Reciprocating Compressors

In reciprocating compressor systems, the flow of fluid through the suction and discharge valves is intermittent, resulting in periodic pressure fluctuations which are known as acoustical pulsation. The magnitude and shape of this pulsation are influenced by the physical, geometrical, and mechanical characteristics of the compressor system. Although pulsation cannot be entirely eliminated, it can be effectively attenuated through a comprehensive acoustical pulsation analysis.

The acoustical pulsation analysis involves calculating pressure pulsations, pulsation-induced shaking forces, and pressure drops resulting from the specific attenuation measures. Typical attenuation strategies include design of pulsation suppression devices and/or orifice plates and modification of piping layouts. By performing an acoustical pulsation analysis to optimize these measures, the pressure pulsations, shaking forces, and pressure drops can be reduced to levels within the API 618 pulsation guidelines.

API 618 Design Approaches

API 618 (6th Ed.) specifies two design approaches for pulsation analysis of reciprocating compressor packages: Design Approach 2 (DA2) and Design Approach 3 (DA3). These approaches are defined as follows:

• DA2 — Acoustic Simulation and Piping Restraint Analysis.
• DA3 — Acoustic Simulation and Piping Restraint Analysis plus Mechanical Analysis (with Forced Mechanical Response Analysis if necessary).

The table below outlines the criteria for selecting the appropriate design approach based on the cylinder power and discharge pressure of compressors.

The API 618 (6th Ed.) DA3 Steps details and the comparison with API 618 (4th Ed.) pulsation design study procedure are summarized as follows:

API 618 Pulsation Guidelines

(1) Maximum allowable pulsation in piping system

Per API 618 (6th Ed.) Clause 7.11.7.3.1, the allowable pulsation limit in the piping system with absolute line pressures above 3.5 bara (50 psia) is specified as,

Where,

For absolute pressures less than 3.5 bara (50 psia), the above equation shall be multiplied by

(2) Maximum allowable pulsation at compressor cylinder flange

Per API 618 (6th Ed.) Clause 7.11.7.2, the allowable pulsation limit at the compressor cylinder flange is specified as,

Where,

(3) Maximum allowable pressure drop

Per API 618 (6th Ed.) Clause 7.11.8.1, the steady flow pressure drop shall not exceed 0.5% of mean absolute line pressure at pressure ratios less than or equal to 1.4. At pressure ratios greater than 1.4, the value is specified as,

Where,

If the pulsation control device includes an integral moisture separator, the steady flow pressure drop shall not exceed 0.6% of mean absolute line pressure at pressure ratios less than or equal to 1.4. At pressure ratios greater than 1.4, the value is specified as,

(4) Maximum allowable piping acoustic shaking force

Per API 618 (6th Ed.) Clause 7.11.7.4, the allowable piping non-resonant shaking force limit is specified as,

Where,

The shaking force guideline is applied to non-resonant vibration. Shaking forces near resonance shall be reduced well below the guideline specified above.

In the above equation, SFk and SFpmax are specified as,

Where,

(5) Maximum allowable pulsation control device acoustic shaking force

Per API 618 (5th Ed.) Clause 7.11.7.4, the allowable pulsation control device non-resonant shaking force limit is specified as,

Where,

The shaking force guideline is applied to non-resonant vibration. Shaking forces near resonance shall be reduced well below the guideline specified above.

In the above equation, SFk is specified as,

Where,

Acoustical Simulation Tools

Several analysis tools are available for acoustical simulations of reciprocating compressor packages. At CCPGE, we use Bentley PULS XM Option 3, BOSpulse, and an in-house developed program to perform the acoustical simulations in accordance with the latest API 618 Standard.

Bentley PULS XM Option 3 utilizes the transfer matrix approach to relate acoustical pressure and acoustical volume velocity across all elements of the model. It calculates pulsation-induced shaking forces along pipe segments and vessels, predicts pressure drops across each element, and compares the calculated pulsations, shaking forces, and pressure drops with the allowable limits specified by API 618 standard.

BOSpulse employs both transient (time domain) and harmonic (frequency domain) solution methods for acoustical simulations of reciprocating compressor systems. This software enables analysts to simulate periodic pressure pulsations in piping systems, compare the results against API 618 limits, and assess the impact of these pulsations on the structural behavior of the system.

At CCPGE, we have also developed an in-house pre- and post-processing program to manage the input and output data from these commercial tools across all operating cases. This program significantly accelerates the simulation process while maintaining high quality to better serve our clients' interests.

Acoustical Pulsation Analysis Example

An example of acoustical pulsation analysis for a reciprocating compressor package with the following specifications is illustrated below. The analysis was performed using Bentley PULS XM Option 3 software in accordance with API 618 Standard.

The calculated acoustical pulsations, represented by the maximum pulsation violation ratio, for the 1st inter-stage system of the compressor package, both before and after design modifications, are shown in the following figures. The design modifications aim to implement pulsation attenuation measures which include the design of suitable pulsation bottles with internal choke tubes and baffles, and the use of orifice plates to mitigate excessive pulsations and shaking forces.