Threw the years we have gathered several general questions regarding our products.
We have tried to answer several topics if you require more information or a topic is not been addressed please don’t hesitate to contact us.
On July 1st 2014 it became a legal requirement for fabricators of structural components used in the construction industry to comply with the requirements of the CPR & CE marking requirements. While there are exceptions to this it is not at all clear to what degree it should be applied to the manufacture & supply of pipe supporting equipment & associated steel work. To be very clear about the situation, Pipe Hangers & Supports as defined by EN 13480-3 section 13 & associated appendices are intended solely for the purpose of supporting & distributing the weight & forces generated by the piping into the primary structure. They also allow for the displacement of the piping during plant operation. The connection point to the steel structure can be by direct bolted or welded attachment or by the placement of secondary steel members to provide a convenient connection point.
All attachments to the primary structure are either pre-fabricated by the structure fabricator or else made at site during the installation of the pipe supporting equipment. The loads & forces imposed on the primary structure by the pipe hangers & supports are known to & considered by the primary structure designer. Pipe Hangers & supports are therefore classified as a ‘second-fit’ to the primary structure & as such do not provide or enhance the structural integrity of the primary structure. They are designed in accordance with the requirements of BS EN 13480-3 which is harmonised with the “Pressure Equipment Directive” (“PED” 97/23/EC). CE marking under the PED is limited to parts that are welded to the pressure containment part. Because of this Pipe Hangers, Supports & associated secondary structural members do not fall under the requirements of either the CPR or EN 1090 when supplied by the pipe hanger manufacturer.
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.
If you are using stand-alone PSDesigner, you just specify pipe size as zero. If you are using the SupportModeler-PSDesigner link, you will need to draw the assembly including the clamp base initially, then delete the clamp base from PDS.
The constant support is modelled parametrically in SupportModeler. It is necessary to specify the correct parameters. If you use the SupportModeler-PSDesigner interface, the correct parameters are specified automatically.
It doesn’t do this at present. This is one area we are now working on.
There is an option to export a 3D DXF image to 3D CAD programs. We would need to import the complete support in one piece and not component by component.
If you are using PDS with SupportModeler, we have an interface where pipe data is read from the PDS model and PSDesigner passes data back to SupportModeler which creates the support in PDS from the SupportModeler PSL parametric library. This interface is available to download from the website.
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.
People around the world use pipe supports and restraints; in fact they spend somewhere in the region of £150million on ‘engineered supports’ each year.
The majority of pipes that we support and restrain are actually long thin pressure vessels operating at high pressures and temperatures, and occasionally at very low temperatures. In general they connect one large piece of equipment to another and facilitate the flow of fluid between the various processes. In some cases we supply supports for pipes that operate at temperatures as high as 850°C and diameters large enough to walk through.
During the operating cycle of the plant there is inevitably a change in temperature; when the plant is not working it is at ambient temperature and when it works it operates at a different temperature. Even changes in temperature between day and night can have significant effects.
Almost all materials expand or contract as their temperature is increased or decreased. A pipe that carries steam from a boiler to a turbine heats up from room temperature to 570°C between not working and working. This change in temperature will cause the pipe to expand by approximately 7.5mm/m, though the change is most prominent in the length of the pipe rather than in its diameter.
Imagine if the pipe could not expand or contract freely, the force generated in preventing the expansion to take place will cause substantial damage to either the pipe or the equipment at each end of it!
Consider the pipe work in a power station and liken it to your own central heating system; fluid is pumped around a closed system. In the boiler water is heated under pressure allowing its temperature to be increased to over five times the normal boiling temperature of water. An escape of steam under these conditions would simply cut a man in half.
This steam passes through the pipe work into the turbine where the pressure drives the turbine and generates the electricity. Inside the turbine the pressure is reduced and the temperature of the steam decreases. It is then sent back to the boiler where it is heated up again and so the cycle continues. The greater the demand on the power station, the higher the operating pressure and temperature will be.
