Almost every process facility requires steam. It is used for heating, drying, power, etc. Steam is an excellent way to convey energy from one process to another. The heat and energy of the steam needs to be measured and controlled into the next stage of the process.
Steam flow and pressure is controlled with an orifice. You can use a fixed orifice, a line sensed pressure regulator, or a full control valve. There is no adjustment with an orifice. A regulator is given a fixed output setpoint, but is not adjustable by the control system. A control valve can vary the orifice size according to the output of the control system which has measured the flow, pressure, and/or temperature from process instrumentation and sensors.
When controlling system, it is always done with an orifice which creates a pressure drop. The higher the flow rate and pressure drop, the noisier the valve will be. There are two problems with noise. First is the loud environment and potential damage to your hearing. The second is that noise is vibration and that vibration can damage the physical components of the valve. 85dB or lower is the ideal noise level. You can avoid high noise by taking the pressure down in small increments rather than one big drop. This can be accomplished within a control valve or through a fixed drop low decibel cassette inline with the control valve.
There is a way to make almost every steam control application low noise and highly reliable. You must select the proper method of controlling and regulating the steam with the proper number of pressure drops. For help with your next steam application, please contact us.
If you are like me, you started your adventure into process measurement with measuring liquid and gas flows, temperatures, and pressures. I got very good at designing systems using the proper instrumentation to measure and control these. However, I didn’t pay much attention to solids measurement. This has since changed. I work with solids control in a large percentage of my projects now. I have come to realize that the expertise gained in liquid control does not always directly translate to solids measurement.
There are 5 main ways to measure solids flow. They are as follows:
Belt speed measurement
Conveyor motor amp draw
Belt Scale Conveyor
Impact Mass Flow
Radiometric Absorption Conveyor
Belt speed measurement is the simplest method, but it is also the most inaccurate. This method just measures the linear speed of your conveyor or screw and estimates the mass of the dry product it conveys per minute. Sometimes this is ok, but it assumes a steady feed rate and steady product density. This method shouldn’t be used for control or inventory tracking.
Conveyor motor amp draw measurement assumes that increased amps is an increase in mass being conveyed. This is a better idea than purely monitoring the conveyor’s speed, but it too has massive drawbacks. It assumes that the conveyor itself is steady state mechanically. Does your conveyor have a consistent friction at all speeds and temperatures? Probably not. Do mechanical systems wear over time causing “calibration” drift? Probably. Does the amp increase vary linearly with mass increase? Probably not. I personally don’t like these systems as they don’t work very well and give a false sense of having measured something well, when in fact you have not.
Belt scale conveyors are the gold standard in mass flow accuracy. They consist of load cells on independently supported rollers under a conveyor belt. There can be from one to 4 of these roller/load cells in series. As solid product rolls over these load cells, it is weighed. This weight and an output from a speed sensor combine to give you an extremely accurate mass flow. These systems, depending on the number of load cells, can give a mass flow reading of up to +/-0.25%. These systems aren’t very expensive in the scheme of things, but they do require a lot of straight space in your belt to install and work properly. They also rely on moving parts and are exposed to the dust from the solids which will require maintenance and periodic recalibration with provided reference weights. Overall, these are the most accurate systems you can get.
Impact mass flowmeters are the next most accurate solids flow measurement device. These require that product be lifted above this “box” and dropped through it down to a bin or secondary conveyor. This “box” consists of a top entry chute and a bottom outlet chute. In the middle of this box is a angled plate attached to a load cell. As solids fall into the box, they strike the load cell plate and then fall off and out of the box. The load cell sensor assembly is sensitive enough to track the rate and mass of the solids going through it and gives you a reliable +/-0.75 to 1.00 percent accuracy spec. These require less maintenance than the belt scales, but still need cleaned periodically as the sensor is exposed to dust and the plate is open to wear from the impact of potentially abrasive solids. These are a great option if you have the vertical space to fit it into your process.
Lastly is relatively unknown method of radiometric absorption. This method uses a radio isotope to emit gamma rays at a known and regular rate. The conveyor and the material being conveyed absorb these gamma rays. The conveyor never changes density, so it absorbs the same amount of gamma all the time. The change in absorption over time is therefore directly related to the change in product mass flow. The levels of gamma rays emitted are so low as to make these safe for install in the field. In fact, (only with Ronan) no special certificate or government regulation applies to most of their installs. These systems do not make contact with the product and are installed outside of the conveyor. This means they require no maintenance as dust and abrasion are not a factor. They have a pretty good accuracy rating of +/-1-2% in most applications.
Now that you know the main ways to measure solids mass flow, it’s time to include this in your plant’s mass balance calculations and inventory controls if you haven’t already. If you have questions about how to implement this in your process or how to correct an install this isn’t living up to your expectations please contact us.
