Design Tips 13

Flare

• Load Summary: Tabulate loads for all scenarios. (1) Single Relief: Blocked outlet, Control valve failure, Gas Blowby, HX Tube Rupture, Reflux Failure, Excess Heat to Reboiler etc (2) Multiple Sources: Power, Cooling Water or Instrument Air Failure and for blowdown valves. See ‘Relief and Blowdown Summary’ in Sizing. API: It is not the largest mass flow but volume flow (low mol weight) that could govern sizing. Transient or declining load like blow down should not decide flare size. Wasted money. You don't have to route LP BDVs to LP header; can route to HP header too if capacity is available and vice versa for HP BDVs. Similarly, a single LP RV contingency like Gas Blowby can be routed to HP Flare

• Liquid Full Vessel Under Fire: Can burst under thermal expansion. C3 and light ends can lead to BLEVE. It will take long to vapor depressurize due to large vapor generation. Drain (or liquid blowdown) inventory in a fire zone with Remote Operated Valve (ROV) to unit blowdown drum or Flare KOD. Add an electrical heater to avoid LT issues and switch it on during liquid removal

• Purge Gas: Air ingress via (1) Open pipe LP tip (2) Vacuum pulled when a hot discharge cools or condenses or (3) Lighter than air purge gas fills stack; buoyancy effect. Purging maintains a positive pressure in flare header + stack

• Purge Gas - Tip: Wind eddies downwash air thru tip and create an explosive mixture in stack. API 521 purge rate maintains min tip velocity to reduce air ingress via tip. Valid for open pipe without seals. Tip seals reduce purge rate. Fluidic: 0.01-0.02 m/s; mol seal: 0.002-0.02 m/s; no seal 0.1-1 m/s

• Purge Gas: After a hot release, as gas cools, it pulls in vacuum. May condense if steam or heavies. See https://cdn.digitalrefining.com/data/articles/file/757959842.pdf. Hot releases are common in Refineries and Petchem plants. Sweep gas based on header volume, density of hot and cooled gas. As header metal absorbs heat, sweep gas demand is usually less. P-T compensated purge can reduce sweep gas demand. Water Seal Drum avoids air ingress and flame flash back to upstream as vacuum in header pulls up water in WSD maintaining a barrier to air via tips

• Purge Gas: If purge gas is lighter than air (mol wt < 29), then as it fills the stack, it creates buoyancy (-ive pressure) at stack base due to density difference. Can ‘theoretically’ suck in air. Based on stack height, calculate the buoyancy effect = H*(ρ-air minus ρ-purge). Maintain a purge rate so that its friction loss thru stack height exceeds buoyancy to have a +ive pressure at stack base

• Purge Gas: Sonic tips have small holes thru which air can’t easily get in; not given any seal. Operators increase purge rate to see a visible flame to avoid burn back - difficult to convince them. Rate can be reduced to see a flame at night and not during day

• Purge Gas: For a vent with a flame arrestor and fed by several sources, purge gas is not required. FA prone to blockage. May lead to high back pressure. Provide access for periodical cleaning. Review/ delete or provide a bypass with RD/ glycol seal are other options

• Purge Gas: RVs pass or leak and add to purge flow. RD upstream of RV, avoids RV passing. If leak is excessive as observed by tail pipe sweating/ external icing and noise, replace the RV. In some service with tail pipe hydrate, heat trace the tail pipe

• RV Leaks: See API 527 RV Seat Tightness. Leak rate = 0.6 to 1.5 SCF/day. See EPA http://www.epa.gov/gasstar/documents/testandrepairpressuresafetyvalves.pdf. “Small leaks will grow larger as the leak point erodes”. Suggests 95 (15 ~ 300) SCFH. EPA mentions a Dehydrator RV. It usually operates at 1,000 psig. Sonic flow. W α Pin*0.09*(0.01 ~ 0.3) SCFH/psia. Use this magic number or RV sizing formula for a leak size of 0.4 mm, wiredraw cut on RV seat or seat not sitting properly on the nozzle, to estimate leaks. https://www.ipieca.org/resources/energy-efficiency-solutions/units-and-plants-practices/passing-valves-leakage/#reference-7 gives worst leaking RVs (4 out of 87) 8,800-22,000 SCFH. Way high. API 527 provides higher rates for higher pressure

