Numerous approaches are currently employed to avoid excessive vibrations that can occur in the connected piping, instrumentation, etc. These include:
1. Relying on vendors to design and supply adequate suppression dampers along with the compressor, without stating any specifications. Vendors may use digital calculations or proprietary empirical correlations. This is normally 5–7% of line pressure peak-to-peak pulsation at the bottle outlet unless otherwise specified.
2. Relying on vendors to design and supply suppression dampers, based on a specified maximum pulsation amplitude at the outlet of the damper. The specified pulsation may be 1–5% of line pressure and determined by estimating pulsation affects on the piping system.
3. Specifying the size of pulsation dampers based on empirical correlations within the Company.
4. Specifying that the vendor obtain an analog study of the pulsation dampers and associated piping, for machines not specified by API 618.
5. Specifying one of the three design approaches as outlined in API 618, as summarized below.
Most manufacturing and many producing locations employ analog studies (from SWRI) when purchasing new equipment. These studies are expensive, however, and may not always be appropriate. Analog studies have the disadvantage of not being very flexible. If changes in piping, vessels, or operating conditions are made after the analog study, the entire analog must be redone at additional cost. For example, Warren Petroleum, which employs a large inventory of integral and high-speed separable compressors used in varying applications, uses all of the previous approaches, as they determine to be applicable.
API 618 thoroughly specifies the requirements for pulsation suppression devices in terms of pressure drop, pulsation level, and mechanical design. Three design approaches are listed in API 618:
Design Approach-1 does not include a simulation study. Empirical factors and experience are applied, and everyone involved hopes for the best.
Design Approach-2 calls for a simulation study which reveals the pulsation amplitudes and frequencies, but does not predict how much the piping is going to vibrate.
Design Approach-3 is an extension of Design Approach 2 where the structural design of the piping is reviewed to determine its vibration tendency with respect to excitation by the gas pulsations.
Some compressor and damper vendors have digital computer programs to simulate compressor systems. They are faster, and may be less expensive than analog studies, and have proven to be reliable. Digital programs also have the advantage of saving a file which can be easily modified when changes are made.
For large, critical projects, it is recommended that SWRI continue to do this analytical work, because although digital programs have some advantages, SWRI has the overwhelming amount of simulation experience.