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Anonymous Poster

Tightening of bolts by hydraulic tensioner

09/06/2009 1:00 AM

Dear all,

I'm using hydraulic bolt tensioner (1500 bar) for tightening of bolts. The required torque is 5000 Nm but the said bolt tensioner uses bar/psi is there any relation between Nm and bar. How can I prove that I've achieved 5000 Nm using this tensioner.

Your supoort will highly be appreciated as my job is at stake due to this tangle.

Salam Khan

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#1

Re: Tightening of bolts by hydraulic tensioner

09/06/2009 2:21 AM

Let us be clear about one thing

The End result as a fastener I want is the amount of stretch (in fact the pre-stress) I have achieved on the bolt vis a vis the yield point of it (usually from design aspect it is between 60 to 80%, may even vary depending on the application or rather the applied stress on service- additive, subtractive, fluctuating...)

The best method is the bolt tensioning, followed by the strain measurement, as we do in our critical application, but it is not feasible every where. And these cases we do the stretching and rest left to God .

The torque tightening is only an approximate method to achieve this stretch. You tighten the bolt, due to it stretches, and that depends on all the lot of factors, thread frictions, lubricants....

Even in my old Dieter book (Mech Engg design) he has touched on this aspect, and has mentioned if a number of bolts in a flange are tightened by torque (assumedly inder same condition, same wrench, etc) the stress retained on the bolt after torque tightening varied as much as 30% to 70% of the desired value.

So there it goes. I am sure if your boss understand, then the method you are using, he will appreciate to be the almost correct one, and far better than the torque method. That is used only for the speed of operation, and where the application is not that critical.

Note: The half section joint bolts, foundation bolts etc of the large turbines and generators are either stretched mechanically or thermally (we have both procedures at our place).

The psi that you are using can easily be converted to force (x cyl area) to bolt stress (based on the effective bolt diameters available in net , also the frormulae available)

And based on the material and YS of it, the required force can be calculated.

BTW: At our place, we put an extra scale on the pressure gauge (calibrate it in terms of Tons) and specify in our literature- shop process, customer OM manuals etc) in terms of Tons and not psi. Only ensure that the gauge is married to the cylinder and not to the pump.

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#2

Re: Tightening of bolts by hydraulic tensioner

09/06/2009 9:53 AM

Hello Salem,

I'll provide you with a way of calculating the proper Bolt Tensioner operating pressure in a minute. I'll also tell you how you can verify that the proper load has been applied. However, it's best if you first understand the following:

Bolt tensioners do not apply "torque" to fasteners; they apply tension. Torque is very inaccurate (and hence potentialy dangerous) when used to try to control a bolt's tightness. Here's a link expanding on this significant statement: Torque Danger

Bolt tensioners have likely been specified for the assembly of this joint because:

  • this is a critical joint and the degree of "tightness" must be very accurate. It's difficult (and often impossible) to achieve accurate bolt load even when using calibrated torque wrenches - Torque Wrench Calibration
  • the use of bolt tensioners allows consistent joint compression because all bolts can be tightened simultaneously. Individual bolt tightening results in wide load scatter with some of the bolts perhaps being too loose and others perhaps being too tight- (due to elastic interactions). Tightening one (or only a few bolts) at a time can also result in gasket damage. Here's another link describing bolt tensioning - Bolt Tensioning Introduction

The 5000 Nm torque spec was calculated by an engineer after he had determined the required bolt load. In so doing, he had to guess at what the actual K Factor might be when the joint was to be assembled. If the designer's guess was incorrect (which is likely because nobody can predict what may happen in the future), the wrong bolt load would be applied and the joint may fail. The problem with you trying to determine the correct bolt load from the given torque spec is that you now have to guess which K Factor was used when the designer guessed. You can probably see that the initial innacuracies of torque can actually be compounded!