• Acid Gas Flare Header: Sulfur and Iron Sulphide deposits formed as a result of SO2 + H2S slow Claus reactions can block up to 80% header flow area. Avoid routing SO2 streams to Acid Gas Header; add RDs upstream of RVs in SO2 sources; spec blind SO2 maintenance vents

• Acid Gas to LP Flare tip. If Acid Gas is routed to LP Flare tip upon incinerator trip, sudden release of acid gas to LP flare may result in LP Flare flame out - low heating value or transient pressure fluctuation or unburnt H2S release and fatality. Check with LP Flare supplier

• H2S headers: Require periodical isolation, washing and cleaning

• Ammonia: Segregate ammonia streams from H2S and CO2 streams to avoid solid blockage due to NH4S or (NH4)2CO3

• High CO2 Flare Header: Simulations show that dry ice (solid CO2) plugs can form and block flare piping when high CO2 streams are let down. But tests done by ExxonMobil indicate no significant blocking. Google <Successful Demonstration of Relieving CO2-Solid-Forming Streams through a Pressure Relief System>; <Dry ice formation in CO2 Flare headers>

• Water Seal Drum (WSD): Vertical drums preferred; can be part of stack. Caution: Drum corrosion issues if part of stack. H/D of 2 or less as long as min height between inlet to outlet; inlet to LAHH and various levels. Horizontal drum causes pulsation and noise. Good to have continuous ‘visible’ overflow of water to check seal from a distance. Caution (1) Water may evaporate and seal lost (2) Light hydrocarbon can get discharged to OWS with water forming vapor cloud. Steam coil in WSD could keep water warm and evaporate light ends to flare. A funnel at seal top can continuously skim HC. Water seal is not possible or required in HP flares and in cold countries - danger of water freezing. Some may heat water with steam but you end up with - what if steam fails scenario. WSD water outlet is sized for a low velocity to help HC vapor separation in WSD

• Vent to Safe Location: Perform dispersion calculations for max flow to confirm (a) Electrical area classification (b) LEL at grade (c) Operators won’t get a blast on their face when they walk around the plant. 3-5 m above an area an operator is likely to be present. In offshore: mid-point of flare boom 

• Vent: (1) HC: Atmospheric pressure gases viz compressor secondary seal - to Hazardous and non-hazardous open drain caissons (2) DBB bleed as in fuel gas - Local (3) H2S streams: DBB bleed routed away via a hose rather than a fixed vent system (4) Instrument/ plant air/ N2 vessel RV outlets - Local. Check for noise at working areas (5) N2 blanket gas outlet, no-HC present - local (5) Hot streams, steam, hot water drum PCV outlet - local (6) Toxic gases viz analyser vent etc - to LP Flare (7) Flammable gases, HC tank - to AP Flare or dedicated vent (8) Small chemical injection tank - local (9) Water Degassing - to LP Flare as gas blowby from upstream is possible

• Dispersion: Google - "Atmospheric Dispersion Modelling" and “Stability Classes”. Read API Manual on Disposal of Refinery Wastes. Calculations done to find Ground Level Concentration (GLC) of a component ejected via stack. Still air allows stack plume to rise adding to stack height; high winds flatten plume rise but dilute and reduce GLC. Based on atmospheric conditions + day/ night + solar input (light to moderate), 6 stability classes A thru F, each with a set of wind speeds are taken. Class A (1-2 m/s); Class C (3 to 25 m/s), Class F (2-3 m/s). Range: 1-25 m/s. Stability = Ability of atmosphere to resist/ enhance vertical motion or turbulence. Class A is unstable and F is very stable. A set of equations used for each class decides downwind dispersion - in the crosswind direction (sig Y) and the vertical direction (sig Z). For each stability class wind speeds calculate GLC max. Example: Class A: Max GLC = 2% LEL at 1.5 m/s 500m downwind. Class C: GLC = 10%LEL at 20 m/s 100 m. Summation gives the highest GLC

• Preliminary Estimate: 3 pilots. Each 1.5 Nm3/h fuel gas. Flame front generator: 4 Nm3/h gas + 40 Nm3/h instrument air. Water seal water flow: 5 m3/h. Purge Gas flow: See ‘Flare System’ in Training

• Flare Pilot Ignition: Provide 2 different systems. See API standard 537 on redundant/ back up ignition source. “Spark ignition is often preferred as it is easily automated. A manual compressed air flame front generator is commonly installed as backup system because of its ultimate reliability and serviceability”