Reliability of aging static equipment (1): Best practice for Valve Sealing

Process plants in Europe and the US are facing some specific challenges. Most of them have installations that are in operation already since the 50’s or 60’s of the last century. They face increasing competition from overseas and have to run their plants longer between plant stops. Furthermore they have to deal with higher profit expectations and with more stringent Health, Safety and Environmental regulations.

Costs for maintaining plants are rising and it is a challenge to reduce maintenance costs to ensure sufficient profit margins. To tackle recurring issues it is very important to identify root causes and therefore understand very well the equipment and its failure modes. It is crucial to understand what the important factors are to consider with regards to sealing aging static equipment. This can help plants to make the right decisions about their aging assets.

Between 50-60% of Fugitive Emissions come from valves. Data published by the European Industrial Emissions Directive and the U.S. Environmental Protection Agency are in agreement of that. Even though product loss is expensive it is legislative compliance and health and safety requirements that really drive the reduction of plant leakage. Piping, Vessels and Heat Exchangers are other assets that can cause mayor headaches with regard to reliability and this equipment can account for a major part of the maintenance budget.

There are techniques available to make older static equipment comply with current emission regulations and performance expectations. All static equipment have their specific challenges with regards to keeping them leak free, whether they are Block Valves, Dynamic Valves, Pipe Flanges or Pressure Vessels.

In this post we will be talking about valves. In an upcoming post we will discuss the best practices for flange sealing.

Leaking Steam Valve

Small valves that are not performing usually are replaced by new valves. Low emission valves, certified according to API624, ISO15848-1 or TA-Luft, are readily available off the shelf and the buying cost of small new valves often doesn’t justify repair.

Larger valves might require a bigger investment and in order to get them compliant with current regulations an upgrade of the valve might be the most cost effective option. To make an older valve seal according to the latest requirements with respect to fugitive emissions we have to look closer to the mechanisms that affect the seal.

Valve packing is a contact seal and its mechanism relies on maintaining a very small gap between two surfaces, just like with every other seal. In addition a force is required that keeps the surfaces together. In the case of a valve stem packing, it is the packing that has the conformability and elasticity to adapt itself to the surface of the valve stem to maintain a narrow sealing gap. The gland bolts in combination with the internal elasticity of the packing material supply the force to maintain the seal. Failure of the seal has in all cases to do with the fact that one or both of the requirements cease to be met.
It is not just a matter of increasing sealing force to improve performance because the undesirable downside of this is that stem friction increases when the sealing force increases.

The engineering challenge is to find the correct sealing force that forms the balance between having a good seal and acceptable stem friction.

Valve condition and design

Older valves sometimes have extremely deep stuffing boxes. Deep stuffing boxes were once thought to seal better but in reality they cause more packing relaxation, high stem friction and low sealing performance. The ideal number of packing rings in a stuffing box is 5. Deep stuffing boxes can be easily improved by installing a metallic or carbon filler bushing.

Gland bolts or studs are crucial in applying the right gland force on the packing set and therefore creating the seal. Old corroded and plastically deformed bolts cannot perform this function. Therefore it is of crucial importance to exchange the studs on an older valve at each replacement of the packing set.

Gland studs and nuts need to be lubricated with a lubricant with a known K-factor or coefficient of friction. Unlubricated bolts have a coefficient of friction that can vary +/- 40%. Lubricated bolts have a variation of +/- 20%. Lubricants need to have a small variation between the wet K nut factor and the dry K nut factor to assure that re-torqueing of the valve after some time can happen reliably. They should neither be easily washed off. Nickel Anti Seizes or similar are the lubricants to use.
Gland studs have in general very low elasticity. Thermal cycling, pressure surges, packing relaxation, wear or extrusion may cause loss of gland force. In these cases Live Loading can be applied to majorly improve performance by assuring the right gland force over a longer period.

 

Cartridge Live Loading

The valve stem and stuffing box condition is crucial to the correct functioning of the valve. Pitting corrosion can occur due to a galvanic reaction between the graphite packing and the valve stem. Therefore good graphite valve packing has a passive corrosion inhibitor to prevent these issues.

Galvanic Stem Corrosion

Stem run out should be within certain limits. Stems that are in bad conditioned should be either replaced or reworked.

The stuffing box bottom should be flat and have no angle. The same applies to the bottom of the gland nose.

The valve packing

Increasing legislative requirements with regards to fugitive emissions have led to enormous improvements in the sealing technology for valves over the last 10 years. Packing emission testing standards are available so that packing can be compared. Modern low emission packing can bring even older valves up to the newest emission requirements.



Reinforced Graphite Packing

Conclusion

So even if valve are aging there is no need to be non-compliant with current industry standards. There are techniques available to seal valves and make them 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.