Criteria for sizing process and utility lines and specification of insulation and trace heating in the design Oil & Gas projects Criteria for sizing process and utility lines 1. Codes and References Flow of Fluids through Valves, Fittings and Pipe, Crane Technical Paper 410. Gas Processors Suppliers Association (GPSA) - Engineering Data Book, 1980. API RP14E Recommended Practice for Design and Installation of Offshore Production Platform Piping Systems, Fifth Edition, October 1, 1991. Designing Piping for Gravity Flow, P. D. Hills, Chemical Engineering, September 1983. Guide for Pressure Relieving and Depressurizing Systems, API RP 521, 5th Edition January 2007 API RP 520, Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, Part II, Fifth Edition, August 2003. Criteria for sizing process and Utility lines(continued) 2. SIZING CRITERIA FOR SINGLE PHASE LIQUID LINE 2.1 Maximum Liquid Velocities Where solids are expected, a minimum velocity of 0.8m/s should be maintained to avoid solids deposition. Criteria for sizing process and Utility lines(continued) 2.2 Pumped Liquids Generally, The available NPSH shall be calculated for all pump suction lines, based on low liquid level (i.e. minimum operating level) in the suction vessel, minimum fluid density and referenced to grade at the pump. Design margin between the available and required NPSH shall be referred to the pump specifications. 2.2.1 Centrifugal Pumps Criteria for sizing process and Utility lines(continued) 2.2.1 Reciprocating Pumps Due to pulsation and acceleration effects, the allowable flowing velocities for reciprocating pumps are lower than centrifugal pumps. The acceleration head effect is proportional to pipe length, velocity, pump speed and a factor depending on the number of plungers in the pump (the acceleration effect decreases as the number of plungers is increased). Installation of pulsation dampers on both the suction and the discharge lines shall be considered to potentially eliminate suction acceleration head losses. Criteria for sizing process and Utility lines(continued) 2.3 Liquids Flowing By Pressure General Limits is section 2.1 are still valid When the flow is regulated by a control valve; the pressure drop across the control valve should be at least 30% of the total system pressure loss (frictional and static losses). liquids at, or close to the bubble point, flashing upstream of control valves should be minimized and a maximum velocity of 4.6 m/s is recommended. 2.4 Gravity Flow Lines The minimum flow velocities to prevent solids deposition in sewage and open drain lines Drains will be a minimum 2" and 4" for lines containing dirty fluid and drain headers respectively. Lines should have a minimum slope of 1:100, however for dirty fluids or fluids containing particulates, the slope should be increased to a minimum of 1:50. Criteria for sizing process and Utility lines(continued) 3. SIZING CRITERIA FOR SINGLE PHASE Gas LINE These criteria exclude flare and vent lines, which are covered in Section 5. 2.1 Sizing Criteria for Gas Lines Piping with gas at its dew point and/or with some droplets shall be sized as a single phase gas line. Criteria for sizing process and Utility lines(continued) 2.2 Utility Gases For plant air, instrument air and inert gas distribution, the maximum pressure drop to be used is 0.1 bar / 100 m. 4. SIZING CRITERIA FOR GAS/LIQUID 2 PHASE LINES 4.1 Pressure Drop Complex computer software or hand calculations estimate pressure drop and evaluate the flow regime. Liquid slugging flow regimes should be avoided. 4.2 Erosional Velocity The mixture velocity (Vm) should not exceed the erosional velocity given by API RP 14E Criteria for sizing process and Utility lines(continued) 4.2 Erosional Velocity The value of the empirical factor (C) is contentious and depends upon a number of different factors. The factors suggested in API RP 14E are 122 for continuous service and 153 for intermittent service for all material, but may be considered overly conservative. One of references I mentioned consider the following values for services where the fluid does not contain particles; i.e. sand free: Criteria for sizing process and Utility lines(continued) 5. SIZING CRITERIA FOR RELIEF LINES All flare lines (including headers, sub-headers and individual relief lines and tail pipes) shall be designed to meet the criteria ρv2 <200,000 kg/m.s2. 