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2.7 Flow Measurement and Indication
Flow measurement on analyzer systems falls into three main categories:
1. Measuring the flow precisely where the accuracy of the analyzer depends on it
2. Measuring the flow where it is necessary to know the flow rate but it is not critical (e.g., fast loop flow)
3. Checking that there is flow present but measurement is not required (e.g., cooling water for heat exchangers).
It is important to decide which category the flowmeter falls into when writing the specification, as the prices vary over a wide range, depending on the precision required.
The types of flowmeter available will be mentioned but not the construction or method of operation, as this is covered in Section 1.
2.7.1 Variable-Orifice Meters
The variable-orifice meter is extensively used in analyzer systems because of its simplicity, and there are two main types.
Glass Tube
This type is the most common, as the position of the float is read directly on the scale attached to the tube, and it is available calibrated for liquids or gases. The high-precision versions are available with an accuracy of .1 percent full-scale deflection (FSD), whereas the low-priced units have a typical accuracy of 1 percent FSD.
Metal Tube
The metal tube type is used mainly on liquids for high-pressure duty or where the liquid is flammable or hazardous. A good example is the fast loop of a hydrocarbon analyzer. The float has a magnet embedded in it, and the position is detected by an external follower system. The accuracy of metal tube flowmeters varies from 0.10 percent FSd to 0.12 percent FSd, depending on the type and whether individual calibration is required.
2.7.2 Differential-Pressure Devices
On sample systems these normally consist of an orifice plate or preset needle valve to produce the differential pressure, and are used to operate a gauge or liquid-filled manometer when indication is required or a differential pressure switch when used as a flow alarm.
FIG. 13 Flexible impeller pump. Courtesy of ITT Jabsco. (a) Upon leaving
the offset plate the impeller blade straightens and creates a vacuum, drawing
in liquid-instantly priming the pump, (b) As the impeller rotates it carries
the liquid through the pump from the intake to outlet port, each successive
blade drawing in liquid, (c) When the flexible blades again contact the offset
plate they bend with a squeezing action which provides a continuous, uniform
discharge of liquid.
FIG. 14 Peristaltic pump.
Courtesy of Watson-Marlow. The advancing roller occludes the tube which, as it recovers to its normal size, draws in fluid which is trapped by the next roller (in the second part of the cycle) and expelled from the pump (in the third part of the cycle). This is the peristaltic flow-inducing action.
FIG. 15 Lute-type pressure stabilizer. Courtesy of Ludlam Sysco.
2.7.3 Spinner or Vane-Type Indicators
In this type the flow is indicated either by the rotation of a spinner or by the deflection of a vane by the fluid. It is ideal for duties such as cooling water flow, where it is essential to know that a flow is present but the actual flow rate is of secondary importance.
2.8 Pressure reduction and Vaporization
The pressure-reduction stage in a sample system is often the most critical, because not only must the reduced pressure be kept constant, but also provision must be made to ensure that under faulty conditions dangerously high pressures cannot be produced. Pressure reduction can be carried out in a variety of ways.
2.8.1 Simple Needle Valve
This is capable of giving good flow control if upstream and downstream pressures are constant.
Advantage: Simplicity and low cost.
Disadvantage: Any downstream blockage will allow pressure to rise.
They are only practical if downstream equipment can with stand upstream pressure safely.
2.8.2 Needle Valve with Liquid-Filled Lute
This combination is used to act as a pressure stabilizer and safety system combined. The maintained pressure will be equal to the liquid head when the needle value flow is adjusted until bubbles are produced (FIG. 15).
It is essential to choose a liquid that is not affected by sample gas and also does not evaporate in use and cause a drop in the controlled pressure.
2.8.3 Diaphragm-Operated Pressure Controller
These regulators are used when there is either a very large reduction in pressure required or the downstream pressure must be accurately controlled (FIG. 16). They are frequently used on gas cylinders to provide a controllable low pressure gas supply.
