Description

The specification selection of galvanized steel pipes for low-pressure fluid
transportation needs to be determined comprehensively based on the fluid type,
pressure, flow rate and installation environment. The following are common
selection reference standards and specification recommendations:

1. Commonly used standards

1,Chinese Standard (GB/T 3091-2015),

Suitable for welded galvanized steel pipes for conveying low-pressure fluids
such as water, air and gas.

Pressure level: Usually the design pressure is ≤1.6MPa (needs to be checked
according to wall thickness).

2,International standards (such as ASTM A53)

Classification: Type E (resistance welding), Type F (furnace welding), Type S
(seamless pipe), galvanized for use in low pressure systems.

2. Recommended general specifications

1.Nominal diameter (DN) range

Low-pressure systems usually use DN15 ~ DN150 (1/2"~6"), and the specific
selection depends on the flow requirements:

DN15-DN25 (1/2"-1"): Household water supply branch pipes and gas inlet
pipes.

DN32-DN50 (1¼"-2"): Building main pipes, small irrigation.

DN65-DN100 (2½"-4"): Fire water pipes, HVAC main pipes.

DN125-DN150 (5"-6"): Industrial low-pressure water supply and regional gas
supply.

2.Wall thickness selection

Ordinary low pressure system: Choose ordinary wall thickness (SCH40), such as
DN20 galvanized pipe with a wall thickness of 2.75mm.

Higher pressure or underground laying: Choose thickened wall thickness
(SCH80), such as DN50 wall thickness 3.5mm.

Select the wall thickness according to the actual project conditions.

Refer to GB/T 3091 wall thickness table:

Nominal diameter (DN）	Outer diameter (mm）	Normal wall thickness (mm）
15	21.3	2.75
20	26.9	2.75
25	33.7	3.25
32	42.4	3.25
40	48.3	3.5
50	60.3	3.5

3. Refine the selection by fluid type

1,Domestic water supply/fire water

Material: Hot-dip galvanized steel pipe (GB/T 3091).

Specifications: DN15-DN100, wall thickness ≥2.75mm (ensuring pressure
≥1.0MPa).

2,Gas transportation

Standard: Must comply with GB/T 3091-2015 "Galvanized welded steel pipe for
gas".

Specifications: DN15-DN50, with thicker wall thickness (e.g. DN25 wall
thickness ≥ 3.2mm).

3,HVAC Pipes

Hot water/steam: DN20-DN65, it is recommended to use seamless galvanized pipe
(GB/T 8163) for better temperature resistance.

4. Precautions for special scenarios

Underground laying

The wall thickness needs to be increased (such as SCH80) and anti-corrosion
reinforcement should be done (such as wrapping anti-corrosion tape).

Corrosive environment

Choose hot-dip galvanizing + epoxy coating for double protection, or use
stainless steel pipes instead.

Threaded connection

Pipes with a DN≤50 can be connected by threads, and pipes with a DN≥65 are
recommended to be connected by flanges or grooves.

5. Selection Process Example

Determine flow and pressure：For example, the household water supply demand
flow rate is 1.5m³/h and the pressure is 0.4MPa.

Calculate pipe diameter: According to the flow rate (1-2m/s is recommended
for water), choose DN20 (3/4").

Check wall thickness: Check GB/T 3091, DN20 ordinary wall thickness is
2.75mm, which is sufficient to bear pressure.

6. Alternatives

If you have higher requirements for hygiene or corrosion resistance, you can
consider:

Plastic lined galvanized pipe: Lined with PE/PP, suitable for direct drinking
water.

Plastic coated steel pipe: Internal and external epoxy coating, resistant to
acid and alkali corrosion.

It is recommended to finalize the specifications based on the actual needs of
the project and refer to local regulations (such as the "Design Standard for
Building Water Supply and Drainage" GB 50015).