Reliability of aging static equipment (2): Best practice for Flange Sealing

The majority of flanges in a modern process plant function without issues. However there are a number of flanges that cause major headaches.

Emergency Repaired Heat Exchanger Flange

A flange gasket needs a certain stress over a certain gasket width to be able to generate the pressure drop that is required in sealing a flange. This gasket stress needs to stay between a certain minimum and a certain maximum value. The gasket stress is generated by tightening the flange bolts to about 30-70% of their yield strength. The failure that can occur and make a flange leak have either to do with the fact that the sealing gap is not maintained or the fact that the sealing force falls under a certain minimum.

Gasket Stress

Bad flange surfaces, bad bolts or improper tightening of bolts are common causes of flange failure. If the flange condition is bad the flange should be replaced or repaired. Repair can be done by machining the flange or to coat the flange with a special coating. Large flanges can be machined in situ with special equipment.

Corroded bolts or plastically deformed bolts cannot perform their function properly and will not provide sufficient sealing force on the gasket. It is recommended to replace flange bolts at each gasket replacement. Live Loading can help to increase the elasticity and compensate for the effects created by thermal cycling, loss of gasket thickness and as well might compensate some of the issues that are the result of yielded bolts and bad bolt tightening.

Flange Live Loading Installation

Correct flange tightening values are derived from theoretical calculations. In the petroleum industry the method described in Section VIII of the ASME Boiler and Pressure Vessel Code is commonly used. In Europe EN1591-1 is winning ground to become a reliable method to calculate the ideal bolt force on circular flanges.

When a torque wrench is used at installation, it is important to use the lubricant with a K-factor or coefficient of friction that was used in the calculation.

Stud Lubrication

The correct stress should be established on the entire surface of the gasket. Thus all bolts should be tightened simultaneously or successively by using a tightening method that gradually build up the stress in a cross pattern. This is done by applying sequentially 30% - 60% and 100% of the final torque in a cross wise pattern, and finally 100% in a circular manner.

Conclusion

So even if equipment is aging there is no need to be non-compliant with current industry standards. For valves and flanges there are techniques available to make them seal and perform well. This means that plants can be upgraded to meet current emission legislation without major capital investments but by extending the life of the equipment that is currently in place.

Hans Dekker

hans.dekker@Chesterton.com

 Senior Application Engineering at A.W. Chesterton Company and he is supporting the Stationary Equipment business segment in the EMEA region. Hans is a Chairman of the Packings Division for the European Sealing Association.

How Split Seals Can Help You to Save on Maintenance Costs

Seal maintenance on large rotating equipment such as large pumps, can be a time-consuming and costly exercise. As a maintenance manager, you want to find out if using split seals can make your team spend less time on maintaining seals, and if they can help you to reduce costs.

What are Split Seals?

Split mechanical seals are mechanical seals whereby all the parts are split into at least two halves. Like standard non-split mechanical seals, they are used on rotating equipment, such as pumps, mixers and agitators. And like standard mechanical seals, they seal the rotating shaft of the equipment against its housing.

Split seals were first used on the submarine main propeller shafts back in 1954. But only in 1986, were split seals introduced to the process industries as a standard, off –the-shelf available sealing technology.

Since the first generation split seal was introduced, many technological improvements have been made and integrated into the latest generation of split seals. These improvements have greatly expanded the window of operation and application of split seals.

"split seals can perform the same duty as standard mechanical seals. The benefit is the fact that the equipment does not need to be dissassembled for installation of a split mechanical seal."

Why use Split Seals? 

The fact that there is no need for equipment disassembly is the single biggest benefit of using split seals.  Split seals eliminate the need for removing anything from the pump except the seal. As split seals can be installed, in place and typically by one installer, without removing the pump, motor or coupling, they simplify the repair process and eliminate the associated costs with typical solid seal replacement.

But there are some important additional cost savings that  you achieve by installing split seals on your rotating equipment, especially when your equipment is now packed:

• Installing split seals will eliminate sleeve wear and associated maintenance interventions and costs.
• There is no more need for packing adjustments adjustments.
• Split seals will eliminate gland leakage and associated housekeeping costs, and corrosion of your assets.

The benefit  increases with the size of the equipment.  While there may not be a benefit to using split seals on small pumps, the savings start to add up with increasing shaft sizes. For single stage centrifugal pumps, a positive Return On Investment (ROI) typically starts at shaft sizes is greater than 2.5”/65 mm.  However, on specialty equipment and double ended pumps, the ROI starts at even smaller shaft sizes.

Where Can I Use Split Seals?

Split seals can be used on large pumps that pump water based fluids.  These include (raw) water intake pumps, effluent pumps, boiler feed pumps, cooling water pumps, sewage pumps, brine pumps, fan pumps, mixer, agitators and so on.

By Marco Hanzon
Marco Hanzon is Marketing Manager at A. W. Chesterton Company. Before getting involved in the Marketing of Mechanical Seals, Marco worked as an In-Field Support Engineer for mechanical seals.