This auxiliary screen is used to enter stress intensification factors, or fitting types for up to two nodes per spreadsheet. If components are selected from the drop list, CAESAR II automatically calculates the SIF values as per the applicable code (unless overridden by the user). Certain fittings and certain codes require additional data as shown. Fields are enabled as appropriate for the selected fitting.
The next element (80-90) is the flanged check valve. This CAESAR II element would include the flanged valve and the mating flanges as these piping components are much stiffer than the attached pipe. If the length and weight of this “rigid” element were known, this data could be entered directly by entering the length in the DY field, enabling the Rigid box and then entering the Rigid Weight in the Auxiliary Data area. Here, for lack of better data and for convenience, the CAESAR II Valve/Flange database will be accessed to generate this input automatically. This data is made available through the Model-Valve menu option or by clicking the Valve/Flange Database button on the toolbar. This command will bring up the window shown below.
Flanged Gate Valve
Restrains
Supports are provided to the piping to resist various loads. The loads can be classified into three categories. They are: primary loads, secondary loads and occasional loads. The response of the piping to various loads is different. The primary load is also known as sustained load. The primary loads are due to the self- weight of the piping, its contents, insulation, refractory, inner casing, outer casing, internal pressure and external pressure. The secondary loads are due to temperature change and relative settlement of foundations. The occasional loads are due to wind, earthquake, water hammer, steam hammer, safety valves blowing jet reactions, surge load, blast load and accidental loads.
If the piping is not provided with adequate supports, it will be over-stressed and excessively deform. Over-stressing will cause premature failure. Excessive deformation will impair the performance of the piping.
Anchor
+Y Restrains (shoe Support) o Vertical Pipe Line o Horizontal Pipe Line Guide
o Vertical Pipe Line o Horizontal Pipe Line Line Stopper ( Axial Stopper) Line Stopper with +Y Restrain Limit stop
Limit Stop With + Y Restrain Rod Hanger
Spring hanger
o Constant Spring Hanger (CSH)
o Variable Spring Hanger (VSH)
The CNode, or connecting node number, is used only when the other end of the hanger is to be connected to another point in the system, such as another pipe node.
ANCHOR
An anchor is rigid restraints providing full fixation, i.e., permitting neither translator movement (in X-, Y- and Z- direction) not rotation (around X-, Y- and Z- axis). An anchor provides a fixed reference point of constant position and rotation. Through which effects from the pipe on opposite sides cannot be transmitted. This makes the anchor a convenient terminal point for defining stress analysis problem. A pipe anchor is a rigid support that restricts movement in all three orthogonal directions and all three rotational directions. This usually is a welded stanchion that is welded or bolted to steel or concrete
+Y Restrain (Shoe Support)
Restraints are provided in the piping primarily to transfer the Sustain loads to the supporting structure. Restraints are usually oriented in any one of the coordinate axes of the plant. Inclined restraints are also used. Usually the restraints are double acting. Struts and ties, which are single acting, are also used. A single acting restraint is a device, which carries only tension or compression.
+Y Restrain ( Shoe Support) +Y Restrain (Shoe Support)For For Horizontal Pipe Line Vertical Pipe Line
Guide Support
The following are some important facts pertaining to Guides in CAESAR II. Guides are double-acting restraints with or without a specified gap.
Guides may be defined using the global system coordinates or with the restraint type GUI.
A guided pipe in the horizontal or skewed direction will have a single restraint, acting in the horizontal plane, orthogonal to the axis of the pipe.
A guided vertical pipe will have both X and Z direction supports.
Guide Support for Horizontal Pipe Line
Guide Support for Vertical Pipe Line
Axial Stopper (Line Stopper)
Translational restraints may be preceded by a (+) or (-). If a sign is entered, it defines the direction of allowed free displacement along the specified degree of freedom. (i.e. a +Y restraint is restraint against movement in the minus –Y direction and is free to move in the plus Y direction).
Axial Stopper (Line Stopper) For Horizontal Pipe line
Limit Stops
Limit stops are used to limit the stresses in the piping and to reduce the anchor reaction. The behavior of the limit stops is non-linear. The limit stop has zero rigidity up to certain movement. After this predetermined movement, the limit stop comes into action. The active rigidity of the limit stop can be finite or infinite. This depends on the construction of the limit stop.
Limit stop for a similar situation in a power plant. There should not be any problem if the pipe stresses are within limits and if the load on the stop is also reasonable. U may be providing this stop to limit the load on some component.
The following are important facts pertaining to Limit Stops:
Limit stops are single- or double-acting restraint whose line of action is along the axis of the pipe.
The sign on the single-directional restraint gives the direction of unlimited free movement.
Limit Stops/Single Directional Restraints can have gaps. The gap is the distance of permitted free movement along therestraining line of action.
A gap is a length, and is always positive. Orientation of the gap along the line of action of the restraint is accomplished via the sign on the restraint.
Connecting Nodes (CNode) may be used with any Limit Stop model. Limit Stops may be defined using the restraint type LIM.
