only be slight in short configurations but on long runs of pipe a change in
metallurgy results in a different coefficient of expansion - either increasing or decreasing the total
amount of thermal growth. The support/anchor/guide call out will also be affected.

07 - METHOD OF CONSTRUCTION - Is the current general construction different than the
original construction?

CONCERN; Changing from socketwelded or threaded construction to buttwelded or flanged or
vice versa will affect the load calculations. A buttwelded 8" line will not weigh the same as an 8"
line that is of flanged construction - two 8"-300# weld neck flanges and bolting weighs 152#
(69kg).

08 - COMPONENT LOCATION - Is there any difference in inline component location in the
current configuration from the locations in the original analysis?
CONCERN; If a valve has been moved to the horizontal from the vertical or vice versa, or has a
valve moved from one end of a horizontal run to the other end, or if a reducer changes in a similar
manner - this will affect the concentrated loading in different parts of the line and definitely at
support locations , and the equipment nozzle loads.

09 - COMPONENT DESIGN - Have any of the piping components current design changed from
the their design in the original analysis?
CONCERN; Have wafer check (split disc) valves become swing check valves (8"-150# wafer
checks weigh 71# vs 390# for 8"-150# swing checks)? Have the cooling water valves at
exchangers changed from butterfly valves to gate valves (6"-150# butterfly valves weigh 45# vs
240# for 6"-150# gate valves)? Have gate valves become bellows seal gate valves (bellows seal
gate valves weigh 1 1/2 to 2 times as much as a standard gate valve)? All of these changes
result in increased weight. Did the handwheel on a gate valve get replaced by a motor operator?

10 - CONFIGURATION/ROUTING - Is the current configuration the same (If not, modification(s)
must be clearly specified/noted on isometrics) as that used for the original analysis?
 
 
CONCERN; Changes in shape of the system will affect bend leg (elbows especially), stress

analysis and calculations. Dimensional changes of only a "few inches", positive or negative, will
affect the analysis. Indicate all changes on isometrics, leave the decision to stress . Prior
discussions with stress engineer is also recommended to assess the degree of impact. Stress
may have quick decisions on small differences.

11 - INSULATION TYPE/EXTENT - Is the current insulation the same as that in the original
analysis?

CONCERN; If the insulation changes type such as from polyurethane to calcium silicate the
density changes which changes the weight , in this instance the insulation weight is tripled. If the
extent of the insulation changes from partial to full, or if the valve bodies or the flanges are
insulated when they originally were not - there will be increased weight.

12 - SYSTEM/PROCESS OPERATION - Is the current process (cycling) the same as for the
original analysis?
CONCERN; If a pair of pumps or other equipment are changed to where they are now in a
sparing design, stress for that piping will have to be re-analyzed. Information on process
operation will not be given to you. You have to have initiative and talk to process engineers.
Process operations result to determination of the number of cases for stress analysis. The
number of cycles also affects the limits or allowables of calculated stresses (ASME B31.3,
paragraph 302.3.5).

13 - BRANCH CONNECTION - Is the current design philosophy for branch connections the
same as for the original analysis?
CONCERN; If a stub-on connection changes to needing a reinforcing pad, or becomes a
reducing tee, or a weldolet, etc. , areas of congestion (layout) may be affected. if the material for
the project requires a long lead time - an MTO change could cause unnecessary delay in
fabrication/installation. MOST IMPORTANT IS THAT THE CORRECT CONSTRUCTION IS
USED! For different types of branch connections, the stress intensification factors differ .
Example, 8" sch.40 : TEE in-plane=1.63 & i out-plane=1.84; WELDOLET in-plane=2.23 & I outplane=
2.23; UNREINFORCED STUB-IN in-plane=3.96 & I out-plane=4.95.

14 - SUPPORT/ANCHOR/GUIDE LOCATION - Do the current locations match the locations
shown in the original analysis?
CONCERN; The locations must be as close to the original analysis locations as possible - the
approved system was approved only with those locations -anything much different is to be
reanalyzed. Modification(s) must be clearly specified/noted on isometrics. All support types
indicated on approved stress isometrics must be reflected on AFC isometrics for final stress signoff.
In cases of any support(s) which will not be automatically reflected by PDS (i.e., supports not
reflected because line is uninsulated and/or directly on steel), design needs to manually indicate
and add on isometrics for sign-off.