2.8.4 Vaporization
There are cases when a sample in the liquid phase at high pressure has to be analyzed in the gaseous phase. The pressure reduction and vaporization can be carried out in a specially adapted diaphragm-operated pressure controller as detailed above, where provision is made to heat the complete unit to replace the heat lost by the vaporization.
2.9 Sample Lines, Tube and Pipe Fitting
2.9.1 Sample Lines
Sample lines can be looked at from two aspects: first, the materials of construction, which are covered in Section 1.5, and, second, the effect of the sample line on the process sample, which is detailed below.
The most important consideration is that the material chosen must not change the characteristics of the sample during its transportation to the analyzer. There are two main ways in which the sample line material can affect the sample.
Adsorption and Desorption Adsorption and desorption occur when molecules of gas or liquid are retained and discharged from the internal surface of the sample line material at varying rates. This has the effect of delaying the transport of the adsorbed material to the analyzer and causing erroneous results.
FIG. 16 Diaphragm-operated pressure controller. Courtesy of Tescom.
Water and hydrogen sulfide at low levels are two common measurements where this problem is experienced.
An example is when measuring water at a level of 10 ppm in a sample stream, where copper tubing has an adsorption/ desorption which is twenty times greater than stainless steel tubing, and hence copper tubing would give a very sluggish response at the analyzer.
Where this problem occurs it is possible to reduce the effects in the following ways:
1. Careful choice of sample tube material
2. Raising the temperature of the sample line
3. Cleaning the sample line to ensure that it is absolutely free of impurities such as traces of oil
4. Increasing the sample flow rate to reduce the time the sample is in contact with the sample line material.
Permeability
Permeability is the ability of gases to pass through the wall of the sample tubing. Two examples are:
1. Polytetrafluoroethylene (PTFE) tubing is permeable to water and oxygen.
2. Plasticized polyvinyl chloride (PVC) tubing is permeable to the smaller hydrocarbon molecules such as methane.
Permeability can have two effects on the analysis:
1. External gases getting into the sample such as when measuring low-level oxygen using PTFE tubing. The results would always be high due to the ingress of oxygen from the air.
2. Sample gases passing outwards through the tubing, such as when measuring a mixed hydrocarbon stream using plasticized PVC. The methane concentration would always be too low.
2.9.2 Tube, Pipe, and Method of Connection Definition
1. Pipe is normally rigid, and the sizes are based on the nominal bore.
Typical materials:
Metallic: carbon steel, brass, etc.
Plastic: UPVC, ABS, etc.
2. Tubing is normally bendable or flexible, and the sizes are based on the outside diameter and wall thickness.
Typical materials:
Metallic: carbon steel, brass, etc.
Plastic: UPVC, ABS, etc.
Methods of joining
Pipe (metallic):
1. Screwed
2. Flanged
3. Welded
4. Brazed or soldered
Pipe (plastic):
1. Screwed
2. Flanged
3. Welded (by heat or use of solvents)
Tubing (metallic):
1. Welding
2. Compression fitting
3. Flanged
Tubing (plastic):
1. Compression
2. Push-on fitting (especially for plastic tubing) with hose clip where required to withstand pressure.
General The most popular method of connecting metal tubing is the compression fitting, as it is capable of with standing pressures up to the limit of the tubing itself and is easily dismantled for servicing as well as being obtainable manufactured in all the most common materials.
3. Typical Sample Systems
3.1 Gases
3.1.1 High-Pressure Sample to a Process Chromatograph
The example taken is for a chromatograph analyzing the composition of a gas which is in the vapor phase at 35 bar (FIG. 17). This is the case described in Section 1.4.3, where it is necessary to reduce the pressure of the gas at the sample point in order to obtain a fast response time with a minimum wastage of process gas.
