In this series, we're discussing the flow control challenges faced by users of abrasive or aggressive fluids. The
first post described several applications for these challenging fluids, and briefly introduced a couple of concerns encountered by these users. In this post, we'll review these concerns in more detail and summarize a few flow control options available for these difficult applications.
Material compatibility is a major concern when measuring the flow of aggressive fluids like acids. There are several alternatives to ensure the wetted materials in process instrumentation ‘get along with' the process fluid. Some options include: the use of high-alloy or exotic metals like Hastelloy C, applying a chemically-resistant lining to the wetted flow path, or even using instruments constructed entirely out of chemically-resistant plastics. In addition to the instruments that provide a way to measure flow, instruments that provide a control function (like valves) should also be specified with appropriate consideration for material compatibility.
A secondary consideration that is sometimes overlooked when specifying instruments for aggressive fluids is the failure mode of the item - as an aggressive fluid leak can pose significant safety risks to personnel and equipment. Understanding how the wetted flow path is constructed, and how the process fluid is contained inside the flow path are key parts of this consideration. Every threaded component and/or o-ring seal on the flow path is a weak point that increases the risk of an aggressive fluid leak and decreases the installed life of the instrument.
When working with abrasive fluids like slurries, a flow effect called agglomeration can be a huge challenge to overcome. Since slurries are made of solid particles suspended in a liquid, agglomeration occurs when the solid particles deposit or ‘clump' at some point in the flow path. In polishing processes like chemical mechanical planarization, (CMP) deposits onto the actual part being polished directly reduce the yield of the polishing process. Similar deposits commonly occur at a couple of other points in the flow path that can result in process downtime, such as: piping elbows or other sections of the flow path where the flow direction changes, (including direction changes inside instrumentation) or on flow measurement instruments with moving parts. Minimizing the occurrences of these 2 scenarios in a system that will control abrasive fluid flow is a solid design strategy.
Peristaltic pumps are a common flow control method for both aggressive and abrasive fluids. These pumps provide a wide flow control range, can be designed to minimize agglomeration, and can also be made from materials compatible with a range of aggressive fluids. The primary concern with this method of flow control for OEM's and some other system designers is the high initial cost of these pumps. Fortunately, there are other configuration options available for such users.
Manual aggressive fluid flow control can be achieved by using instrumentation to provide a flow measurement along with a control valve made out of suitable materials. In addition to the options listed in the 2
nd paragraph above, a differential pressure element can be assembled to provide a measure of flow using an orifice or similar flow restriction with
pressure transmitters designed for aggressive fluids. This would also provide the user with a measure of system pressure. We welcome any further thoughts or concerns you have about these applications in the comments. If you'd like to discuss one of these applications in more detail give us a call.