15 - SUPPORT/ANCHOR/GUIDE TYPE - Are the current types the same as those in the
original analysis?
CONCERN; During the course of a project, the supports/anchors/guides are subject to change
for one reason or another. if such a change takes place it is important that the newly-designed
support/anchor/guide meet the original requirements of the line. Did the variable spring hanger
become a constant spring hanger (becomes larger and could interfere with nearby structural steel
or piping, etc.)? Did the directional anchor become a full anchor, etc.? Modification(s) must be
clearly specified/noted on isometrics.

16 - EQUIPMENT DESIGN LIMITS - Is the current design of the related equipment the same as
used in the original analysis?
CONCERN; Sometimes a stress analysis is conducted using preliminary design criteria, when a

vendor is selected sometimes allowable loads on nozzles, shells, housings are less than that
used in the original analysis. The vendor data will have to be inspected to see if any changes will
cause the need for reanalysis of the line. Vendor drawing squad checks should passed both
design and stress to ensure that all items , i.e. dimensions, nozzle details and ratings, nozzle
loads, support details, support loads, etc., are all covered and communicated. Stress
communicates with equipment engineers and/or vendors.

17 - EQUIPMENT DESIGN/CONSTRUCTION - Is the current design/construction of the
connecting equipment changed from the design/construction used in the original analysis?
CONCERNS; During the course of a project the construction of the equipment can have changes
both internal and external that can affect the associated piping. One example is when the internal
construction of fired heaters changes - the tube design is either reconfigured or the internal tube
supports are moved. This could result in a different allowable load on the nozzle/connection than
that with which the line was originally analyzed. This is closely related in effect (if not the same)
to #16 above. Examples of special equipment design which will affect stress analysis are
refractory lining, stress relief requirement, excessive wind drifts. For horizontal equipment, i.e.,
shell and tube heat exchangers, horizontal vessels and drums, location of fixed and sliding
supports or saddles have effects on stress analysis and calculations.

18 - SHIPPING METHOD (IF MODULAR CONSTRUCTION) -
Is the current shipping method on the dressed vessel or module the same as the method of shipping used in the original analysis?

CONCERNS; When a dressed vessel or modularized piping system is shipped from the
fabrication area to the jobsite, the method of shipping may affect piping design and stress
analysis. When the first analysis is conducted, the project may not have a finalized idea of how
the vessel or module are being shipped. Some of those who make the decision may not fully
understand there are factors that will impact design and stress - so what happens is
construction/project makes a decision based on experience on the best available information and
sometimes it winds up being different than what will actually be used. When a module is shipped
some piping systems require temporary bracing and supporting to protect them from the effects
of handling - especially pump piping, polyvinylchloride (PVC) piping, fibreglas reinforced piping
(FRP), and any piping connected to equipment that is considered sensitive. In shipping, piping
and equipment can possibly sustain forces that approximate those encountered in operation.
Also, similar bracing may be required for dressed vessels and their connecting piping. Loads for
temporary vessel dress-out supports are calculated by stress.

19 - STRUCTURAL STEEL/CONCRETE REGARDING ATTACHMENT OF
SUPPORT/ANCHOR/GUIDE - Is the current design of the structural members serving as
attachment points changed so as to affect the supports used in the original analysis?

CONCERNS; The types of supports involved would be hangers, attachment to the underside of
beams, attachments to the sides of columns - were they once steel and now are concrete and
vice versa. Spring hanger drawings refer to the attachment surface - these must reflect the actual
method of attachment, whether welded, clamped, cinch-anchored, or drilled. Structural
movements (due to seismic , wind drifts) will affect stress analysis and calculations. These data
come from and supplied by the structural engineers.

20 - CLIMATIC DATA - Has the current weather data been revised to anything other than what
was used in the original analysis? Project will be the source for any information relating to
changes in data. Solar temperature, ambient temperature may have effects on stress analysis
and calculations.

21 - SEISMIC DATA - As in the climatic data, Project will be the source of information. Usually
established at the very start of the project, data coming from structural engineers.
 
22 - LINE LIST DATA - This is duplicated somewhat in items #1 through #7, #11 & #13. Anything

that causes the current design parameters to differ from the original used in the analysis must be
investigated for effects on analysis. Very important!

23 - P&ID - This works with items #1 through #7, #11, #13 & #22. Equally important!

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