dp cell

http://en.wikipedia.org/wiki/DP_cellA DP cell is a device that measures the differential pressure between two inputs.[1]

Example:

To measure the pressure difference between a container (or vessel) and the surrounding atmosphere, you may connect 'Hi' port of the DP-cell to a fitting that enters the vessel, using suitable tubing. The 'Lo' port, you leave open to the atmosphere (open air, or possibly through a buffer or dessicant chamber). The DP-cell will indicate the relative difference between the pressure of the vessel (container) and the atmospheric pressure.

This signal is often wired to an indicator that reads out locally, or remotely in a control room, and/or as a control (or feedback) signal to a valve, pump, or other control element to maintain a set pressure, or limit a maximum pressure. Typically, the signal is 4-20 mA DC loop current,[2] where, usually, 4mA represents the minimum differential pressure and 20mA represents the maximum differential pressure. Alternatlively, the signal may be a variable voltage, or digital information stream.Sebuah sel DP adalah perangkat yang mengukur perbedaan tekanan antara dua input. [1]

contoh:

Untuk mengukur perbedaan tekanan antara wadah (atau kapal) dan suasana sekitarnya, Anda dapat menghubungkan 'Hai' port dari sel DP-ke pas yang masuk kapal, menggunakan tabung cocok. The 'Lo' port, Anda meninggalkan terbuka untuk atmosfer (udara terbuka, atau mungkin melalui buffer atau ruang dessicant.Selain). DP-sel akan menunjukkan perbedaan relatif antara tekanan kapal (kontainer) dan tekanan atmosfer.

Sinyal ini sering kabel ke indikator bahwa membacakan secara lokal, atau jarak jauh di ruang kendali, dan / atau sebagai kontrol (atau umpan balik) sinyal ke katup, pompa, atau elemen kontrol lainnya untuk menjaga tekanan set, atau membatasi maksimum tekanan. Biasanya, sinyal 4-20 mA DC loop arus, [2] di mana, biasanya, 4mA mewakili tekanan diferensial minimum dan 20mA mewakili tekanan diferensial maksimal. Alternatlively, sinyal mungkin tegangan variabel, atau aliran informasi digital.http://www.spiraxsarco.com/resources/steam-engineering-tutorials/the-boiler-house/methods-of-detecting-water-level-in-steam-boilers.aspOn a steam raising boiler there are three clear applications for level monitoring devices:

* Level control - To ensure that the right amount of water is added to the boiler at the right time. * Low water alarm - For safe boiler operation, the low water alarm ensures that the combustion of fuel does not continue if the water level in the boiler has dropped to, or below a predetermined level. For automatically controlled steam boilers, national standards usually call for two independent low level alarms, to ensure safety. In the UK, the lower of the two alarms will 'lockout' the burner, and manual resetting is required to bring the boiler back on line. * High water alarm - The alarm operates if the water level rises too high, informing the boiler operator to shut off the feedwater supply. Although not usually mandatory, the use of high level alarms is sensible as they reduce the chance of water carryover and waterhammer in the steam distribution system.

Fig. 3.16.1 - Operating levels for water controls and alarms Fig. 3.16.1Operating levels for water controls and alarmsArrow TopMethods of automatic level detection

The following Sections within this Tutorial discuss the principal types of level detection device which are appropriate to steam boilers.Basic electric theory

The way in which electricity flows can be compared with a liquid. Liquid flows through a pipe in a similar way that electricity flows through a conductor (see Figure 3.16.2).Fig. 3.16.2 - Analogy of an electrical circuit - with a water circuit Fig. 3.16.2Analogy of an electrical circuitwith a water circuit

A conductor is a material, such as metal wire, which allows the free flow of electrical current. (The opposite of a conductor is an insulator which resists the flow of electricity, such as glass or plastic). An electric current is a flow of electric 'charge', carried by tiny particles called electrons or ions. Charge is measured in coulombs. 6.24 x 1018 electrons together have a charge of one coulomb, which in terms of SI base units is equivalent to 1 ampere second.

When electrons or ions are caused to move, the flow of electricity is measured in Coulombs per second rather than electrons or ions per second. However, the term 'ampere' (or A) is given to the unit in which electric current is measured.

* 1 A = A flow of 6.24 x 1018 electrons per second * 1 A = 1 coulomb per second

The force causing current to flow is known as the electromotive force or EMF. A battery, a bicycle dynamo or a power station generator (among other examples) may provide it.

A battery has a positive terminal and a negative terminal. If a wire is connected between the terminals, a current will flow. The battery acts as a pressure source similar to the pump in a water system. The potential difference between the terminals of an EMF source is measured in volts and the higher the voltage (pressure) the greater the current (flow). The circuit through which the current flows presents a resistance (similar to the resistance presented by pipes and valves in a water system).

The unit of resistance is the ohm (given the symbol W) and Ohm's law relates current, voltage and resistance, see Equation 3.16.1:Equation 3.16.1 Equation 3.16.1

Where:I = Current (amperes)V = Voltage (volts)R = Resistance (ohms)

Another important electrical concept is 'capacitance'. It measures the capacity of the charge between two conductors (roughly analogous to the volume of a container) in terms of the charge required to raise its potential by an amount of one volt.

