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Level and Flow Control Applications

A range of level control systems and methods areused in industry. Systems may be based on theuse of floats, probes or even more sophisticatedtechnology. This tutorial studies the use of probesto provide adjustable & non-adjustable on/offcontrol, and modulating control of liquids. Simpleflow control applications are also considered.

Use the quick links below to take you to the mainsections of this tutorial:

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Level Control ApplicationsThe control of liquid levels, for example in a process tank, is an important function. An example would be a hotwater tank where water is removed, perhaps for washing down, and the level needs to be restored ready forthe next wash cycle.Control of water level and alarms for steam boilers is specifically excluded from this Tutorial, and the reader isreferred to Block 3 (The Boiler House), which deals with the subject in depth.Many different types of level control systems are used in industry, covering a wide range of processes. Someprocesses will be concerned with media other than liquids, such as dry powders and chemical feedstock. Therange of media is so wide that no single instrument is suitable for all applications.Many systems are available to serve this wide range of applications. The following list is not exhaustive but, inmost cases, the final control signal will be used to operate pumps or valves appropriate to the application:

Float operated types- a float rises and falls according to the change in liquid level and operatesswitches at predetermined points in the range.Solid probe types- these measure conductivity or capacitance and are discussed in more detail in thefollowing pages.Steel rope capacitance types - a flexible steel rope is suspended in the liquid, and the change incapacitance is measured relative to the change in water level.Ultrasonic types- a high frequency acoustic pulse is directed down from a transducer to the surfaceof the medium being measured and, by knowing the temperature and speed of sound in air, the time ittakes for the pulse to rebound to the sensor is used to determine the level.Microwave radar types- similar in principle to the ultrasonic type but using high frequencyelectromagnetic energy instead of acoustic energy.Hydrostatic types- a pressure transmitter is used to measure the pressure difference between theconfined hydrostatic pressure of the liquid head above the sensor and the outside atmosphericpressure. Changes in pressure are converted into a 4-20 mA output signal relative to the headdifference.Differential pressure types- similar to hydrostatic but used where the application being measured issubjected to dynamic pressure in addition to static pressure. They are capable of measuring smallchanges in pressure in relation to the output signal range. Typical applications might be to measurethe level of water in a boiler steam drum, or the level of condensate in a reboiler condensate pocket.Magnetic types- a float or cone is able to rise and fall along a stainless steel probe held in the tankfluid being measured. The float can interact magnetically with switches on the outside of the tankwhich send back information to the controller.Torsion types- a moving float spindle produces a change in torsion, measured by a torsiontransducer.

It is important that the level control system is correct for the application, and that expert advice is sought fromthe manufacturer before selection.

It is not within the scope of this Tutorial to discuss the pros and cons and potential applications of all theabove control types, as the types of level control systems usually employed in the steam and condensate loopand its associated applications are float and solid probe types. The operation of float types is fairlyself-explanatory, but conductivity and capacitance probes may require some explanation. Because of this, thissection will mainly focus on conductivity and capacitance probe-type level controls.

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Methods of achieving level controlThere are three main methods of achieving level control:

Non-adjustable on/off level control.Adjustable on/off level control.Modulating level control.

Non-adjustable on/off level control (Figure 8.3.1)The final control element may be a pump which is switched on/off or a valve which is opened/closed.

Two main types of on/off level control systems are usually encountered; float operated types and types usingconductivity probes. Float type level controls either rely upon the direct movement of a control valve, or uponelectrical switches being operated by a float moving on the surface of the liquid. Conductivity probes (seeFigure 8.3.1) may have several probe tips; the control points being located where the separate tips have beencut to different lengths.

Fig. 8.3.1 A four tip level probe

Adjustable on/off level control (Figure 8.3.2)Again, the final control element may be a pump which is switched on/off or a valve which is opened/closed.

One method used to adjust the control points is that of a capacitance probe (see Figure 8.3.2). The probe willmonitor the level, with control points adjusted by the controller. Capacitance probes are not cut to length toachieve the required level and, of course, the whole probe length must be sufficient for the complete controlrange.

