Design Tips 5

Storage Tanks

• Flame Arrestors: Flame arrestors provide restricted path + large metal mass to cool any propagating flame. In CDD, ODD or Diesel Storage or Compressor Seal Gas. Flame propagation possible only in “open” systems where air infiltrates into a HC tank due to inbreathing. Flame flash back likely when flame speed is greater than gas exit velocity

• Flame Arrestors: API 2000. FA, not necessary with PV venting to atmosphere as flame speeds are less than vapor velocities across PV seats. Note: Flash back possible during low unstable outflow or if there is an external flammable vapor cloud that ignites with sufficient overpressure to lift the vacuum pallet

• Flame Arrestors: No FA for PSV outlets vented. PSVs do leak and may have a lingering flame during thunderstorm. A flame snuffing connection, fired by CO2 cylinders or steam should do

• Flame Arrester: Atmospheric Tank. DP, say 2 kPag (200 mm WC). PSV/ Blow-off hatch at 200 mm WC. Vent ΔP = 100 mm for loading/unloading; 150 mm for max flow - loading/unloading + thermal. Vent ΔP: (1) friction (2) FA, if any and (3) 1 velocity head exit loss

Heat Exchangers

• Hot fluid flows down. Cold fluid rises from bottom. Baffle cut orientation to allow vapor separation and avoid sand/mud accumulation

• Design Duty: May not match flow and properties in data sheets. (1) Feedstock changes, especially in refineries. Light Vs Heavy crudes (2) ± 10-15% error in simulation co-relations (3) Overdesign factor

• Tube Rupture: Relief and protection depend on fluids - gas or liquid; whether HP is on shell or tube side. For HP gas on tube and LP liquid on shell, LP pressure will spike up quickly, unable to push + accelerate liquid, like pushing a herd of elephants. As good as a blocked outlet. Dynamic Simulation helps to find surge pressure. Provide a pair of RDs in series on shell. Fluid side RD is for tube rupture; Flare side RD avoids varying flare header backpressure on first RD, giving it a constant backpressure. RDs act fast. Typical opening times - Rupture pin: 2milli seconds; Rupture disk: 5ms; RV 25ms. Notes: (1) RDs can prematurely fail. RD on Cooling Water (CM) failed due to pressure surge when CW pump started, sending CW to flare. Second RD on reverse rupture on high flare header pressure. See Safety Alerts (2) LP fluid in tubes: 2 sets of RDs on either end (3) VHP: 2 sets of RDs on either end of shell. For VHP go for PCHEs. 0.4-4 mm channel hole. Narrow channels create high ΔP and reduce relief rate. Smaller PSV/header

• Tube Rupture: Add a check valve at LP inlet to stop backflow to other users. SDV at LP outlet to close on PAHH. If HP gas JT effect can form ice/ hydrate blockage on LP, a SDV at HP inlet to close on LP PAHH. Since gas leaks are more likely, HC detectors in CW Tank/ Tower

• Sizing: Many vendor designs go by software output without operating and maintenance considerations. Long tube bundles save first cost - but tough to insert a tube aligned to so many holes in many baffles

• Sizing: In HX sizing one has to start with an assumed configuration - tube diameter, length, number of tubes per pass etc. Right assumptions based on experience results in the right velocity/ heat transfer coefficient and lower area. Otherwise, lower velocity results in lower coefficient and higher area. Get thermal design done or verified by experienced personnel

• Kettle Reboiler: Go for square pitch for easy vapor release

• Design Temperature: Hot side DT for cold side? Little impact on stress values to 200°C and -5% for every 55°C rise. If hot fluid is < 400°C, less than hydrotest margin. Cold side cold DT, temperature reached on blowdown or venting auto-refrigeration or during start-up has a real impact. See Safety Alert. During start-up, cold fluid may go below MDMT - brittle rupture, release and explosion. Spot at design stage

• Seawater Coolers: Usual to keep HP fluid on tube and seawater on shell. May lead to corrosion of CS tube sheets behind titanium cladding on shell side. HC release/ explosion. Go for seawater on tube side

• Sparing: Wellfluids with sand and muck foul and plug HX. Keep a standby or a clean bundle in warehouse.

• Cleaning: Cleaning in sonic baths help full recovery and save fired fuel. Plan early to limit bundle size - length, diameter and layout to suit cleaning facilities

Fired Equipment

• Read first ‘Fired Heaters - Introduction’, ‘Fired Heaters - Design’ and ‘Fired Heaters - Operations’ in Training to know all about heaters, their components, design and operational issues

• Sizing: Many vendor designs go by software output without operating and maintenance considerations. Mismatched firebox to burner results in flame lick, hot spots, coking, tube rupture and reduced run lengths. Get thermal design done or verified by experienced personnel

• Design Duty: May not match flow and properties in data sheets. Fat over fat. (1) Feedstock changes, especially in refineries. Light Vs Heavy crudes (2) 15-20% margin to factor preheat HX fouling (3) ± 10-15% error in simulation co-relations (4) Instead of hot upstream feed, provision for cold feed from storage and (5) Overdesign factor

• ΔP Vaporizing Service: Get EFV Vs Enthalpy from system designer - Equilibrium Flash Vaporization or % of Vaporization against Pressure and Temperature, Superimposed with Enthalpy charts

• Absorbed Duty: (1) Thermocouples, flow meters are not properly calibrated. Convection exit temperature may read less than inlet temperature (2) Fuel flow or Xs Air reading errors. Absorbed duty may appear > delivered by fuel (3) Plant dynamics: Can’t get an instant snapshot of all variables. Process flow changes continuously. Fuel / air flow response is time-delayed to catch up with process changes (4) Errors in Orsat. See ‘Fired Heaters - Performance Orsat Checker’ in Training. Fuel C/H ratio based on O2 and CO2 may fall outside of likely fuel - methane (0.3) to residue (H/C=0.11) (5) Tramp Air

• Material of Construction (MOC): Decided by owner/operator based on range of feedstock or by a metallurgist. 4 groups (1) Carbon Steel (2) Carbon - Moly (3) Chrome - Moly and (4) Chrome - Nickel (SS). Cr increases resistance to oxidation and corrosion by H2S, S compounds. It has low improvement on rupture strength. Mo improves creep resistance. Ni does not contribute to oxidation or corrosion or creep. Without Ni, high Cr alloys become brittle. Air/ steam require CS below 425°C. C-Mo to 600-650°C and SS for higher temperature. HC streams allow CS if sweet (no sulphur); 5 Cr if sour as in crude topping; 9 Cr in vacuum/ visbreaker/ coking services with heavy ends and higher temperature. If naphthenic acid, SS 361L. 25Cr-20Ni in high temperature services as in ethylene cracker or steam:HC reforming