Saturday, August 11, 2012

Piping Design - Material Specification

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Basis of Specification

The following documents shall form the basis of project piping specifications: 

• Basic piping specifications from process department or process licensor. 

• Service summary indicating service against basic material of construction (MOC) for each service. This is available either from process discipline or process licensor. 

• For utility piping, the project piping specifications for these services may be based on in-house experience. However, the client’s project manager shall be consulted in such cases before proceeding.


Design Criteria

1. Design Pressure

The design pressure of each component in a piping system shall be the severe condition of the following: 

a. 1.1 x maximum operating pressure as given in line list or service summary. 

b. Design pressure as given in the line list or the service summary by the Process Licensor. 

c. Design pressure of equipment to which it is connected. 

d. Set pressure of a pressure relieving device which protects the system.

Shut-off discharge pressure of a centrifugal pump, not protected by a pressure relieving device. If the shut-off discharge pressure is unknown, it may be determined by the largest of the following: 

i. 1.2 times the differential pressure at nominal flow plus the maximum pump suction pressure. 

ii. 1.1 times the pump’s discharge pressure at normal flow. Full vacuum for a system operating below atmospheric pressure.

2. Minimum Wall Thickness

a. The required pipe wall thickness shall be determined in accordance with the ASME B 31.3 using the design pressure and temperature mentioned above. The calculated wall thickness shall include mechanical allowance, including manufacturing minus tolerance and weld joint efficiency factor plus the corrosion allowance.

Unless otherwise specified, the minimum corrosion allowance shall be: 

Carbon Steel Alloy Steel: 1.5 mm 

Ferritic Steel: 1.5.mm 

Austenitic Stainless Steel for general purpose: 0 mm 

The pipe wall thickness determined according to the above procedure shall be checked for ambient and mechanical influences and other loadings described in the ASME B 31.3 in addition to the process pressure temperature requirements.

The following minimum pipe wall thickness shall be used for carbon and low alloy steels of process and utility piping including the items mentioned above.

NB

Welded Joint

Threaded Joint

112“ and smaller

Sch 40

Sch 80

2" to 6"

Sch 40

8" to 12"

Sch 40

14" and over

Sch 40

3. Design Temperature


Design temperature as given in the line list for each pipeline. Design temperature as given for each service in the service summary by Process Discipline or Process Licensor.

When maximum operating temperature (T) is given in line list or service summary, the design temperature (T) shall be as follows :

T = T 0+ (+20) 0 C, When T0 is between 0 to 200 0 C

T = T 0+ (+10 to + 20 ) 0 C , when T0 is over 200 0 C

T = T 0+ (-5) 0C, when T 0 is 00 C and under

Where = T = Design Temperature

T 0 = Maximum Operating Temperature

The design temperature of a piping system shall be the design temperature of connected equipment.

For un-insulated piping, the design temperature may be determined in accordance with ASME B 31.3.

The reducing co-efficient for piping components not specific in the ASME B 31.3 shall be 95% for fluid temperatures over 370 C .

4. Fittings

Long radius (R= 1.5 D) but welding elbows shall be used wherever possible. Unless otherwise specified, flanged elbows shall not be used.

Pipe bends may be used in place of elbows, which shall not be used. Pipe bends may be used in place of elbows, but the minimum-bending radius shall be 5 times the nominal pipe size.

Miter bends may be used within the limitations specified in the ASME B 31.3.



Generally for utility services (except steam) miter bends shall be used for pipelines 8" NB and above.

Branch connections shall be preferably made by fittings such as tees, half couplings or weldolets. If the ranch connections are made by welding the branch pipe directly to the run pipe, the required reinforcement shall be designed in accordance with the ASME B 31.3.

Fittings of 2 “ NB and larger shall be the butt weld type and fittings of 1 12 “ NB and smaller, socket weld or threaded type.

As far as possible, use of threaded fittings shall be avoided, except in the case of galvanised piping for instrument air and drinking water services.