Fig. 8.3.2 A capacitance level probe


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Modulating level control(Figure 8.3.2)The final control element may be a valve that is adjusted to a point between fully open and fully closed, as afunction of the level being monitored. Modulating level control cannot be achieved using a conductivity probe.Capacitance probes are ideal for this purpose (see Figure 8.3.2).

In systems of this type, the pump can run continuously, and the valve will permit appropriate quantities of liquidto pass. Alternatively, the final control element may be a variable speed drive on a pump. The speed of thedrive may be adjusted over a selected range.

Alarms - are often required to warn of either:

A high alarm where there is a danger of the tank overflowing and hot liquid being spilled, with theattendant danger to personnel.A low alarm where there is a danger of the tank water level becoming too low, with the potential todamage a pump drawing from the tank, or running out of liquid for the process.

Installation of floats and probes in turbulent conditionsIn some tanks and vessels, turbulent conditions may exist, which can result in erratic and unrepresentativesignals. If such conditions are likely to (or already) exist, it is recommended that floats or probes be installedwithin protection tubes. These have a dampening effect on the water level being sensed. The rest of thisTutorial concerns itself with probes rather than floats for level control applications.

Non-adjustable on/off level controlDescriptionNon-adjustable on/off level control uses a conductivity probe connected to an electronic controller. The probetypically has three or four tips, each of which is cut to length during installation to achieve the requiredswitching or alarm level (see Figure 8.3.3).

When the tip of the probe is immersed in liquid it uses the relatively high conductivity of the water tocomplete an electrical circuit via the tank metalwork and the controller.When the water level drops below the tip, the circuit resistance increases considerably, indicating tothe controller that the tip is not immersed in the liquid.In the case of a simple 'pumping in' system with on/off level control:

- The valve is opened when the tank water level falls below the end of a tip.- The valve is closed when the water level rises to contact another tip.- Other tips may be used to activate low or high alarms.

AdvantageA simple but accurate and relatively inexpensive method of level control.

Applications:The system can be used for liquids with conductivities of 1 S/cm or more, and is suitable for condensatetanks, feedwater tanks and process vats or vessels. Where the conductivity falls below this level it isrecommended that capacitance based level controls are used.

Point to note:If the tank is constructed from a non-conductive material, the electrical circuit may be achieved via anotherprobe tip.


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Fig. 8.3.3 General arrangement of a non-adjustable on/off level control system for a tank

Adjustable on/off level controlDescription:An adjustable on/off level control system consists of a controller and a capacitance probe (see Figure 8.3.4),and provides:

Valve open/closed control plus one alarm point.Alternatively two alarms - high and low.

The levels at which the valve operates can be adjusted through the controller functions.

Advantage:Adjustable on/off level control allows the level settings to be altered without shutting down the process.

Disadvantage:More expensive than non-adjustable on/off control.

Application:Can be used for most liquids, including those with low conductivities.

Point to note:Can be used in situations where the liquid surface is turbulent, and the in-built electronics can be adjusted toprevent rapid on/off cycling of the pump (or valve).


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Fig 8.3.4 General arrangement of an adjustable on/off level control system for a tank

Modulating level controlDescriptionA modulating level control system consists of a capacitance probe and appropriate controller, which provides amodulating output signal, typically 4-20 mA. Refer to Figure 8.3.5. This output signal may be used to affect avariety of devices including:

Modulating a control valve.Operating a variable speed pump drive.

Advantages:

Because the probe and controller only provide a signal to which other devices respond, rather thanproviding the power to operate a device, there is no limit on the size of the application.

1.

Steady control of level within the tank.2.

Disadvantages:

More expensive than a conductivity probe system.1.More complex than a conductivity probe system.2.Supply system must be permanently charged.3.Less suitable for 'stand-by' operation.4.Possibly greater electricity consumption.5.

Point to note:To protect the supply pump from overheating when pumping against a closed modulating valve, are-circulation or spill back line is provided to ensure a minimum flowrate through the pump (neither shown inFigure 8.3.5).

Fig. 8.3.5 General arrangement of a modulating control system maintaining the level in a tank

Steam flow control applicationsThe control of steam flow is less common than pressure and temperature control, but it is used in applicationswhere the control of pressure or temperature is not possible or not appropriate to achieving the processobjectives. The following sections give more information on measuring and controlling the flow of steam.

