Swagelok Orifice Fittings: What do you know about flow control?
by Jason Wynne, on Thu, Aug 21, 2014 @ 08:08 AM
How do you measure what's in a pipeline a few thousand miles long? What you need is an orifice plate.
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Flow measurement. For some, it’s a fascinating topic and rewarding career path that employs the skills and knowledge of an Engineering degree. For others, there are enough brain-numbing equations and white papers written on the topic to induce an afternoon nap. So let’s lighten it up a bit and let you in on what all those engineers know…and orifice plates and fittings are the key.
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 every day. What you need is an orifice plate.
An orifice plate is a restriction in the flow of a pipe. 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 two things happen:
- The pressure of the media in the pipe reduces immediately following the orifice restriction, and
- 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.
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 which 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 in viscosity to the equation and you have a proven method of measurement that works on all sizes of piping and tubing.
Now you are 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?"
So now you are 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 orifice restriction will be observed again further down the pipe, usually 5-8 times the diameter of the pipe downstream of the orifice. Thank you, Mr. Bernoulli.
I already know your next question: "How do I measure the pressure drop created by an orifice plate?" That’s a blog topic for another day. In the meantime, congratulations! You are a flow measurement expert. (Well, at least you know more than the guy you sit beside at lunch.)
For more information on restricted orifice fittings available from Swagelok, please give Edmonton Valve a call at 780-437-0640 or download literature here: