Ralph and I must have started work in nearby workshops.
For heat exchanger bonnets flogging spanners and sledge hammers were almost always used. I believe they give better results than torque wrenches because the flogger (yes) can see how much the nut turns with each blow. You can buy spanners with a hexagon on one end and an anvil on the other. They sell well. With a torque wrench, one tends to concentrate on the dial too much. To make a tight joint, bolt tension is the desired result. It is too easy to believe that tension is the direct proportional result of torque. This is not true. A damaged thread, dirt, dust or a change in frictional coefficient (lubricant) can cause significant departures. My opinion is that feel is far more important than a gauge reading, unless the gauge is one that measures bolt extension. For many structural steel bolts, the rule is to snug it down (tighten it till it seats hard) and then tension it by turning the nut a certain angle, say 1/8 turn, hopefully more. If a bolt is properly snugged (seated) then an additional angular turn will produce a known extension and therefore hopefully a known tension in a known bolt. I prefer this method, although it is obviously useless when soft gaskets are used. With critical gaskets I tighten to the "crush" value from the gasket manufacturer, but reseat according to the manufacturers tension values, normally set by bolt extension or pretension. Heated bolts are very slow for assembly unless on ideal assemblies such as a turbine casing. The only way I know to be absolutely sure of producing a known tension is to use necked bolts which are designed to stretch in a certain area. These are fairly unusual and expensive.
For general purpose joints, I leave it to the fitters. They know what works and save hours of senseless calculations. However a series of tables showing bolt torques for various flange/gasket/ bolt lube combinations would be very useful. This site would be a good place to provide such information. I do not believe that operating pressure should be included as this is included in the flange rating. A required tension per bolt would be invaluable. If someone else has done it, lets "borrow" it.
I use superbolts in tight spaces such as the joint in a mill girth gear. They are handy but very slow, and I still like to check the bolt extension where possible (often not). For those who do not know, a superbolt has around six small bolts threaded through the main bolt head and bearing on a hardened washer. Tightening the small bolts tensions the main bolt.
Obviously "special" joints such as those requiring belville washers and the like will still require individual attention.However it would be useful to have a set of tables dealing with the other 98%.
Cheers
Steve McK
> ok. lets take it a little further. whats the max. torque that can be
> obtained using a torque wrench - about a thousand foot-lbs. if you pull a
> 100 lb at the end, the hand wrench extension will be 10 ft - good enough
for
> a 1 inch dia bolt. double the bolt dia and you needed at least 3x the
torque
> that a 1 inch bolt can handle (torque req't increases by the third power
of
> its dia) requiring too long an extension. the torque values are fine but
> achieving it with large dia bolt is another story (heat exchanger channel,
> rotating equipment casing, etc). use of hydraulic torque or tensioner is
> also not the answer since it too has some severe limitations ( antoher
mthod
> i remember is the use of torch to heat up the bolt).
>
> hand wrench & sledgehammer method although crude, sometimes work quite
> reasonably well. by the way, what happen if someone use a machine bolt
> rather than a stud bolts in a flange joint? is it permissible?
>
> -----Original Message-----
> From: Geoff Stone DD&D Australia [mailto:blenrayaust@yahoo.co.uk]
>
> Where it gets dodgy is when the bolts are dirty, galvanised or from an
> unknown
> source. Also 316 ss bolts may give you a problem if you dont use "never
> sieze"
> and galling occurs.
>
>
>
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Received on Mon Mar 25 04:29:00 2002
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