reciprocating compressor loading and unloading system

Forsthoffer, W.E., “Forsthoffer’s Rotating Equipment Handbooks”, Vol. An over temperature switch activates an alarm and switches off the heater. Inlet throttling. If the main motor starts while over-lubrication time has not elapsed, motor space heaters and lube oil heaters will be stopped at the same time. The advantages and disadvantages of each method, as well as their applications, are presented. For a single-cylinder compressor stage, valve unloaders can achieve three-step unloading that provides nominal cylinder capacities of 100, 50 and 0%; and two-step unloading that provides cylinder capacities of 100 and 0%. The oil pumps can be started at minimum ambient temperature, whereas the compressor only when the lube oil temperature is above the minimum temperature recommended by the manufacturer (about 15°C). Before the barring device is enabled to start, the hydro jacking of compressor and main motor have to be started, and the minimum pressure must be obtained. Hydro jacking device. RELIANCE ON ANY INFORMATION SUPPLIED IN THIS PUBLICATION IS SOLELY AT YOUR OWN RISK. It should be noted that, none of pre-lubrication time, low-oil-temperature, or the lube-oil-pressure alarms should be activated. When the compressor runs ‘loaded’, it is actually compressing air and pumping it into the system. The compressor should be barred over by a barring device (for two to three revolutions) to ensure that liquid is not present in cylinders. Barring devices (gear), hydro-jacking systems, frame lubrication systems and cylinder lube oil systems are explained. Moreover, a pneumatic system should be equipped with an air-supply inlet filter and oiler, and a four-way control valve for operation in forward or reverse directions. If a cylinder may be operated with unloaders, it needs to be analoged in both its operating mode and its Minimum cooling water flow to the cylinders and packings is one of the allowed conditions for starting the main motor. Moreover, the operator can enable the oil heater by pushing a button in the DCS. It is, however, inefficient because excess compressor capacity is expanded across the control valve in the recycle line. The 0% performance step is only used in compressor start-up and shutdown sequences. Check the operability of all voltage circuits and the shut down system. For this purpose, the first stage bypass valve shall be enabled to control suction pressure automatically. Rapid valve movement causes rapid load change on the compressor. When neither the low-lube-oil pressure alarm nor the low low-lube-oil pressure alarm is activated, the hydro jacking pump and subsequently the main motor are permitted to start. All lines to and from unloaders are to be AISI 316 L stainless steel [6, 7]. Selecting the correct reciprocating gas compressor and designing a proper piping system can make the daily operations of a plant very efficient. 1. In this regard, in each stage, 25% of master compressor capacity (one step) will be decreased and 25% capacity of slave machine will be increased subsequently. For this purpose, one oil pump is usually added in series after the lubrication pump. In most process applications, compressor capacity control is established by a suction pressure controller acting on the bypass valve and step capacity control (0, 25, 50, 75 and 100%). For a single-cylinder compressor stage, valve unloaders can achieve three-step unloading that provides nominal cylinder capacities of 100, 50 and 0%; and two-step unloading that provides cylinder capacities of 100 and 0%. Certain articles contain the author's personal recommendations only. Maintenance mode. 8. Manual barring may be achieved by turning the flywheel. Accumulated oil in the cylinders has to be removed by turning the barring device before another starting sequence commencement. After startup, all functions will be carried out through the DCS automatically. A trip is an irregular state that requires an immediate and automatic shutdown of the compressor in order to avoid damage to the equipment and personnel. Auxiliary water pumps can be started, as long as the water-pressure or low-flow alarm is activated in the DCS. For safety reasons, the heater is interlocked and must be switched off when the low low oil level is activated. In order to obtain the desired discharge pressure for each stage, the bypass valve setting must be performed manually. In a closed cooling cycle, the system is pressurized through two electrically driven water pumps. One is used for normal operation (main pump) and the second is in standby mode as an auxiliary pump for necessary conditions. A jacking oil pump assures lasting oil film existence between bearing and journal (shaft) surfaces at the instant a rotation commences. The main drive motor should be started only if it has been purged and the pre-lubrication time of the crank gear and cylinder have passed. It is of high importance to close the bypass valves slowly and gradually to stabilize the conditions and prevent overshooting of discharge pressure. To avoid unnecessary bypass flow, the step capacity control should perform down-stepping, decreasing the compressor performance by 25%. Manual barring devices should have a locking feature, whereas pneumatic and electrical