# Engineering Calculator » Pipe Support Span

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## Guideline for supports location

INERTANCE provides the pre-design calculator for pipe support span length against pulsation excitation from running speed of positive displacement equipment. This calculator is prepared for the initial piping support design of reciprocating compressor, to minimize the modification of the piping system due to mechanical study. After the initial piping arrangement is set-up, the final support location shall be determined by acoustic and mechanical study results.

Separation margin is defined as margin between the harmonic forcing excitation and the natural frequency of a compressor, pump or piping system element. Generally a 10% actual separation margin is available, and further margin for 10% uncertainty in natural frequency predictions. Therefore, total 20% of design separation margin is recommended.

For double acting compressor, the significant pulsation-induced forces at one and two times of rotational speed will be generated. Hence, resonance shall be avoided at these frequencies, suggesting predicted mechanical natural frequencies should be at least 20% above two times of running speed.

## How to use this calculator

See below how to use this support span calculator

1. Input the equipment operating speed (RPM).
2. Press ‘Tab’ on keyboard to move next space.
3. To specify the equipment type with single acting or double acting. Support span length will be calculated for single acting type based on one (1) time of running speed, for double acting type based on two (2) times of running speed.
4. To specify the separation margin to be specified. (Typically 20% margin is recommended as per API 618 5th, para. 7.9.4.2.5.3.2)
5. To specify the pipe support type with simply supported (S) or fixed supported (F).

The piping support span length should be determined to avoid any coincidence of excitation frequencies with mechanical natural frequencies of the compressor, pump and piping system. The minimum mechanical natural frequency of piping system element shall be designed to be greater than 2.4 times maximum rated speed of equipment. It should be noted that the meaning of ‘2.4 times of speed’ is to be above twice running speed, because there is almost always sufficient excitation energy at the first and second orders to excite resonances to a dangerous level.

## Pipe Support Span Calculator

In order to ensure that the gas and auxiliary piping system is free from equipment running speed, INERTANCE recommends that a bump test of the piping to be performed prior to start-up to determine the piping natural frequency. Even-if piping support span length has been determined as per this guideline, site construction tolerance and unknown site adjustments should be considered. In addition, the mass flow rate of process fluid may be considered for high accuracy estimation.

The results of this calculator shall be used only for reference, and shall not be checked as guaranteed points. Final support arrangement from positive displacement equipment shall be finally determined via Design approach 3 procedure for acoustic pulsation and mechanical analysis by INERTANCE.