What is the standard velocity of gas in a transmission pipeline?

Answer 1

BigInch (Petroleum)

6 Jul 07 14:35

I wasn't happy with my answer above, so I decided to dig it out of the Saudi ARAMCO specs. (L032) Here's what they use.

5 Exception to the maximum velocities are proprietary piping (e.g. metering skid, surge relief skid, etc.) or piping requiring flow balance in branch segments (e.g., firewater spray/sprinkler systems). Where velocities are not otherwise limited by Table 1, the maximum fluid velocity in carbon steel piping shall be limited to the following:

5.1.1 Single-Phase Gas lines

For in-plant piping, except during a relief and flare flow, the maximum velocity in gas lines shall be limited to 18.3 m/s. In-plant noise may be a problem when velocities in gas lines exceed this limit. Higher velocities are acceptable when the piping layout configuration is relatively simple and has a minimum number of fittings and valves subject to review and approval of the Engineering Specialist in the Consulting Services Department.

For cross-country pipelines, when noise is not a concern, the maximum gas velocity is an economic balance between acceptable pressure drops, the desired gas flow rates and other factors.

Flow velocity in gas lines shall not be less than 4.6 m/s to minimize accumulation of water at the bottom of the pipe.

5.1.2 Liquid lines

Flow velocity in single-phase liquid lines for services other than shown in Table 1 shall be limited to 4.6 m/s.

Flow velocity shall not be less than 1 m/s to minimize deposition of solids and accumulation of water at the bottom of the pipe.

Higher flow velocity may be used in special cases or in intermittent services subject to review and approval by the Engineering Specialist in the Consulting Services Department.

5.1.3 Gas/Liquid two-phase lines

Except for liquid relief and blowdown lines, flow velocities in flowlines and other lines transporting gas and liquid in two-phase flow shall not exceed the fluid erosional velocity as determined by equation (1):

(1)

where:

Ve = fluid erosional velocity, feet/second

dm = density of the gas & liquid mixture at operating pressure and temperature, lbs/ft³

dm =

where:

P = operating pressure, psia

Sl = liquid specific gravity at standard conditions (water=1; use average gravity for hydrocarbon-water mixtures)

R = gas/liquid ratio cu-ft/barrel at standard conditions

T = operating temperature, OR

Sg = gas specific gravity at standard conditions

(air = 1)

Z = gas compressibility factor, dimensionless

C = (empirical constant)

C = 100 for continuous service

C = 125 for non-continuous service

(for solid-free fluids where corrosion is not anticipated or when corrosion is controlled by inhibition or by employing corrosion resistant alloys, values of "C" up to 150 to 200 may be used for continuous service. When "C" values higher than 100 for continuous service are used, periodic surveys to assess pipe wall thickness should be considered).

Once the erosional velocity is known, the minimum cross-sectional area, A, required to avoid fluid erosion is determined from equation (2):

A = (2)

Where:

A = minimum pipe cross-sectional flow area required, square inch per 1000 barrels liquid per day.

The minimum velocity in two-phase lines should be about 10 feet per second to minimize slugging of separation equipment. This is particularly important in long lines with elevation changes.

5.1.4 Steam lines

For insulated steam lines, the maximum velocity for continuous service shall be as follows:

Low Pressure Steam, 50# to 150# 175 ft/sec

Medium Pressure Steam, 150# to 400# 130 ft/sec

High Pressure Steam, 400# to 600# 100 ft/sec

For vent steam, the maximum velocity is limited to 200 ft/sec.

5.2 The maximum allowable fluid velocity in 90-10 CuNi piping varies according to the size of the line as shown in Table 2.

5.3 For sizing of firewater systems, the maximum velocity of the water, based on the nominal capacity of the outlets (hydrants and monitors), shall not exceed two times the maximum velocity listed in Table 1 for the material of the pipe