Dear Friends,

I would like to have your views on modeling snubber stiffness in stress analysis of piping -

E.G Snubber design load : 57000 N based on Default translation stiffness in Caesar II (1.75 E11 N/mm)

Movement during Operating: 133 mm (Compression)
Snubber part no. based on M/s Lisega catalogue - 306316 having details as mentioned below:
Nominal load (Fn) : 100 kN Emergency load 141 kN Stroke: 300 mm

However, if one refers Operational Performance details of snubber as shown in Page 3.9 of M/s Lisega 2010 catalogue we can find :

Rigidity of the snubber is specified = 2Fn/Sb

Fn = Nominal load of snubber
Sb: Piston rod travel at Fn (At room temp.)

Corresponding to Stroke 300 mm of piston (Travel range 3) , As per M/s Lisega catalog (Sb <=8 mm)

Case 1: Sb =8 mm Stiffness of snubber, K = 2 x 100000/8 = 25000 N/mm

Case 2: Sb =4 mm Stiffness of snubber, K = 2 x 100000/4 = 50000 N/mm

Case 3: Sb =3 mm Stiffness of snubber, K = 2 x 100000/3 = 66667 N/mm

In systems where snubbers have been provided to take care of high stresses due to Occasional loads based on default rigid stiffness in Caesar II, if the snubber stiffness values are incorporated we may find it difficult to qualify Code stress criteria.

Please note that most of the vendor catalogs I have come across do not indicate the stiffness for each part no. corresponding to the nominal load and it would be difficult to qualify Code stress criteria considering worst case of Sb mentioned in catalog. e.g. in the above case K = 25000 N/mm for 100 kN Nominal load (It is 1/4th Nominal load)

Loads on neighboring supports also are increased because of introduction of snubber stiffness in the model.

Query:

1) What is the preferred method to model snubbers in cases snubber was provided in a line to qualify stress as per Code criteria?

2) Different vendors may have different values of stiffness which may not even be documented in the catalog.In such cases what is the preferred method for finalizing stress lines which have high code stress which are dependant on introduction of snubber in the line

Best Regards

Benoy Abraham