TechArchive

Original article date: September 1998

Finally the Government has confirmed the long-awaited expectation that it would introduce a ban on the import supply and use of all asbestos-containing products. The sealing industry expects a ban on asbestos to be in force by the end of 1998 or early in 1999. So what now?

Comprehensive ranges of alternative gasket materials have been under development for many years and Beldam Crossley now claims to have a range which will match virtually every application where compressed asbestos fibre (CAF) is now employed.

But selecting a gasket material represents only half the story when aiming for optimum performance in a flange. Correct installation procedures and maintenance are equally important. To fit them correctly use a sensible “opposite pattern” procedure for tightening bolts or studs. Each bolt should be secured finger tight and then tightened to 30% of final torque then 60% then to final level.

Clean and lubricate bolt/stud threads prior to assembly wherever possible as this can result in 100% more stress on the gasket using the same bolt torque due to lower thread friction.

Generally the thinner the gasket the better the performance. 1.5mm thick material will usually be the correct choice for Class 150 standard flanges up to 600mm diameter. Use 1 on Class 300 flanges up to 600mm diameter. 3mm should only b needed for larger flanges or old/damaged flanges. For normal cut gasket applications a flange surface finish of 3.2um to 12.5um Ra (125 to 500uin) is acceptable.

Select a bolt torque which will create a gasket stress somewhere between the minimum and maximum recommended. It is usually best to use a bolt torque which represents somewhere between 50 and 80% of the yield stress of the bolt material at the operating temperature. But the 50% minimum may be reduced to 25% for higher tensile bolt materials.

Jointing compounds are not needed to effect a seal and they can often impair gasket performance. Their use should be avoided. And anti-stick finishes can be provided in other ways.

Re-tighten after one cycle or up to 24 hours but not after a longer period. This is particularly important for fibrous gaskets because they may become brittle in service and crack on re-tightening.

More gasket problems are caused by under-tightening than by over-tightening. Unfortunately asbestos-based gaskets were very tolerant in this respect but the asbestos-free grades ar eless so. Only on smaller flanges (eg 25mm and below) will over-tightening be more likely than under-tightening.

Most gaskets are not designed for re-use. A gasket is a precision component often containing a polymer binder. It needs good storage conditions to ensure that it is not degraded before it is fitted. And never re-use a gasket – the small saving involved may well result in extra cost later if a leak occurs.

Beldam Crossley is offering a special deal with the following elements:

• A free stores survey to highlight and identify all asbestos-containing sealing products.
• Offer specific advice on asbestos-free gaskets for critical flange applications including ideal thickness bolt torque etc.
• A comprehensive report to be submitted for less critical applications giving general recommendations and advice regarding asbestos-free substitutes.
• Formal in-house product training covering the technical aspects of gaskets such as gasket selection correct installation fugitive emissions reduction and troubleshooting.
• Free sample gaskets for evaluation on plant.
• Free removal and dispersal of any surplus asbestos-containing sealing product.
• 30% discount from the first order for asbestos-free gaskets.

• Beldam Crossley
• P Childs
• 01204 494711

September 1998