A pair of conductors has a large capacitance if they need a large amount of charge to raise the voltage between them by one volt, just as a large vessel needs a large quantity of gas to fill it to a certain pressure.

The unit of capacitance is one coulomb per volt, which is termed one farad.Conductivity probes

Consider an open tank with some water in it. A probe (metal rod) is suspended in the tank (see Figure 3.16.3). If an electrical voltage is applied and the circuit includes an ammeter, the latter will show that:

* With the probe immersed in the water, current will flow through the circuit. * If the probe is lifted out of the water, current will not flow through the circuit Fig. 3.16.3 - Operating principle of conductivity probes - single tip Fig. 3.16.3 Operating principle of conductivity probes - single tip

This is the basis of the conductivity probe. The principle of conductivity is used to give a point measurement. When the water level touches the probe tip, it triggers an action through an associated controller.

This action may be to: o Start or stop a pump o Open or close a valve o Sound an alarm o Open or close a relay

But a single tip can only provide a single or point action. Thus, two tips are required with a conductivity probe in order to switch a pump on and off at predetermined levels, (Figure 3.16.4). When the water level falls and exposes the tip at point A, the pump will begin to run. The water level rises until it touches the second tip at point B, and the pump will be switched off. Fig. 3.16.4 - Conductivity probes arranged to switch - a feedpump on and off - two tip Fig. 3.16.4 Conductivity probes arranged to switch a feedpump on and off - two tip Fig. 3.16.5 - Conductivity probe - in a closed top tank Fig. 3.16.5 Conductivity probe in a closed top tank

Probes can be installed into closed vessels, for example a boiler. Figure 3.16.5 shows a closed top metal tank - Note; an insulator is required where the probe passes through the tank top.

Again: o With the probe immersed, current will flow. o With the probe out of the water, the flow of current ceases.

Note: An alternating current is used to avoid polarisation and electrolysis (the splitting of water into hydrogen and oxygen) at the probe. A standard conductivity probe must be used to provide low water alarm in a boiler.

Under UK regulations, this must be tested daily.

For a simple probe there is a potential problem - If dirt were to build up on the insulator, a conductive path would be created between the probe and the metal tank and current would continue to flow even if the tip of the probe were out of the water. This may be overcome by designing and manufacturing the conductivity probe so that the insulator is long, and sheathed for most of its length with a smooth insulating material such as PTFE / Teflon®. This will minimise the risk of dirt build-up around the insulator, see Figure 3.16.6. Fig. 3.16.6 - Dirt on the insulator: the problem and the solution Fig. 3.16.6 Dirt on the insulator: the problem and the solution

The problem has been solved by: o Using an insulator in the steam space. o Using a long smooth PTFE sheath as an insulator virtually along the whole length of the metal probe. o Adjustable sensitivity at the controller.

Special conductivity probes are available for low level alarms, and are referred to as 'self-monitoring'. Several self-checking features are incorporated, including: o A comparator tip which continuously measures and compares the resistance to earth through the insulation and through the probe tip. o Checking for current leakage between the probe and the insulation. o Other self-test routines.

Under UK regulations, use of these special systems allows a weekly test rather than a daily one. This is due to the inherently higher levels of safety in their design.

The tip of a conductivity probe must be cut to the correct length so that it accurately represents the desired switching point. Conductivity probes summary

Conductivity probes are: o Normally vertically mounted. o Used where on/off level control is suitable. o Often supplied mounted in groups of three or four in a single housing, although other configurations are available. o Cut to length on installation.

Since the probes use electrical conductivity to operate, applications using very pure water (conductivity less than 5 µ Siemens / cm) are not suitable. Fig. 3.16.7 - A typical conductivity probe - (shown with four tips) - and associated controller Fig. 3.16.7 A typical conductivity probe (shown with four tips) and associated controller Capacitance probes

A simple capacitor can be made by inserting dielectric material (a substance which has little or no electrical conductivity, for example air or PTFE), between two parallel plates of conducting material (Figure 3.16.8). Fig. 3.16.8 - A capacitor Fig. 3.16.8 A capacitor

The basic equation for a capacitor, such as the one illustrated in Figure 3.16.8, is shown in Equation 3.16.2: Equation 3.16.2 Equation 3.16.2

Where: C = Capacitance (farad) K = Dielectric constant (a function of the dielectric between the plates) A = Area of plate (m²) D = Distance between plates (m)

Consequently: o The larger the area of the plates, the higher the capacitance. o The closer the plates, the higher the capacitance. o The higher the dielectric constant, the higher the capacitance.

Therefore if A, D or K is altered then the capacitance will vary!

A basic capacitor can be constructed by dipping two parallel conductive plates into a dielectric liquid (Figure 3.16.9). If the capacitance is measured as the plates are gradually immersed, it will be seen that the capacitance changes in proportion to the depth by which the plates are immersed into the dielectric liquid. Fig. 3.16.9 - A basic capacitor in a liquid Fig. 3.16.9 A basic capacitor in a liquid Fig. 3.16.10 - Output from a capacitor in a liquid Fig. 3.16.10 Output from a capacitor in a liquid

The capacitance increases as more of the plate area is immersed in the liquid (Figure 3.16.10). A simple capacitor can be made by inserting dielectric material (a substance which has little or no electrical conductivity, for example air), between two parallel plates of conducting material (Figure 3.16.8).

