Types of Steam Flowmeter

8/10/2019 Types of Steam Flowmeter

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Types of Steam FlowmeterThe operation, advantages and limitations of different types of steam flowmeter,

including orifice plate, variable area and vortex shedding devices.

There are many types of flowmeter available, those suitable for steam applications include:

• Orifice plate flowmeters

• Turbine flowmeters (including shunt or bypass types)

• Variable area flowmeters

•

Spring loaded variable area flowmeters

• Direct in line variable area (D!V") flowmeter

• #itot tubes

• Vorte$ shedding flowmeters

%ach of these flowmeter types has its own advantages and limitations& To ensure accurate andconsistent performance from a steam flowmeter, it is essential to match the flowmeter to the

application&This Tutorial will review the above flowmeter types, and discuss their characteristics, theiradvantages and disadvantages, typical applications and typical installations&

Orifice plate flowmeters

The orifice plate is one in a group 'nown as head loss devices or differential pressureflowmeters& !n simple terms the pipeline fluid is passed through a restriction, and the pressuredifferential is measured across that restriction& ased on the wor' of Daniel ernoulli in *+(see Tutorial -&.), the relationship between the velocity of fluid passing through the orifice is

proportional to the s/uare root of the pressure loss across it& Other flowmeters in thedifferential pressure group include venturis and no00les&

1ith an orifice plate flowmeter, the restriction is in the form of a plate which has a holeconcentric with the pipeline& This is referred to as the primary element&

To measure the differential pressure when the fluid is flowing, connections are made from theupstream and downstream pressure tappings, to a secondary device 'nown as a D#(Differential #ressure) cell&



2ig& -&+&

Orifice plate

2ig& -&+&.Orifice plate flowmeter

2rom the D# cell, the information may be fed to a simple flow indicator, or to a flowcomputer along with temperature and3or pressure data, which enables the system tocompensate for changes in fluid density&

!n hori0ontal lines carrying vapours, water (or condensate) can build up against the upstreamface of the orifice& To prevent this, a drain hole may be drilled in the plate at the bottom of the

pipe& 4learly, the effect of this must be ta'en into account when the orifice plate dimensionsare determined&

4orrect si0ing and installation of orifice plates is absolutely essential, and is well documentedin the !nternational Standard !SO 5 6*&



2ig& -&+&+

Orifice plate flowmeter installation

Installation

" few of the most important points from !SO 5 6* are discussed below:

Pressure tappings Small bore pipes (referred to as impulse lines) connect the upstream anddownstream pressure tappings of the orifice plate to a Differential #ressure or D# cell&

The positioning of the pressure tappings can be varied& The most common locations are:

• 2rom the flanges (or carrier) containing the orifice plate as shown in 2igure -&+&+&This is covenient, but care needs to be ta'en with tappings at the bottom of the pipe,

because they may become clogged&

• One pipe diameter on the upstream side and 7&5 $ pipe diameter on the downstreamside&This is less convenient, but potentially more accurate as the differential pressure

measured is at its greatest at the vena contracta, which occurs at this position&



!orner tappings These are generally used on smaller orifice plates where space restrictionsmean flanged tappings are difficult to manufacture& 8sually on pipe diameters including or

below D957&

2rom the D# cell, the information may be fed to a flow indicator, or to a flow computer along

with temperature and3or pressure data, to provide density compensation&

Pipewor" There is a re/uirement for a minimum of five straight pipe diameters downstreamof the orifice plate, to reduce the effects of disturbance caused by the pipewor'&

The amount of straight pipewor' re/uired upstream of the orifice plate is, however, affected by a number of factors including:

• The ratio; this is the relationship between the orifice diameter and the pipediaameter (see %/uation -&+& ), and would typically be a value of 7&*&

%/uation -&+&

• The nature and geometry of the preceding obstruction& " few obstruction e$amplesare shown in 2igure -&+&-:

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Orifice plate installationsTable -&+& brings the ratio and the pipewor' geometry together to recommend the numberof straight diameters of pipewor' re/uired for the configurations shown in 2igure -&+&-&

!n particularly arduous situations, flow straighteners may be used& These are discussed inmore detail in Tutorial -&5&



Table -&+&

<ecommended straight pipe diameters upstream of an orifice plate for various ratios and preceding obstruction

#dvantages of orifice plate steam flowmeters$

• Simple and rugged&

• =ood accuracy&

• >ow cost&

• 9o calibration or recalibration is re/uired provided calculations, tolerances andinstallation comply with !SO 5 6*&

%isadvantages of orifice plate steam flowmeters$

• Turndown is limited to between -: and 5: because of the s/uare root relationship between flow and pressure drop&

• The orifice plate can buc'le due to waterhammer and can bloc' in a system that is poorly designed or installed&

• The s/uare edge of the orifice can erode over time, particularly if the steam is wet ordirty& This will alter the characteristics of the orifice, and accuracy will be affected&<egular inspection and replacement is therefore necessary to ensure reliability andaccuracy&

• The installed length of an orifice plate flowmetering system may be substantial; a

minimum of 7 upstream and 5 downstream straight unobstructed pipe diameters may be needed for accuracy&

This can be difficult to achieve in compact plants& 4onsider a system which uses 77 mm pipewor', the ratio is 7&*, and the layout is similar to that shown in 2igure -&+&-(b):

The upstream pipewor' length re/uired would be ? +6 $ 7& m ? +&6 m

The downstream pipewor' length re/uired would be ? 5 $ 7& m ? 7&5 m

The total straight pipewor' re/uired would be ? +&6 @ 7&5 m ? -& m

Typical applications for orifice plate steam flowmeters$



• "nywhere the flowrate remains within the limited turndown ratio of between -: and5: &

This can include the boiler house and applications where steam is supplied to many plants, some on line, some off line, but the overall flowrate is within the range&

Turbine flowmeters

The primary element consists of a multi bladed rotor which is mounted at right angles to theflow and suspended in the fluid stream on a free running bearing& The diameter of the rotor isslightly less than the inside diameter of the flowmetering chamber, and its speed of rotation is

proportional to the volumetric flowrate&

The speed of rotation of the turbine may be determined using an electronic pro$imity switchmounted on the outside of the pipewor', which counts the pulses, as shown in 2igure -&+&5&

2ig& -&+&5

Turbine flowmeter

Since a turbine flowmeter consists of a number of moving parts, there are several influencingfactors that need to be considered:

• The temperature, pressure and viscosity of the fluid being measured&

• The lubricating /ualities of the fluid&

• The bearing wear and friction&

• The conditional and dimensional changes of the blades&

• The inlet velocity profile and the effects of swirl&



• The pressure drop through the flowmeter&

ecause of these factors, calibration of turbine flowmeters must be carried out underoperational conditions&

!n larger pipelines, to minimise cost, the turbine element can be installed in a pipewor' bypass, or even for the flowmeter body to incorporate a bypass or shunt, as shown in 2igure-&+&6&

ypass flowmeters comprise an orifice plate, which is si0ed to provide sufficient restrictionfor a sample of the main flow to pass through a parallel circuit& 1hilst the speed of rotation of the turbine may still be determined as e$plained previously, there are many older units still ine$istence which have a mechanical output as shown in 2igure -&+&6&

4learly, friction between the turbine shaft and the gland sealing can be significant with thismechanical arrangement&

2ig& -&+&6

ypass or shunt turbine flowmeter

#dvantages of turbine flowmeters$• " turndown of 7: is achievable in a good installation with the turbine bearings in

good condition&

• "ccuracy is reasonable (A 7&5B of actual value)&

• ypass flowmeters are relatively low cost&

%isadvantages of turbine flowmeters$



• =enerally calibrated for a specific line pressure& "ny steam pressure variations willlead to inaccuracies in readout unless a density compensation pac'age is included&

• 2low straighteners are essential (see Tutorial -&5)&

• !f the flow oscillates, the turbine will tend to over or under run, leading toinaccuracies due to lag time&

• 1et steam can damage the turbine wheel and affect accuracy&

• >ow flowrates can be lost because there is insufficient energy to turn the turbinewheel&

• Viscosity sensitive: if the viscosity of the fluid increases, the response at lowflowrates deteriorates giving a non linear relationship between flow and rotationalspeed& Software may be available to reduce this effect&

• The fluid must be very clean (particle si0e not more than 77 Cm) because:

o 4learances between the turbine wheel and the inside of the pipe are verysmall&

o %ntrained debris can damage the turbine wheel and alter its performance&

o %ntrained debris will accelerate bearing wear and affect accuracy, particularlyat low flowrates&

Typical applications for turbine flowmeters$

• Superheated steam&

• >i/uid flowmetering, particularly fluids with lubricating properties& "s with allli/uids, care must be ta'en to remove air and gases prior to them being metered&