devices should incorporate appropriate safety switches or measures for interlocking with the main driver. Additionally, the LIB shows whether local motor-control stations are also available for local starting and stopping of auxiliary equipment in maintenance mode. He is specialized in reciprocating, centrifugal and screw compressors, gas and steam turbines, process pumps, engines and electric machines. She obtained B.S.ME. The cylinder lubrication timer should be programmed as a cumulative timer. Moreover, this pressure shifting may cause an overload in the last stage. | Privacy Policy | Diversity Inclusion & Equity, The Digital Difference from the Leader in Thermal Mass Flow Measurement, In a report 1 issued by the American Council for…, By exploring strategies for process integration, such as waste-heat recovery…, New offerings, including software products, consulting services, research insights and…, Reverse-osmosis (RO) desalination membranes use a polyamide selective layer formed…, Pixelligent Technologies Inc. (Baltimore, Md. Cylinder delivered capacity can be reduced by increasing the cylinder clearance volume; this is done by a clearance pocket. The main objective of the capacity control is to maintain constant suction pressure (PT1). If the product-handling system can handle small traces of oil, a fully lubricated unit should be considered. The compressor is ready to start if none of the following alarms are active: • Barring gear is in safe position (barring gear disengaged), • Compressor has been barred over for a minimum of 1 min (mandatory if cylinder over-lubrication time has elapsed before), • Crank-gear lube-oil pump pre-lubrication time has passed, • Cylinder pre-lubrication time has passed. Ammonia Refrigeration Training VideoThe internals of a cut-away recip compressor showing loading and unloading capacity. After a period of time (adjustable between 5 to 15 min), in each performance step, solenoid valve activation on unloaders will be automatically switched over. Note that the crankcase oil temperature should not exceed 70°C (160°F). Depending on the manufacturer and user concerns, an interlock system could be varied, but interlocks (alarm conditions) below are considered in most applications. To start two compressors, when the master machine is stable at 100% capacity, the slave machine should be started in 0% load. Print If an unloader valve also exists, the bypass valves are fully closed and unloader devices open the inlet valves, until compressor reaches the rated speed at 0% capacity. Check if all utilities including instrument air, hydraulic oil (for high-pressure valves actuation) and nitrogen for purging of pressure packings are available. Corken's single-stage reciprocating gas compressors work well in low pressure recovery. If it is not reset, it will accumulate the total time of the cylinder lubrication pump running while the main motor is not running up to the over-lubrication time limit. Per API recommendation, the maximum predicted discharge temperature on each stage should not exceed 150°C (300°F). Before the compressor is allowed to be started, the following checks and conditions have to be fulfilled in the field: Check if the cooling-water supply valves are open (for inter-stage gas- and oil-system coolers). In general it is not recommended because of limitations in performance accuracy and fatigue consequences on the valve plate and seat [7]. Two of the more common types of unloaders are depressor-type unloaders and plug-type unloaders. There are two main reasons why compressor capacity regulation is used. Opening the pocket reduces the cylinders inlet volumetric flow by trapping additional gas in the larger clearance volume at the end of the piston stroke. (Purge time is approximately between 30 to 60 min). The compressor main motor must be started within the over lubrication time (approximately 5 min) to prevent accumulation of oil in the cylinder. A compressor is provided with several local instrument boards (LIBs), with gages for indicating pressure, temperature, flowrate and level of process gas, lubrication oil and other utilities (such as water, air and nitrogen), during compressor operation in all modes. The machine should be capable of starting with operating suction pressure on each stage. Moreover, for approximately 2 min, the low low-pressure trip on the compressor suction will be overridden to allow a stable condition without tripping the machine. When the compressor runs ‘unloaded’, it is still running, but it is not compressing any air. Note that in compressors, which are only started by bypass method, the starting procedure is performed with bypass valves fully open. This stop sequence is similar to automatic trip stop except that no post lubrication is needed by frame and cylinder oil pumps. The finger type unloader is not recommended when other types are viable. At compressor normal stop, post lubrication time starts for approximately 2 min, and the cylinder oil pump switches off automatically afterwards. Loading sequence for a compressor with two double-acting cylinders, Table 2. The cylinder lubrication system is pressurized by electrically driven multiple-plunger-type oil pumps (plunger per point) or by the divider block method. At partial loads, the unloading device