What is Isokinetic Sampling?
Sampling from fluid streams of air, flue gas, steam, or any media that contains entrained particles is a very tricky affair.
If the fluid is homogenous, the sampling is relatively simple since the fluid has the same consistency throughout the flow area. This is not the case with fluids having entrained particles. Particle concentration changes because of the flow pattern inside the fluid stream.
Let us consider the example of measuring particulate emission from a coal fired power plant. The norm in most of the countries around the world is around 50 mg/Nm3. Non-compliance results in stiff penalties and even closure of the plant. To prove that the plant is running at these levels, particulate samples from the flue gas ducts are analyzed. The key step is getting the correct sample. There are two major problems in getting a correct sample.
- The large cross section area of the flue gas duct results in flow segregation due to many reasons. Taking a large number of samples from points across the duct avoids the effect of this segregation.
- The sample is drawn out of the flue gas duct by suction from each point through a sampling tube. If the sampling velocity at the point of sampling is less than the fluid velocity, then all the particles, especially the smaller size particles, will not enter the sampling tube. If the velocity is more, then more particles will enter the tube, again especially the smaller particles. Both conditions produce samples with wrong concentration. Ideally, the flow of the sample through the sampling system should be such that the velocity at the sampling point inlet is the same as the velocity of flue gas at that point. This is called Isokinetic Sampling.
Where is Isokinetic Sampling used?
Isokinetic sampling is widely used in dust and particle measurements in power plants, furnaces, kilns, and scrubbers. Ambient air pollution measurements also make use of this method.
Other applications in coal-fired power plants include sampling of pulverized coal and sampling steam to check the purity before in enters the turbine.
Crude oil sampling at transfer points to check the impurities is another application.
How to do Isokinetic Sampling
An ideal isokinetic sampling system consists of a probe that inserts and traverses along the section of a duct or pipe. It has a velocity measuring point at the tip, the sampling tube, and a temperature measurement point. The probe includes a sampling flow adjustment valve. After measuring the velocity and making the sampling flow adjustments, samples are drawn. This gives an isokinetic sample at each point. A typical probe is shown in the image at right.
Sampling of steam and other fluids at high temperatures and pressure make use of fixed nozzles sized to get isokinetic flow.
Isokinetic sampling is an elaborate process used for collecting samples that have commercial or legal implications. This is also the primary sampling method for the calibration of online instruments.