Swagelok Orifice Fittings: What Do You Know About Flow Control?

How do you measure what's in a pipeline that is thousands of miles long? What you need is an orifice plate. Today we review its function, answer a couple of frequently asked questions about implementation, and link to free resources.

You’ve got a process pipe. It has gas or liquid going through it. You need to know how much is going through per minute or hour or day or year. The trouble is, the pressure within the pipe varies, like a garden hose. Higher pressure = more water, less pressure = less water. So how do you account for these pressure fluctuations and accurately measure volume?

One way would be simply empty the contents into a tank and measure that way, but if your pipe goes a few thousand miles (like a pipeline) and takes days or weeks to make its journey, then you need a more instant method to measure. After all, commodity pricing changes daily.

What you need is an orifice plate — a restriction in the flow of a pipe.

Example orifice plate data sales drawing.

Imagine an hourglass. You flip the hourglass over and the sand runs through the restricted flow path from one side to the other. In the hourglass example, the amount of sand in the hourglass equals one hour of sand flowing through the restricted orifice.

Since the amount of sand stays constant, and gravity is the force to move the sand, the output is always the same. Your pipe sees different variables; increased or decreased pressure, fluid viscosity variables, changes in temperature, and more.

In the early 1900’s a lot of work was done with orifice plates and fittings. Basically what they found is that when you restrict the flow of a pipe with an orifice plate this happens:

1. The pressure of the media in the pipe reduces immediately following the orifice restriction, and
2. The velocity of the media in the pipe increases immediately after the orifice restriction. The relationship between these two effects is proportional to the size of the orifice. A large orifice will induce a lower pressure drop and a lower increase in velocity, while a small orifice creates a large pressure drop and a higher increase in velocity.

Bernoulli’s Principle

When you measure the pressure ahead of the orifice restriction and immediately after, you can calculate the velocity and measure flow across that point in the pipe. Still with me? This effect is known as Bernoulli’s Principle. It states that there is a relationship between the pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure decreases and vice versa. Add viscosity to the equation and you have a proven method of measurement that works on all sizes of piping and tubing.

Now you might be thinking: “Why did I build this pipe line just to restrict the flow to measure it. Am I not defeating the original design of my pipe? I’ve affected my flow, this seems very inefficient!"

Not so. The original pressure and flow observed ahead of the restriction will be observed again further down the pipe, usually 5-8 times the diameter of the downstream pipe. Thanks Mr. Bernoulli.

In the meantime, congratulations! You are a flow measurement expert. (Well, chances are that you know more than the person you sit beside at lunch.)

Getting help with orifice fittings

We offer technical support, product selection assistance, consulting with local Swagelok Field Advisors, fully built assemblies, and a range of business support. At our contact page we offer four ways to get in touch with us:

We can work with you on any fluid system challenges you are facing. Not only do we have three certified Field Advisors on-call, but we also can tap into our network of 50 Swagelok Field Engineers North America.

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