Vibration pulsation

However, the apparatus that gives the most precise measurement of ps — °p0 works in a completely different manner. This apparatus was devised by Kratky et al.13 It consists of a tube, one end of which is set in a vertical wall, and of a system which excites transversal vibrations of this tube (see Fig. 5,14). The tube is filled with solution. The tube-plus-solution device constitutes a system which has its own vibration pulsation to. The mass per volume of solution ps is obtained from the observed value of to. The whole thing can be compared to a vibrating beam and the elastic forces are practically independent of the solution. 

Most rotating equipment includes electric motors or steam dryers that generate noise at a constant frequency. Air cooler fans are a source of noise that can be reduced by lowering the fan speed and increasing the number of blades. Pump motor noise can be reduced by including a shroud or fan cover that is accurately lined. Centrifugal compressor noise reduction can be achieved by blade design and the use of compressor pulsation noise reduction, silencers, and vibration isolation. 

When a pulsation frequency coincides with a mechanical or acoustic resonance, severe vibration can result. A common cause for pulsation is the presence of flow control valves or pressure regulators. These often operate with high pressure drops (i.e., high flow velocities), which can result in the generation of severe pulsation. Flashing and cavitation can also contribute. 

The no-load test is a very informative method to determine the no-load current, core and pulsation losses, friction and windage losses, magnetizing current and the no-load power factor. The test also reveals mechanical imbalance, if any, performance of the bearings, vibration and noise level of the motor. 

Loose casing and support Pressure pulsations Vibration transmission Gear inaccuracy Valve vibration Dry whirl Blade passage... 

Nimitz, Walter W., Pulsation and Vibration, Part I. Causes and Effects, Part II. Analysis and Control. Pipe Line Industry, Part 1, August 1968, pp.. 6-79. Part II, September 1968, pp.. 19 2. 

Screw compressors of the dry type generate high frequency pulsations that move into the system piping and can cause acoustic vibration problems. These would be similar to the type of problems experienced in reciprocating compressor applications, except that the frequency is higher. While volume bottles will work with the reciprocator, the dry type screw compressor would require a manufacturer-supplied proprietary silencer that should take care of the problem rather nicely. 

Reciprocating compressor pulsations were covered in Chapter 3, but neeti to be mentioned with the discussions on reliability. Problems with reciprocating compressor pulsations and the potential for acoustic and mechanical resonances are very similar to those experienced with helical-lobe compressors. The significant difference is the frequencies are much lower and the number of discrete frequencies per compressor aie much less. However, piping vibrations can occur and there is always a... 

Slow speed requires high degree of vibration and pulsation suppression. Vane-Type Rotary Compressors... 

Each cylinder should have suction and discharge pulsation bottles to dampen the acoustical vibrations caused by the reciprocating flow,... 

Because of the reciprocating action of the piston, care must be exer-ci.sed to size the piping to minimize acoustical pulsations and mechanical vibrations. As a rule of thumb, suction and discharge lines should be sized for a maximum actual velocity of 30 ft/.sec (1,800 ft/min) to 42 ft/sec (2,500 ft/min). Volume 1 contains the necessary formulas for determining pressure drop and velocity in gas piping. 

Mechanical vibration of pipe is handled in the same manner as for reciprocating pumps (Volume 1, Chapter 12). Normally, if the pipe support spacing is kept short, the pipe is securely tied down, the support spans are not unifoiTn in length, and fluid pulsations have been adequately dampened, mechanical pipe vibrations will not be a problem. It is good practice to ensure that the natural frequency of all pipe spans is higher than the calculated pulsation frequency. The pulsation frequency is given by ... 

When specifying compressor packages to API 1 IP, it may be necessary to specify certain sections of API 618 to ensure satisfactory installations. An example of this would be the supply of multiple compressors to be located in pipeline booster stations. In this case, an analog or digital pulsation and vibration study per API 618 Section 3.9 would be advisable to improve reliability and to minimize system problems and potential damage caused by gas pulsations and interaction between the individual compressor packages. 

In many instances, the design of suction and discharge pulsation dampening drums (or bottles) for reciprocating compressors is based on piston displacement and volumetric efficiency, and this design normally will suffice to reduce peak pulsation to approximately 5% of the line pressure. In special or other cases, experience has shown that operational difficulties (vibrations, meter pulsations, etc.) may indicate that the peak pulse pressure of 5% line pressure is inadequate. Thus, the pressure in pulsation-reduction design selection is... 

For the application and design of a reduced pulsation-vibration system, the acoustic computer technique developed by Southwest Research Institute for the Gas Machinery Research Council in cooperation with the Pipeline and Compressor Research Council is considered the most... 

As von Nimitz points out, only cyclic stresses are directly related to failure probability. These stresses are often produced by pulsations in the fluid system, by mechanical vibrations produced by the mechanical movement of certain equipment components, and as a result of the fluid pulsations. Figure 13-8 lists the sequence of events that leads to most failures of equipment and piping. 

Figure 13-8. Relationship of pulsations to vibrations to cycle stresses. (Used by permission von Nimitz, W. W. Lecture of Reference 13, Part 1, Table 1, proceedings of the 1974 Purdue Compressor Technology Conference.)...

Vibration control is one of the key objectives behind any pulsation suppression. Therefore, the end result of much effort is to reduce the magnitude of the measured mechanical vibration. Figure 13-9 provides frequency-variable allowable vibration level criteria. A typical fixed maximum peak-to-peak vibration movement of 8 mil is a reasonable reference however, for many critical applications, a vibration of 2-4 mil is all that can safely be tolerated. Figure 13-10. 

To size a pulsation dampener so that vibrations may be held to a very minimum (size dampener for discharge side only for this example) ... 

---