Compressors cylinder clearance

Reciprocating compressor capacity may easily be adjusted by changing compressor speed, changing compressor cylinder clearance, unloading compressor cylinder inlet valves, recycling gas from unit discharge to unit suction, or a combination of these methods. All these methods may be accomplished either manually by the operator or automatically by the control panel. 

Note that this requires or assumes that compressor cylinder clearance Vp be established. For studies, it may be assumed and the effects calculated. Values range from 5-35% clearance on actual cylinders. Special designs are used for smaller or larger values. 

The major source of fugitive emissions from a gas compressor cylinder is the piston rod packing. Other sources of fugitive emissions are around the cylinder valve covers, unloader covers, unloader actuator packing, and clearance pocket gasket and actuator packing. 

Figure 12-17B. Effects of clearance volume on performance efficiency of reciprocating compressor cylinder (valve design effect). (Used by permission Livingston, E. H. Chemical Engineering Progress, V. 89, No. 2, 1993. American Institute of Chemical Engineers, Inc. All rights reserved.)...

A twin-cylinder, single-acting compressor, working at 5 Hz, delivers air at 515 kN/m2 pressure, at fee rate of 0.2 m3/s. If the diameter of the cylinder is 20 cm, the cylinder clearance ratio 5% and the temperature of the inlet air 283 K, calculate the length of stroke of the piston and the delivery temperature. 

When multiple compressors are specified, only those required for the initial plant should be purchased. Adequate space must of course be allocated to meet expansion needs. If only one compressor is specified, one large enough to meet expansion needs should be specified. For centrifugal compressors the capacity can easily be varied over a wide range. The designer should, however, check to see that the initial requirements exceed its minimum capacity, which is called the surge point. In the case of reciprocal compressors, either a clearance volume can be added to the compressor cylinders or the suction valve can be removed. See reference 39 for a discussion of this. 

Equation (8.5) does not account for the volumetric efficiency (VE) loss of reciprocating-type compressors. Volumetric efficiency is simply the clearance allowed in the compressor cylinder head in which this compressed gas volume is allowed to mix with the inlet gas to be compressed in the next compression stroke. Thus this compressed gas in the cylinder clearance is recycling, which makes the output of the compressor less efficient, the larger this clearance volume. Volumetric efficiencies usually range fron 6 to 20%. An average reciprocating compressor VE should be 10%. For conservative cost calculating, use 15% of overall GHP for the VE efficiency. 

The centrifugal-type compressor is advantaged here, since it doesn t have a VE efficiency (cylinder clearance) to account for. There are no cylinders in a centrifugal-type compressor. More and more major oil/gas producers are therefore installing centrifugal compressors in their... 

Fig. 33.—Cards from high-pressure cylinder of a two-stage compressor with clearance...

The best way to unload a reciprocating compressor is to increase the cylinder clearance. When the piston completes the end of its stroke, the space between the piston face and the cylinder head is called the clearance. Increasing the clearance reduces the scfd flow of gas to a compressor running at a constant speed and a fixed suction pressure. The effect of adjusting the cylinder clearance is proportional to the suction stroke volumetric efficiency (Evs). Evs is calculated as follows ... 

Physically, the cylinder clearance is adjusted with a device similar to a rising-stem valve handle. Turning the handle clockwise pushes the adjustable cylinder head toward the piston, which increases the flow of gas through the compressor. This is the exact opposite of what normally transpires when we close a valve by turning the valve handle clockwise (see Fig. 25-1). 

For a constant speed driver such as motor, capacity of a reciprocating compressor can be controlled by Unloading inlet valve and/or unloaditig pockets or reservoirs at the cylinder. Lift suction valve (suction valve unloading) will prevent compression and open pocket to cylinder (clearance unloading) will increase cylinder clearance volume. Either action will reduce compressor capacity. If gas demand is intermittent, on-ofT control of motor can be used to control capacity. 

Figure 3-1. Labyrinth piston compressor. This non-iubed piston s circumferential labyrinths operate with a close clearance to the cylinder wall instead of rubbing. Courtesy of Sulzer)...

The second reason for modification of the displaced volume is that in real world application, the cylinder will not achieve the volumetric performance predicted by Equation 3.4. It is modified, therefore, to include empirical data. The equation used here is the one recommended by the Compressed Air and Gas Institute [1], but it is somewhat arbitrary as there is no universal equation. Practically speaking, however, there is enough flexibility in guidelines for the equation to produce reasonable results. The 1.00 in the theoretical equation is replaced with. 97 to reflect that even with zero clearance the cylinder will not fill perfectly. Term L is added at the end to allow for gas slippage past the piston rings in the various types of construction. If, in the course of making an estimate, a specific value is desired, use, 03 for lubricated compressors and. 07 for nonlubricated machines. These are approximations, and the exact value may vary by as much as an additional. 02 to. 03... 

