Relief valve outlet line sizing

The outlet line from the relief valve is designed based on the Mach number. Generally, the Mach number in between 0.5 to 0.75 is maintained. However, the outlet line size shall not be smaller than the outlet flange of the relief valve. 

Maximum allowable velocity is calculated, based on an empirical relationship of erosional velocity [11]. 

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 from 183 to 244 may be used for continuous flow, and values up to 305 can be used for intermittent service. 

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For conventional relief valves venting to a closed system, the line is sized to keep the back pressure within 10% of the relief valve set pressure. This is subject to a maximum velocity of 0.75 Mach. API 521 recommends a velocity up to 1 Mach. However, proper analysis of piping integrity is required for a velocity close to Mach 1 such high velocity should not be used for the design of a new system. Outlet line sizing for balanced-bellows and pilot-operated relief valves is based on a maximum velocity of 0.75 Mach. 

When the relieving scenarios are defined, assume line sizes, and calculate pressure drop from the vent tip back to each relief valve to assure that the back-pressure is less than or equal to allowable for each scenario. The velocities in the relief piping should be limited to 500 ft/sec, on the high pressure system and 200 ft/sec on the low pressure system. Avoid sonic flow in the relief header because small calculation errors can lead to large pressure drop errors. Velocity at the vent or flare outlet should be between 500 ft/sec and MACH 1 to ensure good dispersion. Sonic velocity is acceptable at the vent tip and may be chosen to impose back-pressure on (he vent scrubber. 

Safety features for the tank include a pressure relief-valve system on the tank roof, to be opened when draining or adding to the tank contents. A bursting disc system also allows for pressure release in the event of pressure build-up. Amanhole providesaccessto the tank for internal maintenance. The product inlet line is standard nominal pipe size 1.5, schedule number 40S. This inlet is sized for the maximum production flowrates. The product outlet line is of standard nominal pipe size 6, schedule number 120. This line is sized such that a standard-size road tanker may be filled in 15 minutes. 

Inlet line Nominal pipe size 1.5. schedule number 40S Outlet line Nominal pipe size 6, schedule number 120 Manhole, bursting disc, pressure relief valve on roof No insulation... 

For relief valves venting to the atmosphere, the outlet line is sized for a maximum velocity of 0.75 Mach at atmospheric pressure. 

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