Flow control systemTypical applications:

Feed-forward systems on boiler plant, where the rate of steam flow from the boiler will influence othercontrol points, for example: feedwater make-up rate, and burner firing rate.

1.

Rehydration processes, where a measured quantity of steam (water) is injected into a product, whichhas been dried for transportation or storage. Examples of this can be found in the tobacco, coffee andanimal feedstuff industries.

2.

Batch processes, where it is known from experience that a measured quantity of steam will producethe desired result on the product.

3.


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The selection and application of components used to control flowrate require careful thought.

Fig. 8.3.6 General arrangement of a flow control system

The flowmeter (pipeline transducer)The flowmeter is a pipeline transducer, which converts flow into a measurable signal. The most commonlyused pipeline transducer is likely to relate flow to differential pressure. This pressure signal is received byanother transducer (typically a standard DP (differential pressure) transmitter) converting differential pressureinto an electrical signal. Some pipeline transducers are capable of converting flowrate directly to an electricalsignal without the need for a DP transmitter.

Figure 8.3.6 shows a variable area flowmeter and standard DP transmitter relating differential pressuremeasured across the flowmeter into a 4 - 20 mA electrical signal. The standard DP transmitter is calibrated tooperate at a certain upstream pressure; if this pressure changes, the output signal will not represent the flowaccurately. One way to overcome this problem is to provide a pressure (or temperature) signal if the medium issaturated steam, or a pressure and temperature signal if the fluid is superheated steam, as explained in thenext Section. Another way is to use a mass flow DP transmitter, which automatically compensates for pressurechanges.

The possible need for a computerIf steam is the fluid in the pipeline, then other temperature and/or pressure sensors may be necessary toprovide signals to compensate for variations in the supply pressure, as shown in Figure 8.3.7.


Multiple inputs will mean that an additional flow computer (or PLC) containing a set of electronic steam tablesmust process the signals from each of these flow, pressure and temperature sensors to allow accuratemeasurement of saturated or superheated steam.

If a flow computer is not readily available to compensate for changes in upstream pressure, it may be possibleto provide a constant pressure; perhaps by using an upstream control valve, to give stable and accurate


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Using a mass flow DP transmitter

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pressure control (not shown in Figure 8.3.7).

The purpose of this pressure control valve is to provide a stable (rather than reduced) pressure, but it willinherently introduce a pressure drop to the supply pipe.

A separator placed before any steam flowmetering station to protect the flowmeter from wet steam will alsoprotect the pressure control valve from wiredrawing.

By using a mass flow DP transmitter instead of a standard DP transmitter, the need for a computer to provideaccurate measurement is not required, as shown in Figure 8.3.8.

This is because the mass flow transmitter carries its own set of steam tables and can compensate for anychanges in saturated steam supply pressure.

However, a computer can still be used, if other important flowmetering information is required, such as, thetimes of maximum or minimum load, or is there is a need to integrate flow over a certain time period.

A controller is still required if flowrate is to be controlled, whichever system is used.


The controllerEven if the output signal from the DP transmitter or computer is of a type that the control valve actuator canaccept, a controller will still be required (as for any other type of control system) for the following reasons:

The output signal from certain flowmeters/computers has a long time repeat interval (approximately 3seconds), which will give enough information for a chart recorder to operate successfully, but may notoffer enough response for a control valve. This means that if the controller or PLC to which thetransmitter signal is being supplied operates at higher speeds, then the process can become unstable.

1.

PID functions are not available without a controller.2.Selecting a set point would not be possible without a controller.3.The signal needs calibrating to the valve travel - the effects of using either a greatly oversized orundersized valve without calibration, can easily cause problems.

4.

SummaryIt is usually better to install the flowmetering device upstream of the flow control valve. The higher pressurewill minimise its size and allow it to be more cost effective. It is also likely that the flowmeter will be subjectedto a more constant steam pressure (and density) and will be less affected by turbulence from the downstreamflow control valve.

In some cases, the application may be required to control at a constant flowrate. This means that features,such as high turndown ratios, are not important, and orifice plate flowmeters are appropriate.

If the flowrate is to be varied by large amounts, however, then 'turndown' becomes an issue that must beconsidered.

The subject of Flowmetering is discussed in greater depth in Block 4.

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