3.1 Piping specifications

Piping specs

What are these? Whenever a plant is designed to run a certain process, piping to carry fluid will be involved. This spec will be developed by the piping engineer for any user to reference and select the specified pipe material, thickness, rating and many for the fluid service in future. The content of piping specifications varies through out different contractors, so dont worry if one item is missed out, however the critical ones shall be present.

If there are no company standard piping specs available and one needs to be specifically developed for the project, it should preferably be developed from an existing database for similar services. If otherwise consult the experienced materials , reliability, maintenance and operations specialists, the more the merrier to eventually form the most probable opinion based on experience to mete it on trueform.

This is a vital document for the following activities during projects, so have it ready during the early periods of the design phase in a project.
P&ID & Detailed engineering - Specifying pipes, fittings and indication of line numbers
Equipment list - Specifying line code number, valve code numbers
Proj QC - Verification on conformance to codes
Maintenance - Specifying pipe and fitting replacement
and many more.

What information to look for or include in the piping specs

These are what you would normally find on the piping specification table from top to bottom

1. Fluid service

• Types of process fluid this specification can handle, the number of fluids are not limited in each pipe spec, to reduce the pages! 

2. Flange pressure rating and type

• The ratings are derived usually from ANSI or DIN/ISO data tables, select one that could meet the pressure and temperature specified on the PFD or design specifications.

3. Design temperature/pressure of the pipeline

• This may not be necessary to include as it may confuse designers with equipment design temp/press. This item needs to be verified with thickness calculation and process engineers to minimize change logs.

4. Corrosion allowance

• This is derived from corrosion rates for similar pipe material and process fluid from existing data. Corrosion allowance(mm) = Corr. Rates(mm/yr) X Plant Design Life(y)

5. Pipe material

• This is usually specified in its ASTM name, so procurement could use them to source accurately, suppliers will understand in an instant. Eg. CS ASTM SA106, Gr. B SMLS refers to Astm code 106 grade B seamless carbon steel pipe. 
• The pipes comes in std length of 6m. Other lengths are also available but uncommon and subjected to increased lead time. 

6. Pipe size and thickness

• This will come in a table format, as pipe sizes and thickness will vary to meet design pressure requirements. It is developed from the calculations in ASME B31.3 process piping codes: t=[PD/2(SE+YP) +tc+tf ] x [100/(100-tf)]

t= Thickness of pipe, in

P=Design pressure, psi

D= OD of pipe, in

S=Yield strength of pipe at design temperature, psi (Based on material)

E=Weld efficiency, dimensionless (from 0.85 to 1 depending on joint method)

Y= Derating factor, dimensionless (usually 0.4 for steel material below 900F)

tc = Corrosion allowance, in

th = Thread depth, in (If threaded connections are used)

tf = tolerance factor, dimensionless (Usually 12.5% depending on pipe maker)

• If designing for pipeline projects, you may refer to B31.4 for less stringent calculation, for marine systems refer to ABS/LR/DNV or the class design codes, as calculations may differ. However the principles remains the same. 

7. Fitting size and rating

• Similar to pipe calculations but different calculations, will be specified in a specific post in future.

8. Flange size, spec and rating

• Specified type of flange and its rating to meet required pressure requirement at its design temperature. 
• Type of flange face and joint to equipment/pipes, will be specified in a specific post in future. 

9. Valve size, specs and rating

• There are different valve material, types and configuration for each range of the sizes.  Select the suitable type for the service, ie. On/off, throttling, low pressure loss, quick opening, etc. Explain on later posts.

10. Brand connection types & its fittings.

• Different size pipes(run) may use different joints types which offers similar strength at a lower cost. Reason being a socket type joint on. 1" pipe may be strong, but not on a 4" pipe. The recommended connections are specified in the last section
• Tees are used for same size connections.
• Reducing tees for a reduced branch
• O-lets, weld type, socket type, thread type allows to stub on a pipe to the main with better strength.
• Stub in, most economical way of connecting branch pipes.
• stub out for one size smaller branch

Sample piping specification for reference to the above explanations.
Source: ligo.org