5.1 Flare Headers and Sub Headers The peak flowing velocity in flare headers should be a less than 0.6 Mach. The peak flowing velocity in sub-headers and tail pipes should be less than 0.8 Mach May be limited by back-pressure requirements. Sonic velocity is calculated using: Criteria for sizing process and Utility lines(continued) 5.2 Individual Relief Lines and Tail Pipes 7.2.1 Pressure Safety Valve Lines: Inlet lines The inlet line size shall be at least the size of the PSV inlet nozzle the pressure loss shall be less than 3% of the gauge set pressure of the PSV at valve rated PSV capacity. Outlet lines The outlet line size shall be based on the required relieving rate and shall be designed considering the following criteria: Minimum size should be 2” and the outlet line shall not be smaller than the outlet flange of the relieving device. Piping should be sloped towards the flare header with no pocket, preferable top entry Connection shall be done at 45°. Criteria for sizing process and Utility lines(continued) Outlet lines (continued) The maximum PSV back pressure should be less than 10% of the set pressure for a conventional (spring loaded) valve, 30% for a balanced bellows valve or 50% for a pilot operated valve. The flowing velocity in the line from the PSV to the downstream header should be a maximum of 0.8 mach. Sonic velocity must be avoided by stepwise increasing the pipe size. The actual back pressure at the PSV outlet shall be checked to be consistent with back pressure limitations. Acoustic induced vibration fatigue analysis shall be considered and carried out. Criteria for sizing process and Utility lines(continued) 5.2.2 Depressurization and Blow down Lines Design of the lines upstream of the restriction orifice shall be based on a flow velocity that gives ρv2 < 200,000 kg/m.s2. The blow down line size and orifice shall be sized based on the peak depressurization flow rate from the blow down analysis. The flow velocity downstream the restriction device shall be a maximum of 0.7 Mach. Acoustic induced vibration fatigue shall be considered. Insulation and Trace heating 6. Insulation and Trace heating As a general guideline, the application of insulation should be minimized to avoid potential corrosion beneath the insulation and to simplify access for maintenance. Insulation and other coatings that are required for fire protection are excluded from the scope of this document. 6.1 Hot Insulation Applied where heat losses from piping and/ or equipment to the surroundings are to be minimized for the following reasons: ✓ Maintenance of the heat balance to ensure optimum operation of the process system and to ensure that intended operation is maintained. ✓ Limiting heat losses in heat exchangers and heater systems to optimize heat input to an exchanger unit. ✓ Minimize the ambient temperature cooling effect keeping the process gas temperature as possible near to operating temperature, so that the temperature from the cold blow down shall be kept with the existing system metal design limits. Insulation and Trace heating(continued) 6.2 Heat Tracing Electric trace heating shall be applied when; stagnant flow conditions may result in hydrate formation, which may prevent flow resuming (e.g. standby pumps) or block instrument impulse lines. condensate system level instrumentation/connections to vessels to mitigate against wax formation at low ambient temperatures. water system level instrumentation / connections to vessels to mitigate against ice formation at minimum ambient temperatures. 6.3 Personnel Protection Equipment and piping which normally operate at 60°C or above and which frequently have personnel in close proximity shall be insulated or have personnel protection guards installed, located within 2.1m vertically above and 1.5m horizontally from any platform, ladder, deck or passageway. Insulation and Trace heating(continued) 8.4 Cold Insulation Cold conservation insulation shall be used where the pipe operates at a temperature below ambient and water condensation may form on the outside of the pipe. This is particularly important in areas where the water condensation may drip on to electrical equipment, operations personnel and walkways inside control buildings and equipment rooms. Also, applied for cryogenic processes. 8.5 Acoustic Insulation Acoustic insulation should be provided on piping systems where the continuous noise exceeds 85 dBA measured at 1 m from the piping. No acoustic insulation is required where the noise limit above is exceeded on an intermittent basis, e.g. relief and blow down lines. Equipment which produces continuous noise in excess of 85 dBA measured at 1 m should be provided with an acoustic enclosure or acoustic blanket.
