Orifice plates abound at many plants because they provide a simple and inexpensive way to measure flow rate. An orifice plate induces a pressure drop in a fluid that should accurately correlate to its flow rate. If the orifice plate meets established standards for its geometry and is correctly installed in the process line, then, as long as we know the properties of the fluid, we can get flow measurements to within 1% accuracy.
However, in practice, many factors can lead to flow rate errors of up to 15% or more. These include mis-location of the instrument, improper installation, damaged plates, blockages and unexpected flow regimes (laminar versus turbulent or two-phase versus single-phase).
One of my very first troubleshooting assignments was attempting to determine fuel consumption and efficiency for a number of large thermal cracking furnaces. The complex fuel gas system at the plant had evolved over decades. Fuel compositions varied over time and in location — and composition wasn’t continuously measured. Additionally, branches of the system used different flow meter types; some branches lacked any flow meters and, thus, required mass balance calculations based on data from meters installed elsewhere. Adding to the challenge, meter maintenance was sporadic and poorly documented. Apparent errors for fuel consumption on some heaters exceeded 30%. In other cases, two meters in series on the same pipe gave readings that differed by more than 15%.
Every time an eager new-hire engineer was between assignments in the plant, that person was tasked with trying to sort out the fuel system. Long-established history had shown the assignment to be thankless and progress only occurred in small steps; my experience bore that out. Nevertheless, I did learn some important lessons. Today, we’ll focus on those related to orifice plates.
First, most orifice plate errors, except for installing one that’s too small, tend to give flow rates lower than actual.
Backwards installation of an orifice plate is a classic error. For a reasonable beta ratio (~0.5) and in turbulent flow, expect an error of 12–15% too low a flow rate. Orifice plates have a specific installation direction that should appear on the tab on the orifice plate along with dimension information. If you can reach the orifice plate, this is easy to check. Accessing the plate may be a problem, though. Accurate measurement may require a length of straight pipe upstream equivalent to up to 90 upstream pipe diameters for sufficient flow conditioning (see: “Think Straight About Orifice Plates”). This length of straight run often only occurs in pipe-racks, making getting to the orifice plate difficult and time consuming. Additionally, in rare cases, the plate stamping is on the wrong side, which easily can lead to backwards installation.
Orifice plates should be flat but can undergo buckling or bending during manufacture or transport. Badly buckled plates are easy to spot, so rarely get installed. Somewhat buckled ones often are installed, though. If the plate is buckled toward the upstream direction, the flow rate will err high, while one buckled toward the downstream direction will err low. When installed in bolted flanges, the buckling usually decreases due to the flange forces on the plate. However, don’t count on this. Errors due to buckled and bent plates usually are 7–8% or less.
Plates have a specific edge geometry on the orifice. Pay particular attention to the shape of the edge and measure the dimensions carefully to make sure the plate is acceptable. If the sharp edge isn’t manufactured correctly or is damaged, the discharge coefficient of the orifice changes. This tends to result in low flow errors in the 4–6 % range. If enough wear occurs, then the entire orifice size changes and errors can get much larger.
Deposits may accumulate on the plate surface or edge. Deposits also may build on the pipe on either side of the plate. Errors from these problems can reach nearly any value. A surface deposit only on the plate likely will lead to an error of 4% or less. Deposits on pipe that interrupt flow to or from the orifice can cause errors exceeding 15%, which can be high or low depending upon where the deposits are.
Problems with orifice plates often prevent plants from closing material balances within acceptable error ranges. However, they aren’t necessarily the only causes. As in all troubleshooting, start by assessing available data and only use correct information.
ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can email him at ASloley@putman.net