How to determine Flange Bolts Material and Grade? 1

[DjazAutomation]

Hi everybody,

I want to know how to determine the bolts material for a plastic flange connection (PE100, or simply polyethylene). Actually, this connection is done by using stubs and backing flanges. I know that the material will define how much torque the bolt can withstand (As well, the diameter has to be taken into account).

Let's consider that I have ANSI 1'' class 150 Flanges. I am using a profiled gasket.
The manufacturer give the following information:
- Recommended tightening Torque: 20 Nm or 14.75 foot pound(Just an example).
- Zinc plated Bolts or stainless steel bolts are recommended.

I already know that low and medium carbon steel are available as well as other materials. For each kind of material different grades are also available.

My problem is I don't how to find the right steel grade for my application (Low, medium carbon steel, steel alloy, other?).

Can anyone tell me how to proceed and how to use available material tables.

Thank you very much.

Djaz.

 

[adamuk]

The bolt torque is calculated to withstand the pressure in service and also meet the initial bolting up condition to achieve a given gasket compressive stress so in order to determine the joint type you first need to establish the design conditions.

I would choose a gasket with a low seating stress (e.g. graphite sheet with tanged insert but it depends on the corrosiveness of the process fluid). Then calculate the required bolt torque, then determine the bolting to use to achieve this.

I would assume that you could use a wide range of bolt types and materials as typically plastic piping is only used in low pressure applications.

Sorry I can't give you the answer as I would require more information - this sort of thing you should really get some design input rather than just posting on an internet forum but the above method should point you in the right direction.

Adam Potter MEng CEng MIMechE

http://www.ax-ea.co.uk

 

[DjazAutomation]

Thank you very much,

In fact, the gasket type is already determined according to our design conditions (Pressure, Temperature, Fluid). We've taken into account these parameters and checked them with the manufacturer's (George Fischer) solutions. Each solution has its own requirements regarding espacially the compressive stress that you mentioned.

So, the manufacturer recommends for the choosen gasket and for operating conditions that are more severe than ours, a tightening torque of 20 Nm or 14.75 foot pounds. They recommend also UNC (Coarse thread) bolts. The diameter is determined according to the flange holes diameter. The lengh of the bolt is easily determined by taking into account Flanges Thickness, Gasket Thickness, Nut thickness, Washers thinkness and an additional safety distance.

At this point, I have all the required data for the bolts but I don't know which material to choose.
Unless I am mistaken, for each kind of material(Grade), a maximum torque, that we should not exceed, is given.

For example, if I Have a required tightening torque of 20NM(14.75 foot pounds) with 1/2'' Bolt, Should I choose a material like :
- Grade 0, 1 or 2 low carbon steel with 47 foot pounds maximum torque for a 1/2'' Bolt.
Or
- Grade 3 Medium carbon steel with 69 foot pounds maximum torque for a 1/2'' bolt.
Or
- others.

In summary how much margin should I have between my required tightening torque and the maximum allowed to have a good and safe design ?

Please note that I am not considering the corosion resistance than can be reached by zinc plating or any other special coating or material.

Sorry, Maybe I didn't explain well what I am looking for.

Thanks again for your remarks.
Waiting for your support.

Djaz.

 

[adamuk]

Just go with grade 0 or 1. There's a good design margin between the specified max. torque and the point at which the bolt actually yields.

Adam Potter MEng CEng MIMechE

http://www.ax-ea.co.uk