Design Explained 1: High Pressure System   

The Weymouth equation is used for calculating flows and pressure in high pressure gas system. The Weymouth equation does not use Moody friction factor, but uses a pipeline efficiency factor, E. The equation is developed empirically for compressible, turbulent flow with high Reynolds number in long pipelines. The Weymouth equation is one of the oldest and most common equation (as compared to the Panhandle A & B equations) in high pressure gas calculations. The Weymouth equation is considered to be more conservative than the Panhandle A & B equations.

The Weymouth equation in English units is presented as follows (with elevation effects ignored):

It is worth noting that the Weymouth's formula has expressed Moody friction factor as a function of the pipe diameter, i.e. f = 0.008/d1/3.

For NFPA / IFGC formula, refer to NFPA 54 or International Fuel Gas Code.

Design Explained 2: Low Pressure System  

For pressure less than 10 kPa (1.5 psi), the following equation presented in SI units from NFPA / ASHRAE is used to determine gas flow capacity. AGA's International Fuel Gas Code (IFGC) uses the same equation.

 Q = 0.0001d 2.623 [dP/(C*L)]0.541

where  Q = flow rate at 15 oC and 101 kPa (l/s)

                 d = inside diameter of pipe (mm)

              dP = pressure drop (Pa)

               C = factor for viscosity, density and temperature

               L = pipe length (m)

Comparisons: Weymouth vs NFPA / IFGC for Lower End of High Pressure System 

Given:  

Inlet pressure = 5 psi (gauge), Pressure drop = 3.5 psi, Flowrate = 84656 cfh, Gas = Natural Gas, Specific gravity = 0.6, Pipeline temperature = 60 oF, Pipe = smooth wall.

Gas Sizing Example 1: High Pressure System  

Calculate the outlet (downstream) pressure in a 17.5 inch (444.5 mm) natural gas pipeline, 20 miles (32.2 km) long. The gas flow rate is 6,250,000 cfh (176,875 cmh) at a flowing temperature of 70 oF (21.1 oC). The inlet pressure (absolute) is 1014.7 psia (6996.2 kPa) and the gas gravity is 0.6. High Density Polyethylene (HDPE) pipe is used, Assume the pipeline efficiency is 0.98, and compressibility factor is 0.85. Ignore elevation effects.

The following results are computed by pocketGAS program:

Note: Standard atmospheric conditions used in pocketGAS program = 520 oR (15 oC) & 14.7 psi (101.3kPa). 

Gas Sizing Example 2: Low Pressure System  

A restaurant serving about 300 persons requires a total gas demand of 1000 cfh (7.8611 l/s) for its cooking equipment. The total pipe length measured from the gas meter to the restaurant kitchen is 60 ft (18.29 m). Natural gas with 0.60 specific gravity is used. The pressure drop from the gas meter to the kitchen shall not exceed 0.3 in. w.g. (74.6 Pa). Determine the required pipe size.

The following results are computed by pocketGAS program:

Natural Gas:

Medium gas pressure system with pressure available at point D = 60 psi. The 1st regulator is set to deliver 5 psi at the outlet of the regulator. The upstream piping system shall deliver pressure no less than 20 psi at point A to the 1st regulator. Size the gas pipe line from D to A.

Information available:

Building 1 estimated gas demand = 20,000 cfh

Building 2 estimated gas demand = 20,000 cfh

Building 3 estimated gas demand = 30,000 cfh

Pipe length D-C (including fittings) = 250 ft

Pipe length C-B (including fittings) = 200 ft

PIpe length B-A (including fittings) = 200 ft

Working illustrations (by no means this is the only approach):

Allowable pressure drop from D to A = 60-20 = 40 psi.

Total pipe length from D to A = 650 ft.

Allocate pressure losses equally from D to A. Therefore, pressure loss = 40/650 = 0.0615 psi per ft.

(Note: you can distribute the pressure losses any way you wish as long as the sum of the total pressure drop does not exceed the allowable pressure drop.)