Static Bonding And Continuity In Chemical Transfer Hoses

A road tanker sits under a loading gantry at midday, the pump starts, and product begins moving through a two inch hose at full rate. Nothing looks unusual. Inside the hose, charge is separating at the wall and travelling downstream with the liquid, and where that charge ends up decides whether the transfer is routine or an incident report.

Electrostatic ignition remains one of the least visible hazards in liquid transfer, and the controls that manage it live inside equipment people handle every day.

 

Where the charge comes from

Charge separation happens whenever two dissimilar materials move against each other and then part company. Liquid flowing along a hose wall does exactly that, leaving one polarity behind on the wall and carrying the other away with the product. The effect grows with flow velocity, with the surface area the liquid contacts and with anything that disturbs the flow.

What happens next depends on the liquid. Conductive products such as water and most alcohols relax their charge quickly to any earthed metal they touch. Low conductivity hydrocarbons, toluene and purified solvents among them, hold onto it, sometimes for minutes after flow has stopped, which is why a receiving tank can still be charged when a gauger opens a hatch.

The hose and its hose fittings sit in the middle of the path that charge has to follow to get back to earth. A rubber hose wall is an insulator, so unless the assembly is built to provide a conductive route, the metal fittings at each end can be isolated from one another by the very component joining them. Some operations generate far more charge than a routine transfer.

• Pumping at high velocity, particularly through small bore lines.
• Splash filling into a partly empty tank or drum.
• Passing product through a fine filter immediately upstream of the hose.
• Handling two-phase mixtures, such as product carrying water droplets or entrained air.
• Clearing a line with compressed gas at the end of a transfer.

 

Bonding and earthing describe different jobs

Bonding connects two conductive items so they sit at the same electrical potential and cannot spark across the gap between them. Earthing connects that bonded assembly to the general mass of earth so accumulated charge has somewhere to go. A tanker bonded to a gantry with neither connected to earth is better than nothing and still leaves charge sitting on the system, while an earthed gantry and an unbonded tanker leaves a potential difference exactly where the operator is standing. Singapore’s Ministry of Manpower addresses both in its circular on safe storage and handling of flammable substances, which asks for grounding and bonding systems together with annual continuity tests carried out by competent persons.

 

How a chemical hose carries the path

EN 12115, the European standard covering rubber and thermoplastics hoses for liquid or gaseous chemicals, describes two common approaches using designations that appear on the hose layline. An M hose uses metallic conductors, typically the reinforcing helix, bonded to the fittings at both ends, giving a resistance of no more than 10 squared ohms between them. An omega hose achieves the path through conductive rubber compounds instead and permits up to 10 to the sixth ohms. A /T suffix on either designation means the wall itself dissipates charge, which is specified where the assembly sits wholly or mostly inside a hazardous area.

That distinction has practical consequences. A metal helix will carry stray current from cathodic protection or a welding return path as readily as it carries static, so jetty and terminal operators often prefer the conductive compound type on ship to shore duties. Conductive compound assemblies in turn depend on the compound remaining intact, which makes physical damage and ageing a continuity problem as well as a pressure one.

 

Continuity testing in practice

Continuity is a property of the whole assembly rather than the hose on its own. Resistance is measured between the two end fittings with a suitable low voltage instrument and compared against the limit for the hose type. EN ISO 8031 sets out the test methods for determining electrical resistance and conductivity of hoses and hose assemblies, and it is the method most specifications point towards.

Frequency comes from the site’s own assessment and from the regulator. An annual test is the baseline many operators here work to, with additional checks after any repair, reassembly or suspected damage. An assembly that has been recoupled, shortened or refitted has effectively become a new assembly and needs testing before it returns to service.

Record keeping is what turns a test into evidence. A usable record for each assembly covers four things.

• A unique identification number marked physically on the assembly, not held only in a file.
• The hose type and its electrical designation as originally supplied.
• The measured resistance and the instrument used to obtain it.
• The date, the result against the stated limit, and who carried out the test.

 

Where it goes wrong

Failures cluster around the ends. Reusable fittings refitted on site without restoring the bond to the helix, painted flange faces at the gantry, corroded bonding clamps and bonding cables with broken strands hidden inside intact insulation account for a large share of failed continuity tests. None of these are visible from a walk past, which is the argument for scheduled testing alongside the visual and pressure checks that make up industrial hose safety best practices on most sites.

 

Conclusion

Static control is a system property. The assembly, the clamps, the gantry steelwork and the receiving vehicle all have to form one connected path, and a single high resistance joint anywhere along it undoes the rest. Treating continuity as a scheduled measurement instead of an assumption is what keeps that path honest between shutdowns.

Reviewing the assemblies on your loading bay, or specifying new ones for a hazardous area? Get in touch with Pharmchem Engineering to talk through hose construction, end connections, electrical designation and the documentation your site needs, and we will help you match the assembly to the duty.