Dear friends

I have been doing seismic analysis of high temperature piping. I have choice of using snubbers or seismic arrestor with gap. Well, can any one explain the relative merits and demerits (other than cost) and their apllicability for high temperature piping?

Thanks in advance
Sels

I am not sure your question is snubber with gap. Snubber/shock arrestor systems are not installed with gap, actually they do not have the feature of gap.
Please le me know what's your question.

Dear Sun Wee

I am aware that snubbers are not installed with gap. The following are my questions...
1) what is the working principle of shock or seismic arrestor?
2) Can shock arrestors be used in high temperature pipe lines to resist earthquake induced vibrations?

Snubber is a typical type of shock arrestor. Generally hydraulic type snubber can be used up to 150~200 oF with continuous operation and 250~350 oF with max. short term temperature but the maximum applicable temperature is subject to the vendor designs and types.
This temperature limitation is to protect elastomer seal of reservoir and hydraulic fluid and ensure proper operation of snubber assembly in long-term use.

Q1 : what is the working principle of shock or seismic arrestor ?

Please get the vendor information or go to website. There will be lots of information about snubber.

Q2 : Can shock arrestors be used in high temperature pipe lines to resist earthquake induced vibrations?

Of course, snubber can be used in elevated temperature service pipe line(over 1000 oF). However, I would like to recommend you to check surrounding temperature(around cylinder) is enough lower than limitation due to heat transfer from hot pipe. Also, you to install cylinder side far away from pipe, that is, to weld cylinder side rear bracket to steel structure to minimize heat transfer from hot pipe.
I had used hydraulic snubber several times up to 1450 oF pipe system of EB/SM and FCC plants.

Sels,

I think most people are familiar with snubbers or shock absorbers - ie those that are essentially a hydraulic cylinder that restricts fluid flow under rapid movement.

For your other component - "seismic arrestor with gap" - I am wondering if you are referring to a product that is produced by Lisega - they make an "Energy Absorber" - Type 32. These devices use bronze cones inside an austenitic stainless housing and convert forces into deformation of the housing.

They can also be set up with gaps, so match the description you have provided. They are certainly much simpler than a snubber and you would therefore expect lower maintenance costs.

They allow gaps to be set between 20 and 30mm, (depending on model), so would not be suitable if for example your thermal movement exceeded this amount.

regards

Mr.Ross Sinclair is right… Lisega’s Energy Absorber has a feature of gap installation for thermal movement. However, it seems that Lisega’s Energy Absorber is a kind of mechanical snubber and it is ideally designed for the shock absorber of water/steam hammering and pipe whip in nuclear power plant. I am wondering if this assembly must be re-adjusted or-re-installed after each dynamic event same as other mechanical snubber.
I would not like to recommend this assembly with gaps for the seismic event in high temperature system because earthquake has both positive and negative movement and repeats vibration within few millisecond or few hours. Meanwhile, it is not easy to forecast the exact thermal movement to set positive/negative gaps of this assembly.

Long time years ago, I had involved in snubber reduction study in Containment Building of PWHR nuclear power plant because the maintenance cost of snubber was too high in radioactive area every two year. It was totally doubtful if mechanical snubber worked properly. Some hydraulic snubbers had leakage problem also.
In these days, the quality of hydraulic snubber has been apparently improved in view of leakage and maintenance cost.
The best way is not to use snubber. If it is unavoidable, I would like to recommend hydraulic snubber in lieu of shock arrestor with gap in complicated piping system with large thermal movement.

Dear friends

I express my sincere thanks to all.

sels

The use of a mechanical snubber in lieu of a gapped support (or limit stop or gapped ebar) carries more risk to the piping system than having no snubber at all.

This was a finding by Sandia national laboratories. Mechanical snubbers jam and when they do the thermal strain imposed on the system may be very high.

The ebar is a gapped device with a simple deformable metal element.

Information on Visco-dampers as alternative to snubbers can be found in an article by Dr. S.Saha in the Dec 2005 issue of Hydrocarbon Processing.

Regards