Limit Stops provide double or single-acting support parallel to the pipe axis. Limit Stops may have gaps and friction.
The positive line of action of the Limit Stop is defined by the FROM and TO node on the element.
Allowable Support Spans
The allowable support span is defined as the maximum permitted unsupported span between any two adjacent supports on a horizontal straight piping. The loads on the piping induce direct stress (axial tension or compression), bending stress, torsion-stress, shear stress and linear and angular deformation. The torsion-stress, shear stress and angular deformation are not usually limited. Popular piping codes limit the bending stress of steel pipes to 15,850 KPa (2,300 psi) and the linear deformation to 2.54 mm (0.1 inch). The suggested pipe support span for commonly used steel piping is given in the following Table.
Table – Suggested Pipe Support Span
NPS – inch
(DN – mm) Water Service m (ft) Steam, gas or air service m (ft) 1 (25) 2.13 (7) 2.74 (9)
6 (150) 5.18 (17) 6.40 (21) 8 (200) 5.79 (19) 7.32 (24) 12 (300) 7.01 (23) 9.14 (30) 16 (400) 8.23 (27) 10.70 (35) 20 (500) 9.14 (30) 11.90 (39) 24 (600) 9.75 (32) 12.80 (42)
For the vertical runs of the pipes, a support span of four times that allowed for horizontal runs can be permitted. It is preferable to avoid providing supports on the pipes inclined in the vertical plane. It is preferable to provide a support at each location of direction change of the pipe.
Hangers
Hangers are special types of ties. They are always vertical and carry tensile loads.
Rigid Support and Flexible Support
The supports may be rigid or flexible in construction. Flexible supports are used where loads are to be carried, at the same time, accommodate movement. The movements may be due to the thermal expansion of the piping or connected equipment movements. The load variation in the variable load hanger from cold to hot is usually limited to 25%.
Variable Load Hanger (VLH)
Variable load hanger is a special type of hanger, which accommodate the vertical thermal movements, while carrying the vertical load. Usually variable load hangers are made of helical springs. The load varies from cold condition to hot condition.
Constant Load Hanger (CLH)
Constant load hanger is a special type of hanger, similar to the variable load hanger. There are several types of constant load hangers. The load variation in the constant load hanger from cold to hot is usually limited to 0%.
Ten Dos the following leads to a good engineering practice
1. Use minimum number of supports 2. Limit the use of flexible supports
3. Provide supports near the already provided columns and beams 4. Provide necessary clearance for thermal movement
5. Consider all the primary, secondary and occasional loads in the design 6. Provide access for valves and fittings
7. Provide additional loops to satisfy flexibility requirements
8. Provide guides to resist occasional loads like wind and earthquake 9. Provide an ergonomically acceptable design
1. Avoid too many anchors 2. Avoid too long a span 3. Avoid too thin a pipe 4. Avoid large local stresses 5. Avoid too many fittings
6. Avoid too many flexible supports 7. Avoid supports on horizontal bends
8. Avoid supports on pipes inclined to the vertical 9. Avoid bunching of too many pipes
10. Avoid large vertical or horizontal loops
VARIABLE LOAD HANGERS – SELECTION & SETTING PROCEDURE FOR BOILER AND PIPING APPLICATIONS
Scope
This procedure deals with the selection and setting of the variable load hangers (VLH) for boiler (pressure parts and non-pressure parts) and piping applications from the presently available list of VLH being manufactured his may be used for pressure vessels and heat exchangers also.
General
The description, range and type of VLH are described in the write-up on "VARIABLE SPRING HANGERS". The details like selection procedure, shop and site setting information’s are described herein.
Selection:
Before selecting a particular VLH, the designer is advised to acquaint himself/herself with the various aspects of the VLH by perusing the write-up on "VARIABLE SPRING HANGERS".
SPRING SPRING LOAD – COMPRESSION WITH NO LOAD WITH LOAD DIAGARAM (Fully Compressed)
Figure
A - Pre-compression (initial compression)
B - Additional compression in cold condition
C - Vertical thermal movement of connected equipment (cold to hot – either downward or upward) = ∆Y D - Working range of the VLH E - Unused range of compression
F - Total possible compression
G - Additional compression in hot condition
H - Free height
J - Fully compressed height
P - Maximum load carrying capacity of spring-corresponding to a compression of 'F'.
Note Figure-1 describes the terminology used in VLH for an application where the vertical thermal movement of the connected equipment is downward (cold to hot). When the vertical movement from COLD to HOT is upwards, the marks 'COLD POSITION' and 'HOT POSITION' indicated in Figure-1 should be mutually interchanged and following changes in terminology should be effected.
B – Additional compression in hot condition. G – Additional compression in cold condition.
Some of the dimensions are constants for all the load groups. They are indicated in Table-1.
Sl.No. Dimension Unit TRAVEL
RANGE=80 mm TRAVEL RANGE=160 mm 01. A mm 56 112
02. D mm 80 160 03. E mm 24 48 04. F mm 160 320
All other dimensions (B, C & G) are application – specific and are to be calculated for each VLH.
