CENTRIFUGAL COMPRESSORS FOR HYDROCARBON SERVICE

Compressor packages shall be designed and constructed to operate for a minimum of 3 years uninterrupted site duty, and shall be designed for a 20 year service life. No proto-type equipment or equipment with less than five (5) years of proven experience shall be used. The design life shall be achievable with a minimum on site maintenance and a maximum availability.

Spare parts and Tool shall include Start-up and commissioning spares, two years spare. Tools and fixtures required to disassemble, assemble or maintain the unit, shall be included in the bid and furnished as part of the initial supply of the compressor (such as torque wrenches, hydraulic devices, etc, and normal wrenches).

The scope of instrumentation to be clarified with Company based on proposed schematic diagrams, prior to order.

As a rule of The Centrifugal Compressor package for hydrocarbon service shall be designed to API 617.

In case of conflict among this specification, data sheets, referred codes & standards and statutory requirements & regulations, the Contractor shall bring the conflict into the notice of Company in writing for resolution before proceeding with the work.

The compressor shall comply with compressor data sheet and this specification. All components shall be suitable for the operating conditions stated on the compressor data sheet.

Material shall be new, free from defects.

All equipment supplied shall be finished machined.  On site only pure assembly work is acceptable.  Piping not completely prefabricated is to be marked on the Piping arrangement drawing(s).  All spare components are to be machined at manufacturer’s shop with suitable tolerances in order to allow replacement without any re-machining.

The compressor and its auxiliaries shall be suitable for outdoor installation (without roof) unless otherwise stated in the data sheet. All equipment supplied must be suitable for start-up and for operation at minimum ambient site temperature as indicated in the specification’s data sheets.

Driver shall be Gas turbine, the specification please see Gas Turbine below.

At normal speed and normal suction condition, the volume capacity at the surge point shall not exceed 70% of the normal operating point. The rise in pressure ratio from the normal operating point to the surge point at normal speed shall not be less than 5%. The head developed at 115% of the normal operating flow at normal speed shall be not less than 85% of the head developed at normal operating point.

Noise emission data shall be provided as well as data of other emissions exhausted to the atmosphere (gas leakages, oil vapour, etc) with the proposal. Please see bellow for specification of noise.

Facility for optical alignment shall be provide and the manufacturer’s supervisor shall be present during initial alignment check at site.

Process gas compressors will be run-in on air in the field. Run-in on air means mechanical test run in the field where compressor is priming and discharging to atmosphere. Compressor manufacturer to propose procedure for run-in on air (flows, discharge pressures and temperature, speed, horsepower requirement, necessity of bleed connections, safety screens, etc) and to provide performance curves and temperature limitations, etc, for this operation. If specified in the specification’s data sheet, process gas compressors will be operated for several weeks on air for pre-start-up equipment drying.

All casing drains shall have isolation valves.  All casing drains down stream of isolation valve shall be connected to common drain header terminated with a flange connection at the edge of skid/base plate.

Compressor shall be designed to withstand the external loading defined bellow:

•             Vertical component. Combined forces and moments due to all piping connections or to any one piping connection resulting in a vertical reaction (either upward or downward) at any support point of at least one-half the dead weight reaction of the compressor at the support point.

•             Horizontal transverse component. Combined forces and moments due to all piping connections or to any one piping connection resulting in a horizontal transverse reaction at any support point of at least one-third the total dead weight reaction of the compressor at the support point.

•             Axial component. Combined axial forces of all piping connections or to any one piping connection resulting in an axial force on the compressor casing of at least one-sixth the compressor weight.

Shaft shall be made of one-piece, heat treated steel that is suitably machined. Shafts shall be made of forged steel.

Impellers shall be assembled on the shaft with fit and a key. Other methods shall not be acceptable without Company approval. Impellers shall be designed to limit the maximum stress at maximum continuous speed to a value not exceeding 70% of the material yield strength. Proven impellers shall be provide.

Balance line sizing shall consider also noise generation due to high gas velocities in the balance line. During mechanical run test in manufacturer’s shop balance line (s) must be installed.

If the compressor is driven by turbine particular attention concerning bearing design must be given to turbine turning gear speed, which may be very low.  Compressor manufacturer to coordinate with turbine manufacturer.

Nonpressurised bearing housings shall be provided with a 25 mm vent connection equipped with the standard breather cap or closed with a steel plug, if no venting is necessary.

Torsional Analysis, A composite torsional vibration analysis shall be performed for all compressor units.  For compressor units driven by electric motor or turbine through a gear box, both the compressor and driver manufacturer shall perform an independent torsional analysis. The input and results of this analysis shall be prepared for submission to Company.

