Static Discharge in Hoses

[goldenspark]

On the theme of static discharge hazards, we have another problem that is related.

We are transporting a mineral oil in silicone rubber hosing and are experiencing static build-up.

The hose joins two stainless steel fittings which are earthed, but the static is discharging from the middle of the hose to a nearby earthy point. The hose is only a couple of foot long.

The suspicion is that our linear flow velocities are just too high, but of 9 equipments only 2 seem to exhibit this phenomena.

I have heard that point discharges such as this can puncture the hose and lead to leaks. The oil is not flammable.

Any comments would be welcome!

 

[25362]

It is apparent that not all of the nine hoses are of equal characteristics or not all transported fluids having equal properties. The streaming current leading to a voltage accumulation on a given fluid is differently estimated for laminar and turbulent flow conditions.

The sparks resulting from the electric breakthrough may indeed puncture the hose wall and lead to leakage. To reduce the breakthrough voltages in such short hoses for a given hydrocarbon, try to keep linear flow velocities below 1 m/s.

 

[quark]

Just keep in mind that electric current prefers least resistance path. Just check the resistance at the couplings to the ground and try to improve it. (by improving the earth pit conditions)

Sparking occurs if and only if the static electricity is discharged abruptly. If you take care of this then no problems.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[goldenspark]

Thanks for those replies.

The flow is laminar (Re ~700) but the linear velocity is about 2.3 m/s.

We are about to investigate why 2 of 9 seem to be affected, it is possible there is a subtle geometry / flow difference between them.

One option is to put a high resistance discharge path, such as a conductive coating, to continuously leak the charge back to where it came may be a practical solution. Possibly a static discharge conductive braid (high resistance, not metallic, as this may make things worse).

 

[Thomasjl]

Goldenspark

Make sure the hose is kept away from any aerating possibility, keep it in the air, put rubber blocks underneath or something. Then the static electricity remains in the liquid until it is drained at the SS joints. Simple as that.

Good luck Thomas.

 

[hacksaw]

you can usually specify grounded hose. consist of metallic ground wiire together with proper compounding--reduces resistivity to better drain off the charge. Velocity control is the key driving force.