The situation is somewhat different in the case of plates immersed in a conductive liquid, such as boiler water, as the liquid no longer acts as a dielectric, but rather an extension of the plates.

The capacitance level probe therefore consists of a conducting, cylindrical probe, which acts as the first capacitor plate. This probe is covered by a suitable dielectric material, typically PTFE. The second capacitor plate is formed by the chamber wall (in the case of a boiler, the boiler shell) together with the water contained in the chamber. Therefore, by changing the water level, the area of the second capacitor plate changes, which affects the overall capacitance of the system (see Equation 3.16.2). Fig. 3.16.11 - Capacitance in water Fig. 3.16.11 Capacitance in water

The total capacitance of the system therefore has two components (illustrated in Figure 3.16.12): * CA, the capacitance above the liquid surface - The capacitance develops between the chamber wall and the probe. The dielectric consists of both the air between the probe and the chamber wall, and the PTFE cover. * CB, the capacitance below the liquid surface - The capacitance develops between the water surface in contact with the probe and the only dielectric is the PTFE cover. Fig. 3.16.12 - Components of a capacitor signal - (not to scale) Fig. 3.16.12 Components of a capacitor signal (not to scale)

Since the distance between the two capacitance plates above the water surface (the chamber wall and the probe) is large, so the capacitance CA is small (see Equation 3.16.2). Conversely, the distance between the plates below the water surface (the probe and the water itself) is small and therefore, the capacitance CB will be large compared with CA. The net result is that any rise in the water level will cause an increase in capacitance that can be measured by an appropriate device. Fig. 3.16.14 - Typical capacitance probe - (shown with head) Fig. 3.16.14 Typical capacitance probe (shown with head)

The change in capacitance is, however, small (typically measured in pico farads, for example, 10-12 farads) so the probe is used in conjunction with an amplifier circuit. The amplified change in capacitance is then signalled to a suitable controller.

Where the capacitance probe is used in, for example, a feedtank, (Figure 3.16.13) liquid levels can be monitored continuously with a capacitance probe. The associated controller can be set up to modulate a control valve, and / or to provide point functions such as a high level alarm point or a low level alarm.

The controller can also be set up to provide on / off control. Here, the 'on' and 'off' switching points are contained within a single probe and are set via the controller, removing any need to cut the probe. Since a capacitance probe must be wholly encased in insulating material, it must not be cut to length. Fig. 3.16.13 - Typical control using a capacitance probe in a feedtank - (not to scale) Fig. 3.16.13 Typical control using a capacitance probe in a feedtank (not to scale) Float control

This is a simple form of level measurement. An everyday example of level control with a float is the cistern in a lavatory. When the lavatory is flushed, the water level drops in the cistern, the float follows the water level down and opens the inlet water valve. Eventually the cistern shuts and as fresh water runs in, the water level increases, the float rises and progressively closes the inlet water valve until the required level is reached.

The system used in steam boilers is very similar. A float is mounted in the boiler. This may be in an external chamber, or directly within the boiler shell. The float will move up and down as the water level changes in the boiler. The next stage is to monitor this movement and to use it to control either: o A feedpump (an on / off level control system)

or

o A feedwater control valve (a modulating level control system)

Because of its buoyancy, the float follows the water level up and down. o o At the opposite end of the float rod is a magnet, which moves inside a stainless steel cap. Because the cap is stainless steel, it is (virtually) non-magnetic, and allows the lines of magnetism to pass through it.

In its simplest form, the magnetic force operates the magnetic switches as follows: o o The bottom switch will switch the feedpump on.

o The top switch will switch the feedpump off.

However, in practice a single switch will often provide on / off pump control, leaving the second switch for an alarm.

This same arrangement can be used to provide level alarms.

A more sophisticated system to provide modulating control will use a coil wrapped around a yoke inside the cap. As the magnet moves up and down, the inductance of the coil will alter, and this is used to provide an analogue signal to a controller and then to the feedwater level control valve. Fig. 3.16.15 Float control Fig. 3.16.15 Float control

Float control application Vertically or horizontally mounted, the level signal output is usually via a magnetically operated switch (mercury type or 'air-break' type); or as a modulating signal from an inductive coil due to the movement of a magnet attached to the float. In both cases the magnet acts through a non-magnetic stainless steel tube. Fig. 3.16.16 Magnetic level controller in a chamber Fig. 3.16.16 Magnetic level controller in a chamber

Differential pressure cells The differential pressure cell is installed with a constant head of water on one side. The other side is arranged to have a head which varies with the boiler water level.

Variable capacitance, strain gauge or inductive techniques are used to measure the deflection of a diaphragm, and from this measurement, an electronic level signal is produced.

Use of differential pressure cells is common in the following applications: o o High-pressure water-tube boilers where high quality demineralised water is used.

o Where very pure water is used, perhaps in a pharmaceutical process.