5. Flanges

The number of flanges in piping systems shall be kept to a minimum and should be installed only to facilitate maintenance and inspection and where construction or process conditions dictate. For instance:
  • Where pipelines are connected to flanged equipment and valves. 
  • Where frequent dismantling of piping is required. 
  • Where clearance for dismantling equipment such as compressors and reactor heads is required. 
  • Where steel piping is connected to non-metallic or non-ferrous piping.

 6. Flange Types

6.1 Slip-on Flanges

Slip-on flanges may be used where the following requirements are met :

i. Carbon steel piping

ii. Pipes handling non-toxic fluids

iii. Pressure - temperature conditions are within the ANSI 300 LB rating.

iv. Design temperature exceeds minus 20 0C

Slip-on flanges of austenitic stainless steel may be used within the limitations of item ii. through iv. above, if justified from the cost point of view.

6.2 Welding Neck Flanges

Welding neck flanges shall be used in all instances where slip-on flanges, socket weld flanges and screwed flanges are not permitted.

6.3 Socket Weld Flanges

For 112“ NB or smaller, socket weld flanges may be used within the limitations specified in paragraph 4.3.4 but S.W flanges shall not be used for piping under IBR purview.

6.4 Flanges Facing

Flat face flanges shall be used for connecting flanges to flat face cast iron or bronze piping components and equipment. In this case, gaskets shall cover the whole flange face.

Raised face flanges shall in general be used:

i. For flanges of 600 LB or lower rating in process services.

ii. For flanges of 1500 LB or lower rating in utility services.

iii. Regardless of the above limitations a) and b) for design temperatures not exceeding 4500C.

Ring joint type flanges shall be used for flanges of 1500 LB rating or higher, or for design temperatures exceeding 450 0C. The flanges can also be used for lower ratings, for service conditions which requires higher degree of tightness. Small and large tongue and groove faced flanges shall be used for services requiring higher degree of tightness and for all lethal services.

7. Bolting

Stainless steel galvanised cadmium plated nuts and bolt studs should be specified in cases where:

1. The spillage of pipeline fluids on the bolting shall corrode them. 

2. The plant atmosphere may contain gases which may corrode the bolting.

The basic documents indicated above may not specify this clearly. However this needs to be verified during the preparation of piping specifications and finalised in consultation with clients and project manager.

8. Gaskets

a. Specific approval from the Project Manager, Client Process Licencor, Process Discipline (as the case may be) shall be obtained if material specifications are different from those specified in the basic documents. 

b. Full-face gaskets shall be specified as far as possible, where both RF and Flat Face Flanges are used in the same piping class (e.g. cast iron valves in carbon steel piping with raised face (RF) flanges).

9. Valves

The types of valves specified shall be as per either P&I Diagrams or as per basic piping specifications from process licencor or Jacobs H& G Process Discipline.

Types of valves may be revised as per the client’s requirements. However, this shall be done only after the review of suitability of the same for the intended service and proper documentation of the change required.

Ball valves may be used in place of gate or plug valves with the following limitations.

i. Operating conditions are within the permissible pressure-temperature range of seat materials.

ii. The Fire safe type is used for flammable services.

Large diameter check valves shall be provided with anti-knock device if swing check valves are specified. 
Check list for valve requisitions may be referred to if required, to specify special requirements for valves in the project piping specifications.

10. Branch Connection Chart

The standard branch connection chart shall be used to specify use of tees, etc. for a specific piping class.

When the details following, order or preference shall be kept in mind to facilitate procurement and reduce cost of materials (arranged in reducing order of preference i.e. (a) is most preferred).

a. Pipe to pipe branch. 

b. Use of half couplings 

c. Use of weldolets, sockolets. etc. 

d. Seamless tees.

However requirements of basic documents shall not be diluted in this process.

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2 comments :

  1. "Stainless steel galvanised cadmium plated nuts and bolt studs should be specified in cases where:"

    This is not possible. We don't galvanize SS bolts as zinc attacks SS. Cadmium plated is not categorized as galvanized based on ASTM A153 standard. Cadmium plated should be treated as a separate item as per ASTM B766 standard.

    Stainless steel bolts, Galvanized steel bolts and cadmium plated bolts should be treated as separate items. 

    ReplyDelete
  2. Thank you for the correction sir. I really appreciate that.

    ReplyDelete

 

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