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1. Introduction
1.1 Importance of Sampling
Any form of analysis instrument can only be as effective as its sampling system. Analysis instruments are out of commission more frequently due to trouble in the sampling system than to any other cause. Therefore time and care expended in designing and installing an efficient sampling system is well repaid in the saving of servicing time and dependability of instrument readings. The object of a sampling system is to obtain a truly representative sample of the solid, liquid, or gas which is to be analyzed, at an adequate and steady rate, and transport it without change to the analysis instrument, and all precautions necessary should be taken to ensure that this happens. Before the sample enters the instrument it may be necessary to process it to the required physical and chemical state, i.e., correct temperature, pressure, flow, purity, etc., without removing essential components. It is also essential to dispose of the sample and any reagent after analysis with out introducing a toxic or explosive hazard. For this reason, the sample, after analysis, is continuously returned to the process at a suitable point, or a sample-recovery and disposal system is provided.
1.2 Representative Sample
It is essential that the sample taken should represent the mean composition of the process material. The methods used to overcome the problem of uneven sampling depend on the phase of the process sample, which may be in solid, liquid, gas, or mixed-phase form.
1.2.1 Solids
When the process sample is solid in sheet form it is necessary to scan the whole sheet for a reliable measurement of the state of the sheet (e.g., thickness, density, or moisture content). A measurement at one point is insufficient to give a representative value of the parameter being measured.
If the solid is in the form of granules or powder of uniform size, a sample collected across a belt or chute and thoroughly mixed will give a reasonably representative sample.
If measurement of density or moisture content of the solid can be made while it is in a vertical chute under a constant head, packing density problems may be avoided.
In some industries where the solids are transported as slurries, it is possible to carry out the analysis directly on the slurry if a method is available to compensate for the carrier fluid and the velocities are high enough to ensure turbulent flow at the measurement point.
Variable-size solids are much more difficult to sample, and specialist work on the subject should be consulted.
1.2.2 Liquids
When sampling liquid it is essential to ensure that either the liquid is turbulent in the process line or that there are at least 200 pipe diameters between the point of adding constituents and the sampling point. If neither is possible, a motorized or static mixer should be inserted into the process upstream of the sample point.
1.2.3 Gases
Gas samples must be thoroughly mixed and, as gas process lines are usually turbulent, the problem of finding a satisfactory sample point is reduced. The main exception is in large ducts such as furnace or boiler flues, where stratification can occur and the composition of the gas may vary from one point to another. In these cases special methods of sampling may be necessary, such as multiple probes or long probes with multiple inlets in order to obtain a representative sample.
1.2.4 Mixed-Phase Sampling
Mixed phases such as liquid/gas mixtures or liquid/solids (i.e., slurries) are best avoided for any analytical method that involves taking a sample from the process. It is always preferable to use an in-line analysis method where this is possible.
1.3 Parts of Analysis Equipment
The analysis equipment consists of five main parts:
1. Sample probe.
2. Sample-transport system.
3. Sample-conditioning equipment.
4. The analysis instrument.
5. Sample disposal.
1.3.1 Sample Probe
This is the sampling tube that is used to withdraw the sample from the process.
1.3.2 Sample-Transport System
This is the tube or pipe that transports the sample from the sample point to the sample-conditioning system.
1.3.3 Sample-Conditioning System
This system ensures that the analyzer receives the sample at the correct pressure and in the correct state to suit the analyzer. This may require pressure increase (i.e., pumps) or reduction, filtration, cooling, drying, and other equipment to protect the analyzer from process upsets. Additionally, safety equipment and facilities for the introduction of calibration samples into the analyzer may also be necessary.
1.3.4 The Analysis Instrument
This is the process analyzer complete with the services such as power, air, steam, drain vents, carrier gases, and signal conditioning that are required to make the instrument operational.
(Analysis techniques are described in Part 2 of this guide.)
