Design Tips 16

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 handle 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’. Low temp can cause icing on LT rod attached to a ball in a float transmitter, prevent 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 sludge 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. In 4”-6” size, it will not make any cost difference

• 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. SRB and Microbiologically induced corrosion (MIC)

• See ‘Closed Drain Valving Philosophy’ in Training

Fuel Gas

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

• 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 trips. Based on study results, if required add PCV to flare, say 30-40% of a large demand to cushion out pressure fluctuations on a large load trip to avoid Fuel Gas trips

• Size Fuel Gas System for n+1 GT to start standby GT

• Superheater: Locate heater upstream of Filter Sep to trap mill scale

• Filter Sep: SS piping downstream of Filter Separator to avoid mill scale and damage to burner tips

• Fuel Gas KOD: Size generously 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. Add feed forward control as appropriate (Scott Newton)

• Heat Tracing: During start-up heat picked up by fuel gas piping will cool down superheated fuel gas, leading to liquids to burner guns, damaging GT blades. Provide a start-up 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 Generator and Diesel FW pump via a bypass across the Diesel Supply Pump. If all the users can get flow by gravity, then single 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 coefficient (hi) Vs pumping ΔP at desired temperature. Water, the best to 200° C. Carbon Steel tubes in HM Service are prone to corrosion leaks and fire

• 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 to Expansion Tank 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

• Expansion Tank: Water trapped after hydro-test/ flushing can expand enormously when hot HM enters tank. “Steam explosion”. Consider oil as hydrotest medium to keep water out. P&ID Caution Box: “Water trapped after hydro-test/ flushing can expand enormously, resulting in “Steam Explosion. Drain and test sample during start-up”

• 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: FAL at inlet and 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 during start-up. System hydraulics + static pressure differential usually results in unbalanced flow. Or go for individual FCVs

Gas Turbine

• Gas Turbine: Are in standard discrete sizes. High design / summer ambient temperature derate available power. If calculated compressor power is less than GT site derated power, consider a case with a lower suction pressure to utilize available GT power. GTs can provide more power during nights and winter. Compressor costs are low c.f. GT cost. A generously sized compressor will allow lower backpressure on wells and increase recovery. Cooler night temperature may allow more export gas with line pack. Finalize early

• 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, you may not need Black Start Fuel Gas system and worry about Fuel Gas nuisance trips