In these applications, the conductivity of the water is very low, and it can mean that conductivity and capacitance probes will not operate reliably. Fig. 3.16.17 Level control using a differential pressure cell (not to scale) Fig. 3.16.17 Level control using a differential pressure cell (not to scale)

Other types of modulating control systems may occasionally be encountered. However, in order to comply with (UK) Health and Safety Executive (HSE) or insurance company demands, most boilers use one or other of the systems described above.Pada ketel uap membesarkan ada tiga aplikasi yang jelas untuk perangkat pemantauan tingkat:

    * Kontrol Level - Untuk memastikan bahwa jumlah yang tepat air ditambahkan ke ketel pada waktu yang tepat.    * Alarm air rendah - Untuk operasi boiler aman, alarm air rendah memastikan bahwa pembakaran bahan bakar tidak berlanjut jika level air dalam boiler telah menurun, atau di bawah tingkat yang telah ditentukan. Untuk steam boiler dikontrol secara otomatis, standar nasional biasanya panggilan untuk dua alarm independen tingkat rendah, untuk memastikan keamanan. Di Inggris, semakin rendah dari dua alarm akan 'penguncian' kompor, dan manual reset diperlukan untuk membawa boiler kembali on line.    * Alarm air Tinggi - Alarm beroperasi jika air meningkat terlalu tinggi, menginformasikan operator boiler untuk mematikan pasokan air umpan. Meskipun biasanya tidak wajib, penggunaan alarm tingkat tinggi adalah masuk akal karena mereka mengurangi kemungkinan air carryover dan waterhammer dalam sistem distribusi uap.

Gambar. 3.16.1 - tingkat operasi untuk kontrol air dan alarm Gambar. 3.16.1Operasi tingkat untuk kontrol air dan alarmPanah AtasMetode deteksi tingkat otomatis

Para Bagian berikut dalam Tutorial ini membahas jenis utama dari perangkat tingkat deteksi yang sesuai dengan boiler steam.Dasar teori listrik

Cara di mana listrik mengalir dapat dibandingkan dengan cairan. Cairan mengalir melalui pipa dengan cara yang sama bahwa listrik mengalir melalui sebuah konduktor (lihat Gambar 3.16.2).Gambar. 3.16.2 - Analogi dari sebuah rangkaian listrik - dengan Gambar sirkuit air. 3.16.2Analogi dari sebuah rangkaian listrikdengan sirkuit air

Konduktor adalah material, seperti kawat logam, yang memungkinkan aliran bebas dari arus listrik. (Kebalikan dari konduktor adalah isolator yang menolak aliran listrik, seperti kaca atau plastik). Arus listrik adalah aliran 'biaya' listrik, dibawa oleh partikel kecil yang disebut elektron atau ion. Biaya diukur dalam coulomb. 6,24 x 1018 elektron bersama-sama memiliki muatan satu coulomb, yang dalam hal unit dasar SI adalah setara dengan 1 detik ampere.

Ketika elektron atau ion disebabkan bergerak, aliran listrik diukur dalam coulomb per detik daripada elektron atau ion per detik. Namun, istilah 'ampere' (atau A) diberikan ke unit di mana arus listrik diukur.

    * 1 A = Aliran 6.24 x 1018 elektron per detik    * 1 A = 1 coulomb per detik

Kekuatan yang menyebabkan arus mengalir dikenal sebagai gaya gerak listrik atau EMF. Sebuah baterai, dinamo sepeda atau stasiun pembangkit listrik (di antara contoh lainnya) dapat menyediakannya.

Sebuah baterai memiliki terminal positif dan terminal negatif. Jika kawat dihubungkan antara terminal, arus akan mengalir. Baterai bertindak sebagai sumber tekanan yang serupa dengan pompa dalam sistem air. Perbedaan potensial antara terminal sumber EMF diukur dalam volt dan semakin tinggi tegangan (tekanan) yang lebih besar saat ini (aliran). Rangkaian di mana arus mengalir menyajikan perlawanan (mirip dengan resistensi disajikan oleh pipa dan katup dalam sistem air).

Satuan resistansi adalah ohm (diberi simbol W) dan hukum Ohm berkaitan arus, tegangan dan resistensi, lihat Persamaan 3.16.1:Persamaan Persamaan 3.16.1 3.16.1

Dimana:I = Arus (ampere)V = Tegangan (volt)R = Resistance (ohm)

Konsep lain yang penting listrik adalah 'kapasitansi. Mengukur kapasitas muatan antara dua konduktor (kira-kira analog dengan volume wadah) dalam hal biaya yang dibutuhkan untuk menaikkan potensinya dengan jumlah satu volt.

Sepasang konduktor memiliki kapasitansi yang besar jika mereka membutuhkan sejumlah besar biaya untuk menaikkan tegangan antara mereka dengan satu volt, hanya sebagai kapal besar membutuhkan

jumlah besar gas untuk mengisinya sampai tekanan tertentu.

Satuan kapasitansi adalah satu coulomb per volt, yang disebut satu farad.Konduktivitas probe

Pertimbangkan sebuah tangki terbuka dengan air di dalamnya. Penyelidikan (logam batang) yang tersuspensi dalam tangki (lihat Gambar 3.16.3). Jika tegangan listrik diterapkan dan sirkuit termasuk ammeter, yang terakhir akan menunjukkan bahwa:

    * Dengan probe direndam dalam air, arus akan mengalir melalui sirkuit.    * Jika probe diangkat keluar dari air, arus tidak akan mengalir melalui sirkuit      Gambar. 3.16.3 - Operasi prinsip konduktivitas probe - Gambar ujung tunggal. 3.16.3      Operasi prinsip konduktivitas probe - tip tunggal

      Ini adalah dasar dari probe konduktivitas. Prinsip konduktivitas digunakan untuk memberikan pengukuran titik. Bila tingkat air menyentuh ujung probe, memicu tindakan melalui controller terkait.