The sample is taken from the process line using a low volume sample probe (Section 2.1.2) and then flows immediately into a pressure reducing valve to drop the pressure to a constant 1.5 bar, which is measured on a local pressure gauge. A pressure relief valve set to relieve at 4 bar is connected at this point to protect downstream equipment if the pressure-reducing valve fails.
After pressure reduction the sample flows in small-bore tubing (6 mm od) to the main sample system next to the analyzer, where it flows through a filter (such as shown in Section 2.2.3) to the sample selection system.
The fast loop flows out of the bottom of the filter body, bypassing the filter element, and then through a needle valve and flowmeter to an atmospheric vent on a low-pressure process line.
The stream selection system shown in FIG. 17 is called a block-and-bleed system, and always has two or more three-way valves between each stream and the analyzer inlet.
The line between two of the valves on the stream which is not in operation is vented to atmosphere, so guaranteeing that the stream being analyzed cannot be contaminated by any of the other streams. A simple system without a block and-bleed valve is described in Section 3.2.1 below.
After the stream-selection system the sample flows through a needle valve flowmeter and a miniature in-line filter to the analyzer sample inlet. The analyzer sample outlet on this system flows to the atmospheric vent line.
3.1.2 Furnace Gas Using Steam-Injection Probe Inside the Flue
This system utilizes a Venturi assembly located inside the flue (FIG. 18). High-pressure steam enters the Venturi via a separate steam tube, and a low-pressure region results inside the flue at the probe head. A mixture of steam and sample gas passes down the sample line. Butyl or EdPdM rubber-lined steam hose is recommended for sample lines, especially when high-sulfur fuels are used. This will minimize the effects of corrosion.
At the bottom end of the sample line the sample gas is mixed with a constant supply of water. The gas is separated from the water and taken either through a ball valve or a solenoid valve towards the sample loop, which minimizes the dead volume between each inlet valve and the analyzer inlet.
The water, dust, etc., passes out of a dip leg (A) and to a drain. It is assumed that the gas leaves the separator saturated with water at the temperature of the water. In the case of a flue gas system on a ship operating, for example, in the Red Sea, this could be at 35°C. The system is designed to remove condensate that may be formed because of lower temperatures existing in downstream regions of the sample system.
FIG. 17 Schematic: high-pressure gas sample to chromatograph. Courtesy of
Ludlam Sysco.
At the end of the loop there is a second dip leg (B) passing into a separator. A 5 cm water differential pressure is produced by the difference in depth of the two dip legs, so there is always a continuous flow of gas round the loop and out to vent via dip-leg (B).
FIG. 18 Schematic: furnace gas sampling. Courtesy of Servomex.
The gas passes from the loop to a heat exchanger, which is designed so that the gas leaving the exchanger is within 1 K of the air temperature. This means that the gas leaving the heat exchanger can be at 36°C and saturated with water vapor. The gas now passes into the analyzer which is maintained at 60°C.
The gas arrives in the analyzer and enters the first chamber, which is a centrifugal separator in a stainless steel block at 60°C. The condensate droplets will be removed at this point and passed down through the bottom end of the separator into a bubbler unit (C). The bubbles in this tube represent the bypass flow. At the same time the gas is raised from 36°C to 60°C inside the analyzer.
Gas now passes through a filter contained in the second chamber, the measuring cell, and finally to a second dip leg (d) in the bubbler unit. The flow of gas through the analyzer cell is determined by the difference in the length of the two legs inside the bubbler unit and cannot be altered by the operator.
This system has the following operating advantages:
1. The system is under a positive pressure right from inside the flue, and so leaks in the sample line can only allow steam and sample out and not air in.
2. The high-speed steam jet scours the tube, preventing build-up.
3. The steam maintains the whole of the sample probe above the dew point and so prevents corrosive condensate forming on the outside of the probe.
4. The steam keeps the entire probe below the temperature of the flue whenever the temperature of the flue is above the temperature of the steam.
