Dry fasteners provide secure attachment without personnel safety and environmental issues.

Wherever secure sealing under pressure (as well as fastening) has been required, dry, embedded O-ring sealing fastener technology (with optional thread locking) versus wet, liquid adhesive thread locking have been the choices for pump manufacturers and users for more than half a century. 

Preventing fluid/gas fastener leakage that originates from within and seepage from external contaminants require a reliable, adjustable sealing barrier. This is especially true when field servicing and employee health are considered. 

The embedded O-ring provides 360-degree, metal-to-metal contact. 

Every fastening assessment should include a review of the options against the following criteria:

  • What psi/vacuum rating (internal/external) is considered operational, and what are the temperature conditions?
  • To what chemicals (including cleaning agents and lubricants), gas or other liquids will the fastener be exposed?
  • Will there be concern about field servicing, especially spare parts availability?
  • Will there be any preference for either a liquid or dry mechanical sealing method during production?
  • Will excessive vibration be a consideration and therefore requiring additional thread locking? If so, which method (liquid or dry) best addresses installation time, future maintenance and economic considerations?
  • Will the fastening happen as part of a high speed automated assembly operation, or will manual fastening be the rule?
  • Will the main sealing concern be limited to installation, with little or no concern about warranty, or are there longer range potential maintenance considerations? 

The Liquid Method

The liquid method, which is best suited for large automated production applications, is typically an anaerobic adhesive/sealant used with other applied chemicals. This is because most are designed as two-step processes involving prep and curing. These can be difficult to remove during field servicing and can create possible environmental disposal issues because of the 
chemical residues. 

This type fastener replacement generally requires a new fastener and sealant reapplication. Because these sealants are harsh chemicals, physical contact and inhalation of the vapors can present a health hazard. They can cause eye, skin and respiratory tract irritation, as well as environmental disposal problems. 

A fastener with an embedded O-ring

According to NSF International (formerly the National Sanitation Foundation), that develops standards and criteria for health-related equipment, products and services worldwide, these sealants should not be used in situations in which they might contact food. Of course, this could also be a problem with other materials and liquids. Applying the correct number of anaerobic drops and thread location is essential for sealing integrity. 

The Dry, Embedded O-Ring Method

The self-sealing screw/bolt, a dry embedded O-Ring method, presents none 
of the problems associated with the liquid method. It also provides complete 360-degree, metal-to-metal, full-clamping contact. 

Table 1. O-ring materials

This is because the O-Ring, generally silicone rubber (A-A-59588, Class 3B, Grade 50), nests in an asymmetrical, protective groove located under the head of the screw and or bolt. See Table 1 for other compatibility material choices. 

This simple, safe technique 
results in: 

  • Maximum sealing up to 20,000 psig/vacuum 
  • Sustains harsh environmental operating temperatures from -160 F to 500 F (-106 C to higher than 260 C) 

Fasteners with coating applied to the threads

These stainless steel fasteners can easily be reused multiple times without appreciable O-Ring degradation, making them ideal candidates for field serviceability. No chemicals are required during the installation process. 

Therefore, there are no health issues. No assembly or special training is required either because the fasteners are delivered pre-assembled, just insert and torque. They also work well for bleeding applications. A wide choice of drive styles is available. 

For extreme screw, bolt or nut vibration applications, several thread locking options are possible: 

Adjustable polymer (normally Nylon) embedded pellets

An adjustable polymer vertical strip, (both located in the lower half of the thread area) 

Alternatively, a pre-applied dry coating process which is fused to the thread surface is available. All the options are adjustable while maintaining their full sealing capability. 

Self-Sealing Benefits 

Self-sealing fasteners can be found throughout many industries—including oil and gas, marine and offshore, food and chemical processing plants, construction, mining and laboratories of all kinds. They are appropriate for use in any application, indoors or outdoors, that requires high-pressure wash-down and where exposure to the elements and extreme temperatures are present.

Table 2. Wet versus dry characteristics and benefits

Pumps & Systems, April 2012