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

Dear Friends,

In addition to the above, I would also like to add some more details :

1) With varying Sb values the Stiffness of the snubber in the example above varies from 25000 N/mm (Corresponding to Sb=8 mm) to 200000 N/mm(Corresponding to Sb= 1 mm).

Hydraulic snubber is generally provided in systems when-
1) Stress in Sustained + Occasional case (Seismic, Safety valve discharge etc.) are exceeding Code allowable values and can be qualified only if snubber were to be provided in the line.

We often come across cases where stresses during Occasional cases like Seismic (Bi-directional), Steam hammer(Uni-directional) and safety valve discharge(Uni-directional) are high which can be reduced only by adding some rigid restraints to take care of stresses due to loads in sustained + occasional case. However, providing rigid restraint in a line would constrain movement in Normal Operating case as well which would lead to high thermal stress and large Operating loads. In cases where modification in pipe route is not possible to add flexibility for expansion case, Hydraulic snubbers are provided even though they are very costly compared to Rigid struts.

2) Loads at terminal point of nozzles are exceeding allowable values in Occasional case which can be qualified only if the snubber was provided

e.g. In Boiler pressure part headers connected to critical piping (Main steam and Hot Reheat pipe) subjected to steam hammer loading in header axial direction. Hydraulic snubbers are provided near to the bends to take care of the steam hammer loads and avoid undue loading on Boiler Pressure part header. The excess loading in Occasional case may damage the tubes connected to Pressure Part headers.

In both the cases effectiveness of the snubber is necessary to ensure that loads and stresses are within allowable values.

Design intent of providing the snubber was to safeguard the pipe and maintain terminal point nozzle loads within allowable values in this case. If we refer the catalog, we can see that snubber capacity (Nominal load -100 kN) is of nearly twice the Design load (57 kN) however, when we apply the stiffness, effectiveness of the snubber changes with the corresponding stiffness values (Range 25000-200000 N/mm)

Best Regards

Benoy Abraham