The analogy with the central heating system; when your heating comes on or goes off you hear all sorts of creeks and bumps as the system heats-up or cools-down. That is simply because the piping is expanding and contracting between fixed points; the noises are due to the pipe moving against the joists and floor-boards of your house.
On a large, coal fired power station such as Drax in Yorkshire the boiler may be as tall as a ten storey building and the turbine will be perhaps 500m away from the boiler. The length of pipe could quite easily be 1km between the two. When you consider the amount of the expansion mentioned above, the whole pipe will grow in length by 7.5m.
Peel away the insulation around the pipe when it is hot and you will actually see the pipe glowing a dull cherry red – at this temperature the metal from which the pipe is made becomes like plasticine. If it is not supported correctly it will sag and deform; this will cause problems to the subsequent operation of the plant. Drainage slopes will become disturbed, excessive forces will be transferred to the boiler and turbine connections and eventually the power station will not be able to operate.
An example of what can go wrong under such situations occurred at Money Point power station in Ireland some years ago. Steam was released into pipe work where a pool of water had gathered; the pressure of the steam forced the water through the pipe causing severe damage to the pipe, the supports and even the building structure. A very costly repair followed!
Spring Selector is a C++ stand-alone programme that is used to select variable and constant effort supports.
In the centre of the dialogue box all possible spring selections are shown for the load and movement selected. If you would prefer to choose one of the other spring sizes shown, you can select it by clicking on it.
In the top left of “Spring Selector” a drop-down box gives the choice of spring types. The representation of the type of spring below the drop-down box alters when a different selection is made.
To the right of the list of spring sizes, the coloured arrow indicates where the operating movement of the spring is in relation to its total operating range (i.e. the grey area, with the overtravel shown at either end for variable effort supports). In the example shown the bottom of the arrow shows the preset position, and the top of the arrow shows the operating position. The direction of the arrow indicates that there is an upward movement. For a downward movement the arrow will point downwards (and the preset and operating positions will be reversed). The optimal position of the arrow is in the middle of the total operating range.
Our support products are included in the following programmes:
- SupportModeler programme for 3D modelling of hangers supplied by Pelican Forge, which is owned by Intergraph Corporation.
- Caesar II stress analysis programme produced by COADE. PSG’s variable effort support data is included in Caesar II Version 5.00 under ‘Pipe Supports’, and will also be in the new Version 5.20 when this is released. In Caesar II Versions 4.50 and earlier, PSG’s supports are listed under the name ‘Comet’ (this used to be PSG’s brand name). To find out more about Caesar II please click here.
- Autopipe 3D piping analysis programme, supplied by Rebis.
- Triflex piping analysis software supplied by Software Solutions.
- Logisterion P10 piping analysis software.
A sway brace is a special type of variable effort restraint and is built around a standard or non-standard spring. It is used to restrain piping or equipment and is not intended to support. The construction of the sway brace enables a pre-loaded spring which sustains both compression and extension displacement to provide a pre-determined restoring force.
For example, a pipe that is exposed to cross wind will sustain high transverse force during strong winds. If the pipe is subject to thermal expansion and contraction it will have a certain amount of flexibility. If allowed to displace freely during strong winds the pipe may become unstable and possibly sustain permanent deformation.
By installing a sway brace the pipe can be held in position during the application of forces less than the pre-set force within the spring. At higher forces the pipe will be allowed to displace but the further from its neutral position it is pushed the greater the restoring force will become.
When the storm recedes the sway brace will push or pull the pipe back to its neutral position.
Specifying Sway Braces: Like all devices that exert a restoring force to a pipe the magnitude of force that can be applied and the amount of acceptable displacement will be decided by the allowable stresses within the pipe. This information will be defined by piping engineer during his analysis of the system.
The level of pre-load within the sway brace shall be defined by the minimum force required to restore the pipe to its neutral position; it may be a function of the dead weight of the piping and the magnitude of frictional resistance thus created at sliding surfaces or it may be the amount of force required to restore an unstable, out-of-balance mass.
For simplicity, if we consider a pipe crossing a bridge structure, thermal expansion of the pipe is predominantly in the axial direction and so the pipe is carried on three sliding supports each having a coefficient of friction of 0.1. The total supported mass of the pipe is 10,000kg. Therefore the frictional resistance in the transverse plane is 1000kg.