Vibration and Balancing, the final balancing of the rotating element shall be carried out with the coupling half installed. Along with the compressor a fully integrated vibration monitoring system complete with the cabinet (series and protection as defined by Purchaser suitable for service) shall be provided. Vibration probes and system shall be inaccordance with API 670.

Drivers shall be sized and rated to develop at least 110% of the horsepower or as specified in data sheet (whichever is higher) at the maximum compressor operating conditions, including either gear or hydraulic coupling losses, or both. Steam turbine drivers for compressors shall be in accordance with API 611 or API 612 as called for in the individual turbine specifications.  Auxiliary drive turbine shall be capable of continuously developing 110 % of the horsepower required at the relief valve pressure of the driven equipment, at the corresponding speed under minimum steam inlet and maximum back pressure.

The coupling (whether hydraulically or mechanically fitted) shall be designed for easy removal. Devices shall be provided by the manufacturer for the mounting and removal of this coupling.

DESIGN PRESSURE PHILOSOPHY

Pressures vessels shall be designed for maximum operating pressure, expected upset or settle out conditions, plus a safety factor, which shall be at least the greater of the following:

·         10% of the maximum operating pressure for pressures below 1450 psig, but in any case not less than 50 psi.

·         5% of the maximum operating pressure for pressures above 1450 psig, but in any case not less than 150 psi.

An appropriate margin shall be applied between design pressure and the flange rating of associated piping and equipment to avoid over-stressing of equipment nozzles. A minimum margin of 5% is recommended or 15 psi, whichever is the greater.

Maximum operating pressure for individual equipment is discussed below:

a.     Separators

Normally 1.05 times of the maximum pressure under normal operation as a minimum will be considered.  The design pressure should relate to the top of a vessel (top tangent line in vertical drums) and therefore will not include any liquid head in the vessel.

b.    Compressor Suction Scrubbers

The design pressure of the suction side of the compression system is determined as the maximum settle-out pressure plus a 5% margin. Settle-out conditions will be based on the suction system at the high pressure trip setting and the discharge system at relieving conditions.

c.     Storage Tanks

Square or rectangular storage tanks operating at atmospheric pressure shall be designed for the highest density fluid contained and water as a minimum. An additional design pressure of 0.5 psi minimum will be added where tanks are blanketed. Tanks shall be designed for 0.03 psi vacuum.

Nomination of design pressure shall consider the location and elevation of the tank vent and the potential that liquid overflow from the vent may occur.

NGL storage spheres will operate based on the vapour pressure at the NGL run-down temperature. For design pressure, 105% of the maximum operating pressure will be considered as a minimum.

d.    Pumps and Compressors

Equipment not protected by a PSV and located downstream of a pump or a compressor shall be designed for the shut off pressure according to the following criteria:

·         Pumps

Pump design pressure will be calculated as:

⁻          the maximum suction pressure based on suction vessel PSV set pressure and NLL, plus

⁻          Pump shut-in head at the maximum rated speed.

Where vendor data is unavailable, the shut-in head may be estimated as 1.25 times the differential pressure developed by the pump at maximum rated speed.  Maximum rated speed for variable speed drives is to be specified as 1.1 times the speed for the normal differential pressure at maximum rated flow.

When there is a large difference in delivery and suction pressures consideration shall be given to whether the delivery pressure can be imposed on the suction line in which case the suction line shall be rated for the same pressure as the discharge line.  The spec break will be taken at the first block valve on the suction line.

·         Reciprocating Pumps

The design pressure at the discharge of a reciprocating pump, (eg in chemical injection systems) shall be set at 10% above the maximum expected operating pressure, overpressure protection being provided by an independent pressure relief valve.  The pump shall be capable of pumping at the design pressure of the discharge vessel or system.

·         Centrifugal Compressors:

Compressor design pressure will be calculated as:

⁻          the maximum suction pressure based on suction vessel PSV set pressure, plus

⁻          1.15 times the differential pressure developed by the compressor at maximum continuous speed.

Consideration may be given to designing the whole compression loop for the full discharge pressure if this provides a simpler, safer design without excessively increasing the cost of the system.

e.     Heat Exchangers

The design pressure of heat exchangers (unless the process system dictates otherwise) shall be the greater of the following:

·         Maximum operating pressure plus safety margins as per separators.

·         Pump design pressure as calculated in 5.1.3 or 5.1.5

·         Consideration of ASME/API RP 521 (modified) - 10/13 rule concerning tube rupture in shell and tube heat exchangers. Design pressure of the lower operating pressure side shall be 77% of the higher operating pressure side.

f.     Piping

The design pressure of the piping shall not be less than:

·         Set pressure of the relief valve when mounted on the line plus static head.

·         Set pressure of the relief valve mounted on equipment plus static head and friction loss.

