1. Following notes are from "Handbook of Natural Gas Transmission and Processing", by S Mokhatab, W A Poe, J G Speight, pages 418 - 421, Section 11.6 - Design Considerations on sales gas pipelines, subsection 11.6.1 - Line Sizing Criteria, Elsevier, 2006.α. Most cost effective gas pipelines should have a pressure drop between 3.50 and 5.83 psi/mile. However for those pipelines (short ones) in which pressure drop is of secondary importance, the pipe could be sized on fluid velocity only.
β. In systems with CO2 as low as 1-2 %, velocity should be limited to 50 ft/s or lower, for it is difficult to inhibit CO2 corrosion at higher velocities.
γ. In most transmission pipelines recommended gas velocity is 40 - 50% of the erosional velocity.
As a rule of thump, pipe erosion begins when velocity exceeds the value of C/SQRT(ρ) in ft/s, where ρ = gas density (in lb/ft3) and C = empirical constant (in lb/s/ft2) (starting erosional velocity).
δ. C=100 in most cases. API RP 14E (1984) suggested C=100 for continuous and 125 for non continuous service. In addition C = 150 to 200 may be used for continuous, non corrosive or corrosion controlled services without solid particles present.
2. Attached "NGvel.xls" estimates starting erosional velocities, in function of operating pressure, for either the NG described by cnu879 or pure methane at 15 oC. The low erosional velocity corresponds to C=100, the high one to C=200 per API 14E. Recommended gas velocities are considered as 50% of corresponding erosional velocities, low is represented by the orange line
of the diagram, high by the red line
. Recommended max allowable velocities by Norsok Standard P-001 are also presented for comparison (green line
α. According to above, allowable max gas velocity under the conditions specified by cnu879 would be 7.4 - 14.7 m/s, versus 28.4 m/s given by Norsok Standard. Indeed, recommended velocities are quite low for high pressure pipe lines.
β. If operating pressure of the gas were 0.2 barg (which could more or less occur in a city distribution network, with no polyethylene pipes
), allowable max gas velocity would be 64 - 127 m/s, versus 60 m/s per Norsok Standard.
γ. The lower the pressure the higher the max allowable velocity. Reasonable, since density decreases with pressure (yet density * estim max allowable velocity decreases with pressure
Recommended gas velocities should report corresponding operating pressure.
δ. Norsok Standard gives much higher velocities for pressures higher than about 15 barg (approaching starting erosional velocities for C=200). For lower pressures it gives a flat value of 60 m/s.
3. Questions to promote clarification of the matter.
"Handbook of Natural Gas Transmission and Processing" gives a method to specify max allowable gas velocity, depending on conditions.
3.1 What is considered as starting erosional velocity in normal NG pipe lines, per API 14E ? Shall we use C=100, or C=200 in the formula (in lb/s/ft2)?
C=100 when there may be condensate, or corrosion? How is it that the gas contains solid particles?
3.2 A flat allowable max velocity at low operating pressures (60 m/s per Norsok standard) seems reasonable to apply in the case of NG pipe line. Can you confirm applicability and recommend flat value? The Handbook of Natural Gas does not seem to mention something similar.
3.3 Any other comment on the above or the "NGvel.xls"? 4. Note: For additional info that could be useful, refer to the links below.