1.3.5 Sample Disposal
The sample flowing from the analyzer and sample conditioning system must be disposed of safely. In many cases it is possible to vent gases to atmosphere or allow liquids to drain, but there are times when this is not satisfactory.
Flammable or toxic gases must be vented in such a way that a hazard is not created. Liquids such as hydrocarbons can be collected in a suitable tank and pumped back into the process, whereas hazardous aqueous liquids may have to be treated before being allowed to flow into the drainage system.
1.4 Time Lags
In any measuring instrument, particularly one which may be used with a controller, it is desirable that the time interval between occurrence of a change in the process fluid and its detection at the instrument should be as short as possible consistent with reliable measurement. In order to keep this time interval to a minimum, the following points should be kept in mind.
1.4.1 Sample-Transport Line Length
The distance between the sampling point and the analyzer should be kept to the minimum. Where long sample transport lines are unavoidable a "fast loop" may be used. The fast loop transports the sample at a flow rate higher than that required by the analyzer, and the excess sample is either returned to the process, vented to atmosphere, or allowed to flow to drain. The analyzer is supplied with the required amount of sample from the fast loop through a short length of tubing.
1.4.2 Sampling Components
Pipe, valves, filter, and all sample-conditioning components should have the smallest volume consistent with a permissible pressure drop.
1.4.3 Pressure Reduction
Gaseous samples should be filtered, and flow in the sample line kept at the lowest possible pressure, as the mass of gas in the system depends on the pressure of the gas as well as the volume in the system.
When sampling high-pressure gases the pressure reducing valve must be situated at the sample point. This is necessary, because for a fixed mass flow rate of gas the response time will increase in proportion to the absolute pressure of the gas in the sample line (i.e., gas at 10 bar A will have a time lag five times that of gas at 2 bar A). This problem becomes more acute when the sample is a liquid that has to be vaporized for analysis (e.g., liquid butane or propane).
The ratio of volume of gas to volume of liquid can be in the region of 250:1, as is the case for propane. It is therefore essential to vaporize the liquid at the sample point and then treat it as a gas sample from then on.
1.4.4 Typical Equations
1. t S L
=
t = time lag
S = velocity (m/s)
L = line length (m)
2. General gas law for ideal gases:
10 8314 T pv M W 5
#
# =
p = pressure
T = abs. temperature (K)
o = volume (1)
W = mass (g)
M = molecular weight
3. Line volume:
d V 4 I 2 r =
d = internal diameter of tube (mm)
VI = volume (ml/m)
4.
100 6 t F LVI
# =
L = line length (m)
VI = internal volume of line (ml/m)
F = sample flow rate (1/min)
t = time lag (s) (For an example of a fast loop calculation see Section 3.2.2, Table 1.)
1.4.5 Useful Data internal volume per meter (VI) of typical sample lines:
1 8 in od # 0.035 wall = 1.5 ml/m
¼ in od # 0.035 wall = 16.4 ml/m 3 8 in od # 0.035 wall = 47.2 ml/m
½ in od # 0.065 wall = 69.4 ml/m
½ in nominal bore steel pipe (extra strong)(13.88 mm id)
= 149.6 ml/m 3 mm OD # 1 mm wall = 0.8 ml/m 6 mm OD # 1 mm wall = 12.6 ml/m 8 mm OD # 1 mm wall = 28.3 ml/m 10 mm OD # 1 m wall = 50.3 ml/m 12 mm OD # 1.5 mm wall = 63.6 ml/m
1.5 Construction Materials
Stainless steel (Type 316 or 304) has become one of the most popular materials for the construction of sample systems due to its high resistance to corrosion, low surface adsorption (especially moisture), wide operating tempera ture range, high-pressure capability, and the fact that it is easily obtainable. Care must be taken when there are materials in the sample which cause corrosion, such as chlorides and sulfides, in which case it is necessary to change to more expensive materials such as Monel.
When atmospheric sampling is carried out for trace constituents, Teflon tubing is frequently used, as the surface adsorption of the compounds is less than stainless steel, but it is necessary to check that the compound to be measured does not diffuse through the wall of the tubing.
