Design & Simulation / Fluid Handling / Reliability & Maintenance

Don’t Let Orifice Calculations Put You In A Hole

Significant mistakes can occur even for such a simple device

By Andrew Sloley, Contributing Editor and Scott Schroeder, Senior Consultant, Advisian

The right calculation method for orifices depends on the system.

Are results reasonable? That’s a question that all calculations should face. However, defining what’s reasonable can be difficult. Determining that requires both understanding the basic principles of the process as well as experience in application, as three recent orifice calculations show. All had errors that could have caused problems. Fortunately, the mistakes were caught and corrected. A look at these examples illustrates several elements of arriving at a reasonable result.

In our first case, piping attached to a high-pressure vapor/liquid separator had a design pressure that couldn’t handle the full pressure in the drum. If the control valve feeding the line failed open, it was possible that high flow could cause the line pressure to be exceeded. One standard approach to prevent this is to put a restriction orifice in the line (upstream of the specification change to the lower pressure) that restricts the flow rate.

Orifice calculations for single-phase flow are relatively straightforward. So, after a quick chat, orifice specification was handed off to a junior engineer. The engineer came back with an orifice size of 1.65 in. The orifice was going to be installed in a 2-in. NPS schedule 160 line. The line diameter is 1.687 in.

This isn’t a reasonable result. How did the calculation go wrong? The culprit could be one of a number of factors: misunderstanding of the situation, a calculation error, use of an inappropriate method, or a poorly defined task set. Some questioning identified the cause. No one had made clear that the purpose of the orifice was to restrict flow. The junior engineer hadn’t understood that and, so, made the calculation assuming the entire flow rate after the control valve failure would go down the line. This completely contradicts the actual intent of the orifice. Here, the objective of the orifice — to restrict flow — wasn’t communicated well, if at all.

For orifices, the wrong calculation method also can lead to unreasonable results, as occurred in our second example. The right calculation method for orifices depends on the system. Is it liquid flow, gas flow or flashing flow? Will the fluid attain sonic velocity? Methods are available for each situation. Many commercial software packages will calculate orifice pressure drops and sizing as needed.

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Much of the software for orifice sizing comes from flow instrument vendors. These vendors are most interested in the pressure drop at the location of the tap for the pressure drop reading to get a flow rate. The pressure tap is located at the vena contracta. This is the point downstream of the orifice where the flow velocity is the greatest and pressure drop is the highest. Downstream from this point, the pressure rises again as the fluid slows down to the bulk velocity in the piping.

When using control element sizing software for orifices, it’s important to know if you need the pressure drop for an instrument (with a tap located at the vena contracta) or the permanent pressure drop downstream. When putting in an orifice as a flow restrictor, the downstream pressure drop is what’s important. Using the vena contracta pressure drop instead of the permanent pressure drop results in an over-sized orifice. Flow rates will be higher than expected at the flow restriction conditions. If the flow restriction is in place for safety reasons, this may have serious consequences.

The third recent example with restriction orifices illustrates software misuse. Rather than doing an orifice calculation, an engineer sized orifices with a conventional process flowsheet simulator. The flow was handled as feed through a pipe element in the simulation. The engineer specified a short pipe segment with a restricted diameter as the pipe element. The software adjusted the diameter until it reached a target flow rate and pressure drop. That final diameter of the pipe segment was used as the orifice diameter. How close the result is to a proper orifice calculation depends upon the Reynolds number in the pipe segment, the length used and the diameter. All three must be taken into account to make this approach work. It’s simpler, easier and more accurate to use the orifice equations or the correct software.

Orifices may appear simple. In fact, they are simple. But even simple things require understanding and experience to get right.


ANDREW SLOLEY is a Chemical Processing contributing editor. Scott Schroeder is a Senior Consultant with Advisian. You can email them at asloley@putman.net and Scott.Schroeder@advisian.com.