GIMBAL EXPANSION JOINTS
 
Employment
 
Gimbal expansion joints
Unlike axial and articulated compensators which operate as a single unit, angular and gimbal type units must be used in sets of two or three joints, thus forming two-pin and three-pin systems.
The longer the distance between the angular compensators the greater is the expansion that can be taken up.
The axial reaction forces due to internal pressure are absorbed by hinge bars. The hinge axis passes through the bellows centre line.
Gimbal type compensators are fitted with hinged gimbal rings to take up angular deflections at right angles to the axis unit. Angular or gimbal joints are therefore modular units. Only through the above mentioned devices it's possible for us to solve all problems due to compensation since angular or gimbal joints are able to absorb any value of pressure reducing the same to a very moderate thrust.
Angular or gimbal joints are perfectly suitable to compensate either winding connections or wide pipeline systems. Making use of two gimbal joints "CS" and an angular joint "RS" we even obtain a spatial isostatic flexible system, able to absorbs expansions along all the three orthogonal axis. Their own performance depends upon bellows angular rotation which allowable values are quoted in the following tables.
 
 
Pipe guiding
 
Flexible joints, contrary to the axial ones, produce moderate thrusts against anchor points. In case of short pipe connections among central plant equipments, slides seem to be even useless. The dead weight of connecting pipe sections between joints has to be supported in order not to hinder movements.
In case of long pipelines, axial slides are to be installed before the first joint and after the last one. Such slides must facilitate free pipeline expansion. The distance between slide and the first joint of pipe network must not be longer than two times nominal diameter plus half movement. In these pipe systems the weight of connecting pipe sections between the joints has to be neutralized by simple roller sliding stands (see supports).
In vertical systems the weight of connecting pipe sections can be neutralized, if it's necessary, through counterweights or spring suspensions which let the joints free to expand and shrink.
 
 
Cold draw gap
 
On assembling a pipeline, flexible joints are generally inserted with a 50% of cold-draw-gap percentage. Cold-draw-gap is not to be done starting from a single flexible joint (one by one) but involving the whole expansion system and above all when required anchor points have been determined only.
As regards working temperatures over 450 C, cold-draw-gap must be wholly executed (i.e at the rate of 100%). Should pipelines work at operating temperatures over 100 C., it's enough for you to arrange for an assembling cold-draw-gap equal to 50% of its total expansion lenght taking besides into no account, pipeline temperature during its installation. When pipes have to be subjected to working temperatures covering a thermic values range, starting from the one near room up to temperatures of less than 100 C., in this case pipe temperature cannot be neglected anymore since pipeline is just partially already expanded.
At this point it's necessary to adjust cold-draw-gap expansion value in connection with the share already expanded. When you have to put cold-draw-gap into execution we suggest you to assemble the expansion joints in rest condition afterwards you shall have to cut a piece of pipe equal to the total cold-draw-gap value already calculated .
It's easy enough for you to realize flexible expansion joints cold-draw-gap since their own resistance is very low. As you can see, the movement outlined by the flexible expansion joint is an arch which height "h" depends on the value of compensation and on the distance from the centre of joints. Then, suitable lateral clearances in the slides near expansion joints are to be realized in the ratio of the arch height value, so that a free movement may be allowed to pipeline.
 
 
Anchor points
 
If we make use of flexible joints with thrust already neutralized, contrary to what happens to axial expansion joints, we see that anchor point has to support just the expansion joint resistance only. Said resistance covers either bellows strength moment or the pin frictions. As a matter of fact internal pressure thrust is absorbed by rods and hinge pins.
 
 
 
 
 
 
 
 
 
Technical data


CS/CF




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