Pulsation Control

The intermittent personality of the reciprocating compressor becomes evident when the subject of pulsations is broached. Because discharge flow is interrupted while the piston is on the suction stroke, pressure pulses are superimposed on the discharge system's mean pressure. At the suction side of the system, the same type of interruption is going on, causing the suction pressure to take on a non-steady component. The frequency of the pulses is constant when the speed is constant, which is the most normal condition. The pulses are literally that, not sinusoidal in characteristic; therefore, if the frequency spectrum is analyzed, it will be found to contain the fundamental frequency and a rich content of harmonics. When a forcing phenomenon is superimposed on a system with elastic and inertial properties (a second order system), a resonant response is likely to occur. This is particularly true when the band of exciting frequencies is as broad as the type of system under consideration. The gas system meets the criteria of the second order system, as gas is compressible (elastic) and has inertia (mass). If left unchecked, and a resonant response were to occur, the pressure peaks could easily reach a dangerous level. Because the oscillations are waves, standing waves will form, and interference with valve action may occur, adversely affecting the cylinder performance.

While a single, low pressure compressor may require little or no treatment for pulsation control, the same machine with an increased gas density, pressure, or operational changes may develop a problem with pres sure pulses or standing wave performance deterioration. As an installation becomes more complex, such as with an increase in the number of cylinders connected to one header and the use of multiple stages, the possibility of a problem can increase.

When an installation is being planned, it is recommended that the API Standard 618 be reviewed in detail. The pulsation level for API 618 at Design Approach 1, the outlet side of any pulsation control device regardless of type, should be no larger than 2% peak-to-peak of the line pressure, or the value given by the following equation, whichever is less

where

Figure 3-26. Manifokl-type volume bottles are used where cylinders are operated in parallel, as on this two-stage, motor driven compressor. (Courtesy of Dresser-Rand)

P% = maximum allowable peak-to-peak pulsation level at any discrete frequency, as a percentage of average absolute pressure.

P|jlse. = average absolute line pressure.

The objective of this approach is to improve the reliability of the system without having to design acoustical filters. For many systems, this is al! that is needed. API 618 contains a chart that recommends the type of analysis that should be performed, based on horsepower and pressure.

The pulsation control elements can have several forms, such as plain volume bottles, volume bottles with baffles, bottles and orifices, and proprietary acoustical filters. See Figure 3-26 for an example of a compressor with a set of attached volume bottles. Regardless of which device or element is selected, a pressure loss evaluation must be made before the selection is finalized because each of these devices causes a pressure drop.

For those installations where a detailed pulsation analysis, API 618 Design Approach 2 or 3, is required, several consulting companies offer these services. Until the 1980s, the most common method was to perform the pulsation analysis on the analog simulator of the Pipeline and Compressor Research Council of the Southern Gas Association. The

Figure 3-26. Manifokl-type volume bottles are used where cylinders are operated in parallel, as on this two-stage, motor driven compressor. (Courtesy of Dresser-Rand)

procedures used in planning a new installation were to include the pulsation study in the contract with the compressor vendor. During the analog pulsation study, the isometric piping drawings were used to create a lumped model of the piping connected to the compressor cylinders. The purchaser's representative was required to be present for the analog study and had to be familiar with the piping requirements for the compressor area. This was necessary so that decisions relative to the space available and location for the bottles, as well as feasibility of piping modifications could be made during the study. The representative helped to expedite the completion of a final configuration for the piping system and bottle location since the analog components were disassembled after the study was completed. The analog method is still used, although much less frequently.

With the advent of modern workstations and faster PC computers, the solution of the differential equations of motion for acoustical waves in piping system on a digital computer has become feasible. In current prae tice, pulsation design studies using digital computer technology can produce the same results as obtained with a dynamic simulation on the analog system. The results from digital simulation satisfy the requirements of API 618. The digital computer has the advantage of data file storage. With storage capability and the ability to readily manipulate the data, it is not as necessary to have immediate decisions made. Piping changes that are recommended for acoustical control can be evaluated in a more comprehensive manner taking into account safety, cost, maintenance, and operational considerations. An additional benefit is realized if system changes are anticipated at a later time. The data files can be retrieved and the system rerun with the changes to the thermophysical properties or m the piping system itself without the need to remodel the entire system.

The interpretation of the results and the quality of the design from the pulsation study, whether performed on the analog simulator or with digital computer simulation, depends quite heavily on the experience and skill of the analyst performing the study. A purchaser of a compressor system who may be a novice at this type of analysis should give serious consideration to using the services of a competent consultant.

For the purpose of quick estimates or field evaluation of existing systems, consider the curve in Figure 3-27. This curve is not meant to supersede a comprehensive analysis as previously discussed. It should be used in checking vendor proposals or in revising existing installations where a single cylinder is connected to a header without the interaction of multiple cylinders. While not a hard rule, the curve should be conservative for

use a 1 for the speed and N to determine the piston displacement lor a single revolution. Apply the multiplier to the piston displacement per revolution. The product is the bottle volume, Vol, for use in Equation 3.22 This equation will yield the bottle diameter.

To complete the solution for the volume bottle dimensions, assume 2:1 elliptical heads and use the following relationship:

I ,b = volume bottle length db = volume bottle diameter

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Responses

  • Anna
    What is pulsation control in compressor?
    3 years ago

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