EXAMPLE:
Design load (Hot load) W = 5400 Kg
Vertical thermal movement (Cold to Hot) ∆y = 40 mm (downward) (∆y = c)
(When ∆y is less than 1.6 mm, a rigid rod without springs may be used) Try load group '12' and travel range 80 mm.
Spring rate (spring stiffness) K = 62.50 Kg/mm Load variation from hot to cold ∆W = K x ∆y
= 62.5 x 40 = 2500 Kg ∆W 2500
% load variation = --- x 100 = --- x 100 = 46% W 5400
% allowable load variation is 25%, as the actual load variation (46%) is more than the allowable load variation (25%), this selection is not acceptable. (Please note that the allowable load variation is taken as 25% based on ASME-B 31.1, 1992 recommendations. Some customers / consultants specify a lower value of % ∆W, in which case the latter is governing. The tender specifies an allowable load variation of 6% of the cold load).
Try load group '12' and Travel Range '160 mm'. Spring rate K = 31.25 Kg/mm
K∆y v 31.25 x 40 x 100
% load variation = --- x 100 = --- = 23% <25% W 5400 OKAY
Please note that a VLH with a travel range of 160 mm is more expensive than a VLH with a travel range of 80 mm, for the same load group.
If the % load variation for a VLH is more than 25%, use VLH with lower spring rate. If such a VLH is not available, use a constant load hanger (CLH) – spring loaded or dead weight loaded.
SETTING
The following information’s are required by the shop / site, over and above the size and type of the VLH, for setting and monitoring the VLH during operation.
1, Vertical thermal movement of the equipment (cold to hot) = ∆y (upward or downward) = c.
2. Additional compression in cold condition (B or G).
The setting details for the VLH, selected in the aforesaid example are given below: c = ∆y = 40 mm (downward).
W 5400 Total compression in hot condition = --- = --- K 31.25
∆1 = 172.8 mm Total compression in cold condition = ∆1 - ∆y
= 172.8 – 40 ∆2 = 132.8 mm Additional compression in cold condition = ∆2 – A
= 132.8 – 112 = 20.8 mm = B Additional compression in hot condition = G = B + C
mm and 0 – 160 mm for VLH with travel ranges: 80 mm and 160 mm respectively. This aspect should be checked for every VLH.
When ∆y = 40 mm (upward) W 5400 ∆1 = --- = --- = 172.8 mm K 31.25 ∆2 = ∆1 - ∆y = 172.8 + 40 = 212.8 mm G = 2 – A = 212.8 – 112 = 100.8 mm B = G – ∆y = 100.8 – 40 = 60.8 mm
The values of additional compression in cold condition (B or G) and vertical thermal movement from cold to hot (y = c) are to be communicated to shop / site.
Support Thick Calculation
The thickness required to take care of the axial tension due to heavy vertical load is given in Equation-1 Do 1 − πp Do2 2 π Do + 4 W T = --- 1 − --- 2 (p + f) Where
T = minimum required thickness, mm Do = outside diameter of the pipe, mm
p = maximum allowable working pressure, MPa(g) W = axial load, N
Flexible Nozzles
This auxiliary screen is used to describe flexible nozzle connections. When entered in this way, CAESAR II automatically calculates the flexibilities and inserts them at this location. CAESAR II calculates nozzle loads according to WRC 297, API 650 or BS 5500 criteria. When a nozzle node number is input, CAESAR II scans the current input data for the node and loads its diameter and wall thickness and enters it in the Nozzle Auxiliary Data field. Current nozzle flexibility calculations are in accordance with the Welding Research Council Bulletin No. 297, issued August 1984 for cylinder to cylinder intersections.
A valid nozzle node has the following properties:
Only a single element connects to the nozzle node.
The nozzle node is not restrained and does not have displacements specified for any of its degrees of freedom.
Computed nozzle flexibilities are automatically included in the piping system analysis via program generated restraints. This generation is completely transparent to the user. Six restraints are established for each flexible nozzle input. If a vessel node number is defined, then the vessel node acts like a connecting node for each of the six restraints. Vessel nodes are subject to the same restrictions shown above for nozzle nodes.
Note: The user should not put a restrainer on an element between the nozzle node and any specified vessel node. CAESAR II creates the required connectivity from the nozzle flexibility data and any user generated stiffnesses between these two points will add erroneously to the nozzle stiffnesses.
WRC-297 can be applied to a larger d/D ratio (up to 0.5) since the analysis is based on a different, thin shell theory (derived and developed by Prof. C. R. Steele).
Flexible Nozzle - WRC Bulletin 297
Adhere to these requirements when modeling flexible nozzles Frame only one pipe element into the nozzle node.
Do not place restraints at the nozzle node. Do not place anchors at the nozzle node.
Do not specify displacements for the nozzle node. (See the following example for displacements at flexible nozzles.)