1)
Please keep in mind difference between flare stack, flare tip and flare header.
For flare tips see API 521 5.7.2.1.3
The allowable flare burner exit velocity is a function of relief gas composition, flare burner design, and the gas pressure available. These parameters are interrelated. Some flare tips incorporate a flame-retention device or other means that provides a stable burning flame either attached or detached relative to the flare tip. There is evidence [60] [75] [98] [138] [140] [141] [150] that flame stability can be maintained at relatively high velocities depending on the discharge properties and the type of tip used. Experience has shown that a properly designed and applied flare burner can have an exit velocity of more than Mach 0.5, if pressure drop, noise, and other factors permit. Many pipe flares, assisted or unassisted, and air-assisted flares have been in service for many years with maximum Mach numbers of Mach 0.8 and higher.
Some flares are subject to regulations that limit exit velocity. For example, pipe flares applied in the United States as control technology for volatile organic compound (VOC) emissions can have gas exit velocity limited by 40 CFR 60.18 [59]. In some locales, the 40 CFR 60.18 requirements on exit velocities have been extended to “emergency conditions.” The regulations provide guidelines for the determination of the maximum exit velocity as a function of waste-gas characteristics and the type of flare burner employed. It is important to note that there are many flare applications that do not involve VOC control. Such flares are not usually required to meet the exit velocity requirements of the CFR.
As additional guidance you can use ExxonMobil practicies.
Elevated flare tips incorporating various steam injection nozzle configurations are normally sized for a velocity of 400 ft/s (120 m/s) at maximum flow, as limited by excessive noise and the ability of manufacturers to design tips which will ensure flame stability. This velocity is based on the inclusion of steam flow if injected internally, but the steam is not included if added through jets external to the main tip. Flared gas streams containing hydrogen can generally be flared at higher exit velocities. In these cases flare tip manufacturers should be consulted on higher velocity limitations. ...
Available pressure drop may in some cases dictate acceptance of a lower maximum velocity, but at least 250 ft/s (75 m/s) is recommended to insure good dispersion. Flare tips consisting of a simple open-ended pipe with a single pilot are subject to flame lift-off and noise problems at high velocities, and should therefore be designed for a maximum velocity of 1 60 ft/s (50 m/s)
Please pay attention, that there are other limitations caused flare gas flammability limits, heating value and H2S, O2 and inerts content, dispertion and radiation,
For flare pipes (flare header + flare stack) you can use Norsok P-001, see para. 6.3 and 6.1. Again as additional guidance you can use ExxonMobil practicies.
The maximum permissible flow velocity at any point of the PR valve discharge piping is 75% of sonic velocity, regardless of whether the PR valve discharges to atmosphere or to a closed system.
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The maximum velocity in the combined discharge header at design capacity shall not exceed 75% of sonic
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The maximum exit velocity at design capacity shall not exceed 75% of sonic to minimize potential vibration problems and avoid choked flow.
Flare header criteria have been discussed here earlier.
3) Yes, of course. Built-up backpressure can significantly increase. You shall reevaluate all increased common cases.
Edited by shvet, 24 September 2016 - 12:47 AM.