Note:
1. In desert conditions Hanger type springs is used to prevent malfunctioning of Spring due to the accumulation of sand particles
2. for Large diameter tanks, tank bulging plays a important role in nozzle loading and support selection . Spring support can be advised .
1. In desert conditions Hanger type springs is used to prevent malfunctioning of Spring due to the accumulation of sand particles
2. for Large diameter tanks, tank bulging plays a important role in nozzle loading and support selection . Spring support can be advised .
Alignment Check Methodology in Piping Stress Analysis using Caesar II
“Alignment Checking” this term is quite familiar with piping engineers and all construction engineers. During piping installation at construction site it is expected that equipment flange should match perfectly (aligned) with the piping flange so that during bolting no problem occurs. But achieving that perfect alignment is very difficult to achieve. If this alignment for rotary equipments are not proper then there may be several problems in future during operation which may lead to vibration of equipment/piping system or in some situation equipment failure. American Petroleum Institute code API RP 686 provides the data for acceptable deviation from the ideal perfect alignment. As per the code if the vertical and horizontal deviation of piping flange and rotary equipment flange center line is within 1.5 mm and parallelism (rotation) is within 0.0573 degree then the alignment is accepted otherwise means to be devised to bring the deviation within those values. While performing stress analysis of rotary equipment connected piping systems in Caesar II we can very easily ensure this limitation. The following write up will describe the step by step method of doing the same.
Alignment check of nozzle flange shall be performed for all Rotating Equipments like Centrifugal Compressor, Steam Turbine, Centrifugal Pumps, Gear Pumps etc as per following procedure.
Steps for performing Alignment checking:
1-Ensure correct weight of the pipe (with proper thickness), Support weight (dummy pipe), Weight of valves, flanges and any in-line items.
2-Consider Insulation density carefully (equivalent insulation density to be correctly fed with insulation & cladding weight, Check insulation on dummies for cold insulated lines).
3-Model all branch piping (like drip legs etc.) greater than 2 inches.
4-Discuss with piping lead engineer for requirement of any maintenance flanges (Normally for steam turbine or centrifugal connected lines the maintenance flange is recommended) and include it if required.
5-Minimize the sustained load on equipment nozzle as much as possible during static analysis run of the Caesar model.
6-Normal industry practice is to analyse the Alignment checking in separate file. So rename the static file as Filename_Alignment.C2
7-Make the equipment nozzle anchor flexible or remove the displacement if anchor was not modeled.
8-Wherever spring support is used, define spring rate and cold load in case of variable effort spring & Constant effort support load in case of constant effort spring.
9-After performing the above create one additional load case in Caesar II as mentioned below:
WNC+H SUS ( System with spring hanger)
WNC SUS ( System without spring hanger)
10-Set the spring hanger as “As designed”.(Two load cases can be generated for spring As designed and rigid condition)
11-Now run the analysis and check the displacements of the nozzle at the above mentioned load case and limit them within below mentioned values:
Vertical deflection (Normally DY): +/- 1.5 mm
Horizontal displacement (sqrt sum of DX and DZ): +/- 1.5 mm
Parallelism (sqrt sum of RX and RZ) : 0.0573 degree
12-In case the above limitations are not met then re-analyse by readjusting the spring and other supports and do the simulation.
13-Alignment check is to be performed for both inlet and outlet lines.
14-Alignment check must be performed with spring under both in “As designed” and in “locked” condition.
15-To avoid small misalignment in vertical direction first support from rotary equipment nozzle is used either a spring support or an adjustable type support.
1-Ensure correct weight of the pipe (with proper thickness), Support weight (dummy pipe), Weight of valves, flanges and any in-line items.
2-Consider Insulation density carefully (equivalent insulation density to be correctly fed with insulation & cladding weight, Check insulation on dummies for cold insulated lines).
3-Model all branch piping (like drip legs etc.) greater than 2 inches.
4-Discuss with piping lead engineer for requirement of any maintenance flanges (Normally for steam turbine or centrifugal connected lines the maintenance flange is recommended) and include it if required.
5-Minimize the sustained load on equipment nozzle as much as possible during static analysis run of the Caesar model.
6-Normal industry practice is to analyse the Alignment checking in separate file. So rename the static file as Filename_Alignment.C2
7-Make the equipment nozzle anchor flexible or remove the displacement if anchor was not modeled.
8-Wherever spring support is used, define spring rate and cold load in case of variable effort spring & Constant effort support load in case of constant effort spring.
9-After performing the above create one additional load case in Caesar II as mentioned below:
WNC+H SUS ( System with spring hanger)
WNC SUS ( System without spring hanger)
10-Set the spring hanger as “As designed”.(Two load cases can be generated for spring As designed and rigid condition)
11-Now run the analysis and check the displacements of the nozzle at the above mentioned load case and limit them within below mentioned values:
Vertical deflection (Normally DY): +/- 1.5 mm
Horizontal displacement (sqrt sum of DX and DZ): +/- 1.5 mm
Parallelism (sqrt sum of RX and RZ) : 0.0573 degree
12-In case the above limitations are not met then re-analyse by readjusting the spring and other supports and do the simulation.
13-Alignment check is to be performed for both inlet and outlet lines.
14-Alignment check must be performed with spring under both in “As designed” and in “locked” condition.
15-To avoid small misalignment in vertical direction first support from rotary equipment nozzle is used either a spring support or an adjustable type support.
A failure analysis based on ASME Sec-VIII div-2 was done to evaluate ‘Plastic-Collapse’ & ‘Buckling analysis’ using the elastic & elastic-plastic stress methods, by considering the static loads & dynamic loads in combination that act on the 60” pipe header of the flare system. The results were analyzed for the location of buckling failure that occurred during the 1st Failure recorded. The expansion stresses in the pipe header were twice as much higher than that those due to the normal operating temperature loads and were greatest at the time of the failure, when compared to normal operating temperatures. It was found that the failure in the pipe header characterized as the inward plastic collapse could be attribute to 'Buckling' failure mechanism and that may have been caused by multiple static loads creating the pre-stressed state combined with the dynamic load that triggers the actual failure.
The overall vibration characteristics of the 60in flare header piping system, based on the first 15 modes, suggested concerns regarding the presence of maxima / minima and ‘Point of Inflections’ on the 60” section of the main header in the vicinity of Node-3383, which is location of actual failure that occurred during the failure incident. The current configuration of the sulfur lateral branch was also found to provide restraint to the 60" header, such that it is believed to impose unfavorable modes characteristics at the location of actual failure that occurred during the failure incident.
The mode characteristics thus acting on the old (0.375" thickness) header when combined with the high D/t ratio would result in header bending. Under bending, when the cylinder is very long, the flattening of the cross-section leads to large reduction of the effective section modulus of the 60" header, and the instability occurs as a single transverse wave (Buckle) on the compression side of the shell. This phenomenon was attributed to the failure of the 60” sections of flare header piping system. Decreasing the D/t ratio is the most effective way to counter this failure mechanism.
The theoretical solution for the one & two dimensional model of the 60” header cylinder, estimated the critical buckling load under compressive torsional & bending stress to be much higher than what existed in the 60” flare header. However, the modal analysis suggested that given the situation that the flare header piping system would vibrate under a given dynamic condition, it would be much likely that it would do so with a maximum situated at the N-3383, the region of failure incident that occurred. This would be a pre-requisite for a 60” pipe header to buckle at this specific location.