A good clean up

Materials World magazine
,
2 Jun 2015

Ignacio Garcia de Carellan and Cem Selcuk, from the Brunel Innovation Centre and the CleanMine consortium, outlines an innovative approach to the problem of removing hard scale from valves.

The problem of fouling in pipes and valves has been reported in different fluid carrying industries, such as petrochemical, chemical, oil and gas and mining. A number of approaches are in use globally to prevent or reduce the impact of fouling in production, but there are several associated economic and environmental costs. An increase in pumping costs, a reduction in the efficiency of the process, potential blockage, rupture and contamination all impact on the viability of current approaches. Existing cleaning methods generally require a halt in production, and use strong chemicals or pigs to remove this unwanted scale. 

The use of chemicals, due to the accumulation of fouling, and the associated economic disadvantages make non-invasive cleaning a prime improvement. A new method to clean the inner wall of pipelines that avoids the use of chemicals, maintains the cleanliness of the pipeline's inner wall and can be applied during production would provide many benefits to any carrying liquids industry. With this in mind, a consortium of research and technical development centres (RTDs) and small and medium-sized enterprises (SMEs) is working to generate a prototype for the removal of hard scale from valves with an ultrasonic approach that has a minimal environmental impact, which is low cost and applicable in situ. This new idea combines the use of controlled acoustic cavitation with guided wave propagation through the valve for:

prevention and/or removal of fouling accumulations

reduction in operational risk

reduction in cost of maintenance

An ultrasonic approach

High-power ultrasonic cleaning is a well-known technology. It is used for the removal of fouling from different types of objects, such as engine components, metallic surfaces and jewellery. These are immersed in an ultrasonic cleaning bath, containing a solvent with low surface tension, which is induced to vibrate. The compressional waves that travel through the liquid can generate locally at micron level conditions that reach temperatures above 5,000K and pressures above 500bar – this is called acoustic cavitation. Ultrasonic guided waves could allow for in situ removal of fouling in the inner walls of pipes and valves. These days, guided waves are commonly used for inspection of large structures, such as pipes, plates or other engineering constructions. A guided wave is one confined to the same structure in which it is propagated. These waves resonate modes of vibration along the thicknesses of the structure while generating constructive interferences at each reflexion, meaning they can cover large distances because of the low dissipation. 

The ultrasonic approach seeks to make use of these guided waves to clean valves while they are in operation. The principle is based on controlling cavitation effects to remove hard scale without affecting the structural integrity of the valve or its performance. This vibration, when induced at desired modes, is expected to remove fouling when resulting energy is above the cavitation threshold. Optimum frequencies, vibration modes and power levels could be applied through guided waves in valves or other tubular fittings in order to achieve cleaning or prevention of fouling in the first place.

Previous studies performed by the Brunel Innovation Centre (BIC) in Cambridge have proved the efficiency of this technique in other structures, such as plates. The observations show that the energy can be propagated through the structure as a guided wave, as opposed to full dissipation into the cleaning liquid medium as in conventional ultrasonic cleaning. 

Preliminary results on transducer performance in terms of displacement patterns can be seen below, right. For this, a 3D laser scanning vibrometer is used. This equipment can detect and measure the actual vibration of any object.

In the process optimisation of transduction, 3D vibrometry (based on the Doppler Effect) is calculated at each point of the scanning area. This visually captures achievable displacement levels with varying frequencies and power levels when operating a transducer.

The future

The members of the CleanMine consortium are currently in the process of producing a robust portable prototype. This will have to resist the harshest conditions that can be found in the mining industry, such as acidity, alkalinity, humidity, friction, and high and low temperatures. The prototype will be field-tested at prospective end user sites, where a fouled component will be cleaned while in operation. 

The prototype will have a clamping bracelet wrapped around the component to be cleaned, containing an array of high-power transducers. These transducers are designed to be brought into contact at several uniform circumferential points on the valve surface to evenly maximise the area of cleaning coverage. This mechanical assembly will be connected to a custom-built signal generator. The ultrasonic sub-assembly of the prototype will be protected from potentially hazardous environments through a robust cover, as part of the encapsulation design. 

The CleanMine Project is a three-year research programme and it is anticipated that further results will be made public as the project progresses. This new technology, which is already having promising results in the laboratory trials, aims to tackle the problem of fouling for the mining industry through a non-invasive methodology, ultimately reducing the number of times that a process has to be suspended for cleaning. It is anticipated that this will help reduce maintenance costs and operational risk – a worthy aim, as it is estimated that total fouling-related costs for major industrialised nations exceeds US$4.4bln annually.