Process measurements are instantaneous but analyzer responses never are. There's always a time delay from tap to analyzer. Unfortunately, this delay, which represents the time a sample takes to reach the analyzer, often is underestimated or misunderstood.
Using a regulator is one way to address time delay. Regulators control pressure, which in an analytical system closely correlates with time. For gas systems with a controlled flow rate, the lower the pressure, the shorter the time delay.
Delay may occur in any of an analytical instrumentation system's major parts, including the process line, tap and probe, field station, transport line, sample conditioner, stream switching system and analyzer (Figure 1).
Time delay is cumulative. It consists of the total amount of time fluid takes to travel from the latest step in the process line to the analyzer, including time required for analysis in the instrument.
Here, we'll focus on the field station and the regulator's important role in reducing time delay there.
Minimizing time delay begins with proper tap location. It's best to place the tap as close to the analyzer as possible and upstream of sources of delay such as drums, tanks, dead legs, stagnant lines and redundant or obsolete equipment.
In many cases you can't dictate tap location. You may have to make do with an existing spot and, often, with the current analyzer shed position as well.
If the tap is far from the analyzer, use a fast loop to quickly deliver fluid to the analyzer and return the unused portion to the process.
When dealing with a liquid, ensure pressure at the tap suffices to deliver the sample through transport lines or fast loop without a pump, which is expensive and introduces performance variables.
In most systems the probe contributes to time delay — the larger the probe's volume, the greater the delay. So, choose a low-volume probe.
Using a Regulator
Regulators don't suit all systems. For an analyzer requiring a liquid sample it's better to keep the liquid at high pressure to avoid formation of bubbles.
For gas samples use a regulator in a field station to reduce pressure in transport lines. Time delay decreases in direct proportion to absolute pressure — halving pressure halves time delay.
Locate the field station as close to the tap as possible. The sooner the pressure is dropped, the better.
Let's look at three possible regulator applications. Each requires a somewhat different configuration.
In the first application the objective is to reduce gas pressure and pressure drop isn't expected to produce condensation. A simple pressure-reducing regulator will suffice.
This regulator maintains constant pressure at the outlet. A thin metal diaphragm within the unit flexes in response to downstream pressure, allowing a cone-shaped poppet to regulate the size of the orifice through which gas passes. Higher pressure causes the diaphragm to flex up, making the opening smaller. Lower pressure lets the diaphragm relax, enlarging the opening. A dial (handle) on the regulator allows the operator to set outlet pressure.
A metal diaphragm is ideal in applications where inlet pressure doesn't vary sharply. However, where pressure may become erratic or spike, consider a piston-style regulator.
In our second application the objective also is to reduce gas pressure but pressure drop is expected to produce condensation. With a drop in pressure almost all gases lose heat via the Joule-Thomson effect. If the gas is close to its dew point this cooling may result in condensation. In some cases heat loss may be great enough to cause the regulator to freeze up.