For water analysis (e.g., pH and conductivity) it is possible to use plastic (such as PVC or ABS) components, although materials such as Kunifer 10 (copper 90 percent, nickel 10 percent) are increasing in popularity when chlorides (e.g., salt water) are present, as they are totally immune to chloride corrosion.
Table 1 Fast-loop calculation for gas oil sample
2. Sample System Components
2.1 Probes
The most important function of a probe is to obtain the sample from the most representative point (or points) in the process line.
2.1.1 Sample Probe
A typical probe of this type for sampling all types of liquid and gases at low pressure is shown in FIG. 1. It can be seen that the probe which is made of 21 mm OD (11.7 mm id) stainless steel pipe extends to the center of the line being sampled. However, if the latter is more than 500mm od the probe intrusion is kept at 250 mm to avoid vibration in use.
2.1.2 Small-Volume Sample Probe
This probe is used for sampling liquids that must be vaporized or for high-pressure gases (FIG. 2). Typically, a 6mm OD # 2 mm id tube is inserted through the center of a probe of the type described in Section 2.1.1. The probe may be withdrawn through the valve for cleaning.
FIG. 1 Sample probe. Courtesy of Ludlam Sysco.
FIG. 2 Small-volume sample probe. Courtesy of Ludlam Sysco.
FIG. 3 Gas-sampling probe. Courtesy of ABB Hartmann and Braun. 1. Gas intake;
2. Ceramic intake filter; 3. Bushing tube with flange; 4. Case with outlet
filter; 5. Internal screw-thread; 6. Gas outlet.
FIG. 4 Water-wash probe (courtesy ABB Hartmann and Braun).
1. Water intake; 2. Water filter; 3. Gas intake; 4. Gas-water outlet; 5. Connecting hose; 6. Gas-water intake; 7. Gas outlet; 8. Water outlet; 9. Water separator.
2.1.3 Furnace Gas Probes
Low-Temperature Probe
FIG. 3 shows a gas-sampling probe with a ceramic outside filter for use in temperatures up to 400°C.
Water-Wash Probe
This probe system is used for sampling furnace gases with high dust content at high temperatures (up to 1600°C) (FIG. 4). The wet gas-sampling probe is water cooled and self-priming. The water/gas mixture passes from the probe down to a water trap, where the gas and water are separated. The gas leaves the trap at a pressure of approximately 40 mbar, and precautions should be taken to avoid condensation in the analyzer, either by ensuring that the analyzer is at a higher temperature than the water trap or by passing the sample gas through a sample cooler at 5°C to reduce the humidity.
Note that this probe is not suitable for the measurement of water-soluble gases such as Co2, So2, or H2S.
Steam Ejector
The steam ejector illustrated in FIG. 5 can be used for sample temperatures up to 180°C and, because the condensed steam dilutes the condensate present in the flue gas, the risk of corrosion of the sample lines when the steam/ gas sample cools to the dew point is greatly reduced.
Dry steam is supplied to the probe and then ejected through a jet situated in the mouth of a Venturi. The flow of steam causes sample gas to be drawn into the probe. The steam and gas pass out of the probe and down the sample line to the analyzer system at a positive pressure. The flow of steam through the sample line prevents the build-up of any corrosive condensate.
2.2 Filters
2.2.1 "Y" Strainers
"Y" strainers are available in stainless steel, carbon steel, and bronze. They are ideal for preliminary filtering of samples before pumps or at sample points to prevent line scale from entering sample lines. Filtration sizes are available from 75 to 400 nm (200 to 40 mesh). The main application for this type of filter is for liquids and steam.
2.2.2 In-Line Filters
This design of filter is normally used in a fast loop configuration and is self-cleaning (FIG. 6). Filtration is through a stainless steel or a ceramic element. Solid particles tend to be carried straight on in the sample stream so that maintenance time is very low. Filtration sizes are available from 150 nm (100 mesh) down to 5 nm. It is suitable for use with liquids or gases.
