Basics of Analyzer Sample Systems - Parts 1-2

If you had to design and install a process analyzer sample system today, how would you do it? First, remember that an analyzer system includes the sample tap, sample system, analyzer, sample return, signal transmission and control system. If any of these components fail, your company won't gain the economic benefits the system was supposed to produce. And don't forget, it's generally accepted that sample systems are victims of the Pareto principle, which is that 20% of a system consumes 80% of the resources because they're responsible for 80% of analyzer system problems.

While the engineer's golden rule of "keep it simple, stupid" (KISS) also applies to sample systems, this time it also stands for: Know your process conditions; Involve the right people; Simplify the system; and Select the right equipment.

Get the Right People

In addition to process engineers, a project team will involve several other people as well. A likely group will include the following:

A chemist—A representative from the laboratory who will not only provide the stream composition but also know the present method of analysis used on the stream.

Maintenance/Analyzer Technician—A person, or group of people who must be involved from the beginning, not only to gain get a sense of ownership of the process, but also to understand the technology and equipment before it arrives on-site to get commissioned.

Project Manager—A person who coordinates the entire project, gets the funding, arranges for necessary approvals and other important duties as they come up.

Know the Process Conditions

It's important to understand the process conditions, not only at the sample inlet, but also at the analyzer and all along the sample loop. To do this, three basic calculations must be made: 1) dead spaces; 2) system lag time; and 3) system pressure drop.

Using this information, a phase diagram (Figure 1) should be generated for all sample streams. This diagram represents how the fraction of liquids, solids and vapors change as a function of pressure and temperature. It is invaluable when trying to determine if there are condensable products in the stream that can later be vaporized as the pressure decreases. This is similar to checking for cavitation in control valve sizing, only in reverse, since rather than looking for vapor in a liquid, one is looking for a momentary liquid phase in a vapor stream. A process or chemical engineer can generate this diagram, along with a range of pressures and temperatures over which the system may be operating, from the stream composition.

Dead Spaces Often Overlooked

One of the biggest and often overlooked items when designing a sample system is dead spaces or volumes. Dead spaces are parts of the sample system where pockets of fluid can become trapped and can't move along with the remainder of the sample. Perfect places for dead volumes are tee fittings, separators or any other sharp-edged flow change. To minimize its effect, use the following rules:

• Minimize tee fittings in the system;
• Purge the sample system three times for each analyzer cycle;
• Use the smallest size fittings able to do the job within other constraints;
• Use the minimum number of fittings possible, which reduces dead time and minimizes potential leak or failure points; 
• Operate your continuous sample systems in the turbulent flow regime.

For example, the first column of Figure 2 shows a configuration designed to minimize dead volume. The three-way valves eliminate elbows, and when a stream isn't flowing to the analyzer for measurement, it's still flowing to a vent or sample return point, ensuring a continuously fresh sample at every point in the system. The second two columns show the configuration when streams AX-1A and AX-1B, respectively, are being analyzed.

Lag Time Depends on Velocity and Volume

The second item to consider and one of the first things to calculate is the system lag time. System lag time is the sum of the analyzer cycle/measurement time and the sample lag time. Meanwhile, sample lag time is the amount of time it takes for the sample to travel from the sample point to the analyzer sensor. It's simply the volume of the sample system divided by the velocity of the flow and can be calculated using Equation 1.

Where:
t = time
V = sample system volume
L = distance from the sample point to the analyzer sensor
P_a = absolute pressure
Z = compressibility factor
F_s = flow rate under standard conditions
T_a = absolute temperature

Compressibility Is a Factor for Gases at Higher System Pressures

For liquids, compressibility is negligible and the compressibility factor is Z = 1.0. However, in gas systems operating at more than about 35 to 50 psia, compressibility must be considered. For gases, compressibility changes as a function of pressure and temperature according to the rules of the ideal gas law, as shown in Equation 2

Where:
Z = compressibility factor
P_a = absolute pressure
V = volume
n = moles of fluid
R = gas constant
T_a = absolute temperature

The compressibility factory Z can be determined from compressibility charts and the associated reduced temperature Tr and reduced pressure P_r.

The reduced temperature and pressure are calculated as follows:

T_r = T_a/T_c

P_r = P_a/P_c