Discharges of static electricity from hoses are known to cause the ignition of combustible atmospheres during the transfer of material to or from vacuum tankers and road tankers.
There are normally three main reasons why discharges of static electricity from hoses can occur. One reason is that standard non-conductive hoses are incorrectly used to transfer material. Non-conductive hoses are used to transfer material. Non-conductive hoses are capable of accumulating and retaining high levels of static charge which can result in incendive brush discharges from the hose itself, or the charging of isolated conductive objects attached to the hose like a nozzle or coupling that can discharge a spark themselves. It is generally accepted practice within the hazardous process industries that non-conductive hoses should not be used to transfer potentially combustible liquids and powders and numerous standards and industry association publications repeat this recommendation.
Another common reason for static spark discharges from hoses results from connecting conductive hose, or interconnected conductive hose sections, to a vacuum tanker or road tanker that does not have a verified static ground connection. The third most common reason for static spark discharges from hoses is where the conductive components of the hose structure become isolated during normal activity.
Figure 1. Four hose sections joined together in a vacuum tanker operation with an OhmGuard® hose tester testing the first hose section.
Both the second and third modes of electrostatic discharge are the most relevant to the hazardous process industries, and are scenarios where improper use of conductive hoses can lead to the accumulation and discharging of static electricity within a combustible atmosphere.
1.1 Conductive hoses connected to ungrounded vacuum tankers and road tankers.
With no static earthing protection in place a tanker conducting a vacuuming or loading operation will become electrostatically charged as it has no means of preventing the accumulation of static electricity on its tank and chassis. Because the metal connections (couplings) of the hose should be electrically continuous with the tanker, the tanker will also transfer charges to the hose, thereby causing the accumulation of static electricity on the hose as well. The quantity of charge transferred to the hose will be high as unearthed tankers can build up very large electrostatic voltages in a short space of time.
Charge accumulation on the conductive metal components of the hose, like couplings or nozzles, are a particular concern as these are the parts most likely to be closest to any combustible vapours or dusts during operations and may seek to nullify their electrical imbalance by sparking onto objects like operators, tank walls or pipes. If a combustible atmosphere is present in the spark discharge gap ignition of the atmosphere is highly probable.
In one reported incident a vacuum tanker was sucking off-specification toluene from a below grade sump and although the hose was conductive, the tanker to which it was attached did not have a verified static ground connection. The hose itself consisted of a metal wire helix embedded in the hose tubing which bonded the hose couplings but given the high level of voltage induced on the hose via the ungrounded tanker, a static spark was discharged from the metal wire helix of the hose, across the hose tubing and onto the metal rim of the sump. The resulting spark ignited the toluene vapours leading to a fire .
1.2 Damaged conductive hoses connected to grounded vacuum tankers and road tankers.
A more insidious hazard is situations where the tanker has a static ground connection that is verified with either a tanker mounted or gantry mounted earthing system, but the hose(s) connected to the tanker has lost its electrical continuity resulting in the isolation of a metal component somewhere in its structure. A typical example of this would be when the metal wire helix of the hose becomes isolated from an end fitting like a hose coupling or a nozzle.
Metal wire helixes are commonly used to reinforce the hose structure against transfer pressures and bending kinks. Another common function of metal wire helixes is to bond end fittings to provide the necessary end-to-end electrical continuity that will prevent the accumulation of static electricity on the hose. If the metal helix, through normal industrial “wear and tear”, breaks or detaches from hose couplings or nozzles, these components now have the capacity to accumulate enough charge and enough energy to ignite a combustible atmosphere.If a hose section with an isolated coupling is fitted between other hose sections, the other sections are isolated from the grounded tanker also which could lead to multiple components becoming electrostatically charged near to, or within, the potentially combustible atmosphere. In this situation the isolated hose sections will become charged due to contact with the moving liquid or powder.
Figure 2: Examples of a loading gantry mounted (Earth-Rite® RTR) and tanker mounted (Earth-Rite® MGV) static ground verification systems.
