is there a guideline or standard on what is the acceptable way to arrange the trip logic for our Compressor's axial and radial vibration sensors? is it 1-out-of-1, 2-out-of-2, 2-out-of-4, etc or what?

 

Kalan... paraphasing an oldER philosopher, "Ah, what fools we mortals be when asked to chose three!"

Available to you, at no cost, is my paper, "Probabilistic Assessment of Equipment Safety Systems!" It compares the numerous methods you have expressed interest in.

It also discusses the Sacred-Cows of SIS design, one of them being the TMR or 2-of-3 voting scheme.

Regards, Phil Corso (cepsicon [at] aol [dot] com)

 

Despite there are industrial standards (ISO, API etc., based on the dynamic stiffness of pedestal bearings ) for vibration protection philosophy, under certain criteria some companies might deviate and use their own best practices for protection and monitoring of their rotating equipment. Breakeven point must be identified for total investment (purchase installation and maintenance costs). In other words, the use of these vibration monitoring and protection systems must be also economical justified.

Depending on the type of compressor used and the actual mechanical configuration applied, the use of protection philosophy will change. Just to give you some examples;

1. Centrifugal compressor
These compressors are designed for volume increase and usually designed for high operating speed. Minimum protection should consist of 2oo2 axial thrust position monitoring probes. Increasing the use of third probe (2oo3), will result in increasing the reliability of the thrust position monitoring.

The shaft / bearing monitoring is typically performed by using of two radial probes (none contacting, X and Y position) per bearing.

2. Axial compressor
These types of compressors are designed for mass flow and subject to different rotor dynamics.

In addition to same configuration as the centrifugal compressor (radial and axial vibration monitoring), use of seismic probes can be considered on the bearing housings as well as on the casing.

Sometimes overprotecting your equipment might lead to spurious trips and this will lead to possible loss of production (reliability). While under protecting your machinery might lead to catastrophic failure which will also result to loss of production (availability). In both cases the cumulative production availability of your plant will drop.

Implementation of the protection philosophy;
While the radial vibration is typically used for condition monitoring purposes (alarm only), trip function can be considered based on 2oo4 configuration (read this as any 2 probes out of the 2 bearings). This configuration will increase the reliability of your equipment due to the fact that single or two probe failures will not trip your compressor. High radial vibration on the one end of the compressor will also result in the other end (single shaft two bearings).

The axial thrust monitoring can be configured as 2oo3.

While this protection philosophy is deemed to be sufficient for centrifugal compressors, the axial compressor vibration protection configuration might differ; Instead of using the radial probes as trip function (2oo4), one can consider to use this as only alarm function and rely on the seismic protection as 2oo2 per bearing. Seismic protection is a robust protection.

Basically radial probes will detect the vibration faster as the seismic. Vibration frequencies will be damped due the oil film and the bearing housing and as a result the seismic probe will have measurable delay.

Good Luck,
A. Oztas

 

Dear Sir

We are facing similar situation where we have a blower gearbox having 1 of 2 logic tripped due to malfunctioning of one the probe. I request you whether you can send the tech. paper mentioning on "Probabilistic Assessment of Equipment Safety Systems!" Our issue is as following, Gearbox high speed side only one bearing installed with X/Y probes. in Bently 3500 system it is configured like if X or Y probes reaches trip level it will activate relay channel to initiate trip of the machine. We considering to change it to 2oo2 logic

Thanking you.

Best Regards,
Pola Sreenivasulu

 

Pola... send me your e-mail address!

Regards, Phil Corso

(Cepsicon [at] aol [dot] com)

 

Mr. Phil,

I have read your article Compressor's vibration sensor voting logic which gave me very comprehensive approach to understanding of optimizing safety and reliability of equipments especially in rotating turbomachinery. Presently we are evaluating the same for implementing in high speed centrifugal compressor radial vibration probe and axial displacement. However, when we consider axial displacement as a cause of catastrophic damage, then its consequential damage is very high and when we consider the same in radial vibration, its cost of consequential damage is less.

My question is- should we consider risk (Probability times the consequence) in evaluating the voting logic? Could you please reply.

 

Tusar,

The Probabilistic-Risk-Assessment for SIS is a 3-state analysis considering overt and covert failure of components.

Did I also forward EPRI Study # 5416, having to do with Nuclear Plant Turbine Protection Evaluation? That study concluded 2003, with component failure diagnostics, was the way to go and our system was recommended to the industry!

That said, why are you considering 2002? The pie charts associated with Table 3 reveal how much better 2003 is than 2002. (BTW, 2002 was Boeing's choice!)

If you want additional info, please provide details.

Regards,
Phil Corso, [email protected]