Piping Design Basis

The choice of material must account for factors like corrosion resistance, pressure, and temperature tolerance. Similarly, determining the appropriate pipe size is important to maintain fluid flow efficiency and prevent excessive pressure drops. Detailed specifications for flanges and bolts are essential to guarantee secure connections, with ratings aligned to operating conditions. Additionally, careful attention to pipe fittings, including elbows, tees, and couplings, ensures seamless integration within the piping system.

So, understanding the design basis is very important to make your piping design accurate.

Here is the design basis for piping items according to American Codes and Standards;

Piping Material and Size

Minimum corroded thickness should be calculated as specified in the applicable code.

The minimum corrosion allowance used in the determination of pipe watt thickness shall be as follows:

For carbon steel and alloy steel up to 9%Cr: 1,5 mm

For stainless steels and higher alloys: none

The wall thickness given in the piping specifications provides an allowance for corrosion, thread depth, and manufacturing tolerance.

Carbon steel pipes are Grade A or B. Grade B has higher allowable stresses, while Grade A is more ductile and preferred if bending is a concern.

Seamless quality pipe can be used for more rigorous conditions due to higher cost.

Cast iron pipe is used only for underground water and sewer service with limited design temperature.

Tubing connections are the only exception; otherwise, pipes with a diameter smaller than NPS 3/4 shall not be used.

Pipe sizes NPS 1-1/4, 2-1/2, 3-1/2, and odd numbered sizes NPS 5, 7, etc., shall not be used. Equipment fabricated with connections in these sizes must be adjusted with an adaptor.

Flanges

Raised face flanges should be used for services in Class 900 and lower classes and at temperatures 538°C and below, except as noted in the Piping Specification.

For services at more severe conditions than above, ring-type joint (RTJ) flanges shall be used.

The gasket surface finish shall be by ASME B16.5.

The stock finish normally supplied on raised face flanges will suffice for sheet or spiral wound gaskets.

General Practices of Flange construction are as follows:

(1)            NPS 2 and smaller, socket weld.

(2)            NPS 3 and larger welding neck bored to suit pipe.

(3)            Slip-on flanges may be used on a case-by-case basis where approved by engineering. Slip-on flanges are restricted to raised or flat faces, to 315°C design temperature, and Class 300 or lower. For more severe conditions, welding neck flanges shall be used.

Gaskets

Spiral Wound Gaskets

Gaskets shall be as specified in the Piping Specifications.

Gaskets containing asbestos in any form shall not be used.

Compressed sheet gaskets are limited to design temperatures 315°C and lower and to Class 300 and lower flange ratings and shall not be used in Category  Fluid Services.

At more severe process conditions and for flanged joints in the boiler feed water, steam, and condensate services, spiral-wound stainless steel gaskets shall be used for all raised face flanges, unless specified otherwise.

Gasket widths shall be limited by dimensions tabulated in ASME B16.5 Annex E.

Flat gaskets by ASME B16.21 and spiral wound or jacketed gaskets in accordance with ASME_B16.20 meet these requirements.

Where ring joint closure is specified, ring material shall be soft carbon steel for carbon steel flanges, 4-6% Cr 1/2% Mo for ferritic alloy flanges, and 18-8 Cr-Ni for austenitic alloy flanges. The ring shall be an oval cross-section.

The maximum hardness of ring joint gasket material is as per Table 1 ASME B16.20.

Bolting

Carbon steel machine bolts may be used for services at design temperatures of -29°C to 204 °C and with Class 300 or lower flange ratings.

ASTM A 193 Gr. 87 alloy steel bolts should be used at design temperatures of -29°C to 427°C for carbon and low alloy steel flanges; ASTM A193 Gr. 816 shall be used for higher temperatures up to 538°C.

For stainless steel flanges, ASTM A 193 Gr. 87 alloy steel bolts shall be used at design temperatures of -29°C to 370°C, ASTM A453 Gr.660 stainless steel bolts from 370°C to 538°C.

ASTM A193 Gr. B8 stainless steel bolts should be used at design temperature below -29°C.

Fittings

Socket welding fittings shall be used for lines NPS 2 and smaller unless otherwise specified.

Buttwelded fittings should be used for larger sizes.

Threaded fittings should be used in NPS 2 and smaller sizes whose fluid service could be designated in Category D and instrument service, and atmospheric vent or drain piping downstream of the first block valve.

REFERENCE: PIPING TRAINING COURSE BY FWI