The V4 & V5 springs are recent additions to the product range. They will be added to PSDesigner in due course.
Suspended-type springs have +/- 75mm length adjustment. Base-mounted (pusher) springs have +/-25mm height adjustment.
The standard springs are suitable for 2 x design load (constant supports) and 2 x máximum load (variable supports). Greater test loading will require modified designs.
Details of allowable load for hydrostatic test are as follows:
For constant effort supports: 2 x support load.
For variable effort supports: 2 x maximum load for the spring.
If the hydrostatic test load is greater than this, this must be specified on the enquiry and order. PSL will then design special spring housings to accommodate the high test load. Alternatively, additional temporary supports are sometimes used during hydrotest.
I have a situation where I have vertical movements coupled with lateral and axial movements. I am wondering what the threshold of angular movement of the hanger rod relative to the spring can is before I create hinges with weldless eye nuts on both ends of the hanger rod.
Normally the allowable rotation is 5 degrees in all directions. By consideration of lateral movements with regard to the orientation of the various components it is normally possible to maximize the distance between pivot points to suit the direction of maximum displacement. For example, if connecting to a lug plate via a pin, the rotation of the pin should coincide with the direction of largest lateral movement.
There are a wide range of views on what material should be used for springs in low ambient temperatures. Springs manufactured from the materials which are normally used to make coil springs (silico-manganese or low alloy spring steels) will have quite low impact toughness (circa 5J) but the pertinent question is “does this matter?” We have been given the following advice from the Institute of Spring Technology in UK. “For a failure to occur from impact loading, it is necessary for the material to be loaded suddenly and for some plastic deformation to be initiated. A spring is only loaded elastically and so the impact toughness does not matter.” Because a spring is inherently elastic, impact loading is absorbed by elastic deformation of the spring and the shock loading which is necessary to cause brittle failure is not likely to occur. 735A51 (Chromium-Vanadium) steel gives higher impact toughness (typically 12J) than other spring steels, though this is still not particularly tough. We have used this material on some contracts where the springs are being used in low ambient temperatures. There is a cost penalty in using this material, and there may also be some delivery implications depending on the scope of supply.
Spring coils can also be made from stainless steel grades which have much higher toughness, but these materials have a lower allowable stress so the spring will be made from a larger bar diameter and will be to generally larger dimensions at very much higher cost. With regard to experience of failures of springs in low ambient temperatures, we had one instance of a spring coil failure on a support in Siberia, but our investigation into the failure found that it was due to a quench crack which had not been spotted during our spring coil supplier’s crack detection, so this failure would still have occurred if the plant had been in the tropics – the failure was not due to temperature. We would offer spring coils plastic coated for corrosion protection, not galvanized as this process can cause hydrogen embrittlement in high strength steels.
The weight of the pipework doesn’t change when it gets hot, but the pipes move up and down due to thermal expansion. When using simple variable effort supports, the supporting effort necessarily changes as the pipe moves up or down due to the spring rate of the supports. So it is not possible to exactly balance the pipe with the supporting effort from the springs in both the cold and hot conditions. You therefore need to decide in which condition you want to exactly balance the weight of the pipe.
For steam pipes (especially high pressure steam) the material strength is much less at high temperature, so it is usual to have the support loadings exactly matching the weight of the pipe in the hot condition (hot design) and accept the load imbalance and consequent higher terminal point loadings in the cold condition.
For pipes where there is no appreciable reduction in material strength at the design temperature, designers sometimes choose to use the cold design method where the loads balance exactly in the cold condition. This should in theory make the pipework installation easier, as releasing the springs from their locked positions should not result in significant deflection of the pipework since the loads should balance correctly in the cold condition. There will be a load imbalance in the hot condition.
It is good practice to select supports towards the middle of the travel range. Normally one should aim to centralize the cold to hot range rather than just the hot load. Doing this will help to avoid the springs “topping” or “bottoming” out due to any difference between actual and theoretical pipe weight.
The maximum operating temperature of springs, whether coated or not, is limited to 80 degrees C.
Neoprene is a substance that is hazardous to health and has been outlawed in the UK – it is also very poor as a protective coating, having inferior properties to the plastic coating we use.
When running the pipe stress analysis programme, the user will choose a specific support vendor and the analysis program will choose spring rates from that vendor’s catalogue. When using a different vendor, spring rates will be slightly different. This is usually considered acceptable provided the spring rates are reasonably close. If a different vendor is chosen, the analyst has the option of re-running the analysis specifying the selected vendor’s catalogue, but this is not usually considered necessary. Variable effort supports are manufactured using standard spring coils from stock and it is impractical to make one-off designs using non-standard spring rates.