Nov-2013

How to use a regulator to reduce time delay in an analytical system

Process measurements are instantaneous but analyser responses never are. From the tap to the analyser, there is always a time delay. Unfortunately, this delay is often underestimated or misunderstood.

Doug Nordstrom and Mike Adkins
Swagelok Company

Article Summary

Time delay is defined as the amount of time it takes for a new sample to reach the analyser. 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.

Delay may occur in any of the major parts of an analytical instrumentation (AI) system (Figure 1), including the process line, tap and probe, field station, transport line, sample conditioning system, stream switching system, and analyser.

Time delay is cumulative. It consists of the total amount of time it takes for fluid to travel from the latest step in the process line to the analyser, including time required for analysis in the analyser. 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
Minimising time delay begins with the location of the tap. It is best to locate the tap as close to the analyser 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, an expensive component that introduces performance variables.

In many cases, you may not be able to dictate the location of the tap. You may have to make do with an existing tap location, and often, an existing analyser shed location as well.

If the tap is a long distance from the analyser, a fast loop is recommended as a means of quickly delivering fluid to the analyser and returning the unused portion to the process.

In most analytical instrumentation systems, another source of time delay is the probe. The larger the probe’s volume, the greater the delay. Volume will be affected both by the length and width of the probe. Therefore, when attempting to minimise time delay, choose a low-volume probe.

At the field station
In cases where the analyser requires a liquid sample, a regulator in the field station is not employed. It is better to keep liquids at high pressure to avoid the formation of bubbles.

In the case of a gas sample, a field station is employed as 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. The field station is located as close to the tap as possible. The sooner the pressure is dropped, the better.Let’s look at three possible applications for a regulator. With each, the regulator is configured somewhat differently. In the first application, the objective is to reduce gas pressure. The pressure drop is not expected to produce condensation. Therefore, a simple pressure-reducing regulator may be employed.

A pressure-reducing regulator maintains constant pressure at the outlet. A thin metal diaphragm within the regulator flexes in response to downstream pressure, allowing a cone-shaped poppet to regulate the size of the orifice through which the gas passes. As the diaphragm flexes up in response to higher pressure, the opening is smaller. As the diaphragm relaxes with lower pressure, the opening is larger. A dial (handle) on the regulator allows the operator to set the outlet pressure.

A metal diaphragm is ideal in applications where the inlet pressure does not vary sharply. However, in applications where the pressure may be inconsistent or spike, a piston-style regulator may be more appropriate.

In our second regulator application, the objective is to reduce gas pressure. However, in this case, 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, the result from this cooling is condensation. In some cases, the loss of heat may be great enough to cause the regulator to freeze up.
When the Joule-Thomson effect is in play, a heated regulator may be required to keep the temperature of the gas above the dew point. A heated regulator is a pressure-reducing regulator in which the system fluid flows over a heated element. A heater cartridge is required.

You can calculate the number of watts required of the heater cartridge so you buy one in the right power range. Every gas has a Joule-Thomson coefficient, which is plugged into a formula along with the pressure drop and flow rate to produce the number of watts required.¹

In our third regulator application, a liquid must become a gas before it can be analysed by a gas chromatograph or other analyser. In this case, a vaporising regulator is employed.

Vaporising 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 analyser.

The objective of a vaporising regulator is to instantly flash the entire sample into a gas, which requires a lot of heat in just the right location. The heat must be applied at the precise location of the pressure drop.

With vaporising regulators, one must pay close attention to flow. If the flow is too great, the sample will be only partially vaporised and liquids will flow through the regulator and toward the analyser. If the flow is too little, the liquid sample upstream will be vaporised.