FIG. 5 Steam ejection probe. Courtesy of Servomex.
FIG. 6 in-line filters. Courtesy of Microfiltrex.
FIG. 7 Filter with disposable element. Courtesy of Balston.
2.2.3 Filters with Disposable Glass Microfiber Element
These filters are available in a wide variety of sizes and porosities (FIG. 7). Bodies are available in stainless steel, aluminum, or plastic. Elements are made of glass microfiber and are bonded with either resin or fluorocarbon.
The fluorocarbon-bonded filter is particularly useful for low-level moisture applications because of the low adsorption/desorption characteristic.
The smallest filter in this range has an internal volume of only 19 ml and is therefore suitable when a fast response time is required.
2.2.4 Miniature in-Line Filter
These are used for filtration of gases prior to pressure reduction and are frequently fitted as the last component in the sample system to protect the analyzer (FIG. 8).
2.2.5 Manual Self-Cleaning Filter
This type of filter works on the principle of edge filtration using discs usually made of stainless steel and fitted with a manual cleaning system (FIG. 9). The filter is cleaned by rotating a handle which removes any deposits from the filter element while the sample is flowing. The main uses are for filtration of liquids where filter cleaning must be carried out regularly without the system being shut down; it is especially suitable for waxy material which can rapidly clog up normal filter media.
FIG. 10 Coalescer. Courtesy of Fluid data.
FIG. 11 Water-jacketed cooler. Courtesy of George E. Lowe.
Dimensions are shown in mm.
FIG. 9 Manual self-cleaning filter. Courtesy of AMF CUNo.
2.3 Coalescers
Coalescers are a special type of filter for separating water from oil or oil from water (FIG. 10). The incoming sample flows from the center of a specially treated filter element through to the outside. In so doing, the diffused water is slowed down and coalesced, thus forming droplets which, when they reach the outer surface, drain downwards as the water is denser than the hydrocarbon. A bypass stream is taken from the bottom of the coalescer to remove the water.
The dry hydrocarbon stream is taken from the top of the coalescer.
2.4 Coolers
2.4.1 Air Coolers
These are usually used to bring the sample gas temperature close to ambient before feeding into the analyzer.
2.4.2 Water-Jacketed Coolers
These are used to cool liquid and gas samples and are avail able in a wide range of sizes (FIG. 11).
2.4.3 Refrigerated Coolers
These are used to reduce the temperature of a gas to a fixed temperature (e.g., +5°C) in order to condense the water out of a sample prior to passing the gas into the analyzer. Two types are available: one with an electrically driven compressor type refrigerator and another using a Peltier cooling element. The compressor type has a large cooling capacity whereas the Peltier type, being solid state, needs less maintenance.
2.5 Pumps, gas
Whenever gaseous samples have to be taken from sample points which are below the pressure required by the analyzer, a sample pump of some type is required. The pumps that are available can broadly be divided into two groups:
1. The eductor or aspirator type
2. The mechanical type.
FIG. 8 Miniature in-line filter. Courtesy of Nupro.
2.5.1 Eductor or Aspirator Type
All these types of pump operate on the principle of using the velocity of one fluid which may be liquid or gas to induce the flow in the sample gas. The pump may be fitted before or after the analyzer, depending on the application.
A typical application for a water-operated aspirator (similar to a laboratory vacuum pump) is for taking a sample of flue gas for oxygen measurement. In this case the suction port of the aspirator is connected directly to the probe via a sample line and the water/gas mixture from the outlet feeds into a separator arranged to supply the sample gas to the analyzer at a positive pressure of about 300 mm water gauge.
In cases where water will affect the analysis it is some times possible to place the eductor or aspirator after the analyzer and draw the sample through the system. In these cases the eductor may be supplied with steam, air, or water to provide the propulsive power.
