Design Tips 4

Storage Tanks

• Avoid Quick Opening Doors. Have resulted in accidents

• Condensate Filter: In wellfluid service with muck and mud, it is likely to be plugged in no time. Check with client their operating experience

• H2S Service: P&ID Caution Box: "H2S forms pyrophoric compounds - catalysts, coke, metal parts, mill scales, adsorbents, filter elements, Butadiene compounds, polymers and metal packing that are prone to spontaneous fire on exposure to air. Keep metallic parts wet before, during and after opening vessel, piping, and filter etc. Follow company procedures to avoid pyrophoric fire"

• P&ID Caution Box: “Hydrocarbon or chemicals adsorbed in desiccant beds or trapped in sands and sludge can get released, harm or explode while cleaning vessels. Ventilate before entry and while inside. Monitor hydrocarbon/ flammables continuously”

• Utility Connection: Blind flange + check valve + ball valve at nozzle to avoid hose rupture and release. Expecting operators to attach a check valve to the hose, may not work. See Safety Alert

• Vent/ drain spec break: If source > 35 bar (600# piping) (1) Fully rate tail pipe to header. Spec break at header. Check tail pipe pressure in Flarenet. Will exceed 150# rating when a 2" globe valve is fully open. Do not use 1" or ½" vent lines in HP service to reduce flow and tail pipe pressure. Small bore pipes are prone to rupture or break on dropped loads. Minimum 2" pipe

• Vent: Not a RV bypass. To depressurize before draining or take out for maintenance. 2 valves in series, ball and globe. May have a bleed in between to remove residual pressure after venting to LP flare backpressure. Bad practice to install a closed spec blind downstream of vent valve to stop fugitive emission thru a passing valve. Unsafe to expect an operator to swing a spec blind against a single passing valve on pressure (vessel) side and HC/H2S on flare side. Leaking process gas can jettison mill scale and injure operator. Spec blinds are for vessel isolation for manual entry and should be on vessel nozzle

• Drain: Bad practice to install a closed spec blind downstream of drain valve to stop fugitive emission thru a passing valve. See reasons above - vent line. Drain lines are in stagnant and corrosive service, handling muck and sand, prone to seat cutting, corrosion and leak. Spec blind in closed position, may get corroded and lose its integrity. See ‘Closed Drain Valving Philosophy’ in Training

• Drain: 2 ball valves, not a ball and a globe. Globe allows “throttling”, draining under pressure that results in low temperature issues, embrittlement and over-pressurization of drain system and Closed Drain Drum. Resulted in gas blowby into drain header and lifting off Closed Drain Drum (CDD). P&ID Caution Box: “Vent first and then drain under gravity”

• Drain: Perform gas blowby calculations for all sources at 300# and above, one source at a time. Good to have CDD vapor outlet, sized to avoid CDD pressure exceeding its DP during a blowby

• Gas Blowby: To downstream LP Vessel. Design Basis: Agree early with Owner. [A] Flow area: Either LCV or its bypass or both. Both may be open when bypass is opened when LCV malfunctions when undersized or flow more than design or debris/ mill scale reducing LCV capacity. [B] Flow rate (1) Gas alone or Gas + Liquids in proportion to inflow (2) LP Vessel RV sizing: Gas only or Gas and liquid or Gas volume equivalent liquid [C] Source Pressure - NOP, PAH or PAHH or RV. [D]: RV Relieving Pressure at 110% of DP or Hydrotest Pressure. Each client has his own say

• Gas Blowby: Relief flow may reduce if you consider ΔP in LCV outlet + LP Sep at RV Relieving Pressure. At times gas rate can be high, more than normal upstream gas inflow. If gas outflow > inflow, upstream Separator pressure will fall. May not be able to sustain gas blowby rate. Wrong to assume liquid will keep accumulating and send gas only. Accumulating liquids will build level and stop gas blowby. Cv available for gas = Full Cv minus Cv normally taken by liquid. Plant on turndown can increase Cv available for gas. Dynamic simulation helps to get a good picture

• Gas Blowby: If LCV + bypass impacts LP flare size (1) Restriction Orifice (RO) in bypass to limit Cv to that of main LCV (2) 2 x 50% LCVs with independent LICs. No bypass. With varying water cut over years, may help level control too (3) Blowby RV load sent to HP Flare (4) Increase LP vessel DP to HP Sep PAHH/1.3 or same DP. No credit for mechanical stop/ limit on LCV to limit flow. A clamp-on stop may work loose; a welded one may break loose, after a few chattering hits. A new or clueless operator may remove it a few years down the road. Organizations have no memory

