AMMONIA PLANT TPM STEP-4 AMMONIA BOOSTER COMPRESSOR K-1441/1442Description of WRVT510 Compressor The Howden oil injected screw compressor is a positive displacement capacity controlled oil flooded rotary machine. Compression is achieved by the meshing of two asymmetric profile steel rotors with parallel shafts housed in a casing. Each rotor is supported by two plain white metal bearings fitted adjacent to the compression chamber. The action of the rotor is entirely rotary. The accurately machined rotors are called male and female. The male (driving) rotor has four lobes which mesh with six flutes in female (driven) rotor, each rotor having the same outside diameter. Compressor lubricating oil is injected into the bearings and rotor space at a pressure equivalent to discharge pressure plus 2 bars. The bearings therefore also act as shaft seals within the compressor. The compressor input shaft is fitted with a mechanical seal hence the compressor is completely sealed from atmosphere. Rotor end thrust is accommodated by tilting pad thrust bearings. The male rotor which takes the majority of end thrust is also fitted with a balance piston. One side of the balance piston is subjected to oil pressure whilst the other is at gas suction pressure. The balance piston thrust therefore opposes normal rotor end thrust, ensuring that the thrust bearings are lightly loaded. GAS System: Gas is drawn into the compressor via a non return valve and suction strainer. The non return valve is necessary to prevent the compressor being motored in the reverse direction when it is stopped with high gas pressure at the discharge. On leaving the compressor the gas oil mixture enters a combined primary separator / reservoir vessel. The majority of the oil separates from the gas stream on entering this vessel, due to the reduction in velocity, the separated oil falling by gravity into the reservoir base. The gas and entrained oil droplets than pass through a knitted stainless steel wire mesh separator element mounted in the upper portion of the vessel. Almost al the remaining oil is collected by impingement on the very fine wire mesh, from which it falls by gravity into the reservoir base. The gas leaving this separator contains approximately 30 to 50 ppm of oil by weight. The separator / reservoir vessel is also provided with a safety / relief valve. The gas then passes through a gas cooler before entering a high efficiency secondary oil separator. This separator incorporates replaceable high efficiency filter elements which are 100% effective in the removal pf liquid phase oil. Oil carry over d/s of this separator is less than 5 ppm by weight. Separated oil is returned to the compressor through a small pipe fitted with a filter, orifice and isolating valves. Isolating valves and a thermal relief valves are provided for the separator. A non return valve is also fitted downstream of the separator. A gas bypass line with a control valve rated for 10 percent capacity is also provided. Capacity control from 10% to 0% is achieved via this line. An air operated ball valve is also included which opens automatically on shut down to equalize the pressure in the system. Parts in Gas Circuit: 1. Non Return Valve: Suction and discharge non return valves are Wafer type with body and seat manufactured in low temperature steel and disc in alloy steel
2. Suction StrainerSuction strainer casing is a pressure vessel designed for the full system pressure in lowtemperature steel. The strainer element is made from one sheet of 60 mesh corrugatedstainless steel gauze sandwiched between the two sheets of 20 mesh corrugated stainlesssteel gauze formed into a cylinder. The element fits inside a support drum made from 2mm thick stainless steel perforated plate with approx. 50 % open area. The gas flowthrough the element is from inside to outside.3. Primary Separator / ReservoirThis unit has a dual function; it acts as a primary oil separator and also as an oil reservoirfor the lubricating system. The vessel is manufactured in carbon steel and upper and theupper portion of the vessel contains a combined baffle and knitted wire pack separatorelement.4. Gas coolerThe gas cooler is a shell and tube unit and is designed to cool the full compressor gasthroughput to 38 deg C.5. Secondary oil SeparatorThe secondary oil separator is a high efficiency unit designed to reduce oil carry over toless than 5 ppm by weight. The unit contains coalescer elements which should bechanged when the differential pressure across the unit reached 0.5 Kg/Cm2. Theseparated oil returns to the compressor via a connection designed for this purpose.6. Isolating valvesSecondary separator isolating valves are reduced bore ball valves with carbon steel bodyand stainless steel ball and stem.7. Safety / relief valveA safety / relief rated for the full compressor throughput is mounted on the primaryseparator. It