·         Maximum pressure that an item of equipment can generate e.g. design pressure of centrifugal pumps, stalling pressure of reciprocating pumps etc

·         The maximum back-pressure possible (particularly relevant to vent and drain lines where critical flow is possible)

PROCESS EQUIPMENT DESIGN CRITERIA

1.1          Pressure Vessels

1.1.1       Separators/Scrubbers

The following liquid levels and residence times are recommended for 2 phase separator design:

Table 1

2-Phase Separator Design Parameters

Parameters	Vertical Vessel / Column	Horizontal Vessel
Vessel lower tangent or Vessel bottom to LLLL	6” (subject to mechanical constraints on distance to weld lines). (For thick walled vessels, 12”)	(min 12”)
Between HHLL and HLL	1 minute (min 6”)	1 minute (min 6”)
Between HLL and LLL	3 minutes (min 12”)	3 minutes (min 12”)
Between LLL and LLLL	1 minute (min 6”)	1 minutes (min 6”)
Normal Liquid Level (NLL	1 x Vessel I.D. (min 12”)	(min 12”)

Surge volumes between NLL and LLL should based on the following:

Product to storage	2 minutes
Product to a downstream heat exchanger	2 minutes

The requirement volume to handle liquid surges should be determined on a vessel by vessel basis. The surge volume shall be accommodated between NLL and HLL.

For 3-phases vessels allowance is made for separation of the two liquids. For preliminary sizing:

Between bottom of vessel and NIL (water volume)	5 mins (min)
Between NIL and NLL (oil volume)	5 mins (min)

If water volume is small a boot may be used instead of a separate settling compartment. Preliminary sizing of vapour / liquid separators and scrubbers should be performed using standard gas load factors defined as follow:

 

The following values of K should be used for sizing:

VESSEL TYPE AND INTERNALS	K-ft/s
Vertical Knockout Drum (no internals)	0.262
Horizontal Knockout Drum (no internals)	0.328 (bulk separation)
Vertical with Mist Pad	0.361
Horizontal with Mist Pad	0.427
Vertical with Vane Pack	0.656
Horizontal with Vane Pack	0.656
Multicyclone	0.656

The criteria should be applied for preliminary sizing of separators and for cross checking vendor supplied designs. For proprietary vendor internals vendor supplied K values should be considered. For design purposes the maximum allowable operating velocity shall be no greater than 80% of the calculated critical entrainment velocity.

1.1.2       Accumulator / Reflux drums / fractionation columns.

Hold-up time is 5 minutes for half-full accumulators/reflux drums. For a product feeding another tower hold-up is 5-10 minutes.

1.1.3       Surge Vessels

Surge vessel such as Instrument Air Receiver will be sized for 10 minutes hold up based on continuous instrument air consumption and the instrument air header pressure to fall from 110 psig to 80 psig.

Hot oil Expansion Vessel will be sized based on the following:

·         Expansion of the entire inventory of the hot oil circuit from the minimum ambient to the high temperature alarm (TAH) set point.

·         At least one-fourth full by volume at cold conditions and not more than three-fourth full by volume when operating.

·         Capable of providing at least two minutes of retention time below the normal operating (hot) level.

·         Capable of providing minimum liquid head requirements for the Hot Oil Circulation Pumps during start-up and normal operating conditions.

·         Situated so that the cold level of the tank is the highest point in the hot oil circuit to serve as the main venting point of the system.

1.1.4       Pumps for Hydrocarbon & Non-Hydrocarbon Services

A 10% design margin shall be applied in setting the motor capacity (design as rated flow) for a pump in general service.

No margin shall be added to the differential head.

The procedure for pump calculations shall be:

·         Calculate the rated flow as design flowrate

·         Determine the size of the pump discharge piping based on pump rated flowrate. The line pressure drop should be calculated for the design flowrate.

·         Calculate the pump differential head based on the line size determined. Calculate DP at the pump rated flow.

·         Maximum size impellers should not be specified for pumps. Maximum allowable impeller size should be specified as per API Std 610.

·         Calculate the NPSHA at rated flow. A 1 m margin should be added to the calculated available NPSH for suction system design and suction vessel elevation.

Where flow conditions necessitate turndown to less than 30% of rated flow, minimum flow provisions shall be made. For centrifugal pumps this shall consist of a line from the discharge to the suction source. Minimum flow requirements shall be based on the pump vendors recommendations, however in the absence of this data 30% of normal flow shall be used for bypass line sizing. Pumps shall be provided with individual minimum flow recycle lines back to the suction source.

1.1.5       Compressors for Hydrocarbon Service

No design margins shall be applied to the flow-rate or head of compressors. The design flow-rate for compressor shall is based on design production rate.