5. The actual sampling system is intrinsically safe, as no electrical pumps are required.
3.1.3 Steam Sampling for Conductivity
The steam sample is taken from the process line by means of a special probe and then flows through thick-wall 316 stainless steel tubing to the sample system panel (FIG. 19). The sample enters the sampling panel through a high-temperature, high-pressure isolating valve and then flows into the cooler, where the steam is condensed and the condensate temperature is reduced to a suitable temperature for the analyzer (typically 30°C).
After the cooler, the condensate passes to a pressure control valve to reduce the pressure to about 1 bar gauge.
The temperature and pressure of the sample are then measured on suitable gauges and a pressure-relief valve (set at 2 bar) is fitted to protect downstream equipment from excess pressure if a fault occurs in the pressure control valve. The constant-pressure, cooled sample passes through a needle valve, flowmeter, and three-way valve into the conductivity cell and then to drain.
Facilities are provided for feeding water of known conductivity into the conductivity cell through the three-way valve for calibration purposes. The sample coolers are normally supplied with stainless steel cooling coils which are suitable where neither the sample nor the coolant contain appreciable chloride which can cause stress corrosion cracking.
When chlorides are known to be present in the sample or cooling water, cooling coils are available, made of alternative materials which are resistant to chloride-induced stress corrosion cracking.
FIG. 19 Schematic: steam sampling for conductivity. Courtesy of Ludlam
Sysco.
FIG. 20 Schematic: liquid sample to process chromatograph. Courtesy of Ludlam
Sysco.
3.2 Liquids
3.2.1 Liquid Sample to a Process Chromatograph
The example taken is for a chromatograph measuring butane in gasoline (petrol) (FIG. 20). The chromatograph in this case would be fitted with a liquid inject valve so that the sample will remain in the liquid phase at all times within the sample system.
In a liquid inject chromatograph the sample flow rate through the analyzer is very low (typically 25 ml/min), so that a fast loop system is essential.
The sample flow from the process enters the sample system through an isolating valve, then through a pump (if required) and an in-line filter, from which the sample is taken to the analyzer. After the filter the fast loop flows through a flowmeter followed by a needle valve, then through an isolating valve back to the process. Pressure gauges are fitted, one before the in-line filter and one after the needle valve, so that it is possible at any time to check that the pressure differential is sufficient (usually 1 bar minimum) to force the sample through the analyzer.
The filtered sample flows through small-bore tubing (typically, 3 mm od) to the sample/calibration selection valves. The system shown is the block-and-bleed configuration as described in Section 3.1.1. Where there is no risk of cross contamination the sample stream-selection system shown in the inset of FIG. 20 may be used.
The selected sample flows through a miniature in-line filter (Section 2.2.4) to the analyzer, then through the flow control needle valve and non-return valve back to the fast loop return line.
When the sample is likely to vaporize at the sample return pressure it is essential to have the flow control needle valve after the flowmeter in both the fast loop and the sample through the analyzer. This is done to avoid the possibility of any vapor flashing off in the needle valve and passing through the flowmeter, which would give erroneous readings.
The calibration sample is stored in a nitrogen-pressurized container and may be switched either manually or automatically from the chromatograph controller.
3.2.2 Gas Oil Sample to a Distillation Point Analyzer
In this case the process conditions are as follows (FIG. 21):
Sample tap:
Normal pressure: 5 bar g
Normal temperature: 70°C
Sample line length: 73 m Sample return:
Normal pressure: 5 bar g
Return line length: 73 m
This is a typical example of an oil-refinery application where, for safety reasons, the analyzer has to be positioned at the edge of the process area and consequently the sample and return lines are relatively long. Data to illustrate the fast loop calculation, based on equations in Crane's Publication No. 410M, are given in Table 1.