A way to limit the guessing is to assume a bolt load based on a percentage of the fastener's yield strength. For example, if your application contains a gasket, 40 to 55% of yield might be reasonable. Thus, multiply this value by the fastener's Effective Tensile Stress Area to define the load. At this point, you're almost there. However, before you can calculate the tensioner's operating pressure you'll have to determine the Load Transfer Factor (LTF). This will be

  • 1.01 + (Bolt Dia / Clamp Length)

Pump pressure will then be:

  • (Reqd Bolt Load x LTF) / Tool Hydraulic Pressure Area

In order to prove that you've applied the proper preload, the best way to do this is by measuring the resultant stretch of each fastener (since bolt stretch is directly proportional to bolt load). Doing anything less is simply hoping and wishing that you've got the correct load and that the joint won't leak. Here's a link showing an instrument used to verify bolt load: Ultrasonic Bolt Elongation Measurement

If you need further assistance, please provide all details including joint and bolt dimensions, joint and bolt materials, operating pressures and temperatures, gasket details, bolt tensioner model and size.

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#8
In reply to #2

Re: Tightening of bolts by hydraulic tensioner

09/08/2009 7:38 AM

Hello BI,

I have to say you have written a very good piece indeed! With some brain work and the use of the links to help................... Some of the things you mention may not be known by the average Joe. Very good.

GA to you Sir.

Take care.

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Anonymous Poster
#3

Re: Tightening of bolts by hydraulic tensioner

09/07/2009 6:26 AM

To my knowledge hydraulic tigthenig has (almost) nothing to do with bolt torque

Hydraulic tensioning is actually applying a tensile force on the bolt , the bolt is a acting as a spring.

The bolt is extended and then the nut can be hand tight, once the the hydraulic force is removed , the system will act as a spring and compress the 2 flanges and gasket

The applied hydraulic force is a fraction (K) of the bolt Yield strength (Sy)

K = 0.5 ( but could be 0.75 , depending on company practice

If bolt section at root is (A), The hydraulic applied force is

Fhydro= K*Sy* A

The hydraulic pressure you read is related ( proportional ) to this force

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#4

Re: Tightening of bolts by hydraulic tensioner

09/07/2009 6:55 AM

Hi Salam, Normally when you bought a Hydraulic bolt tensioner, the manufacturer suppplies the operating manual. inside the manual, there should be a conversion table that gives you what pressure to apply at given torque and bolt dia. If you dont have OM then check the website of your Tensioners Manufacturer. I think your application is special because torque is quite high, so you better contact your Tensioners tech support or ask the help of the good Engineer who specify the 5000 Nm. I'm interested on your application, can you provide me some detail like sizes of flange connection, no of bolts/bolt materials, clamping height,system operating pressures/temperatures etc. can you tell me the Brand/Model of your tensioner.

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#5
In reply to #4

Re: Tightening of bolts by hydraulic tensioner

09/07/2009 11:08 AM

For the reasons described above, it's impossible for any manufacturer of Bolt Tensioners to provide a meaningful torque-to-tension table. Hence, they don't.

Bolt Tensioner manufacturers do, however, provide a table showing the relationship between pump operating pressure and initial bolt stress for each size of tensioner. If one only has a torque spec and is trying to convert this to bolt stress, one still has to go through the process as I had outlined.

You may be confusing this with typical Torque Wrench manufacturer's charts showing the pump operating pressure to torque relationship for each size of torque wrench. Based on the manufacturer's QA program and the quality of his tools, these relationships are usually very accurate. However, they're quite misleading and hence possibly dangerous, because, as we know, the torque to bolt stress relationship is very dubious and fraught with many uncontrollable variables

A 5,000 Nm torque spec (and whichever intended bolt stress value it implies) is quite reasonable and not at all unusual in a Heavy Industrial application. In any event, it wouldn't be the Tensioner manufacturer who specified this load but rather, the joint's designer.

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#10
In reply to #5

Re: Tightening of bolts by hydraulic tensioner

09/17/2009 1:16 AM

been from long sleep,,, yes you're definitely right, im thinking of hydraulic torque wrench(one we're using for small baby's like up to 3500NM(hytorc). actually at the "shop floor" or "work area" or "battle field" tensioner and torque value don't marry together. when using tensioner, we, most of the times, measure bolt elongation(using dial indicator when we're tightening our generator to steam turbine coupling bolts. --------------------- life is good, when your weary... meditate..

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#11
In reply to #10

Re: Tightening of bolts by hydraulic tensioner

09/20/2009 12:11 PM

Hello Meched,

And I must apologize as well for this tardy response: I was out of the country and away from internet access for the past few days!