&ariable area flowmeters

The variable area flowmeter (2igure -&+&*), often referred to as a rotameter, consists of avertical, tapered bore tube with the small bore at the lower end, and a float that is allowed tofreely move in the fluid& 1hen fluid is passing through the tube, the float s position is ine/uilibrium with:

• The dynamic upward force of the fluid&





• #ressure drop is minimal and fairly constant&

%isadvantages of variable area flowmeters$

• The tube must be mounted vertically (see 2igure -&+& )&

• ecause readings are usually ta'en visually, and the float tends to move about,accuracy is only moderate& This is made worst by paralla$ error at higher flowrates,

because the float is some distance away from the scale&

• Transparent taper tubes limit pressure and temperature&

Typical applications for variable area flowmeters$

• Fetering of gases&

• Small bore airflow metering !n these applications, the tube is manufactured fromglass, with calibrations mar'ed on the outside& <eadings are ta'en visually&

• >aboratory applications&

• <otameters are sometimes used as a flow indicating device rather than a flowmeasuring device&

2ig& -&+&Variable area flowmeter installed in a vertical plane



Spring loaded variable area flowmeters

The spring loaded variable area flowmeter (an e$tension of the variable area flowmeter) usesa spring as the balancing force& This ma'es the meter independent of gravity, allowing it to beused in any plane, even upside down& Eowever, in its fundamental configuration (as shown in2igure -&+&G), there is also a limitation: the range of movement is constrained by the linearrange of the spring, and the limits of the spring deformation&

2ig& -&+&GSpring loaded variable area flowmeters

Eowever, another important feature is also revealed: if the pass area (the area between thefloat and the tube) increases at an appropriate rate, then the differential pressure across thespring loaded variable area flowmeter can be directly proportional to flow&

To recap a few earlier statements

'ith orifice plates flowmeters$

• "s the rate of flow increases, so does the differential pressure&

• y measuring this pressure difference it is possible to calculate the flowrate throughthe flowmeter&

• The pass area (for e$ample, the si0e of the hole in the orifice plate) remains constant&



'ith any type of variable area flowmeter

• The differential pressure remains almost constant as the flowrate varies&

• 2lowrate is determine from the position of the float&

• The pass area (the area between the float and the tube) through which the flow passesincreases with increasing flow&

2igure -&+& 7 compares these two principles&

2ig& -&+& 74omparing the fi$ed area and variable area flowmeters

The spring loaded variable area principle is a hybrid between these two devices, and either:



• The displacement of the float Option

or

• The differential pressure Option .

&&&may be used to determine the flowrate through the flowmeter&

!n Option (determining the displacement of the float or flap )& This can be developed forsteam systems by:

• 8sing a torsion spring to give a better operating range&

• 8sing a system of coils to accurately determine the position of the float&

This will result in a very compact flowmeter& This may be further tailored for saturated steamapplications by incorporating a temperature sensor and programming steam tables into thecomputer unit& See 2igure -&+& for an e$ample of a flowmeter of this type&

2ig& -&+&

Spring loaded variable area flowmeter monitoring the position of the float

#dvantages of spring loaded variable area flowmeters$

•

<obust&• Turndowns of .5: are achievable with normal steam velocities (.5 m3s), although

high velocities can be tolerated on an intermittent basis, offering turndowns of up to-7: &

• "ccuracy is A.B of actual value&

• 4an be tailored for saturated steam systems with temperature and pressure sensors to provide pressure compensation&

• <elatively low cost&



• Short installation length&

%isadvantages of spring loaded variable area flowmeters$

• Si0e limited to D9 77&

• 4an be damaged over a long period by poor /uality (wet and dirty) steam, at prolonged high velocity (H+7 m3s)&

Typical applications for spring loaded variable area flowmeters$

• 2lowetering of steam to individual plants&

• Small boiler houses&

2ig& -&+& .Typical installation of a spring loaded variable area flowmeter measuring steam flow

!n Option . (2igure -&+& 7), namely, determining the differential pressure, this concept can bedeveloped further by shaping of the float to give a linear relationship between differential

pressure and flowrate& See 2igure -&+& + for an e$ample of a spring loaded variable areaflowmeter measuring differential pressure& The float is referred to as a cone due to its shape&