API Std 617 power margins shall be applied.

1.1.6       Compressors for Air / Nitrogen Service

·         A design margin of 10% will be applied to the flow-rate calculated for Air and Nitrogen service compressors.

·         No margin to the head is applied.

·         Compression ratio should be the same in each stage of a multi stage unit.

1.1.7       Turbo-expanders

No design margin shall be applied to the flow-rate or head of turbo-expanders, since the design flow-rate is based on the design production rate. API power margins shall be applied.

1.1.8       Gas Turbines

The design margin over the driven equipment design output should not be less than 10%. Where the design margin for a standard unit is less than 10% consideration should be given to future production rates.

1.1.9       Shell and Tube Exchangers

·         A 10% margin will be added to calculated heat exchange surfaces for shell and tube heat exchangers.

·         No additional margin will be applied to flow-rates, duties or pressure drop.

·         Minimum approach temperature recommended is 20°F.

1.1.10    Air Cooled Exchangers (Fin-Fan)

·         A 10% margin will be added to calculated air cooled exchangers.

·         No additional margin will be applied to flowrates, duties or pressure drop.

·         Minimum approach temperature recommended is 20°F.

·         Air coolers shall be designed for an ambient air temperature of 90°F.

1.1.11    Plate-Fin Heat Exchangers (brazed aluminum)

·         No additional design margin will be added to plate-fin heat exchangers.

·         No additional margin will be applied to flowrates, duties or pressure drop.

·         Minimum approach temperature recommended is 3.5°F.

1.1.12    Plate and Frame Exchangers

·         No additional design margin will be added to plate and frame exchangers.

·         No additional margin will be applied to flowrates, duties or pressure drops.

·         Minimum approach temperature by vendor.

1.1.13    Fired Heaters

A design margin of 10% will be applied to the heat duty of the fired heater.

1.1.14    Dehydration Package Unit

·         Design margin for package-provided equipment units as discussed above shall apply, unless the vendor is able to justify the added margin.

·         For molecular sieve vessels, the vendor will recommend an additional volume margin for the adsorbent bed to prevent saturation break-through.

1.1.15    Hot Oil Package Unit

·         Design margins for package provided equipment units as discussed above shall apply, unless vendor is able to justify the added margin.

1.1.16    Relief Valves

Relief valves shall be designed in accordance with API RP 520 and API RP 521

The allowable accumulation for pressure relieving devices shall be:

·         For general relief 10%

·         For fire case relief 21%

The maximum allowable pressure drop in the inlet line to a relief valve shall be no greater than 3% of the relief valve set pressure.

When multiple relief valves are required to achieve the required relief area, the allowable accumulation for general relief valves shall be 16% with the set pressure for the additional valves set at 5% above the first valve set pressure.

1.1.17    Control Valves

No margin should be applied to the rated flowrate or pressure drop for sizing control valves.

For preliminary estimates control valve size, the following guidelines shall be used. (Instrument discipline is responsible for final control valve sizing and selection).

At pump rated flow the control valve pressure drop shall be the greater of:

·         10 psi

·         15 % of the variable system pressure drop at the rated flow

At pump normal flow, the control valve pressure drop shall be equal to or greater than:

·         15 psi

·         50% of the variable system pressure drop (excluding the control valve). This is typically 30% of the total system pressure drop.

·         5% of the destination pressure.

Control valves in vapour service should normally be specified for a minimum pressure drop of 10 psi at design flow unless otherwise dictated by specific process conditions.

Control valves in non-pumped liquid service shall be specified with due regard to the system hydraulics under all operating modes. In flashing services particular attention shall be given to inlet line size with regard to minimizing flashing at the inlet to the control valve, and to specifying the % liquid flash across the valve.

At maximum flowrate the calculated Cv shall be 80% - 90% of the valve maximum Cv.

At the minimum process flowrate the valve Cv should be greater than 10% of the maximum valve Cv subject to vendor minimum flow requirements.

Acceptable methods for preliminary sizing of control valves by the Process Discipline are:

·         Masoneilan

·         Willis, Masterflow or equivalent for high pressure drop choke style control valves

·         Valtek

Bypasses around control valve sets shall be used on critical services as shown on the P&ID's.

Compressor recycles control valves shall be sized by the compressor vendor. As a guide, the valve Cv should be 2 - 2.4 times the Cv calculated to pass the flow rate delivered at the surge control line with the compressor operating at rated speed.

1.2          Storage Tanks

·         Liquids subject to breathing losses may be stored in tanks with floating or expansion roofs for conservation.

·         Freeboard is 15% below 500 gallons and 10% above 500 gallons.

·         Capacity of product tanks shall depend on the connecting transportation schedule.