If we select a sway brace that delivers a pre-load of 1000kg and has a spring stiffness of 100kg/mm the minimum transverse resistance to sliding is 2000kg increasing by 100kg/mm of displacement.
Assume now that the wind pressure on the pipe exerts a force of 2500kg; the pipe will displace by 5mm. If the pipe is sufficiently flexible and without the influence of the sway brace it may not be able to generate sufficient elastic energy within itself to return back to its neutral position. Subsequent axial loading may then cause further deformation of the pipe because it is not offering a rigid shape to the applied force.
With the sway brace installed the restoring force is at least that which is necessary to overcome friction and so the pipe is returned to its neutral position.
When in the neutral position the sway brace exerts zero restoring force and so the pipe is free once again to move with the thermal cycle.
Any practical combination of pre-load and spring stiffness may be defined and any spring within our standard range of variable and constant efforts supports can be applied to the product.
In our Variable Effort Supports product catalogue we offer a basic range of sway braces but it will normally be necessary to design the device to suit the specific requirements of the customer.
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.
We have two programmes. PSLCAD is an in-house program for drawing PSG’s standard support components within AutoCAD. PSDesigner is an in-house program which automatically designs complete support assemblies. this is a stand-alone program which does not require a separate CAD program. Both PSLCAD and PSDesigner are provided free of charge to customers.
Our PSDesigner programme can be interfaced with Aveva’s PDMS/MDS and Intergraph’s PDS/SupportModeler software. Support assemblies are created from parametric libraries of PSG’s supports and automatically placed in the 3D models.
Do you have a programmed selection system based on the strength calculation of the hanger?
Do you have a programmed selection system based on the strength calculation of the steel beam?
Can 2D-drawings be obtained from 3D-models?
Yes, this is done by Intergraph’s SupportModeler and by Aveva’s MDS. PSG can manufacture to the MDS or SupportModeler drawings.
If the design changes do you revise the 3D model?
No, the 3D model is used by the client. PSG provide the selection/design program which interfaces with the 3D model. Revision of the 3D model is the responsibility of the client.
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.
It is a fact that hydraulic snubbers require maintenance over a period of time in order to replenish the natural leakage of hydraulic fluid. Pipe Supports Limited is working towards a snubber design that will help to minimize the need for such maintenance. However, it cannot be totally eliminated.
In the USA, where there has been very long experience of operating nuclear power plant and where snubbers have been the subject of much discussion and review, there was a move towards mechanical rather than hydraulic snubbers. It was found, however, that a mechanical snubber is also subject to operational problems. The mechanism is prone to wear and fatigue and generally when a mechanical snubber fails in service it seizes and prevents the pipe from being able to move which in turn leads to damage to the piping and connected plant.
Hydraulic snubbers conversely fail in a ‘free’ state – the snubber continues to move with the pipe at the expense of slightly increased drag. However, the resultant stresses on the pipe during normal operation are insignificant. The problem with a ‘failed’ hydraulic snubber is that when the ‘event’ happens it can only provide notional damping by the air that is trapped within it.
Hence in the States there were significant efforts made to minimize the number of snubbers used.
Pipe Supports Group do not intend to manufacture mechanical snubbers; we do, however, have a relationship with a Japanese manufacturer who is willing to supply us mechanical snubbers, although our price will not be as competitive as it would be for hydraulic snubbers.
AWS = American Welding Society standard, which covers welding/welder qualification requirements for use within structural steel fabrications.
All of our procedures/welders are qualified in accordance with ASME IX which is considered to be a superior specification & hence qualification. ASME = American Society of Mechanical Engineers – Section IX applies to welding qualification for Boiler & Pressure vessels.
As our supports have no means of ‘self actuation’ they do not generally fall within the CE requirements. Exceptions are items that are welded to the pressure containment boundary – lugs on pipes or reinforcing pads for example. In such situations the material needs to be produced by a CE accredited mill and the certification needs to carry the CE mark.