An electrically driven gear pump is positioned immediately outside the analyzer house which pumps the sample round the fast loop and back to the return point. The sample from the pump enters the sample system cabinet and flows through an in-line filter from which the sample is taken to the analyzer, through a needle valve and flowmeter back to the process. The filtered sample then passes through a water-jacketed cooler to reduce the temperature to that required for the coalescer and analyzer. After the cooler the sample is pressure reduced to about 1 bar with a pressure-control valve.
The pressure is measured at this point and a relief valve is fitted so that, in the event of the pressure control valve failing open, no downstream equipment will be damaged.
The gas oil sample may contain traces of free water and, as this will cause erroneous readings on the analyzer, it is removed by the coalescer. The bypass sample from the bottom of the coalescer flows through a needle valve and flow-meter to the drain line. The dry sample from the coalescer flows through a three-way ball valve for calibration purposes and then a needle valve and flowmeter to control the sample flow into the analyzer.
The calibration of this analyzer is carried out by filling the calibration vessel with the sample which had previously been accurately analyzed in the laboratory. The vessel is then pressurized with nitrogen to the same pressure as that set on the pressure-control valve and then the calibration sample is allowed to flow into the analyzer by turning the three-way ball valve to the calibrate position.
FIG. 21 Schematic: gas oil sample to distillation point analyzer. Courtesy
of Ludlam Sysco.
FIG. 22 Schematic: water sample system for dissolved oxygen analyzer. Courtesy
of Ludlam Sysco.
The waste sample from the analyzer has to flow to an atmospheric drain and, to prevent product wastage, it flows into a sample recovery unit along with the sample from the coalescer bypass and the pressure relief valve outlet.
The sample recovery unit consists of a steel tank from which the sample is returned to the process intermittently by means of a gear pump controlled by a level switch. An extra level switch is usually fitted to give an alarm if the level rises too high or falls too low.
A laboratory sample take-off point is fitted to enable a sample to be taken for separate analysis at any time without interfering with the operation of the analyzer.
3.2.3 Water-Sampling System for Dissolved Oxygen Analyzer
Process conditions:
Sample tap:
Normal pressure: 3.5 bar
Normal temperature: 140°C
Sample line length: 10 m
This system (FIG. 22) illustrates a case where the sample system must be kept as simple as possible to pre vent degradation of the sample. The analyzer is measuring 0-20 ng/1 oxygen and, because of the very low oxygen content, it is essential to avoid places in the system where oxygen can leak in or be retained in a pocket. Wherever possible ball or plug valves must be used, as they leave no dead volumes which are not purged with the sample.
The only needle valve in this system is the one control ling the flow into the analyzer, and this must be mounted with the water flowing vertically up through it so that all air is displaced.
The sample line, which should be as short as possible, flows through an isolating ball valve into a water-jacketed cooler to drop the temperature from 140°C to approximately 30°C, and then the sample is flow controlled by a needle valve and flowmeter. In this case, it is essential to reduce the temperature of the water sample before pressure reduction; otherwise, the sample would flash off as steam. A bypass valve to drain is provided so that the sample can flow through the system while the analyzer is being serviced.
When starting up an analyzer such as this it may be necessary to loosen the compression fittings a little while the sample pressure is on to allow water to escape, fill the small voids in the fitting, and then finally tighten them up again to give an operational system.
An extra unit that is frequently added to a system such as this is automatic shutdown on cooling-water failure to protect the analyzer from hot sample. This unit is shown dotted on FIG. 22, and consists of an extra valve and a temperature detector, either pneumatic or electronically operated, so that the valve is held open normally but shuts and gives an alarm when the sample temperature exceeds a preset point. The valve would then be reset manually when the fault has been corrected.
References
Cornish, d. C., et al., Sampling Systems for Process Analyzers, Butterworths, London (1981).
Flow of Fluids, Publication No. 410M, Crane Limited (1982).
Marks, J. W., Sampling and Weighing of Bulk Solids, Transtech Publications, Clausthal-Zellerfeld (1985).
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