FAQ: How Do Regulators Cut Delay?
by Jeff Hopkins, on 7/26/18 8:04 AM
Gas travels faster when pressure is lower, so a regulator can speed up a sample in an analytical system
"One way to control time delay is with a regulator. Regulators control pressure, and pressure in an analytical system is closely related to time. In the case of gas systems with a controlled flow rate, the lower the pressure, the shorter the time delay."
- Doug Nordstrom, Swagelok Co and Dean Slejko, Swagelok Co
We've written about the many causes of time delay in sample systems, and how that can hurt the results that come out of an analyzer. The industry standard to aim for is one minute from the process line to the analyzer, but many potential delays lurk along the way.
Today we'll look at one component that can reduce time delay: the pressure regulator. In an analytical system, time is closely related to pressure. In gas systems with a controlled flow rate, the lower the pressure, the shorter the time delay. So it makes sense that a pressure regulator could be a useful tool.
Delay may occur in any of the major parts of an analytical instrumentation system, including the process line, tap and probe, field station, transport line, sample conditioning system, stream switching system, and analyzer. For now, we will focus on the field station and the important role of a regulator in reducing time delay there
Before the field station
Minimizing time delay begins with the location of the tap. It's best to put the tap as close to the analyzer as possible, but it should be upstream of sources of delay, such as drums, tanks, dead legs, stagnant lines, or redundant or obsolete equipment. When sampling a liquid, pressure at the tap should be sufficient to deliver the sample through the transport lines or fast loop without a pump.
You may not always be able to dictate the location of the tap. You may have to make do with an existing tap location, and often an existing analyzer shed location as well. If the tap is far from the analyzer, a fast loop will help you quickly deliver fluid to the analyzer and return the unused portion to the process.
How To Use A Regulator To Reduce Time Delay In An Analytical System (Like this blog, get the tech paper)
At the field station
In cases where the analyzer requires a liquid sample, a regulator in the field station is not needed. It is better to keep liquids at high pressure to avoid the formation of bubbles. But for a gas sample, a field station is a means of reducing pressure in the transport lines. Time delay decreases in direct proportion to absolute pressure. At half the pressure, you will get half the time delay.
Let’s look at three possible applications for a regulator, each with a different configuration. In the first application, the objective is to reduce gas pressure. The pressure drop is not expected to produce condensation, so all you need is a simple pressure-reducing regulator.
In our second regulator application, the pressure drop is expected to cause condensation. With a drop in pressure, almost all gases lose heat, which is known as the Joule-Thomson effect. If the gas is close to its dew point, this cooling could cause condensation. In some cases, the loss of heat may be great enough to cause the regulator to freeze up. A heated regulator may be required to keep the temperature of the gas above the dew point.
In our third regulator application, a liquid must become a gas before it can be analyzed by a gas chromatograph or other analyzer. In this case, you'll want a vaporizing regulator. Vaporizing regulators are tricky but if properly sized and installed, they can be a reliable means of preparing a liquid sample for analysis in a gas analyzer.
Industrial Sampling Systems, the definitive reference guide by expert Tony Waters (book excerpt)
The objective of a vaporizing regulator is to instantly flash the entire sample into a gas, which requires a lot of heat applied at the precise location of the pressure drop. With vaporizing regulators, pay close attention to temperature and vapor flow rate. If the flow is too great, the sample will be only partially vaporized and liquids will flow through the regulator and toward the analyzer. If the vaporizer temperature is too high, the liquid sample upstream will vaporize.
In cases where a liquid is being vaporized, consider a liquid fast loop followed by a slow loop. The objective is to keep the liquid moving right up to the vaporizing regulator.
There's still more to learn about how to find the right settings for heated regulators and vaporizing regulators. We lay out the details in a three-page technical paper that you can download for free. And for the full range of information on the subject, see the Swagelok book Industrial Sampling Systems (2013), the definitive sampling systems reference guide by expert Tony Waters.
Swagelok Grab Sampling Systems Application Guide (Catalog)
More like this:
- View all FAQs posts at this blog (list of blog articles)
- View Swagelok technical articles on analytical instrumentation (page at this website)
- Tony Waters Wrote the Book on Industrial Sampling Systems (blog article)