It's interesting that you measure bolt elongation when bolts are tensioned but not when they are torqued. Since there are many unknowns that affect the actual "tightness" of a "torqued bolt", this important QA step is much more critical on the latter than on the former. Regardless, the use of a dial indicator (or outside micrometer for small coupling bolts) usually provides good results. However, an easier way is to measure the elongation ultrasonically.You simply apply a transducer to the end of the bolt, measure its free length, tighten the fastener, re-measure and, adjust the bolt load (either by torquing, tensioning, heating or even by using a bigger hammer ) until you have the necessary elongation (and hence, bolt load). Here's a link for further details: Bolt Elongation Measurement

I should also note that many turbine shells/casings/half joints have been converted so that the bolts can be tensioned rather than torqued. This allows for rapid joint closure without the need for torque wrenches or bolt heaters. The modification is simple: longer studs so that the tensioners have something to "grab" onto. Since bolting and unbolting are usually Critical Path activities, modifications such as this can result in days being shaved off of the outage schedule!

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#6

Re: Tightening of bolts by hydraulic tensioner

09/07/2009 1:03 PM

Hello Guest,

You have written: "is there any relation between Nm and Bar".

You should have used "Nm^²" . Is there any relation between Nm^². Or Nm²/ (Newton Metres squared).

1 Pa = 1 Nm^²

1 Bar (Force) = 0.9869 (atm)= 100,000 Pa (Pascals) = 1 Pascal = Nm^²

--------------------------------------------------------------------------------------

Note: No not confuse the "Bar" (Force) with, the [Bar atm] (Atmosphere) pressure, (Standard), which is slightly higher at 101.325,024 Pa/Nm

You may find this site useful. It has several pages so if what you want is not on the first click through and look at the second, third etc.

http://www.sengpielaudio.com/calculatorPressure.htm

http://www.sengpielaudio.com/ConvPress.htm

These are different pages on the same site.

Good luck.

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#7

Re: Tightening of bolts by hydraulic tensioner

09/08/2009 4:11 AM

Salam,

You really should go back to your supplier for information.

The tensioner may have been supplied to you for this job for a specific reason where a torque application could damage something.

As stated by others on here, there are variations between torque and tensioning and I am not going to repeat the words of others. Have a look in my post history an I have explained it a few times.

Please go and visit the site of the company I work for: www.hydratight.com

And in particular the bolt-up program: http://www.boltup.com/

It is free to sign up for and to use.

This site is useful too: http://www.joint-integrity.com/

Just be careful with use torque or tensioning methods and make sure you have the right tools for the job at hand.

Kind Regards

Kev Brown

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#9

Re: Tightening of bolts by hydraulic tensioner

09/08/2009 10:25 AM

Hello again Salam,

As I had alluded to above, the most effective procedure when closing a flange is to tighten as many bolts simultaneously as possible. When tensioning, 50% simultaneous tightening is commonly practiced. The adjacent photo of a recent autoclave tool-supply and training project shows an example of this. You'll notice that the pitch diameter of the bolt circle prevents the use of a Tensioner on each adjacent stud due to the tool's diameter. In general, this procedure requires that the first 50% of the fasteners are tightened at a pressure somewhat higher than that used for the second group when the Tensioners are removed and indexed by a factor of "1" to the latter. The reason for this is that the second pressurization tends to relax the loads applied to the initial set of bolts (the same thing happens when torquing albeit to a much greater degree).

Measuring elongation will obviously let you quickly "tune" out-of-tolerance bolt loads but, following the above will reduce the degree of tuning that might be required.

It should be interesting to note that had the autoclave nozzle diameter in this case been smaller, 100% tensioning (and no load transfer!) could have been done. In that case, the necessary stud extension required for Tensioner engagement would have been on every-other stud on both the top and bottom flange of the joint. However, in this case, one such stud arrangement would be offset from the other by an Index Factor of "1". In this manner, each fastener could have been tightened at the same time. The benefits of doing this should be quite clear now.

Having said the above, in some cases, certain circumstances may prevent the use of Bolt Tensioners. Then, even if it is a critical joint, other means such as torquing or perhaps heat-assisted tightening must be utilized. The important thing is to not only be well-versed in each process but, also to be well-aware of their limitations and thus understand the procedures required to compensate for such limitations.

Good luck!

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