There are several tools you can use in flow gauging. In all those flow meters, some have higher popularity. For obvious reasons, a rotameter is the most popular flow meter in the market. It falls under the class of variable area flowmeters. It’s also the easiest meter to use.
What is a Rotameter?
A rotameter is a simple variable area flow meter. It allows fluids and gases to pass through its tapered tube. The meter gauges the volumetric flow rate of the fluid. First, it has to lift and suspend the tube within the tube. Then, you gauge the flow rate by how high the float is suspended.
Rotameters are installed vertically. When the fluid is stagnant, the float rests at the base of the tube. When the fluids start to move, they squeeze into the tube, pushing the float upwards. The scale in the side of the tube reads from the bottom and increases upwards. As the velocity of the fluid increases, more fluid squeezes into the tube, forcing the float higher.
Basic Parts of a Rotameter
A basic rotameter has three parts. Those are all vital to make the rotameter function. The design of these parts is what creates different types of rotameters.
This is the part that holds the float and the scale. It’s made of various materials like glass, metal, or plastic. Glass tubes are standard since it’s easy to see through them. Also, they’re tough and rarely react to corrosive fluids. But some factors may lead you to use plastic or metal tubes. For instance, if your fluid is plain water, a plastic tube would do. It’s also cheaper than the other materials.
Floats are made in different shapes and materials. You use a type of float based on the accuracy needs and the type of fluid you have.
Spherical flow meters are best for easy gauging of low flow rates. However, if you need high accuracy, you should use wedge-shaped floats. Also, they should have sloping notches that make the float rotate, hence, ending friction.
Gases need low-density floats. Metal floats on the other side are not common. They are heavy, and you should use them in high velocity and medium to high viscous fluids.
The scale is attached to the side of the tube. You read the meter from below- just like a thermometer. You read the scale at the float base when it’s suspended. There’s no one-shoe-fit-all scale. Each scale is calibrated against your fluid traits. Thus, it’s usually the last thing to be made for the rotameter. Scales in a metallic tube have housing that protects them from external factors.
The supplier calibrates the rotameter based on your fluid conditions. Thus, you need to know all the fluid traits in advance. If not so, you’ll need a technician to install the rotameter for you. First, they’ll check the fluid and assess the fluid traits. Then they’ll calibrate the rotameter to match the fluid traits- They’ll mate a scale and install it on the metering tube. In most cases, it’s installed on the outer surface of the rotameter.
Factors that Affect Accuracy
Rotameters are calibrated with a guess that fluid traits are constant. But in reality, the fluid will go through changes in heat, pressure, viscosity, amount of gas in a liquid, humidity, etc. When any of the traits vary, there’s an increase in the error gap. As a result, it lowers the accuracy of the reading. Thus, a rotameter would only work best under a controlled environment where all fluid traits are constant.
Other factors that affect the accuracy include pulsation, dirt, misalignment, float damage, unknown reference conditions, and instability with the float. You need to service your rotameter after a period to get rid of errors.
Types of a Rotameter
There are two main types of Rotameters. They are purge meters and orifice bypass rotameters.
They are the primary class of rotameters. They are designed to measure small flow rates in purging gauge piping. Rotameter has a constant flow-up regulator that makes it regulate the flow rate. You can use a needle valve to set the desired purge rate. This tool does not need an external power source. Thus they are useful in urban and rural areas.
As the name indicates, these rotameters are installed around an orifice plate in a bypass line. The rotameter handles large flow rates. The differential pressure in the fluid causes a low flow within the rotameter. You use that as the flow through the main pipe.
These rotameters do not require a power source to operate. Thus, you can use them in remote areas where there’s a shortage of power sources. You can also use the meters in hazardous areas.
The material that makes the type is considered the overall rotameter material. You can select any of the materials depending on the type of the fluids. Below are the three main materials that can make a rotameter tube.
Not every type of glass is suitable to make rotameters. It’d help if you had a rugged and unbreakable glass. That’s why manufacturers use borosilicate glass. It’s the original material that was used to make rotameters before other materials were adopted.
Glass tube rotameters are easy to use. First, you read the scale that is directly attached to the glass tube. Also, you can see the media flowering through it. You can also fit an alarm system if the flow is above or below the readable scale.
You can use glass tubes as a proxy for glass tubes. They’re cheaper to buy and install. Also, they are unbreakable. Thus, you can use it everywhere. Plastics can tolerate ionized water and corrosive liquids. But they have pressure limits of 100psig and heat limits of 150 Fahrenheit.
Type 316 stainless steel can make metal rotameters. The material is corrosion-resistant. Thus, it’s safe to use. They’re suitable for high-heat and high-pressure fluids. You can use them like glass and plastic tube rotameters.
Rotameters are cheap and easy to install. You may not need the help of a technician to install it. Also, the accuracy is relatively fair and meets the needs of many industries. You can gauge a wide range of fluids.