is a cross by style JOS valve with a carbon steel body and is set to relieve at25 Kg/Cm2g. the secondary separator is fitted with a ¾” X 1” thermal relief valve set at25 Kg/Cm2g.8. Gas bypass valveThis is a fisher control type 667-GL globe valve with carbon steel body and stainlesssteel trim. The valve has a type 667 pneumatic actuator and a type 3582 positioner. Thevalve is rated to pass 10 % of the compressor throughput. When capacity controlbetween 0 to 10 % is required, the valve operates automatically, maintaining a constantpreset compressor suction pressure by bypassing excess gas from compressor dischargeto suction.9. Gas pressure equalizing valveThis is an air operated full bore ball valve with carbon steel body and stainless steel balland stem. When the compressor stops, a solenoid valve in the air supply line opens andair is allowed to the valve actuator, opening the valve fully and thereby equalizing thepressure in the systemLUBRICATION SYSTEMThe compressor package incorporates a force feed lubrication system. Electric motordriven oil pumps draw lubricating oil from the reservoir vessel, which is at compressordischarge pressure and passes though a micro filter and the clean, cooled oil then entersthe oil manifold from which connections feed the oil to the compressor services.Manifold oil pressure is maintained at a positive differential above compressor dischargepressure by having the oil pump flow rated higher than the compressor requirement, andbypassing the excess through a control valve, back to the reservoir. The correct oiltemperature to the compressor is maintained by having a temperature control valvemounted at the cooler inlet and another in a cooler bypass line. These valves operatefrom a common controller which senses manifold temperature.A solenoid valve is fitted in a return line from the manifold to the oil reservoir. Solenoidvalves are also fitted in oil drain line from the compressor to the reservoir. These valves
prevent the compressor fill up with oil on startup and also enable oil to be circulatedbefore the compressor is started. Interlock ensures that the compressor can not be starteduntil these valves are closed.Parts in Lubrication System:1. Oil pumps:The oil pumps gear type driven by electric motor. Pumps have steel casing complyingwith API 6142. Oil Cooler:The oil cooler is a two pass water cooled shell and tube unit. It has a carbon steel shelland stainless steel tubes and tube sheet.3. Oil Filter:The oil filter is a duplex unit with changeover valve which allows either one cartridge orthe other to be in use. Elements are replaceable glass fiber type with an absolutefiltration of 25 micron.4. Differential Pressure Control Valve:This is a fisher controls type V500 ball valve with carbon steel body and stainless steeltrim. The valve has a type 1052 actuator and a type 3610J positioner. Oil / gasdifferential pressure is sensed by a differential pressure transmitter and communicated toa controller which controls the differential with this valve.5. Oil temperature control valvesThese are fisher control type V500 ball valves with carbon steel body and stainless steeltrim. Valves have a type 1052 actuator and type 3610J positioner. Valves are controlledby a Foxboro pneumatic temperature controller type 43 AP to maintain the manifoldtemp. at 40 deg C.6. Oil pressure relief valves:To ensure that the oil / gas differential pressure does not exceed a safe limit, a reliefvalve is fitted at the discharge of each pump. Valves are set at 6.7 kg/Cm2 differentialand are capable of passing the full pump capacity.7. Pump Non return ValvesNon return valves are fitted at the discharge of each pump. These are wafer type withsteel body, disc and seat.8. Oil Heater:The heater is a 6 KW FPFR unit complete with integral thermostat set at 30 deg C.9. Oil Level Switch:The oil level switch is a ZR 2132 RO4 chamber type. It is set to switch off the heater ifthe oil level falls close to exposing the heater element.Capacity Control:Capacity control is achieved by means of a sliding valve which is an integral part of thecompressor and a gas bypass valve. The slide valve is operated by a piston within ahydraulic cylinder which is mounted on the compressor. To move the slide valve, oilfrom the lubrication system is fed , via solenoid valve, to one side or other of the pistonwhich is connected to the slide valve. Slide valve movement alters the point on the rotorlength at which compression begins, allowing capacity to be controlled from 100% downto10 % with approximately proportionate power saving. Capacity from 10 % to 0 % iscontrolled by the gas bypass system.The slide valves are controlled manually by ON Load / OFF Load push buttons mountedon the local panel. If the system demand is so that the compressor is operating at 10 %capacity, it is detected by a 10 % limit switch mounted on the compressor hydrauliccylinder.