does not allow the inlet valve to close when the piston is at its bottom dead center position, but rather delays the closing in a controlled way. The capacity control is often determined by the compressor discharge pressure. The barring device operates with a local push button (forward and reverse) on the local panel near the motor. Compressor cylinder lubrication system. Check if the lube oil systems are ready for operation and the manual valves are in the correct position. Generally, 10% increasing flowrate is performed by increasing suction pressure, but in many applications, two compressors are needed to send out enough flowrate downstream of the system. Bypass valve controllers should be activated by the operator when the compressor start sequence has been completed successfully. Inlet valve unloaders are used to deactivate a cylinder end and reduce its capacity to zero. Note that interlocks will only prevent the start sequence from continuing, or will interrupt the start sequence unless all specified conditions are fulfilled; interlocks are not trip signals. All purge and vent connections should be piped up to a single purge and vent connection. 3, Elsevier Science & Technology Books, 2005. Capacity control is obtained when the solenoid valve, fitted in the top cover, closes the access to the two cylinders, positioned under the same top cover. When post lubrication time has passed and the normal stop sequence is accomplished, the oil pump can either be stopped by the operator or continues running. Unloading the frame ends of the cylinders at the same time that the outer ends are unloaded will reduce the flow to zero and is recommended only for start-up due to excessive heat build-up inside the cylinder after extended operation. Posted on January 16, 2021 Written by. Processing & Handling, By Ali Ghanbariannaeeni Nargan Engineers and Constructors Ghazalehsadat Ghazanfarihashemi Sazeh Consultants | September 1, 2012. If the low low-oil-pressure signal lasts more than a specified time (usually 2 s), the compressor main motor should be stopped immediately by a shutdown (trip). Inlet valve unloaders. When barring over time (approximately 5 min) has elapsed, the operator should de-engage the barring device and remove any oil. In EExp” drivers, the motor becomes explosion-proof by pressurizing its cage. To achieve 0%, 50%, and 100% load-unload condition of air compressor, two numbers of unloaders are required to be used. Buradasınız: Ana Sayfa | Genel | screw compressor loading and unloading system. In multistage compressors, a bypass around the first stage or a partial bypass can be used. Before compressor startup, several parameters (such as level, pressure, temperature and flowrates) are checked and compared with set point values, specified by the compressor manufacturer in the set point list (set point lists include all instrument devices’ set values specifying alarm or trip signals). 5. One pump is used for normal operation (main pump) and the second is in standby mode as an auxiliary for necessary conditions. A temperature switch prevents overheating. These methods are bypass recycling, unloader valves, clearance pockets, stepless valves and variable-speed drives. Bloch, Heinz P., A Practical Guide To Compressor Technology, 2nd ed., John Wiley and Sons, 2006. After a certain time, suction pressure will reach stability again and restore the suction pressure to its normal value. Thus, considering two cylinders per compressor, the capacity can be controlled in five stages 100, 75, 50, 25 and 0%. As a rule, capacity control is determined by the compressor discharge pressure. During this time range, it allows multiple startups as long as they are within the over-lubrication time. However, using a divider block system is not recommended because of complexity and low reliability. Start inter-lock system. When a cylinder end is deactivated, the pulsation levels in the piping system can increase significantly. Next, the cylinder lubrication pump will be started if all preconditions are fulfilled. In auto mode, the main oil pump has to be started before starting the compressor main motor. A normal shutdown is manually initiated by the operator from the DCS or LCS, if the compressor needs to be stopped for reasons such as overhauling the machine or unit maintenance. The second reason is to save energy. The unloader actuator should be sized to operate on minimum air pressure as well. This provides complete unloading of one or both ends of the cylinder. 