Plant Cooling Systems
Each summer fin/fan aerial coolers come under maximum thermal load. Operators are aware that the performance of these units deteriorate with age and fouling and many operators are aware that the encroaching summer months (with or without the assistance of El Nino) may result in plant throughput being constrained due to aerial cooler overload. All types of coolers are affected to some degree whether engine glycol cooling systems; compressor inter or after coolers; lube oil cooling systems etc. However the types of units most affected by fouling, air flow shortage and decreasing fan efficiency are condensers. Fractionation plant overhead condensers, refrigeration plant condensers and steam condensers are three categories of coolers most affected by air flow shortage.
Many operators will give these units a "spring cleaning" and hope for the best. However a full field test of critical aerial cooler heat transfer performance and fan efficiency can usefully point the operator in the direction of the most cost effective retrofits (if necessary) or appropriate cleaning programs.
When we field test an aerial cooler, the diagram below shows the minimum parameters and ideal parameters which need to be measured to assess unit performance and extrapolate the performance for changes in ambient and process conditions.
Of all the data parameters to collect, the most difficult to do so accurately are the air flow measurements. At Process Consulting, we use a compensating hotwire anemometer. This has been proven rather more accurate than conventional vane anemometers, particularly in turbulent air flow regimes. We also measure air flow above the bundle rather than into the fan ring as this provides us with a more accurate assessment of air flow vectors and much less variation in air velocities scans. These field test enable us to assess best remedies for the coolers whether they be air side enhancements such as bell-mouths or tip seals or process enhancements such as turbulator rods.
Noise Impact Assessments
Under the AEUB Noise Control Directive ID 94-4, oil, gas and utility operators are required to conduct a Noise Impact Assessment prior to the installation of new or modsified facilities. While this is often not much of a problem in remote areas, this can create potential difficulties in moderately populated rural areas. If there are any residences within approximately 1000 metres of a single gas compressor or approximately 1500 metres of a small/medium gas processing plant, then a Noise Impact Assessment will usually indicate that some degree of extra noise control may be necessary to meet the Directive. In any event, even in remote areas, the facility should be designed not to exceed 40 dBA at 1500 metres.
Noise control included at the design stage of a new or modified facility, will usually add no more than 0.5 to 2.5% to project cost - depending upon the size of the facility and the proximity of the nearest resident. Noise control considered as a retrofit after complaints arise will usually cost two to four times (or more) as much as the same control included with the original design.
In predicting noise levels, operators can often work successfully with silencer suppliers to predict engine exhaust noise with some accuracy. However many operators have difficulty accurately assessing fan noise and engine casing noise. Cooler fans and engine casings are often incorrectly assumed to be point noise sources. This leads to under-estimation of the total noise emissions. Many styles of cooler fans are also highly directional with regard to noise propagation. This effect is sometimes misunderstood with the result that cooler fan orientation is sometimes not optimized at the design stage.
Engine casing noise is often ignored altogether. Building attenuation is often over-estimated resulting in optimistic environmental assessments. In practice, the building attenuation is limited by the open area required for ventilation, which is often inadequate for the task.
This is proven by the need to leave the building doors open in summer to prevent overheating. The open doors, of course, eliminate most of the noise control offerred by the building. The design of compressor buildings and ventilation systems to suppress and minimize the effect of engine casing radiated noise is relatively simple at the design stage and more complicated with field retrofits.
Patching Associates handle some 100+ oil, gas and utility noise control projects annually. Approximately half of these tend to be the more expensive retrofits rather than initial design considerations. We would like to remind our clients that we are available to work with your project or contracting engineer at the design phase in order to eliminate the need for potentially costly retrofits once the facilities are commissioned and subject to potential resident complaints.
