Design Tips 9

Compressors

• Centrifugal for larger flows. Lower flows result in smaller impellers and higher speeds for a given head. Axial for large flows and low heads as in aircraft GT inlets. Compression Ratio = 2.5 to 3 to limit temperature to avoid lube oil damage. Flow control via (1) Suction and discharge throttling (2) Recycle to suction and (3) Variable speed. Assume 75-80% polytropic efficiency for initial sizing. Motor efficiency 95-98% based on size. 90% for gears 

• Recip or rotary for lower flows and high heads. Constant volume + positive displacement. Flow control via (1) Clearance pockets (2) Suction valve lifting (3) Suction throttling (4) Recycle to suction via a cooler and (5) Variable speed. Rotary: Screw, sliding vane and lobe types. Good with dirty gases. Lubricated and non-lubricated based on service. Assume efficiency of 65% for a compression ratio of 1.5, 80% for 3 etc for initial sizing. Motor efficiency 95-98% based on size

• Heavy ends may condense in suction line with cooler ambient or night time + due to ΔP induced cooling. Liquids carried over with gas damage and coke on compressor blades; cause unbalanced loads on compressor shaft and lead to failure/ trip/ damage. Some use a Filter Separator downstream of KOD, especially for Associated Gas at first stage. Heat trace/insulate suction line. Physically carried over liquids due to undersized/ poorly performing Suction KOD cause damage. Do NOT reduce KOD size based on supplier promises. Many compressors are affected by undersized suction scrubbers and poor suction piping layout that results in liquid carry over - with great cost to performance and lost production.  Google “Design of Compressor Suction Scrubbers”, Noijen Clinton - Shell. He calls out “Don’t wreck the expensive compressor with cheap scrubbers or with inadequate engineering”

• Control: Depends on fixed speed motor drive or variable speed motor drive or GT drive. See ‘Compressors in Training’ 

• Design Pressure: Upstream Oil & Gas plants - GT Drive. Surge pressure at 105% speed or as limited by export pipeline rating. Downstream Refinery/ Petchem plants- Standard margin over NOP

• Settle Out Pressure (SOP): Calculate discharge at PAHH/PSV. If suction is at PAHH, discharge PSV would have popped. Try with same compression ratio or with compressor curves in Hysys. Work backwards with last stage discharge PAHH/PSV set point and determine inter-stage pressures to determine settle-out

• Settle Out: 2 stages in a common casing settle-out via internal casing leak. Calculate first, each stage SOP, SOP1 and SOP2 and then common SOP12 for both. Add margins to account for piping volume uncertainties. Be liberal on margin if as-built piping and cooler volumes are unknown. A few operators take design pressure at flange rating to cater to future changes

• Dry Gas Seals (DGS): Duration of a pressurized shutdown without damaging the seal, depends on the fluid. If DGS cools down and drops condensate, as with associated gas, it can char or coke leading to unbalance rotors and compressor damage. Case study. Seal gas goes thru pressure reduction in stages. Check for condensation or hydrates via phase diagram letting seal gas to atmospheric pressure. Sour gas may form sulphur solids on contact with O2. Suppliers do not understand this leading to damaged DGS. Educate the supplier and get proper design. Use treated gas, filtered to 1μ. Superheated gas may condense after a trip and cool down. Check. Select depressurization rate to avoid O ring explosive decompression. Read “Oil & Gas Facilities, p 72-76, Dec 2015

• Seal gas flow at settle-out = Psettleout/ Pnormal suction*Normal demand. Read http://www.ogj.com/articles/print/volume-91/issue-19/in-this-issue/production/constraints-complicate-centrifugal-compressor-depressurization.html

• Gas Turbine: Are in standard discrete sizes. High design 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 you to export more gas with line pack. Finalize early

• Suction: Common to see bucket loads of mill scale and dirt collected in suction filter. Locate PALL downstream

• Check valves: Discharge check valves passing lift off compressor suction KOD PSVs, designed for settle-out pressure. Causes PSV chatter and seat damage. If owner policy requires plant trip on a PSV popping, a suction PCV may help

• Discharge Check Valve: Failure results in bearing or seal failures. Overspeed results in damage and gas release. Combined compressor feed + reverse streams release may overload flare and increase back pressure on low pressure RVs

• Discharge Check Valve: Surging damages swing types. Non-slam - dual plate (wafer and axial - types are at less risk. Fouling and corrosion products impact performance

• Discharge Check Valve: Do not locate it flange to flange to discharge SDV. The in-between pocket will remain pressurized even after blow down. May cause injury while maintaining SDV. Add a bleed valve in the pocket or take off BDV from the pocket

• Discharge SDV: Locate discharge pressure equalization dP transmitter upstream of check valve. The trapped pressure in check valve - SDV pocket leads to false dP and premature opening of discharge SDV

• Isolation: Compressor is maintained as a train. Remove individual in/out valves for PSV/BDV. A common “LO” outlet valve to flare will do. PSV inlet valve may be required to test in-situ or to minimize N2 purging

• Noise: Compressor piping may require noise attenuation insulation

Dehydration

• HC liquid: Heavier HC in feed condense in inlet pipe during winter/ nights. Fine condensate droplets cause foaming. (1) Heat trace/insulate scrubber to contactor piping (2) Locate upstream scrubber close to and/or below contactor or a Filter/Separator upstream of contactor. Don’t go by supplier’s scrubber inside contactor

• Rich Glycol: Check for low temperature at the LCV outlet. Do not derate LCV outlet with PAHH and PSV. Keep this fully rated to Glycol Reflux Coil to Glycol Flash Drum. No cost impact in small bore pipes

• Glycol Flash Drum: Size PSV for gas blowby from Glycol Contactor with a full open LCV/bypass

• Carbon Filter: Size for 10% of not for 100% glycol flow

• Lean Glycol: Instead of Gas/Glycol HX, Glycol cooler can give precise inlet temperature and avoid gas condensing heavies + foaming. Go for differential temperature control as feed temperature may swing

• Lean Glycol: No need for FCV. Recip Pumps. Stroke adjustment to regulate flow. Check with supplier

• Gasket: Do not specify. Supplier to select based on depressurization rate and operating condition

• Insulation: P&ID note: “Inspection windows in insulation. Check Corrosion under insulation (CUI) periodically”