Design Tips 17

Water/ Air

• GRE Pipes: Consider single point responsibility (SPR) for design, supply and installation of GRE pipe system to avoid surge and burst GRE pipe issues

• Water Pumps: P&ID Note near start-up dump PCV: “PCVs are kept open during pump start for xx seconds to avoid start-up pressure surge that can burst GRE pipes; and then slowly pinched closed”. Ask the SPR supplier to find out the time required by transient analysis and means to implement. Start-up failures / ruptures reported in GRE are due to location of bends close to pump discharge and their poor supports. Seawater dump PCV should be conventional type with transmitter (No SAPCV!)

• Water System Design Pressure: Check if the Seawater/ FW system design pressure takes care of pump shutoff head at high sea water level + high tides. Pressure at lower/ cellar deck will be higher than main deck pressure, due to static head

• Safety Shower: Route safety/ eye shower water under the shade of deck/ structural members to minimise solar heat pick-up. Locate their take-offs from lines where water is likely run continuously to avoid hot/ stagnant water on opening the safety showers

• Air KOD: As suggested for fuel gas, 2 parallel PCVs on supply line + another PCV on utility air supply line to maintain backpressure

• Air Receivers: Plant and Instrument Air receivers cushion out pressure fluctuations due to demand fluctuations + as a buffer vessel for say 15-20 minutes supply to allow safe plant shutdown, with pressure declining from NOP, say 10 bar to a min pressure, say 4 bar. Buffer capacity can be met by HP Compressor and HP Storage at say 16 bar. See ‘Utility Air System Design’ in Sizing

• Air Receivers: Optimize size - diameter Vs length based on standard thickness plates that results in the lowest metal mass, subject to site transport limitations

• Air Receiver Bypass: Bypass receiver with a line from inlet valve upstream to outlet valve downstream for online maintenance

Chemicals

• Hydrate Inhibitors: Methanol, Ethanol, MEG and DEG. Storage for 15 days

• No of Drums: Finalize for each type of chemical, lube oil etc to avoid a last-minute surprise during Hazop calling for changes in a frozen layout

• Drum Vents: Depending on diluent / solvent, consider Flame Arrestor or Nitrogen padding. MEG/TEG - N2 blanket to avoid absorbing moisture from air. Methanol vessels with fuel gas or N2 padding

FPSO/ SemiSub

• Motion: Affects performance. Separated oil and water may merge if internals do not consider pitch. Liquid level swings affect alarm/trip setting and column performance. Closed Drain Drum elevation may momentarily go up w.r.t to source. See ‘Hazards - FPSO’ in Training

• Layout: Locate long separators and columns close to centre to minimize movement (3 axis) impact on level settings and performance

Petrochemicals

• C2= Plant: Brittle Fracture. Consider (1) Loss of heating medium (2) Autorefrigeration on pressure loss and (3) autorefrigeration followed by repressurization

• C3-C3 recovery plant pump failures. Avoid reflux pump failure by maintaining adequate Normal Liquid Level (NLL) to ensure bearing lubrication and avoid dry running. Avoid export pump bearing failure by forced lubrication - from discharge to bearing bottom. New additional bearing may help

Hydrogen

• Liquid Hydrogen (LH2): Cold hydrogen can condense water from air. Ice blockage in vents. Consider a rainwater barrier cap on vent tip that opens only when there is release. Add a water collection pot at vent base with automatic drainage. Avoid low point pockets. Keep a Caution Box in the P&ID and near the vent saying, “Do not spray water on the vent stack to avoid water freezing and ice blockage”. Consider Helium as the purge gas. Hydrogen Tech World, “Safety of Hydrogen Vent Systems”

• HTHA: High Temperature Hydrogen Attack can occur in the so called safe region of Nelson curve in API 941. Operating conditions can get severe than design. Avoid CS in HTHA service and go for Cr-Mo steel

• Syn Gas: Compressors. Avoid Wet Gas Seals. Go for Dry Gas Seals. Use Duplex SS Coolers

HAZOP

• Missing pre-Hazop joint review by operations and design leads to poor Hazop. Pre-review comments and contractual issues should be settled ahead. This avoids stalled discussions and teams digging into trenches with a not to yield attitude. Design team knows steady state issues and handles the project for a year. Operations will run it for 20 years and have better insight on transient or start-up issues

• Scribe: Best asset - one who captures on his own essence of discussion + right tag numbers etc without prompting. Frees the team to keep the momentum without dragging the discussion into a drain

• Capture design improvements as observations and assign to owners so that any EPC cost implications can be settled later

• Avoid prescriptive recommendations. Keep it open. Recommendations are NOT mandatory. Review team can provide the best possible solution, based on intent