Flow control can be challenging at times – that fact keeps all of us here at Brooks employed! Many users who need to control gas flow in their process don’t want to spend a lot of time setting up their own flow control scheme, and instead purchase a mass flow controller. Read more…
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.
Here at Brooks, it’s very common for us to work with customers that use our technology in applications that aren’t widely known to the general public. In this series we’ll talk about another of these applications: flow control for abrasive and aggressive fluids. Even though the general public may not know the role that abrasive fluid flow control plays in their daily lives, applications that require this type of flow control are all around us. Many products require this type of control during their manufacturing process, and it is also used in a range of environmental applications like odor control, municipal water treatment, or pH adjustment.
There are a range of applications where reliable abrasive or aggressive fluid flow control is critical, here are a few examples:
Printing Inks: Inks used in printers that we use everyday are made from a range of fluids with different properties. Many color inks contain solvents made from aggressive petroleum distillates, and can also use dissolved titanium dioxide to control color. There are also other fluids that get mixed into these inks like: lubricants that extend the life of the printer heads, waxes that extend the life of the ink on the page, and drying agents that help the ink dry quickly onto new documents.
Slurries: A slurry is made when particles of a solid are suspended in a liquid solution. A common use for slurries is to control the flow of ‘gritty’ solid particles in the slurry across an item to polish the item’s surface, which is a critical step in manufacturing products like processors used in computers and cell phones, or solar panels. In another case, a slurry fills a mold and is turned into a brake pad after it is compressed and dried. Flow control of slurries made from suspended lime are also critical in a range of municipal, environmental, and industrial processes that treat a hazardous compound before disposal.
Metal Pickling: Pickling is a surface treatment process performed on a range of metals to remove impurities or undesirable layers on the surface of the metal. Submerging a metal part into one or more acid baths is commonly used to remove contaminants, rust, or scale to extend the life of metal parts. Pickling can also remove the oxidation layer from copper so it retains its’ color over time; this process is commonly used when making copper jewelry.
What other applications are out there for abrasive or aggressive fluids? We’d love to hear more about your applications in the comments.
As you can tell by the descriptions of these fluids, there are several challenges that users of aggressive or abrasive fluids have to overcome to be successful. Users of these fluids have to ensure that the materials of construction in the equipment and instrumentation they choose for flow control is chemically compatible with these aggressive fluids. Dissolved or suspended solids in a liquid stream can agglomerate (clump) and prevent the system from operating, so users should consider those impacts in their designs for such fluids.
We’ll discuss the range of flow control options available to users of these fluids in our next post.
If you’d like to read a bit more about instrumentation and process control, feel free to check out more of my contributions summarized on my Google Plus profile.
In this series, we’ve been discussing a gas flow control challenge that users face when backpressure changes. In the first post, we discussed several gas flow control applications where this is a concern. In the second post, I described a flow effect called choked flow, which occurs when gas flow through an orifice reaches the speed of sound.
We already know that when gas is flowing through an orifice at the speed of sound, it’s moving faster than the gas can expand on the outlet side. We can get the gas flowing through the orifice at this speed by adjusting the ratio of inlet to outlet pressure. The minimum ratio of these pressures that results in choked flow can be calculated from the isentropic expansion factor of the gas. This ratio is 1.8 to 2.2 for many common gases.
This means that when gas flow control is needed into something with a changing pressure, we can disregard the downstream pressure changes with most gases by using an upstream pressure that is at least 2.2 times the highest downstream pressure. This ratio should always be calculated using absolute pressures. So if a desired mass flow rate needs to be maintained when downstream pressure ranges from 25 to 75 PSIA, the flow will stay steady if the inlet pressure to the orifice is set at 165 PSIA or higher.
Now that we can use choked flow to maintain a mass flow rate into a changing backpressure, what happens if we need to increase the flow rate? Here are two options:
But what if the maximum downstream pressure is higher than 75 PSIA? Our clients operating at higher pressures are having success with the market-leading SLA series mass flow controller. The SLA can operate at pressures up to 4,500 PSIG. It is also capable of operating indoors, outdoors or in hazardous areas, and it provides numerous electrical communication options to meet the needs of a wide range of flow control applications.
If you’d like to discuss an application like this in more detail, you’re welcome to enter some application and contact information into this page to be contacted by your local Brooks product expert. Feel free to give my colleagues and I a call if we can help as well. We can be reached at 215-362-3500, ext 3000.
If you’d like to read a bit more about instrumentation and process control, feel free to check out more of my contributions summarized on my Google Plus profile.
In my last post, we discussed several applications for mass flow controllers where precise flow control is needed despite backpressure changes. I introduced a flow effect called choked flow, which many of our customers use in these applications to ignore downstream pressure changes. This is also referred to as sonic flow or critical flow.
To my flow-savvy readers: You’ll notice that I’m discussing choked flow in conceptual terms rather than demonstrating complex formulas and calculations. Don’t be alarmed! Feel free to post any additional thoughts you have on this topic in the comments below.