2ig& -&+& +

Spring >oaded Variable "rea flowmeter (S>V") monitoring differential pressure



#dvantages of a spring loaded variable area (S)&#* flowmeter$

• Eigh turndown, up to 77: &

• =ood accuracy A B of reading for pipeline unit&

• 4ompact a D9 77 wafer unit re/uires only 67 mm between flanges&

• Suitable for many fluids&

%isadvantages of a variable area spring load flowmeter$

• 4an be e$pensive due to the re/uired accessories, such as the D# cell and flowcomputer&

Typical applications for a variable area spring load flowmeter$

• oiler house flowmetering&

• 2lowmetering of large plants&

2ig& -&+& -Typical installation of a SV>" flowmeter monitoring differential pressure

%irect In )ine &ariable #rea (%I&#* flowmeter



The D!V" flowmeter operates on the well established spring loaded variable area (S>V") principle, where the area of an annular orifice is continuously varied by a precision shapedmoving cone& This cone is free to move a$ially against the resistance of a spring&

Eowever, unli'e other S>V" flowmeters, the D!V" does not rely on the measurement of

differential pressure drop across the flowmeter to calculate flow, measuring instead the forcecaused by the deflection of the cone via a series of e$tremely high /uality strain gauges& Thehigher the flow of steam the greater the force& This removes the need for e$pensivedifferential pressure transmitters, reducing installation costs and potential problems (2igure-&+& 5)&

The D!V" has an internal temperature sensor, which provides full density compensation forsaturated steam applications&

Flowmetering systems will$

• 4hec' on the energy cost of any part of the plant&

• 4ost energy as a raw material&

• !dentify priority areas for energy savings&

• %nable efficiencies to be calculated for processes or power generation&

2ig& -&+& 5 Traditional flowmetering system versus a D!V" flowmetering systemThe D!V" steam flowmeter (2igure -&+& 6) has a system uncertainty in accordance with %9!SO3!%4 *7.5, of:

• A .B of actual flow to a confidence of G5B (. standard deviations) over a range of7B to 77B of ma$imum rated flow&

• A 7&.B 2SD to a confidence of G5B (. standard deviations) from .B to 7B of thema$imum rated flow&

"s the D!V" is a self contained unit the uncertainty /uoted is for the complete system& Fanyflowmeters claim a pipeline unit uncertainty but, for the whole system, the individual



uncertainty values of any associated e/uipment, such as D# cells, need to be ta'en intoaccount&

The turndown of a flowmeter is the ratio of the ma$imum to minimum flowrate over which itwill meet its specified performance, or its operational range& The D!V" flowmeter has a high

turndown ratio of up to 57: , giving an operational range of up to G B of its ma$imum flow&

2ig& -&+& 6 The D!V" flowmeter

Flow orientations

The orientation of the D!V" flowmeter can have an effect on the operating performance&!nstalled in hori0ontal pipe, the D!V" has a steam pressure limit of +. bar g, and a 57:turndown& "s shown in 2igure -&+& *, if the D!V" is installed with a vertical flow directionthen the pressure limit is reduced, and the turndown ratio will be affected if the flow isvertically upwards&





2ig& -&+& G " simple pitot tubeecause the simple #itot tube (2igure -&+& G) only samples a single point, and, because theflow profile of the fluid (and hence velocity profile) varies across the pipe, accurate

placement of the no00le is critical&

9ote that a s/uare root relationship e$ists between velocity and pressure drop (see %/uation-&.& +)& This limits the accuracy to a small turndown range&

%/uation -&.& +

1here:

The averaging Pitot tubeThe averaging #itot tube (2igure -&+&.7) was developed with a number of upstream sensingtubes to overcome the problems associated with correctly siting the simple type of #itot tube&These sensing tubes sense various velocity pressures across the pipe, which are then averagedwithin the tube assembly to give a representative flowrate of the whole cross section&u ? The fluid velocity in the pipeIp ? Dynamic pressure Static pressure ρ ? Density

2ig& -&+&.7 The averaging pitot tube

#dvantages of the Pitot tube

• #resents little resistance to flow&

• !ne$pensive to buy&



• Simple types can be used on different diameter pipes&

%isadvantages of the Pitot tube$

• Turndown is limited to appro$imately -: by the s/uare root relationship between pressure and velocity as discussed in Tutorial -&.&

• !f steam is wet, the bottom holes can become effectively bloc'ed& To counter this,some models can be installed hori0ontally&

• Sensi

•

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Types of Steam Flowmeter