Piston speed

Another value to be determined is piston speed, PS. The average piston speed may be calculated by... 

The basis for evaluation of piston speed varies throughout industry. This indicates that the subject is spiced with as much emotion as technical basics. An attempt to sort out the fundamentals will be made. First, because there are so many configurations and forms of the reciprocating compressor, it would appear logical that there is no one piston speed limit that will apply across the board to all machines. The manufacturer is at odds with the user because he would like to keep the speed up to keep the size of the compressor down, while the user would like to keep the speed down for reliability purposes. As is true for so many other cases, the referee is the economics. An obvious reason to limit the speed is maintenance... 

After all the previous statements, it would seem very difficult to select a piston. speed. For someone without direct experience, the following guidelines can be used as a starting point. Actual gas compressing experience should be solicited when a new compressor for the same gas is being eonsidered. These values will apply to the industrial process type of compressor with a double-acting cylinder construction. For horizontal compressors with lubricated cylinders, use 700 feet per minute (fpm) and for nonlubricated cylinders use 600 fpin. For vertical compressors with lubricated cylinders, use 800 fpm and for nonlubricated cylinders use 700 fpm. 

Calculate the suction capacity, horsepower, discharge temperature, and piston speed for the following single-stage double acting compressor. 

Step 3. Piston speed is calculated using Equation 3.8 converting the stroke to feet by dividing the equation by 12 inches per foot ... 

Several steps can be taken to maximize the run time for the reciprocating compressor. Since wear is a function of rubbing speed, the piston speed can be kept to a minimum. Chapter 3 made recommendations for piston speed. Reliability problems due to valves are reputed to account tor 40% of the maintenance cost of the compressor. Valves are the single largest cause for unplanned shutdowns. Basically, valve life can he increased by keeping the speed of the compressor as low as practical. At 360 rpm, the valves are operated six times a second. At 1,200 rpm, ihc valves operate 20 times a second or 1,728,000 times in a day. It is not difficult to understand why the valves are considered critical. To keep the reliability in mind, valve type, material selection and application considerations such as volume ratio, gas corrosiveness, and gas cleanliness need attention by the experts. One final note is that while lubrication is an asset to the rubbing parts, it is not necessarily good for valve reliability. 

Piping of axials, 248 Piston engine, 53 Piston rings, 68, 69 Piston speed. 57, 58 Pitch, 226... 

The similarity solution for a flow field in front of a steady piston is a special case from a much larger class of similarity solutions in which certain well-defined variations in piston speed are allowed (Guirguis et al. 1983). The similarity postulate for variable piston speed solutions, however, sets stringent conditions for the gas-dynamic state of the ambient medium. These conditions are unrealistic within the scope of these guidelines, so discussion is confined to constant-velocity solutions. 

At = Temperature rise, °F AT,. = Temperature rise, °F/rain t = Piston speed or travel, ft/min V = Liquid velocity, ft/sec v = Average velociry, ft/sec W = Width of channel vtith series pump, ft Wj = Weight of liquid in pump, lb whp = Water or liquid horsepower... 

Figure 12-8A. Piston rings. The piston rod is manufactured from heat-treated stainless steel and is coated with wear-resistant overlays, such as ceramic, chromium oxide, and tungsten carbide applied by plasma techniques. Piston rod cross-head attachment has mechanical preloading system for the threads. Rider rings and seal rings are manufactured from PTFE filled resins fillers are matched to the gas, piston speed, and liner specifications. Typical fillers are glass, carbon, coke, or ceramic. (Used by permission Bui. BCNA-3P100. Howden Process Compressors Incorporated. All rights reserved.)...

Piston speed is a useful guide to set relative limits on compressor cylinder selection. It is difficult to establish acceptable and nonacceptable limits because this is best evaluated with operating experience and compressor manufacturer s recommendations. 

Cg,. = compression efficiency, the product of adiabatic and reversible efficiencies, which vary with the cylinder and valve design, piston speed, and fraction values range from 0.70-0.88 usually. 

Modern twin-headed pumps use two pistons driven by a cam or gear that is shaped so as to make the piston speed constant. Ideally, the output of one such head should be as shown in Fig. 2.2e (Hi), operation of two heads 180°out of phase producing a pulseless flow. In practice, on each drive stroke the change in flow rate is not instantaneous giving an output shown in (iv). To overcome this, the driving cam is arranged to make the piston travel faster on the refill than on the drive stroke, producing an output shown in (v). The flow rate, which is the sum of the output of both heads, is constant. 

Fig. 263. Piston speed during pressing. Fig. 264. Hydraulic pressure in terms of...

Two distinct sections ab and cd of different piston speed can be seen in Fig. 263. The speed for the first section ab is distinctly high. This is followed by a short, transistory section be, that passes into the third section cd in which the speed becomes steady. The diagram in Fig. 264 shows that from the point b a sudden rise of pressure begins and persists for the first two minutes, while curve P in Fig. 265 shows that... 

In the serial arrangement (Fig 3.2), the first piston pushes a volume of solvent twice that of the second piston, due either to the fact that its diameter is greater or that it travels a greater distance. The volume of solvent contained in piston B is released into the column while piston A is filling with solvent. Then as piston A pushes the mobile phase, part of it is used to fill piston B, which regulates the nominal flow. These arrangements demand the use of check valves at the input and output of solvent. To regulate the flow rate, the piston speed is controlled by a motor. 

Operating speeds of larger units are as high as 277 rpm with piston speeds for air service up to 4.3 m/sec. The larger compressors with provision for multiple services reduce the number of motors or drivers and minimize the accessory equipment, resulting in lower maintenance cost. 

Compressors for oxygen service are characteristically operated at lower piston speeds, of the order of 3.3 m/sec. Maintenance of these machines requires rigid control of cleaning procedures and inspection of parts to ensure the absence of oil in the working cylinder and valve assemblies. 

It has been a profound enigma that polymers other than linear polyethylene rarely exhibit any flow instabilities such as flow oscillation under controlled piston speed. As to be discussed in later sections, we can generally understand this... 

Evidence for wall slip was suggested over thirty years ago [9,32,63]. One of the first attempts at a slip mechanism was the performance of a Mooney analysis by Blyler and Hart [32]. Working in the condition of constant pressure, they explicitly pointed out melt slip at or near the wall of the capillary as the cause of flow discontinuity. On the other hand, they continued to insist that bulk elastic properties of the polymer melt are responsible for the flow breakdown on the basis that the critical stress for the flow discontinuity transition was found to be quite insensitive to molecular weight. Lack of an explicit interfacial mechanism for slip prevented Blyler and Hart from generating a satisfactory explanation for the flow oscillation observed under a constant piston speed. 

In summary, the origin of the oscillating flow (sometimes termed slip-stick regime) observed in the constant piston speed mode is the oscillation of the HBC between the no-slip and slip states due to a reversible coil-stretch transition of either adsorbed chains or the first layer of unbound chains entrapped with the adsorbed chains. The experimental demonstration of an abrupt large stick-slip... 

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