Hi

my question is with regards to the safety of the systems connecting to rotary/reciprocating machines
I would like to know what is the safe distance for a part of the system which resonates with the frequency source? do I only have to worry about parts which are directly connected to the pump?

Hello,

Based from my experience, for reciprocating machines you need to coordinate with the pulsation consultant. Normally, the pulsation study if up to a large volume like a vessel (but of course not the pulsation bottle because after pulsation bottle there are still some pressure pulsations that can be transferred to piping).

They normally will give you some criteria regarding what frequency cut-off you need to run a modal analysis (e.g. 1.2 times the 2nd harmonic of the compressor speed). In this case, once they run their pulsation analysis, minimal changes are required such as adding orifice to minimize pressure pulsation to acceptable level. They would give you also dynamic forces which must be restrained by hold-down type of support and must be tightened enough to create a restraining force greater than the dynamic force.

Also T-Post type of support is not recommended, as much as possible run the pipe at grade. You need to inform Civil group also to design a support with a frequency higher than the cut-off frequency to avoid resonance also. I think a sleeper type of support works best. But if unavoidable, need to design a support with enough stiffness.

Any other opinion is highly appreciated. Just correct me if I have some wrong statements.

Cheers,

Hi Borzki,

thanks for your response, this is not exactly the answer I was looking for; I am aware of how to do the pulsation analysis and have done it in the past several times. my question was regarding the resonance of the piping system with the pump; I want to know if for instance the frequency of a part of the system resonates with the frequency of the pump but that particular part is located 50m away from the pump, should I be concerned about it? or to what extent or proximity to the source of the frequency, resonance actually matters?

Hello,

If the source of resonance is the pump mechanical vibration (e.g. pump mis-alignment) and not related to flow induced vibration I think the only concern is near the pump. Is it centrifugal pump or reciprocating pump?. Is it field vibration problem or design stage problem?. If it's a field vibration problem then maybe a harmonic analysis is required or some field criteria.

So far during design stage, I only experience reciprocating machines.

Any other opinion is highly appreciated. Just correct me if I have made wrong statements.

Cheers!!!

Imagine trying to estimate the length of a straight trumpet where sounds (pressure waves) out the end are completely deadened because

a) the flexibility in the tubing walls finally managed to kill enough energy from the high peaks of your sound waves to level it down to a single level
b) losses from the tubing allowed the peaks in your sound wave to shear backwards and downwards and your valleys to shear forwards and upwards until you level out to a steady pressure.

So, my answer would be until the effects are intentionally and by design dampened through a restriction orifice or dampener, consider these effects in activity, until a formalized acoustic analysis is performed.

As a side note, the only thing I've seen that remotely resembles this is where the Engineering Institute Guidelines for FIVs touches on AIVs from high pressure losses and tells you which fittings to look at some distance L from the disturbance. But I wouldn't be quick to draw any kind of conclusions from this info.

Thanks Michael for that opinion. It's really hard to determine where the resonance will stop until a formal acoustic analysis is performed. And sometimes field vibration still happens even though this type of analysis is carried out due to some unforeseen and unexpected scenario or maybe not so stiff support system that reflects the design requirement.

Any other opinion is highly appreciated.


Cheers!!