The sketch to the right shows gas flow through a typical orifice. The green shaded areas are high pressure, low velocity flow areas, and the blue area is a low pressure, high velocity flow area. Inlet gas flow speeds up as it compresses to pass through the orifice, then re-expands and slows back down on the outlet side. The flow rate through the orifice is primarily set by the inlet and outlet pressures, as well as the diameter of the orifice opening. Gas temperature also plays a part.
A gas expands in a space as its molecules collide with whatever else is present. (pipe walls, other gas molecules, etc.) Every gas expands at its own rate, and pressure increases in a gas are a result of squeezing more gas molecules into the same amount of space. Applying these factors to the orifice flow pictured, gas expansion causes some of the molecules that are expanding in the green area on the outlet side to collide with and deflect the “fast” molecules in the blue area. If the pressure rises in the green area on the outlet side, it’s because there are more gas molecules present in the same amount of space.
More molecules in the green outlet area mean that more molecules deflect the “fast” molecules in the blue area. This reduces flow velocity in the blue area, which is what reduces the flow rate passing through an orifice at higher backpressures. If the pressure drops in the green outlet area, it means that fewer molecules are present in that space, which results in fewer deflections of “fast” blue molecules. This causes a higher velocity in the blue area, and thus a higher flow rate when the backpressure drops.
Choked flow occurs when the flow velocity in the blue area reaches the speed of sound. At this velocity, the molecules in the blue area are essentially traveling faster than the molecules in the green outlet area are expanding. So deflection between molecules at the blue/green border doesn’t reduce velocity in the blue area. With a fixed inlet pressure, the outlet pressure can change over a wide range without changing the mass flow rate as long as the conditions to maintain choked flow remain in place.
So how can we reach the conditions needed to maintain choked flow? We’ll cover that in our final post in this series.
Where did the names choked flow, sonic flow, and/or critical flow come from? Please post where you think one of these names came from in the comments. The first poster that correctly lists the reason for each of the names will win a 4 GB jump drive in the shape of a mass flow controller.
If you’d like to read a bit more about instrumentation and process control, feel free to check out more of my contributions summarized on my Google Plus profile.
In gas flow control applications, inlet and outlet pressures are critical factors when configuring a flow controller to ensure that the desired flow rates can be maintained. Increase the downstream pressure following an orifice, and the amount of flow is typically reduced. In this series we’ll talk about a method you can use to specify a mass flow controller that ignores downstream pressure changes to provide reliable mass flow control into a range of pressures.
Repetitive increases and decreases to a gas flow controller’s downstream pressure are common in many of our customer’s applications. How common? Here are a few examples:
Biotechnology: A mass flow controller controls gas flow in a bioreactor to promote a desired biochemical reaction. There are a wide range of reactions or events in bioreactors such as: promoting tissue growth, assisting organisms to produce desired chemicals or medicines, developing enzymes to break down hazardous compounds, and many others. Tight gas flow control of oxygen is needed to help organisms that consume oxygen prosper inside a bioreactor. Many of these processes create other gases, (oxygen converted to carbon dioxide, for example) and different batch sizes or recipes require different gas flows. These factors change vessel pressure without removing the need for precise gas flow control.
Food Aeration: A mass flow controller injects gas into a food item. (Nitrogen is commonly used) As foods like butter, bread dough, chocolate bars, ice cream, and even Oreo cookie stuffing are processed, it’s quite common for a gas to be injected into the food to maintain a target consistency or texture. Different foods and batch sizes change the pressure needed to inject gas into the food. Inaccurate gas flow increases the amount of food rejected for poor quality.
Selective Catalytic Reduction: A mass flow controller injects gas flow into an exhaust stream to break down targeted hazardous gases or compounds for air quality purposes. For example, ammonia vapor is commonly used to breakdown nitrous oxides. The exhaust stream pressure changes as the equipment load changes, and the mass flow controller needs to provide tight mass flow control to break down enough of the compounds. Inaccurate injection gas control reduces air quality.
Vessel Fuel Research: A mass flow controller controls gas that fills a vessel to initiate and control a reaction. Hydrogen is often used for fuel research. A catalyst is placed or gradually fed into a reaction vessel along with the gas(es). The mass flow controller needs to maintain precise mass flow control into the vessel to maintain the desired reaction rate at the same time that the downstream pressure is increasing as the vessel pressure rises. Inaccurate gas control prevents the desired reaction(s) from occurring.
There are definitely other flow controller applications with a variable back pressure that were not included in this list to keep it a manageable size. Please post any you’d like to share in the comments – we’d love to hear more about your applications.
Many of our customers who need a gas flow controller for an application with downstream pressure changes take advantage of a flow effect called choked flow that allows the flow controller to ignore backpressure changes. We’ll talk more about this gas flow effect in the next post.
If you’d like to read a bit more about instrumentation and process control, feel free to check out more of my contributions summarized on my Google Plus profile.
