Design Tips 16

Fuel Gas

• See ‘Fired Heaters - Operations’ in Training on fuel gas/ fuel oil issues

• Black Start Compressors: Sit and rust. Add diesel guns to GT generators or tap from pipeline

• Scrubber: Provide 2 x 60-100% Inlet PCVs with independent PTs on lead-lag with offset set points. Both will remain online. On failure of duty PCV set high, backup PCV will take up immediately

• Scrubber: Dynamic simulation to check pressure fluctuations on start/ stop of a large user (GT). Such load changes may lead PALL/PAHH. Based on study results, if required add PCV to flare, say 30-40% of a large demand to cushion out pressure fluctuations to avoid trips

• Size Fuel Gas System for n+1 GT to start standby GT. If fuel gas train is sized on running GTs ISO (not derated) rating, with a generously sized compressor one can lower backpressure on wells leading to more recovery and line pack during winter and nights

• Superheater: JT effect on piping may result in external icing. Locate heater upstream of Filter Sep

• Filter Sep: SS piping downstream to avoid mill scale pick up and damage to burner tips

• Fuel Gas KOD should be generously sized to avoid liquids from source vessels/ condensation enroute

• Fuel Gas Switch Over: In systems backed up with LPG or Natural Gas, fuel gas heating value may go up suddenly resulting in fuel rich firebox + potential explosion. See Safety Alert on Reformer – Fuel Rich incident. Add feed forward control as appropriate (Scott Newton) 

• Heat Tracing: During start-up heat picked up by fuel gas header will cool down superheated fuel gas, leading to liquids to burner guns, damaging GT blades. Provide a purge/ vent at user points. GT vendors usually provide one. Check. P&ID Caution Box: “Cold fuel gas piping may condense heavies in fuel gas during start-up and damage burner tips and GT blades. Vent gas upstream of GT till fuel gas piping is warmed up to avoid damage to GT components”

• In Fuel Gas systems requiring pre-heat before pressure let down, during start up human error may result in PCV outlet hydrate blockage and delay start up. Review PCV outlet temperature without preheat and approach to hydrate temp. Based on findings (a) Fully rate PCV outlet to vessel nozzle

Diesel Fuel

• Crane Pedestal Tank: Elevation to allow free flow of diesel to users like Diesel Gen and Diesel FW pump. Check and provide a bypass across the Diesel Supply Pump. If all the users can get flow by gravity, then one pump should do

• Tank: Internal (epoxy?) coating on top 1/3 height. Humid air results in water condensation and corrosion of the pedestal that carries crane load. Add bird screen on vent

• Typical Storage Sizing: 1 Crane to operate for 30 hours + Diesel FW Pump/ Emergency Generator for 24 hours + Main Generator 24 hours. 10 m³/h pump

Heating Medium

• Selection: Based on heat transfer hi Vs pumping ΔP at desired temperature. Water, the best to 200° C

• Heat source: Fired heaters or Gas Turbine WHRU. Users: Wellfluid heater, Process HX, Glycol reboiler etc. Fluid: Water, Dowtherm or minerals oils, Hydrotherm or Therminols

• Heater burner cut-off on pump failure. WHRU coils designed to run dry

• Pump: Add a RO + 2” ball valve across discharge check valve to keep standby pump warm and avoid its failure on thermal shock. Add a P&ID Note 

• Expansion Tank: At the highest point and connected to pump suction. Return: Route return into Expansion TankVessel and NOT to pump suction to degas decomposition products and detect any HC leak at users early. Double leg design. Good to have HC detector on Expansion Vessel vapor outlet

• Expansion Tank: Fluid expansion = System volume*ρcold/ρhot. Tank NLL 25% - cold and 75% when hot. Tank size: 2* fluid expansion. Level to suit pump NPSH and to fill a user

• Piping: All welded to minimize flanges to avoid leak + fluid-soaked insulation that offers a large fuel surface. HM temperature may exceed auto-ignition temperature of hydrocarbons. Spiral wound gaskets. Non-adsorbent or closed cell insulation at potential leak points - instruments tapping and joints

• WHRU: FALL at outlet. Inlet FALL does not help in case of tube leak or rupture

• Flow Balance: Good to have an inlet globe valve at each WHRU to flow balance among WHRUs. System hydraulics + static pressure differential usually results in unbalanced flow. Or go for individual FCVs

Gas Turbine

• GT Size: Higher than Compressor power demand. GTs come in discrete standard sizes. Its site rating is decided by hottest air (summer day temperature) and backpressure via exhaust WHRU etc. Compressor flow and power is decided by head curve at the maximum speed and system resistance. Check compressor power with minimum suction pressure, coldest inlet temperature and maximum molecular weight case. See also Compressor section

• Water content fuel gas: Water and steam are regularly injected into GT combustion chamber to suppress NOx, enhance combustion and boost efficiency. Water / steam have a good role in combustion chemistry. Check with GT supplier to know more about impact on a particular machine

• Check if it is cost-effective to have dual fuel guns in GTs, especially in GT generators. If diesel storage is adequate and can be supplied at the pressure required, delete Black Start Fuel Gas Compressors/ system and worries associated with Fuel Gas nuisance trips 

Air/ Oxygen/ Nitrogen

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

• 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

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

• 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

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”

Water

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

• Start-up: 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 to close to pump discharge and their poor supports. Seawater dump PCV should be conventional type with transmitter (No SAPCV!)

• DP: 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

• 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