      Tindakan ini mungkin untuk:          o Memulai atau menghentikan pompa          o Membuka atau menutup katup          o Suara alarm          o Membuka atau menutup sebuah relay

      Tapi tip tunggal hanya dapat memberikan satu tindakan atau titik. Dengan demikian, dua tips yang diperlukan dengan probe konduktivitas untuk beralih pompa dan mematikan pada tingkat yang telah ditentukan, (Gambar 3.16.4). Bila tingkat air jatuh dan menghadapkan ujung pada titik A, pompa akan mulai berjalan. Tingkat air meningkat sampai menyentuh ujung kedua pada titik B, dan pompa akan dimatikan.      Gambar. 3.16.4 - probe Konduktivitas diatur untuk beralih - sebuah feedpump dan mematikan - dua Gambar tip. 3.16.4      Probe Konduktivitas diatur untuk beralih      sebuah feedpump dan mematikan - dua ujung      Gambar. 3.16.5 - Konduktivitas probe - dalam Gambar tangki tertutup atas. 3.16.5      Konduktivitas penyelidikan      dalam tangki atas tertutup

      Probe dapat diinstal ke dalam pembuluh tertutup, misalnya boiler. Gambar 3.16.5 menunjukkan tangki logam tertutup atas - Catatan; isolator diperlukan di mana probe melewati tank top.

      Sekali lagi:          o Dengan probe terendam, arus akan mengalir.          o Dengan probe keluar dari air, aliran arus berhenti.

      Catatan: arus bolak-balik digunakan untuk menghindari polarisasi dan elektrolisis (pemisahan air menjadi hidrogen dan oksigen) pada probe. Penyelidikan konduktivitas standar harus digunakan untuk memberikan alarm air rendah di boiler.

      Di bawah peraturan Inggris, ini harus diuji setiap hari.

      Untuk mengorek ada masalah potensial - Jika kotoran adalah untuk membangun pada isolator, jalur konduktif akan dibuat antara probe dan tangki logam dan saat ini akan terus mengalir bahkan jika ujung

probe yang keluar dari air. Hal ini dapat diatasi dengan merancang dan manufaktur probe konduktivitas sehingga isolator panjang, dan berselubung untuk sebagian besar panjangnya dengan bahan isolasi halus seperti PTFE / Teflon ®. Ini akan meminimalkan risiko kotoran build-up di sekitar isolator, lihat Gambar 3.16.6.      Gambar. 3.16.6 - Dirt pada isolator: masalah dan solusi Gambar. 3.16.6      Kotoran pada isolator: masalah dan solusinya

      Masalahnya telah diselesaikan dengan:          o Menggunakan insulator ruang uap.          o Menggunakan sarung halus panjang PTFE sebagai insulator hampir sepanjang seluruh panjang probe logam.          o sensitivitas Adjustable di controller.

      Konduktivitas probe khusus tersedia untuk alarm tingkat rendah, dan disebut sebagai 'pemantauan diri'. Beberapa diri memeriksa fitur yang dimasukkan, termasuk:          o Tip komparator yang terus menerus mengukur dan membandingkan resistensi terhadap bumi melalui isolasi dan melalui ujung probe.          o Memeriksa kebocoran arus antara probe dan isolasi.          o Lain uji diri rutinitas.

      Di bawah peraturan Inggris, penggunaan sistem ini khusus memungkinkan tes mingguan daripada satu hari. Hal ini disebabkan tingkat lebih tinggi keselamatan inheren dalam desain mereka.

      Ujung probe konduktivitas harus dipotong dengan panjang yang benar sehingga secara akurat merupakan titik peralihan yang diinginkan.      Konduktivitas probe ringkasan

      Probe Konduktivitas adalah:          o Biasanya vertikal dipasang.          o Digunakan mana pada kontrol level / off cocok.          o Sering dikirim dengan dipasang dalam kelompok tiga atau empat dalam perumahan tunggal, meskipun konfigurasi lain yang tersedia.          o Potong memanjang pada instalasi.

            Karena probe menggunakan konduktivitas listrik untuk beroperasi, aplikasi yang menggunakan air yang sangat murni (konduktivitas kurang dari 5 μ Siemens / cm) tidak cocok.      Gambar. 3.16.7 - Penyelidikan konduktivitas khas - (ditunjukkan dengan empat tips) - dan yang terkait Gambar controller. 3.16.7      Penyelidikan yang khas konduktivitas      (Ditampilkan dengan empat tips)      dan terkait kontroler      Kapasitansi probe

      Sebuah kapasitor sederhana dapat dibuat dengan memasukkan bahan dielektrik (zat yang memiliki konduktivitas listrik sedikit atau tidak ada, misalnya udara atau PTFE), antara dua pelat paralel melakukan materi (Gambar 3.16.8).      Gambar. 3.16.8 - Sebuah Gambar kapasitor. 3.16.8      Sebuah kapasitor

      Persamaan dasar untuk sebuah kapasitor, seperti yang diilustrasikan pada Gambar 3.16.8, akan ditampilkan dalam Persamaan 3.16.2:

      Persamaan Persamaan 3.16.2 3.16.2

      Dimana:      C = Kapasitansi (Farad)      K = Dielektrik konstan (fungsi dari dielektrik antara pelat)      A = Luas plat (m²)      D = Jarak antara pelat (m)

      Akibatnya:          o daerah tersebut lebih besar dari piring, kapasitansi tinggi.          o Semakin dekat piring, semakin tinggi kapasitansi.          o kapasitansi tinggi dielektrik konstan, semakin tinggi.

