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.
For now, we will focus on the field station and the important role of a regu lator in reducing time delay there. 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.
Transport lines
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 expen sive 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.
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.
Different configurations
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 main tains constant pressure at the outlet. A thin metal diaphragm within the regu lator 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.
Outlet pressure
A dial (handle) on the regulator allows the operator to set the outlet pressure.
A metal diaphragm is ideal in appli cations 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.
Lose heat with pressure drop
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 condensa tion. In some cases, the loss of heat may be great enough to cause the regulator to freeze up.
Heated regulator
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 coeffi cient, which is plugged into a formula along with the pressure drop and flow rate to produce the number of watts required.
[The formula for calculating required wattage is Pw = Qn Cp DeltaT, where the wattage (Pw) is proportional to molar flow (Qn), heat capacity (Cp) and the amount of cooling from the Joule- Thomson effect (DeltaT).]
Gas chromatograph
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 regu lator 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.
Volume increase
Finally, be sure to set up your vaporising regulator correctly or you will create considerable time delay. As the fluid changes from liquid to gas, volume will increase dramatically. The rate of increase will depend on the liquid’s molecular weight.
Typically, the measured vapour flow after the regulator will be > 300 times the liquid flow before the vaporising regulator.
For example, with a vapour flow of 600 cm/min, liquid flow may be less than 2 cm/min.
Therefore, the liquid will take 25 minutes to travel through 3m of 6 mm tubing. To reduce this time, we must reduce the volume of the tubing preceding the regulator.
Faster response
For example, with only 0.3m of 3 mm tubing, it would take only 30 seconds for the liquid to reach the regulator. To this time, however, we must add time delay in the probe. The narrower the probe, the faster the response.
Another means of attaining a faster response is to move the regulator closer to the analyser with the aid of a second fast loop.
In Figure 2, the regulator is located after the fast loop filter with a second liquid fast loop ensuring that good liquid flow continues right up to the vaporising regulator.
The objective is to minimise slow- moving liquid volume going to a vapor ising regulator.
Time delay
A regulator is a critical tool in addressing time delay in an analytical system. By reducing pressure, you reduce time delay.
The lower the pressure in a gas system, the faster the response time. In general, the sooner the pressure can be dropped in a system, the better.
In cases where a liquid is being vaporised, remember to make intelligent use of fast loops. The objective is to keep the liquid moving right up to the vapor ising regulator.
The field station is one place in a complex analytical system where time delay can be significantly reduced, but the approach to time delay must always be comprehensive.
To reduce time delay, all potential causes of delay in the system must be scrutinised.
[Doug Nordstrom is market manager, analytical instrumentation and Mike Adkins is general industrial valve product manager, both with Swagelok Company.]