The major hazard involved with transferring flammable liquids is static build-up due to charge separation with the potential for discharge resulting in fire and subsequent loss of containment. Certain non-polar liquids can be charged while flowing through piping. Detectable and hazardous charges must be expected if the specific resistance of the liquid exceeds 108 ohms.
The potential for accumulation of static charges may enormously increase if the liquid contains a non-miscible component or a suspended solid. Examples include:
- Crystallisation processes in toluene;
- Quantities of water in toluene.
With the presence of a second phase, velocities less than 1 m/s should be employed.
Measures that can be employed to reduce these hazards include:
- Ensure that the pipe transferring the liquid is COMPLETELY FULL to exclude the formation of explosive mixtures;
- Wherever possible, ensure no contaminants/solids are present;
- Utilize inert gas blanketing;
- When transferring flammable liquids, use an inert gas;
- Avoid mechanical mixing or agitation of low conductivity liquids wherever possible;
- Use of ball valves with earthed metal spheres;
- Employ low transfer velocities. For only partially filled pipes, or pipes that discharge into containers, the velocity is to be limited as follows:
- For chargeable esters: maximum 10 m/s;
- For mineral oil products (e.g., gasoline, petrol, kerosene, paraffin, jet fuel) and for other chargeable liquids (excluding carbon disulphide and ether):
| Nominal pipe diameter, inches | 1.5 | 2.0 | 3.0 | 4.0 | 8.0 | 15.0 | 25.0 |
|---|---|---|---|---|---|---|---|
| Velocity, m/s | 7.0 | 6.0 | 3.6 | 3.0 | 1.8 | 1.3 | 1.0 |
| Quantity, l/min | 600 | 800 | 1100 | 1600 | 3500 | 10000 | 17000 |
If these velocities are adhered to, no hazardous charges will be generated within homogenous liquids. But when suspensions of crystals in non-conductive liquids are conveyed, hazardous charges may always be generated, even at velocities below 1 m/s.
- For ether and carbon disulphide in pipelines up to a diameter of 25 mm, the maximum velocity should not exceed 1m/s. Larger pipes require lower velocities;
- A general rule for all homogeneous liquids (except carbon disulphide and ether) and all pipelines: at velocities below 1 m/s, no dangerous charges will be generated;
- Flanges should be earth bonded;
- Use sub-surface dip pipes or bottom entry filling when discharging into vessels;
- Ensure regular inspection and testing of earth bonding.
Source: UK’s HSE