An additional capacity control method is the unloader. This method can be used in conjunction with clearance pockets to extend the range of control to zero capacity. On double-acting cylinders, unloading the individual sides one at a time will provide a two-step unloading of the cylinder. On multicylinder arrangements, the cylinders can be unloaded one at a time providing as many steps as cylinders operating in parallel. The unloaders can also be used to totally unload the compressor, as is necessary for electric motor driver startup. 

The final element is the compressor guide-vane mechanism. The variable clearance points are adjusted by means of a positioning cylinder that is operated by a servo valve in response to a signal from the flow controller. 

Fixed clearance can also be added to the outer end of the cylinder by adding a fabricated clearance bottle with the desired volume. To change the performance of the cylinder the clearance can be changed by shutting down the compressor, unbolting one bottle, and installing another bottle with a different volume. It is very easy in that respect to add clearance and subtract clearance from a cylinder if the cylinder is set up to receive clearance bottles. 

Late in the field life it is desirable to compress the 100 MMscfd for the example field downstream of the separator from 800 psig at 100°F to l.(XX) psig. An engine-driven separable compressor is available from surplus. The engine is rated for 1,600 hp at 900 rpm. Horsepower is proportional to speed. The compressor frame has six 7-in, bore by 6.0-in. stroke double-acting cylinders with a minimum clearance of 17.92%, a rod load limit of 25,000 lb, and rod diameter of 1.75 in. Assume k = 1.26, Z, -0.88, and Zd = 0.85. 

Piston displacement is the actual volume of the cylinder displaced as the piston travels its stroke from the start of the compression (condition (1)) to the end of the stroke (condition (e)) of Figure 12-12 expressed as fF of volume displaced per minute. Displacement values for specific cylinder designs are available from the manufacturers, Table 12-6. Neerken is a useful reference. Reciprocating compressors are usually rated in terms of piston displacement, which is the net volume in ft per minute displaced by the moving piston. Note that the piston does not move through the clearance volume of Figure 12-12 therefore this volume is not displaced during the stroke. 

Compressor buildings are often elevated to allow the necessary clearance for the installation of drums direcdy below the cylinders. Suction drums are usually located on top of the cylinders. Installation outside the compressor building is considered poor practice, except in special cases. 

In a two-cycle machine, all pistons complete a full cycle each time the crankshaft completes one revolution. Figure 44.9 illustrates the normal action of a two-cycle, or single-action, compressor. Inlet and discharge valves are located in the clearance space and connected through ports in the cylinder head to the inlet and discharge connections. 

During the suction stroke, the compressor piston starts its downward stroke and the air under pressure in the clearance space rapidly expands until the pressure falls below that on the opposite side of the inlet valve (Point B). This difference in pressure causes the inlet valve to open into the cylinder until the piston reaches the bottom of its stroke (Point C). 

In piston-type compressors, the oil serves three functions in addition to the main one of lubricating the bearings and cylinders. It helps to seal the fine clearances around piston rings, piston rods and valves, and thus minimizes blow-by of air (which reduces efficiency and can cause overheating). It contributes to cooling by dissipating heat to the walls of the crankcase and it prevents corrosion that would otherwise be caused by moisture condensing from the compressed air. 

A typical cycle for a compressor with a finite clearance volume can be followed by reference to Figure 8.40. A volume V i of gas at a pressure P is admitted to the cylinder its condition is represented by point 1. 

Air at 290 K is compressed from 101.3 kN/m2 to 2065 kN/m2 in a two-stage compressor operating with a mechanical efficiency of 85 per cent. The relation between pressure and volume during the compression stroke and expansion of the clearance gas is PV1-25 = constant. The compression ratio in each of the two cylinders is the same, and the interstage cooler may be assumed 100 per cent efficient. If the clearances in the two cylinders are 4 per cent and 5 per cent respectively, calculate ... 

HA A single-stage double-acting compressor running at 3 Hz is used to compress air from 110 kN/irr and 382 K to 1150 kN/nr. If the internal diameter of the cylinder is 20 cm, the length of stroke 25 cm and the piston clearance 5%, calculate (a) the maximum capacity of the machine, referred to air at the initial temperature and pressure, ami power requirements under isentropic conditions. 

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