• Gas Blowby: Occurs when operators are fatigued, start-up/ transient conditions when some control valves are on manual, LALL bypassed to establish level etc. As they have 100 things to do with so many alarms and lights flashing, they may forget to switch to auto in one particular vessel. Ensure time available between sand pad and LALL is good to close outlet SDV. Many sizing programs ignore it

• Gas Blowby to CDD: HP vessel say, at 100 bar may route condensate to CDD under on/ off control. Fully rate outlet piping and run it direct to CDD. JT effect - ice/hydrate formation and blockage. Pipe Material of Construction (MOC) to suit Minimum Design Metal Temperature (MDMT). Heat Tracing maintains pipe wall temperature but does not heat bulk fluids inside pipe. Chilling inside the pipe after a blowby is an instantaneous with which any heating arrangement can NOT cope. Add inhibitor injection point downstream

• Sand: Based on sand load, decide sandjet nozzles/ sand disposal options

• Blocked Outlet RV: Earlier sized for gas only assuming oil, gas and water outlets will not close at the same time. Then for 2 streams, Gas + Oil or Gas + Water. While oil can go with gas, downstream Gas KOD cannot handle oil flow! Water can go with oil, but API says no credit for favourable response of a control valve to take additional fluid. Present practice: Size for all 3 streams based on Safety input that PAHH or LAHH can close all 3 outlet SDVs while inlet SDV may fail to close

• Blocked Outlet RV: On high backpressure, inflow may reduce. [In HP gas wells choke downstream pressure may not have any impact on flow]. Model from reservoir to RV to find flow. Individual flowline PAHH will shut wells before Separator RV pops but Safety Integrity Level (SIL) studies may show a few wells may fail to close. Some consider 2 out of 5 wells failing to close, an old rule of thumb. As non-closing wells may be high flow ones, inflow may not be 40% but 60-70% design. High backpressure may reduce to say 50% design. Varies. Some employ a Pressure Control (PC) in Production Manifold to manipulate chokes while a few go for individual flowline PCs. Some close liquid outlet SDVs only on LALL. Not on PAHH/ LAHH. Situation specific. If RV size/ flare loads are high, opt for HIPPS

• Heavy Viscous Oil Relief: Released oil may solidify downstream of RV, losing heat to header pipework or due to heater failure or heater inadvertently switched off. Best to design LP Sep/ Coalescer to upstream HP Sep DP to avoid relief here. Impact on plate thickness and cost is marginal. Liquid relief from upstream HP Sep may have the same issues. If so, HIPPS to close HP Sep inlet SDV can eliminate blocked outlet release

• No Credit for Trip Action: In RV sizing, no credit for trip. Trips could have latently failed. Trip failure is known ONLY when a demand is placed

• RV as Flare Dump PCV: A few use with pilot RVs. Wrong. You don't use a bodyguard for errands

• Passive Fire Protection (PFP): Can delay fire heating and high vessel wall temperature. Thickness based on unattended fire exposure time and allowable metal temperature/rupture stress. Should withstand fire water hose impact without dislodging. Good for large all liquid filled vessels e.g., C3 accumulator

• Corrosion under Insulation (CUI) is a known issue. Provide inspection windows on insulation to monitor CUI. Apply first coatings on the legs

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 in tube and LP liquid in shell, LP pressure will spike up quickly, unable to push + accelerate liquid. As good as a blocked outlet. Dynamic Simulation helps to find surge pressure. Provide a pair of Rupture Disks (RD) 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. For LP fluid in tubes: 2 sets of RDs on either end) VHP: 2 sets of RDs on either end of shell. For Very High Pressure (VHP) gas go for PCHEs. 0.4-4 mm channel hole. Narrow channels create high ΔP and reduce relief rate. Smaller RV/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 HC gas or liquid can leak into Cooling Water, add HC detectors in CW Tank/ Tower

• Sizing: Many vendors design 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 Heat Exchanger (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 leading to 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 Carbon Steel) CS tube sheets behind titanium cladding on shell side. Hydrocarbon 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