Another important consideration is hoses fitted with two metal wire helixes, where one helix is present on the outer surface of the hose and a second helix is present on the inner surface of the hose. In some hose designs the inner helixes are not bonded to the hose end fittings and it is important to ensure that such helixes cannot discharge sparks onto the end fittings or operator, especially when the hose is removed at the end of a transfer operation when a combustible atmosphere may be present in the hose or the area surrounding the hose. A hose fitted with an internal metal wire helix caused a fire through a discharge of static electricity, and in addition to the wire helix being broken, both end couplings were not designed to be connected to the inner metal helix. Quoting from “Avoiding Static Ignition Hazards in Chemical Operations”, AIChE/CCPS, Britton L.G., 1999:
“A fire was reported during draining of toluene from a road tanker through such a hose and after the event it was found that the inner spiral was not only broken but was not designed to be bonded to the end connectors. Two post loading toluene fires occurred with a similar hose as the disconnected hoses were being handled by operators.”
2.0 Industry standards and recommended practice.
To ensure that the hoses used on vacuum tankers and road tankers are not an electrostatic ignition source in a hazardous area there are numerous standards and recommended practices that describe the required electrical continuity of hoses. However, owing to the various hose construction types and established industry sector “norms”, there are a range of electrical continuity values that preclude a “one size fits all” approach to ensuring a hose is safe to use in a potentially combustible atmosphere.
By far, the most common type of hose used on vacuum tankers and road tankers are hoses that contain metal wire helixes that may be sandwiched between layers of hose tubing or may be present on the inner or outer surface of the hose, or both.
The following table lists several standards and industry association publications that outline the conductivity requirements for hoses. The respective recommended values of hose resistance are derived for an equivalent 25 ft. length of hose.
Table 1: Standards and industry publications that address hazards related to electrostatic charging of hoses.
In reality, many companies specify their own internal inspection regime that requires periodic end-to-end electrical continuity testing of their hoses. Periodic testing is normally performed every 6 to 12 weeks by a trained technician who will use a multimeter to measure and record the test results. The normally accepted end-to-end resistance “PASS” benchmark for individual hose sections with metal helixes is 10 ohms or less. Depending on the test results the technician will either allow the hose back into service, schedule the hose for a repair or remove the hose from service altogether. Quoting from CLC/TR: 60079-32-1 (ref. Table 1):
“Due to broken bonding wires or faulty construction, it is possible for one or more of the conductive components of the hose (i.e. end couplings, reinforcing helices and sheaths) to become electrically insulated. If a low conductivity liquid is then passed through the hose these components could accumulate an electrostatic charge leading to incendive sparks. Therefore, the electrical continuity of the hose should be checked regularly. Care should be taken to ensure that all internal metal helices are bonded to the end coupling.”
Although periodic testing of hoses is important, from a static earthing protection viewpoint, it would be safer to test the hoses prior to every transfer operation. In the 6 to 12 week period that the hoses are in use, breaks in end-to-end continuity can, and will, occur. Normally the metal helix that bonds the couplings of the hoses together will either break or loosen from its connection to the coupling.
Figure 3 . An isolated coupling caused by a broken wire helix.
If hoses with breaks in continuity are kept in service there is a strong chance that they will be accumulating static electricity during loading or vacuuming operations thus increasing the probability of static spark discharges when the hose is being used in a hazardous atmosphere.
The ideal procedure for proving a secure static earthing path for all the primary components used in the transfer, i.e. the road tanker and the hose sections connected to the tanker, would be to verify a ground for the tanker via a tanker mounted earthing system (Earth-Rite® MGV), or a gantry mounted earthing system (Earth-Rite® RTR). When the ground path for the tanker is verified, the next operation would be to connect the hose(s) to the tanker and then perform an electrical continuity test through the hose sections back to the tanker. This would ensure that the hose will be capable of transferring static charges through its structure, onto the tanker and down to ground via the static earthing system.