Re: Dimensioned Isometrics

From: <adolphus.o.omodu@exxonmobil.com>
Date: Wed Apr 13 2005 - 01:34:00 EDT

Gents,
I decided to respond to this mail as it interestingly coincided with a discussion we had some 4 days ago in my department.

My experience over the years is that piping isos are normally dimensioned although they are not usually drawn to scale. This is why somebody could describe them as rough guides "normally given to pipe fitters". It is also partly because of this "not usually drawn to scale" issue that Piping General Arrangement drawings are very important alongside the Piping Isos.

The benefits that accrue when isos are dimensioned cannot be overemphasized. Someone has already spoken about Piping stress analysis which are often carried out b/w nodes (with defined length/diameter dimensions) created out of isometric drawings. There are several other design issues like Settle-Out pressures in piping downstream of compressors - where you require pipe lengths and vessel volumes to estimate pressures, instrument tie-in points/locations along plant piping as these often require specified minimum lengths from other fittings for good operation, piping supports locations where you must necessarily indicate distances b/w adjacent supports for flexibility anlaysis etc.

Just to make the point clearer may i just quote verbatim a piping/layout specification that was required from bidding contractors on a major EPC job
(>$ US 500M) that has been successfully completed in Nigeria by one of the
Oil producing majors (Not MPN).

"Produce a complete set of piping isometrics for all piping other than such small diameter minor services (generally 11/2" and below). Isometric Sheet should include a full material take - off (MTO) for each drawing c/w piping spools and fitting dimensions. Isometrics shall include hydrostatic test requiremenst, stress relief limits, field welds and insulation thickness, All welded attachment pipe supports shall be shown"

For me, i'll go for dimensioned isometrics anyday.

Hope i have not taken much of your time.

Adolphus Omodu

                       "Steve McKenzie"
                       <Mechproj@xtra.co.nz>          To:     
<a href="/group/PipingDesign/post?postID=aTDzUf_OTG99prY7psSF0G_z6wKjm8XeUI8_bkFh6ZBjyWmBa9qwCtYhw3lHkF3eDawcUvRmZt98WKTFOTBVqvlA">PipingDesign@yahoogroups.com</a>
                                                      cc:
                                                      Subject: [PipingDesign]
Dimensioned Isometrics
                       04/07/05 01:32 PM
                       Please respond to
                       PipingDesign

Gents+2
I have just had a friendly discussion with a South African colleague who maintains isos are normally dimensioned and are used as part or the formulation of the Schedule of Quantities; BOM; whatever. My experience with isos is that they are comics that we give to the pipefitters as a rough guide. Occasionally I have used dimensioned isos to display fixed and toleranced dimensions around critical plant for shop spooled piping, but in general I just use them as a sort of road map. We are not talking 3D CAD here.
My question is do you guys normally put dimensions on Isos, and if you do what sort of plant do you normally work with?

Cheers

Steve

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