Locking out the danger
Safety interlocking of valves has long been regarded as a absolute necessity with oil, gas or chemicals. And it is now becoming increasingly important in the water and wastewater industries, writes Mike D'Anzieri.
Safety interlocking of valves and associated equipment has been a standard procedure in the oil, gas and chemical-processing industries for years. While many processes in these sectors are automated or semi-automated, there are still many situations, such as the changeover of pressure relief valves, where human intervention is essential.
As the products being handled are often explosive or highly flammable, the consequences of operator error are potentially devastating. The Piper Alpha oil platform disaster of 1988 is one example. Because of this danger, the use of safety interlocks in these industries is standard practice.
While the possibilities of such catastrophes occurring in the water and wastewater industries are less likely, there are still many situations where the interlocking of valves and ancillary equipment is advisable. Although mistakes may not lead to loss of life, they can still result in extensive damage to plant and equipment, as well as expensive downtime.
There are now also heightened fears for the security of municipal water supplies in many countries. In addition to specific safety and security applications, there are many instances where valve and actuator operation can be speeded up, made easier, or reduced to a one-man operation.
Interlocks are available for virtually any type of valve used in the water and wastewater industries, including lever-, gear- and handwheel-operated ball, gate, check, butterfly and diaphragm valves. Applications are wide-ranging and include the unloading of tankers, control of gear-operated valves in pumping stations, and the safe operation of pressure relief and expansion valves.
There are also applications beyond valve interlocking, such as operating electrical switchgear or controlling access to potentially dangerous equipment such as UV or ozone disinfection chambers.
Ex-certified interlocks are also available for use in potentially hazardous areas such as WwTWs where the presence of methane gas can pose an explosion risk.
A simple interlocking system comprises a specially coded key that must be inserted into a valve to operate it. Under normal operating conditions, the valve is either open or closed. Inserting the key allows the wheel or lever to be turned. Once the valve is in its new position, the key is locked in place and cannot be removed until the valve is returned to its normal working condition.
When two valves need to be operated in sequence, two keys are used. The first is locked in place in the first valve and is only released when a second key is inserted, allowing the valve to be operated. This then frees the first key, which is inserted into the second valve, allowing it in turn to be operated.
Interlocks can also be used with actuated valves. Doing this provides the operator with complete manual control of the actuators, allowing them to be incorporated into a key-exchange interlocking sequence with other valves. For actuated valves, there may be two or more means of operating the valve. And all need to be accommodated within the interlock scheme.
Due to the high torque applied during valve actuation, mechanical locking is not practical, and interlocking of the actuator control mechanism is preferred.
A good example of the versatility of interlocks is with the transfer by road tanker of caustic and alkaline compounds to WTWs. Tanker unloading bays, or bunds, normally have a drainage system. And when a tanker is unloading caustic and alkaline, the drain has to be diverted so any spillage does not pass into the water supply.
This diversion is normally carried out by a two- or three-way interlocked valve. The procedure is simple: before unloading the tanker, the valve has to be turned to its divert position, freeing a key that the tanker driver then uses to activate the unloading mechanism. Once the procedure is completed, the driver removes the key and returns it to the valve, which can then be turned back to its normal position.
This simple procedure ensures that there is no possibility of accidentally releasing harmful chemicals into the clean water drain.
Another example of interlocking is with pressure relief valves, which are often installed on closed water loops to protect systems from damage caused by excessive pressure build-up. When maintenance needs to be carried out on these valves, it is important to ensure that an open pressure relief path is maintained at all times.
For this reason, most modern piping arrangements usually include a standby relief valve, allowing continuous operation of the plant while maintenance procedures are carried out. It is essential, however, to ensure that the standby relief valve is opened before the workpiece relief valve is closed – fitting a two-key interlocking system to the valves is an effective way of ensuring that the procedure is carried out correctly.
Several new time- and labour-saving devices also have many uses in the water and wastewater industries. One is the Easi-Drive, a portable valve actuator ideal for use with manual valves in situations where valve size, pressure rating or work environment would otherwise require a dedicated actuator or work crew.
Ideal for isolated or minimum-facility installations, it is particularly useful for moving tight or partially sized valves. An example where the Easi-Drive can be used in the water industry is with pumping stations, which often use large gear-operated valves – the Easi-Drive makes operating these valves a simple one-man operation.
Another useful device is the Flexi-Drive, which allows the remote operation of valves in hard to reach or hazardous locations. It consists of three elements: an operator station (comprising a geared handwheel and mounting facility), a valve station, and a patented cable linkage system joining the two stations together.
Due to the unique nature of the helically wound steel linkage cable, turning the handwheel of the operator station automatically operates the valve station. It can transmit drive to a valve up to 60m away and can accommodate up to 540Ú of bends between the two stations.
This allows it to be passed through walls, floors, tunnels and any other obstacles to reach the valve.
Two Flexi-Drive units were recently supplied to Thames Water for the remote operation of Gate valves located in underground sewage shafts. The valves are located between 8m and 10m below ground, one being used regularly to control sewage flow, the other used only in emergencies to prevent sludge overflows.
For safety reasons, manual operation of the valves was not practical as it would require a confined space underground work crew every time one of the valves needed to be operated. By installing the Flexi-Drive units, the whole procedure has been greatly simplified, and can now be quickly carried out by a single individual simply turning a handwheel on the surface.
In light of recent terrorist attacks around the world, fears have grown about the possibility of terrorists gaining access to water treatment and distribution facilities and contaminating the water supplies. Water companies have responded by various means, including installing surveillance cameras, hiring security guards and fitting more secure locks on plant and equipment.
In the latter case, there are several options for locking off valves, switches and access covers. The simplest of these is a padlock and chain; more secure systems are available, however, which allow a greater level of security while ensuring access is still quick and easy for authorised personnel.
These include devices such as multi-clasp locks, held in place by up to six padlocks, and anti-tamper locks which, once fitted, can only be accessed by authorised individuals. Both of these systems lock the valve, switch or access cover in place (much like an interlock) and prevent it from being operated until the lock or cover had been removed.
Unlike interlocks, however, they do not require any modification to the valve or access cover, so can be fitted or removed as often as required.
While the need for valve interlocking may not be as crucial in the water and wastewater industries as it is in hydrocarbon processing, it is nevertheless important.
In addition to potentially dangerous valve operating procedures, there are also instances where interlocking can provide operators with a greater degree of control over their plant and equipment.
An example is when interlocks are fitted to actuated valves, allowing complete manual control of the actuators. There are also cases where portable valve operating devices such as the Easi-Drive and Flexi-Drive can make it easier to operate valves in isolated, hard to reach or hazardous locations. Finally, simple lock-off systems are useful as security devices in drinking water installations.
Mike D’Anzieri is managing director of Smith Flow Control. T: 01376 517901.
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