      Oleh karena itu jika A, D atau K diubah maka kapasitansi akan bervariasi!

      Sebuah kapasitor dasar dapat dibangun dengan mencelupkan dua pelat paralel konduktif ke dalam cairan dielektrik (Gambar 3.16.9). Jika kapasitansi diukur sebagai pelat secara bertahap tenggelam, akan terlihat bahwa perubahan kapasitansi sebanding dengan kedalaman di mana pelat terbenam ke dalam cairan dielektrik.      Gambar. 3.16.9 - Sebuah kapasitor dasar dalam Gambar cair. 3.16.9      Sebuah kapasitor dasar dalam cairan      Gambar. 3.16.10 - Output dari sebuah kapasitor dalam Gambar cair. 3.16.10      Output dari sebuah kapasitor dalam suatu cairan

      Kapasitansi meningkat sebagai lebih dari area piring direndam dalam cairan (Gambar 3.16.10).      Sebuah kapasitor sederhana dapat dibuat dengan memasukkan bahan dielektrik (zat yang memiliki konduktivitas listrik sedikit atau tidak ada, untuk udara misalnya), antara dua pelat paralel melakukan materi (Gambar 3.16.8).

      Situasi ini agak berbeda dalam hal piring direndam dalam cairan konduktif, seperti air boiler, sebagai cairan tidak lagi bertindak sebagai dielektrik, melainkan perpanjangan dari piring.

      Probe tingkat kapasitansi karena itu terdiri dari probe, melakukan silinder, yang bertindak sebagai pelat kapasitor pertama. Probe ini ditutupi oleh bahan dielektrik yang cocok, biasanya PTFE. Pelat kapasitor kedua dibentuk oleh dinding ruang (dalam hal boiler, shell boiler) bersama-sama dengan air yang terkandung dalam ruangan. Oleh karena itu, dengan mengubah tingkat air, area perubahan kedua pelat kapasitor, yang mempengaruhi kapasitansi keseluruhan sistem (lihat Persamaan 3.16.2).      Gambar. 3.16.11 - Kapasitansi dalam air Gambar. 3.16.11      Kapasitansi dalam air

      Kapasitansi total dari sistem sehingga memiliki dua komponen (diilustrasikan pada Gambar 3.16.12):    * CA, kapasitansi di atas permukaan cairan - kapasitansi berkembang diantara dinding kamar dan probe. Dielektrik ini terdiri dari baik udara antara probe dan dinding kamar, dan penutup PTFE.    * CB, kapasitansi bawah permukaan cair - Kapasitansi berkembang antara permukaan air di kontak dengan probe dan dielektrik hanya penutup PTFE.      Gambar. 3.16.12 - Komponen sinyal kapasitor - (tidak skala) Gambar. 3.16.12      Komponen sinyal kapasitor      (Tidak untuk skala)

      Karena jarak antara kedua pelat kapasitansi di atas permukaan air (dinding ruang dan probe) adalah besar, sehingga kapasitansi CA kecil (lihat Persamaan 3.16.2). Sebaliknya, jarak antara pelat di bawah

permukaan air (probe dan air itu sendiri) yang kecil dan karena itu, CB kapasitansi akan besar dibandingkan dengan CA. Hasil akhirnya adalah bahwa setiap kenaikan tingkat air akan menyebabkan peningkatan kapasitansi yang dapat diukur dengan perangkat yang tepat.      Gambar. 3.16.14 - probe kapasitansi Khas - (ditunjukkan dengan kepala) Gambar. 3.16.14      Khas kapasitansi penyelidikan      (Ditampilkan dengan kepala)

      Perubahan kapasitansi adalah, bagaimanapun, kecil (biasanya diukur dalam farad pico, misalnya, 10-12 farads) sehingga probe digunakan bersama dengan sirkuit penguat. Perubahan diperkuat kapasitansi kemudian memberi sinyal ke controller yang sesuai.

      Dimana probe kapasitansi digunakan dalam, misalnya, feedtank, (Gambar 3.16.13) tingkat cair dapat dimonitor secara terus menerus dengan probe kapasitansi. Pengendali yang terkait dapat diatur untuk memodulasi katup kontrol, dan / atau untuk menyediakan fungsi titik seperti titik alarm tingkat tinggi atau alarm tingkat rendah.

