Design Tips 15

Flare

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

• PSV Leaks: See API 527 PSV 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 PSV. 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 PSV sizing formula for a leak size of 0.4 mm, wiredraw cut on PSV 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 PSVs (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 PSVs 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. 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 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. 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 are 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” 

Drain

• Closed Drain Drum (CDD): Sized for liquid inventory of largest vessel at LALL. One vessel drained at a time

• CDD Pressure: Normally at atmospheric/LP Flare pressure. Size vapor outlet to cater to gas blowby from source vessels, >300# to ensure CDD pressure is below DP. As pump seals are drained to CDD, there should not be any backpressure in CDD; otherwise, pumps experience frequent shaft seal leaks as back flow from drain can contaminate shaft seal chamber. Operators may close the drain valves, hindering a small leak via seal faces for cooling and flushing purposes. Seals may fail prematurely. Back pressure can be due to HP sources draining under an on/off valve

• Heating Coil: Design Temperature and MOC shouldn’t be based on heating coil warming up fluids to keep the contents above MDMT to select CS. Consider LT issues based on heating coil failure or fail to heat in time. Cold fluids could freeze steam in coil or electrical heating could ‘fail on demand’. See Safety Alerts (1) heavy oil fouling prevented heating in cold liquid heater in C2= plant Flare KOD, that had CS based on heating. CS flare header failed and released in plant and fire (2) Low temp caused icing on LT rod attached to a ball in a float transmitter, prevented start of CDD Pumps resulting in overflow in CDD and liquids carry over to flare, burning rain + ground fire

• CDD: Combine with LP Flare KOD. Routinely done. Sizing based on flare and drain demands separately

• Isolation: “LO” valves at inlet and gas outlet to reduce purging/ turnaround time during start-up

• CDD: Add a low sand weir near inlet, to collect debris and sand. Locate manhole upstream of sand weir to scoop collected sand and debris. [Note: Mercury laden gas: P&ID Caution Box near all vessels including CDD: “Mercury laden sludge. Wear PP while scooping out”]

• Drain Lines: It is good to run fully rated drain headers for high pressure sources, all the way to the CDD where one expects blockage from waxy crude and sand, hydrate and icing

• Drain Line: Do NOT provide a check valve. Drain line handles muck, mill scale, sand, sludge, waxy stuff. It is not in continuous service. Check valve can get stuck

• Caisson: Avoid caissons as an open drain oil-water separator. Risk of sweater pollution. Seawater from caisson, routed to process systems can corrode export pipelines