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.

Protective Coating: ARC S1 PW - Food Contact Testing

As we previously announced ARC S1 has been replaced in our product offering with ARC S1PW. ARC S1PW carries NSF standard 61 potable water approval. We are now pleased to announce that we have just completed testing of ARC S1PW following guidelines contained in EN 1935 in relation to food contact materials. Regulation (EC) No 1935/2004 of the European Parliament and of the Council of 27 October 2004 is testing on materials and articles intended to come into contact with food. The principles set out in Regulation (EC) No 1935/2004 require that materials do not: Release their constituents into food at levels harmful to human health Change food composition, taste and odor in an unacceptable way This testing includes overall and specific migration tests in a variety of different foodstuff simulants, organoleptic tests, as well as examining the potential for leaching of bisphenol A, F, and S, according to the requirements of EU Commission Regulation No. 10/2011. ARC S1PW was also tested for content of BADGE, BFDGE and NOGE in sample leachate in accordance with EU Commission Regulation No. 1895/2005. The EU accredited independent test laboratory has provided a statement that according to the above testing, ARC S1PW can be used for direct contact with the following food types: Dry foods Water based food types (foods that have a hydrophilic character) Oil based food types (foods that have a lipophilic character) and alcoholic foods with an alcohol content above 20% and for oil in water emulsions Potable water The only food type that is not recommended for direct contact with is any food with a pH below 4.5. We realise that many areas have their own food testing requirements but believe that this testing may be useful for you particularly if EN 1935 is recognised in your area. To receive the test report, don't hesitate to contact me. Nick Wilson Nick.Wilson@chesterton.com ARC Application Engineer, EMEA at A. W. Chesterton Company

Dual Mechanical Seals for Safety and Containment ( (3/3) - Tank System

Tank Systems

Within Europe the Pressure Equipment Directive governs the manufacture and supply of pressure containing systems and it is the responsibility of the manufacturer and plant operator to ensure that the Tank System is properly certified. Use of Non Certified Tank Systems is punishable by imprisonment.

Regarding the Buffer or Barrier Fluid itself, there are numerous possibilities; however consideration should be given to its properties and its compatibility with the sealed media. The information below provides a guide to the most common fluids being used;

• Water/Glycol mixtures to prevent freezing. A mixture of 60/40 is sufficient for most European countries.
• Light oils and high thermal capability oils.
• A constituent of the process media, or the process media itself.

As discussed the operation of a dual seal and support system can be broken down into two operating modes, Buffer and Barrier. Recommendations are;

Use a Buffer fluid to;

• Create a lower pressure differential from the process to the atmosphere compared to a single seal, also reducing the operating stress of the inboard seal.



Figure4: Buffer hydraulic load distribution

• To identify seal failure during operation.
• Prevent contamination of the process media in the event of seal failure
• Contain process leakage in the support system preventing leakage to the atmosphere/environment

Use a Barrier fluid to;

• Create a lower pressure differential across the inboard faces reducing the operational stress.

  

  Figure5: Barrier hydraulic load distribution

• Prevent process media from crossing the seal faces. Important in Slurry applications.
• Prevent sticky or setting process media from generating high face torques during startup.
• Prevent vaporization of the process across the faces.

Operation considerations

The correct use of a Dual Seal with Buffer or Barrier fluid system can manage the hydraulic load of the inboard seal and increase seal life. It is important to set the pressures either 1 to 2 bar lower than the process pressure for Buffer systems and 1 to 2 bar higher for Barrier systems. The systems should not be operated with the same pressure differentials between Process>Seal>Atmosphere, as this would place both the inboard and outboard seal under the same hydraulic load. It is preferred to have the highest hydraulic load on the outboard seal as it is running on a clean fluid which you have the opportunity to select. Failure of the outboard seal should not lead to contamination of the atmosphere/environment and is detectable and manageable.

The use of level and pressure transmitters on support tanks cannot be over emphasized for both safety and containment. They provide useful operational data and an alert if there are changes to the seals operating condition.

When setting up and installing the support system it is important to fully vent the system before operating the pump. Air entrapment within the seal can cause;

• Dry running of the seal faces. Typically the outboard faces are most affected
• Non function of the Thermosyphon and/or pumping ring
• Reduced fluid volume to carry heat away from the seal faces

Properly operated dual seals and support systems not only provide safety and containment, but also put the seal operator in control of the seals operating mode and fluid film. This fact alone means that dual seals and support systems provide the biggest increase in Mean Time Between Repair for all applications.

Steven Bullen
Steven.Bullen@chesterton.com

Rotating Equipment Segment Manager, EMEA at A. W. Chesterton Company

Vice Chairman, Mechanical Seals European Sealing Association