2. Area Electrical Classification in Research Applications: Is It the Only Mitigative Choice? The compressor should be fitted with a barring device (barring gear), which can be manual, electrical or pneumatic. To prevent this from happening, the bypass characteristic is linear and the stroke travel rate is approximately 2 min; however, this rate can be adjusted during compressor commissioning. At this time, barring over should be enabled from the local panel by pressing two buttons for operation of barring gear in clockwise and reverse direction. Valve unloaders are mechanisms that are held open or bypass one or more cylinders’ inlet valves at each end of double-acting cylinders. The barring device should be designed so that gas forces acting on the pistons cannot cause the compressor either to accelerate or to rotate in the reverse direction. Most reciprocating compressors are specified for constant speed operation to avoid excitation of torsional critical speeds. ©2021, Access Intelligence, LLC. Auto mode. An unloader valve is one of the many small but essential parts and components that ensure the effective, efficient operation of a reciprocating compressor. As the name implies, this control method uses an external bypass around the compressor to recycle gas from the compressor discharge to the inlet, or to the atmosphere in the case of an air compressor. Moreover, if purchase requisition states that the compressor shall run at 0% capacity for a long time, the compressor should run at 25% step capacity, and total flow is recycled by bypass valve thereafter. It blows off the pressure inside the discharge pipe when the compressor stops. At this stage, the frame oil pump is started, and the jacking pumps of the main motor and compressor will be started if pre-lube time, low oil temperature and low-pressure alarms are not active. For this reason, this method is commonly accompanied by the use of inlet valve unloaders or clearance pockets, which reduce compressor capacity in discrete predetermined steps [1–5]. The lowest automatic down-step will be to 25%. Reciprocating compressors are positive displacement machines in which the ... both port and plug unloaders reduce BHP/MMscf/D and significantly improve the reliability of the unloading system. The minimum capacity that can be obtained depends on the number of compression stages. Operation below 75% is normally done by bypass control and inlet valve unloaders or clearance pockets [9–12]. However, the automatic/pneumatic type is preferred. In this mode, all drivers are out of operation, and standby functions are disabled. A minimum number of valve unloader steps should be considered in order to maximize compressor reliability. Because of decreasing discharge pressure, the absolute power input would be reduced in this way, although the mass flowrate is increased as shown in Equations (1) and (2). (There is another type of unloader valve , which can be found at the inlet of rotary screw compressors , also called inlet valve or loading valve). The compressor has been barred over. It should be noted that during compressor operation, the heater shall always be stopped. Note that automatic control capability of all bypass valves will have been deactivated during startup. Lily Collins for L’Officiel Art Global Winter 2020 Issue. However, in actual designation, this limitation is decreased to 135°C (275°F) by compressor manufacturers. Conditions for up- and down-stepping parallel compressors. Gas engine drives, which could operate in a 75 to 100% speed range, make use of variable speed method for flow control. The compressor manufacturer must be consulted if the cylinder is to be run single acting with the frame end unloaded. Permission to start. Additionally, the following conditions should be satisfied: • Barring device in safe position (disengaged). No high-temperature alarm on the main motor. With AERprogress, the…, Kobe Steel, Ltd. (Tokyo, Japan) has turned its Chinese affiliate, Wuxi Compressor Co., Ltd., into a subsidiary after acquiring additional…, Siemens Gas and Power GmbH & Co. KG (Erlangen, Germany) has been selected to supply a range of compression and…, Unlock the Energy Potential of Your Plant, New Offerings Aid Heat Exchanger Design and Operations, Nanoscale uniformity of desalination membranes, Metal-oxide nanocrystals have applications in lubrication and electronics displays, A major project targets sustainable urea production, Magnetic field cures adhesives faster with less energy, A microreactor for continuous formation of Grignard reagents. Hence, frame and cylinder lubrication heater and tracing system should be on (temperature start permissive of motors). Therefore, three-step control is more detrimental to machine components — especially to valves. This publication contains text, graphics, images, and other content (collectively "Content"), which are for informational purposes only. For large reciprocating compressors and motors, manufacturers consider adding an additional system to the equipment in order to lessen the breakaway torque in the bearings in transient conditions (startup, shutdown or maintenance). Main motor purging system.The main-motor purge system is an independent working system. Bloch, Heinz P. and John J. Hoefner, Reciprocating Compressors Operation & Maintenance, Gulf Publishing Co., 1996. 10. and M.S.M.E. At this time, the alarm can be reset in the DCS manually. For the compressor starting procedure to be deemed successful, the position of the manual valves must be as follows: • Gas discharge isolating valves are open, • Blow-off valve and vent valve to safe location, flare, and atmosphere are closed, • Control valves and isolation valve on separator drain are open, • Shut-off valves for pressure instruments are open, • Separator drain valves have been put in auto mode, and their relevant interlocks are activated. Moreover, the cooling water specifications, such as flowrate, temperature and pressure, should be verified with design conditions by local instrument devices.

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