      Controller juga dapat diatur untuk memberikan on / off control. Di sini, poin beralih 'on' dan 'off' yang terkandung dalam probe tunggal dan ditetapkan melalui controller, menghilangkan setiap kebutuhan untuk memotong probe. Sejak probe kapasitansi harus sepenuhnya terbungkus dalam bahan isolasi, tidak harus dipotong memanjang.      Gambar. 3.16.13 - kontrol Khas menggunakan probe kapasitansi dalam sebuah feedtank - (tidak skala) Gambar. 3.16.13      Khas kontrol menggunakan probe kapasitansi dalam sebuah feedtank      (Tidak untuk skala)      Mengapung kontrol

      Ini adalah bentuk sederhana dari pengukuran tingkat. Contoh sehari-hari kontrol tingkat dengan pelampung adalah sumur dalam hajat. Bila kamar mandi sudah bersih, tingkat tetes air di waduk, mengambang mengikuti tingkat air ke bawah dan membuka katup air masuk. Akhirnya sumur menutup dan sebagai air segar berjalan di, kenaikan muka air, pelampung naik dan semakin menutup katup air masuk sampai tingkat yang diperlukan tercapai.

      Sistem yang digunakan dalam boiler uap sangat mirip. Sebuah float sudah terpasang dalam boiler. Ini mungkin dalam ruang eksternal, atau langsung dalam shell boiler. Pelampung akan bergerak naik dan turun sebagai perubahan level air dalam boiler. Tahap berikutnya adalah untuk memonitor gerakan ini dan menggunakannya untuk mengendalikan baik:          o feedpump (on / off sistem kontrol tingkat)

            atau

          o katup kontrol air umpan (sistem tingkat modulasi kontrol)

      Karena daya apung nya, mengambang mengikuti tingkat air atas dan bawah.          o          o Pada ujung batang mengapung adalah magnet, yang bergerak di dalam topi stainless steel.            Karena topi adalah stainless steel, itu (hampir) non-magnetik, dan memungkinkan garis-garis magnet untuk melewatinya.

            Dalam bentuk yang paling sederhana, gaya magnet mengoperasikan switch magnetik sebagai berikut:          o

          o saklar bawah akan beralih feedpump pada.

          o saklar atas akan beralih feedpump off.

            Namun, dalam prakteknya saklar tunggal akan sering memberikan / menonaktifkan kontrol pompa, meninggalkan saklar kedua untuk alarm.

            Pengaturan yang sama dapat digunakan untuk menyediakan alarm tingkat.

            Sebuah sistem yang lebih canggih untuk memberikan kontrol modulasi akan menggunakan kumparan melilit kuk di dalam topi. Sebagai magnet bergerak naik dan turun, induktansi dari kumparan akan mengubah, dan ini digunakan untuk memberikan sinyal analog ke controller dan kemudian ke katup air umpan tingkat kontrol.            Gambar. 3.16.15 Lampung Gambar kontrol. 3.16.15 Lampung kontrol

            Mengapung aplikasi kontrol            Secara vertikal maupun horizontal dipasang, output level sinyal biasanya melalui switch magnetis dioperasikan (merkuri jenis atau tipe 'ber-istirahat'); atau sebagai sinyal modulasi dari kumparan induktif karena pergerakan magnet melekat mengapung. Dalam kedua kasus magnet bertindak melalui tabung baja non-magnetik steel.            Gambar. 3.16.16 pengontrol tingkat Magnetik dalam Gambar ruang. 3.16.16 pengontrol tingkat Magnetik dalam ruang

            Perbedaan tekanan sel            Sel tekanan diferensial diinstal dengan kepala konstan air di satu sisi. Sisi lain diatur untuk memiliki kepala yang bervariasi dengan tingkat air boiler.

            Kapasitansi variabel, strain gauge atau teknik induktif digunakan untuk mengukur defleksi diafragma, dan dari pengukuran ini, sinyal tingkat elektronik yang dihasilkan.

            Penggunaan sel tekanan diferensial adalah umum dalam aplikasi berikut:          o          o Tinggi tekanan air-tabung boiler dimana kualitas air yang tinggi demineralised digunakan.

          o Dimana air yang sangat murni digunakan, mungkin dalam proses farmasi.

            Dalam aplikasi ini, konduktivitas air sangat rendah, dan dapat berarti bahwa konduktivitas dan kapasitansi probe tidak akan beroperasi andal.            Gambar. 3.16.17 Tingkat kontrol menggunakan sel perbedaan tekanan (tidak skala) Gambar. 3.16.17 Tingkat kontrol menggunakan sel tekanan diferensial (tidak untuk skala)

            Jenis lain dari sistem kontrol modulasi sesekali mungkin ditemui. Namun, dalam rangka memenuhi (Inggris) Kesehatan dan Keselamatan Eksekutif (HSE) atau tuntutan perusahaan asuransi, boiler paling menggunakan satu atau lain dari sistem dijelaskan di atas.http://www.spiraxsarco.com/resources/steam-engineering-tutorials/flowmetering/instrumentation.aspA steam flowmeter comprises two parts:

* The 'primary' device or pipeline unit, such as an orifice plate, located in the steam flow. * The 'secondary' device, such as a differential pressure cell, that translates any signals into a usable form.

In addition, some form of electronic processor will exist which can receive, process and display the information. This processor may also receive additional signals for pressure and/or temperature to enable density compensation calculations to be made.