2.5.2 Mechanical Gas Pumps
There are two main types of mechanical gas pump available:
1. Rotary pump
2. Reciprocating piston or diaphragm pump.
Rotary Pumps Rotary pumps can be divided into two categories, the rotary piston and the rotating fan types, but the latter is very rarely used as a sampling pump.
The rotary piston pump is manufactured in two configurations. The Rootes type has two pistons of equal size which rotate inside a housing with the synchronizing carried out by external gears. The rotary vane type is similar to those used extensively as vacuum pumps. The Rootes type is ideal where very large flow rates are required and, because there is a clearance between the pistons and the housing, it is possible to operate them on very dirty gases.
The main disadvantage of the rotary vane type is that, because there is contact between the vanes and the housing, lubrication is usually required, and this may interfere with the analysis.
Reciprocating Piston and Diaphragm Pump of these two types the diaphragm pump has become the most popular. The main reason for this is the improvement in the types of material available for the diaphragms and the fact that there are no piston seals to leak. The pumps are available in a wide variety of sizes, from the miniature units for portable personnel protection analyzers to large heavy-duty industrial types.
A typical diaphragm pump (FIG. 12) for boosting the pressure of the gas into the analyzer could have an all stainless-steel head with a Terylene reinforced Viton diaphragm and Viton valves. This gives the pump a very long service life on critical hydrocarbon applications.
Many variations are possible; for example, a Teflon coated diaphragm can be fitted where Viton may not be compatible with the sample, and heaters may be fitted to the head to keep the sample above the dew point.
The piston pump is still used in certain cases where high accuracy is required in the flow rate (for example, gas blending) to produce specific gas mixtures. In these cases the pumps are usually operated immersed in oil so that the piston is well lubricated, and there is no chance of gas leaks to and from the atmosphere.
2.6 Pumps, Liquid
There are two situations where pumps are required in liquid sample systems:
1. Where the pressure at the analyzer is too low because either the process line pressure is too low, or the pres sure drop in the sample line is too high, or a combination of both.
2. When the process sample has to be returned to the same process line after analysis.
The two most common types of pumps used for sample transfer are:
1. Centrifugal (including turbine pump)
2. Positive displacement (e.g., gear, peristaltic, etc.).
2.6.1 Centrifugal
The centrifugal and turbine pumps are mainly used when high flow rates of low-viscosity liquids are required. The turbine pumps are similar to centrifugal pumps but have a special impeller device which produces a considerably higher pressure than the same size centrifugal. In order to produce high pressures using a centrifugal pump there is a type available which has a gearbox to increase the rotor speed to above 20,000 rev/min.
FIG. 12 Diaphragm pumps.
Courtesy of Charles Austen Pumps.
2.6.2 Positive-Displacement Pumps
Positive-displacement pumps have the main characteristic of being constant flow devices. Some of these are specifically designed for metering purposes where an accurate flow rate must be maintained (e.g., process viscometers). They can take various forms:
1. Gear pump
2. Rotary vane pump
3. Peristaltic pump.
Gear Pumps
Gear pumps are used mainly on high-viscosity products where the sample has some lubricating properties.
They can generate high pressures for low flow rates and are used extensively for hydrocarbon samples ranging from diesel oil to the heaviest fuel oils.
Rotary Vane Pumps
These pumps are of two types, one having rigid vanes (usually metal) and the other fitted with a rotor made of an elastomer such as nitrile (Buna-n) rubber or Viton. The metal vane pumps have characteristics similar to the gear pumps described above, but can be supplied with a method of varying the flow rate externally while the pump is operating.
Pumps manufactured with the flexible vanes (FIG. 13) are particularly suitable for pumping aqueous solutions and are available in a wide range of sizes but are only capable of producing differential pressures of up to 2 bar.
Peristaltic Pumps
Peristaltic pumps are used when either accurate metering is required or it is important that no contamination should enter the sample. As can be seen from FIG. 14, the only material in contact with the sample is the special plastic tubing, which may be replaced very easily during routine servicing.
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