Safe and efficient sludge removal by road tanker
David Knapp of Industrial Flow Control (IFC) describes how efficient sludge removal by road tanker can be achieved with minimal danger to the operator.The equipment for outloading sludge from treatment plants into road tankers hasn't really changed for decades. So why change?
Most loading points are found close to a site's storage tanks and run parallel to the road.
Tankers can therefore be accessed via a stairway to a platform, fitted with a compact, fully balanced folding step unit.
Folding stairs can be used to provide a non-slip self-levelling horizontal walkway onto the tanker barrel top, with automatic height adjustment.
There are several European manufacturers of these step units, with better quality designs that incorporate features such as a rotating step to protect toes during lowering, knee protection guard rails and a foot operated parking lock.
To protect the operator whilst he is working on top of the tanker the folding stairs are provided with a safety cage. This cage should not extend beyond the width of the tanker to minimise areas where an operator could slip through. For a sludge tanker with a single manhole fill point, the safety cage will be at least 1.4m square, to provide a sufficient working area.
Falling from tankers remains a very real hazard for tanker drivers and operators. Whilst increasing numbers of vehicles are being fitted with hand-railings, the fact remains that since 1995, there have been several fatalities and serious injuries in the UK, caused by falls of this nature.
Any loading system must provide maximum protection for operators whilst they are working from the platform or top of the tanker.
Next to be considered is the type of loading arm. Sludge is piped into the tanker from a manhole via a winched or counterbalanced boom.
Current practice favours hard arms of 4" or 6" bore, with all metal construction, in preference to hoses. A well-balanced loading arm is easier than a flexible hose for operators to manoeuvre out from the platform, over the safety cage rail to the fill point. This is particularly relevant where a vapour cone has been fitted to prevent odours and splashing.
The various ancillaries that form a modern loading system would be heavy to handle unless fully balanced.
The key element of a loading arm is the swivel joint. Developments in recent years have increased the reliability of the swivel joint.
The most suitable design for the loading arm that is to operate within a safety cage will have a horizontally hinged boom and a vertically balanced primary arm, with at least four swivel joints.
This style is most easily released from its parking lock and 'walked' to the fill point by the operator.
Traditionally, tanker loading has been via open fill manholes, but increasingly odorous or harmful vapours and fumes now have to be excluded from the immediate loading area.
The introduction of a manhole cone fitted on the drop pipe eliminates vapours escaping to the atmosphere. The cone fits a variety of manhole sizes, and a hose running back along the arm returns the vapour for de-odorising and venting in a safe area. To minimise leakage around the cone the vent system may be blower assisted.
Optionally, the loading arm may be fitted with a pneumatic 'press-down' cylinder to provide a positive pressure on the tanker and maintain a good seal as the tanker lowers on it's suspension during loading.
Several options for controlling the filling process are available. Most methods rely upon tanker driver authorisation by the entry of a PIN number or by swiping a card at the reader mounted on the primary control cabinet installed close to the loading platform.
The driver is then free to draw a load. This can either be preset on a batch controller instrument, or measured out using level switches to prevent overfilling.
Both methods should have a secondary trip switch should the driver erroneously attempt to overfill the tanker.
For sludge sensing, a non-contact type of switch such as an ultrasonic sensor is probably the best option.
IFC's series 3000 batch controller is mounted in a sturdy stainless steel enclosure, with large buttons for gloved hands The 3000 incorporates a hard wired emergency stop.
A typical flow control loop will consist of an electromagnetic flowmeter, converter and display instrument to scale and display the flow total in engineering units, and a control valve to close at the end of the filling operation.
System suppliers are increasingly being asked to provide a full audit trail of loading operations and this is achieved by locally installing an industrial PC in a fully weatherproof enclosure and including software to record all tanker loads identifying the amount drawn, the vehicle driver and the time. Local ticket printing is available as an option as well as data transfer via a modem. Where safe tanker loading demands that not only should certain safety controls be in place, but that they are fulfilled in the correct order, then a mechanical 'keylock' system should be considered.
A typical system may require the tanker to held by a traffic barrier, and only when this is locked in position is the operator allowed onto the platform to lower the folding stairs, and proceed with loading operations.
Each step in the sequence is accessed by a transferable key in an exchange lock unit, which ensures secure control of the loading system at all times.