Finally, the last two rues for pressure drop calculations are:
- If the calculated pressure drop is greater than one-third of the total pressure (i.e., inlet pressure divided by three), then calculation should be done in shorter segments, so the outlet pressure of the segment is greater than one third the inlet pressure of that segment.
- Elevation differences between the inlet and outlet of the sample system must be taken into account for liquid streams (the affect on vapor streams will be negligible). Remember, it takes 10.84 psig to move a column of water up a 25-foot pipe rack. Conversely, and more likely, a water stream gains 10.84 psig on its way down from the same pipe rack. This could make a difference in how you set your pressure relief valves.
The equation to be used is:
DP = pressure drop (feet of liquid or meters of liquid)
r = density (lb/ft3 or kg/m3)
h = height (feet or meters)
g = local acceleration due to gravity (ft/sec or m/sec)
gc = gravity constant (32.17 ft/sec or 9.814 m/sec)
Simplify the Sample System
The easiest way to simplify a sample system is to make sure only the sample you need is collected. Start with the sample tap itself. Taps can be designed in one of the following three ways, and should ideally be installed on vertical pipe runs.
- Continuous, in which a representative continuous slipstream of the process fluid is withdrawn and transported to the analyzer, is the most common sample system. To be representative, the sample probe must extend into the center third of the process pipe.
- Isokinetic and its probe are designed to remove a sample from the stream at the same level of kinetic energy, normally represented as velocity, as the process stream itself. It's used in multiphase streams to insure all components are sampled.
- Discrete. In difficult streams, which are usually extremely hazardous, corrosive or dirty, the most reliable sampling system may be discrete, in which only a small aliquot of the process fluid is transferred as a plug from the sample tap to the analyzer.
To further improve samples from the system, users should seek to minimize dead volume by designing the system so there's always a continuous flow in all lines by selective use of three-way sampling valves. If the stream isn't routed to the analyzer, then route it to either a vent or a recovery system. This also results in a lower lag time. Another way to avoid the problem of dead volume is to insure that the sample system is swept through three times per analysis cycle.
This raises and compounds another common problem with sample systems—the analyzer only requires a very low flow rate. Thus, a fast loop is often used (Figure 3). A fast loop is an external loop with minimal sample conditioning that is cycled to a close proximity to the analyzer and from which the actual sample to the analyzer is taken. A common way to separate the secondary (slow) loop from the primary or fast loop is to flow the sample through a bypass filter. The stream that passes through the filter is the slow or analyzed loop. The fast loop remains unfiltered and also removes any excess material that is trapped or coalesced on the filter.
Another important consideration is material compatibility, not only to the process fluid but also to the ambient atmosphere and plant conditions. Most designers are very aware of the process compatibility and normally specify 316SS as their tubing material, going with more exotic materials only when required. However, 316SS is not a good choice where it can be exposed to seawater. The chlorine in seawater will cause the metal to fail in a short period of time. Another choice, Tygon tubing, should not be used if it could be exposed to sunlight. After exposure to the ultraviolet light in sunshine for three to four years, the tubing becomes brittle and fails.
The only remaining problem is how to move all this material around the sample system. This is ideally done through judicious selection of the sample source and return points. If at all possible, two process points of sufficient differential pressure drop should be selected, so no prime mover is required in the sample system. If a prime mover is required, the normal choices are a centrifugal pump, positive displacement pump or an eductor.
If a positive displacement pump is used, then be aware that it tends to require more maintenance than a centrifugal pump because it has more moving parts, and will likely introduce a pulsating flow to the system. A positive displacement pump also has advantages; it is a constant-volume device, and typically has a much higher differential pressure output.
If an eductor is used, then be sure to check the phase diagram to insure that the process liquid doesn't enter the eductor at less than 25 °F below the bubble point. If it does, experience has shown that there is sufficient pressure drop in the eductor throat to cause the fluid to vaporize (cavitation), and so most of the energy introduced to the eductor to induce flow in the secondary stream will be lost.
Select the Right Analyzer
After doing all the calculations to ensure that your analyzer system will operate properly, it's vital that the sample system be linked to the analyzer itself. In most cases, the analyzer selected will dictate to some degree the type and size of sample system installed. However, if the analyzer is not suitable or able to detect the components of interest in the general surrounding process stream, then all is for naught.
In conclusion, the three Rs of analyzer selection are:
- Reliability—The analyzer must be highly reliable so it will maintain a service factor in excess of 95%.
- Repeatability—The output of the equipment must be repeatable for a given input. It need not be accurate (though, of course, this is desirable), but it must always give the same numeric output for a given calibration or process sample.
- Return—Every analyzer system installation must have an economic return or justification. If it is not used for some form of continuous monitoring or control, then the unit will not get the attention it receives to remain in operation at the required service factors to be considered reliable.
Ian Verhappen is an ISA Fellow, a CAP and principal at Industrial Automation Networks (www.industrialautomationnetworks.com).