EQUIPMENT
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emissions requirements are less than 1g/Nm3 more energy is used to operate the VRU than is recovered by the system, although naturally this value varies case by case. As well as the increasingly stringent emissions
limits, older VRUs suffer loss in operating efficiency over time, even when diligently maintained. The equipment also suffers wear and tear, spares become hard to obtain, and maintenance costs increase. These factors can also lead to the need to upgrade the VRU. The activated carbon material in gasoline
service VRUs has an expected working life of ten years or more. When used with other products, such as crude oil, this lifetime can be expected to be shorter. Normally there is a gradual loss in working capacity as the carbon beds age, although this can be overcome by replacing the carbon. More common is ‘attrition’ of the carbon, which generally manifests itself in the form of ‘dusting’ that covers equipment in the surrounding area. In some cases channels can form within
the carbon bed, allowing vapour to pass through untreated and increasing emissions. While there are ways to break up the channels, eventually the carbon will need to be replaced.
TECHNICAL CHANGES Vacuum pumps are often replaced as part of VRU upgrades. Early VRUs - and some being installed today - used liquid ring vacuum pumps. These pumps generally use an ethylene glycol/water solution, which at some sites has now become incompatible with the terminal’s operation. In certain applications this type of pump can gradually lose capacity to the point where the performance of the VRU may be affected. In both cases replacement of the vacuum
pump system may be the only option. A number of alternatives are now available, such as dry screw vacuum pumps and rotary/sliding vane pumps.
One of the most common reasons for the need to upgrade or retrofit VRU systems
VAPOUR CONTROL SYSTEMS THAT WERE STATE OF THE ART WHEN INSTALLED MAY NOW BE AGEING OR MAY NOT MEET CURRENT REGULATORY REQUIREMENTS
is a change in a terminal’s operational profile. This may be the result of a change in the throughput of the facility, a change in the properties of the stored product (such as a higher vapour pressure), or the relocation of an existing VRU to another terminal where there are different operating conditions. Within reason there are a number of
upgrades or retrofits that can accommodate these types of change. Where upgrading of a VRU is needed, as opposed to retrofit because something additional is required of the system, the replacement of valves, control systems or other equipment is relatively straightforward without the need for significant system changes.
HOW TO EXPAND Where additional capacity is required of the system a number of options exist. For instance, adding vacuum pump capacity can expand volumetric loading capacity and, when combined with other cycle changes within the system, can reduce emissions or help cope with changing product properties. However, adding to vacuum pump capacity
cannot be undertaken in isolation. For instance, the absorber column may have been sized specifically on the basis of the original vacuum pump system and may not be able to cope with
extra capacity. One solution to this is to add a vacuum booster blower operating in series with the existing vacuum pump system to provide as much as 20 or 25 per cent additional capacity to a system or meet tighter emissions at the existing system capacity. Where the existing system already includes a vacuum booster blower, additional capacity can be attained through the pre-conditioning of the vapour stream prior to entering the VRU. Pre-conditioning may include a pre-absorber, acting as either a condenser using chilled absorbent gasoline. If available, using a lower vapour pressure absorbent may help. In both cases the result is a lower inlet hydrocarbon concentration, effectively creating additional capacity in the VRU system. In some cases it may be possible to add a third carbon bed to the system to gain additional capacity from the system. However, this may require additional vacuum pump capacity, significant changes to the vapour inlet and outlet manifolds, and changes to the absorbent circulation system. Where loading system changes arise, for
instance by reducing the available pressure in the vapour collection system, a vapour blower can be added to the system. Vapour blowers might be added to the inlet of the system or »
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