Figure 4.4.1 shows a typical system.Fig. 4.4.1 - A typical orifice plate steam flowmetering station Fig. 4.4.1A typical orifice plate steam flowmetering stationArrow TopDifferential pressure cells (DP cells)

If the pipeline unit is a differential pressure measuring device, for example an orifice plate flowmeter or Pitot tube, and an electronic signal is required, the secondary device will be a Differential Pressure (DP or ΔP) cell. This will change the pressure signal to an electrical signal. This signal can then be relayed on to an electronic processor capable of accepting, storing and processing these signals, as the user requires.Fig. 4.4.2 - Simple DP cell Fig. 4.4.2Simple DP cell

A typical DP cell is an electrical capacitance device, which works by applying a differential pressure to either side of a metal diaphragm submerged in dielectric oil. The diaphragm forms one plate of a capacitor, and either side of the cell body form the stationary plates. The movement of the diaphragm produced by the differential pressure alters the separation between the plates, and alters the electrical capacitance of the cell, which in turn results in a change in the electrical output signal.

The degree of diaphragm movement is directly proportional to the pressure difference.

The output signal from the measuring cell is fed to an electronic circuit where it is amplified and rectified to a load-dependent 4-20 mA dc analogue signal. This signal can then be sent to a variety of devices to:

* Provide flowrate indication. * Be used with other data to form part of a control signal.

The sophistication of this apparatus depends upon the type of data the user wishes to collect.Advanced DP cells

The advancement of microelectronics, and the pursuit of increasingly sophisticated control systems has led to the development of more advanced differential pressure cells. In addition to the basic function of measuring differential pressure, cells can now be obtained which:

* Can indicate actual (as distinct from differential) pressure. * Have communication capability, for example HART® or Fieldbus. * Have self-monitoring or diagnostic facilities. * Have 'on-board' intelligence allowing calculations to be carried out and displayed locally. * Can accept additional inputs, such as temperature and pressure.

Data collection

Many different methods are available for gathering and processing of this data, these include:

* Dedicated computers. * Stand alone PLCs (Programmable Logic Controller systems). * Centralised DCSs (Distributed Control Systems). * SCADAs (Supervisory Control And Data Acquisition systems).

One of the easier methods for data collection, storage, and display is a dedicated computer. With the advent of the microprocessor, extremely versatile flow monitoring computers are now available.

The display and monitoring facilities provided by these can include:

* Current flowrate. * Total steam usage. * Steam temperature/pressure. * Steam usage over specified time periods. * Abnormal flowrate, pressure or temperature, and trigger remote alarms. * Compensate for density variations. * Interface with chart recorders. * Interface with energy management systems.

Some can more accurately be termed energy flowmeters since, in addition to the above variables, they can use time, steam tables, and other variables to compute and display both the power (kW or Btu/h) and heat energy usage (kJ or Btu).

In addition to the computer unit, it is sometimes beneficial to have a local readout of flowrate.Data analysis

Data collection, whether it is manual, semi-automatic or fully automatic, will eventually be used as a management tool to monitor and control energy costs. Data may need to be gathered over a period of time to give an accurate picture of the process costs and trends. Some production processes will require data on a daily basis, although the period often preferred by industrial users is the production week.

Microcomputers with software capable of handling statistical calculations and graphics are commonly used to analyse data. Once the measuring system is in place, the first objective is to determine a relationship between the process (for example tonnes of product/hour) and energy consumption (for example kg of steam/hour). The usual means of achieving this is to plot consumption (or specific consumption) against production, and to establish a correlation. However, some caution is required in interpreting the precise nature of this relationship. There are two main reasons for this:

* Secondary factors may affect energy consumption levels. * Control of primary energy use may be poor, obscuring any clear relationship.

Statistical techniques can be used to help identify the effect of multiple factors. It should be noted that care should be taken when using such methods, as it is quite easy to make a statistical relationship between two or more variables that are totally independent.

Once these factors have been identified and taken into account, the standard energy consumption can then be determined. This is the minimum energy consumption that is achievable for the current plant and operating practices.

The diagram in Figure 4.4.3 plots a typical relationship between production and consumption.

Fig. 4.4.3 - Typical relationship between production and steam consumption Fig. 4.4.3Typical relationship between production and steam consumption

Once the relationship between steam consumption and factory production has been established, it becomes the basis/standard to which all future production can be measured.

Using the standard, the managers of individual sections can then receive regular reports of their energy consumption and how this compares to the standard. The individual manager can then analyse his/her plant performance by asking:

* How does consumption compare with the standard? * Is the consumption above or below the standard, and by how much does it vary? * Are there any trends in the consumption?

If there is a variation in consumption it may be for a number of reasons, including:

o Poor control of energy consumption.

o Defective equipment, or equipment requiring maintenance.

o Seasonal variations.

To isolate the cause, it is necessary to first check past records, to determine whether the change is a trend towards increased consumption or an isolated case. In the latter case, checks should then be carried out around the plant for leaks or faulty pieces of equipment. These can then be repaired as required.

Standard consumption has to be an achievable target for plant managers, and a common approach is to use the line of best fit based on the average rather than the best performance that can be achieved (see Figure 4.4.4).Fig. 4.4.4 - Relationship between production and specific steam consumption Fig. 4.4.4Relationship between production and specific steam consumption

Once the standard has been determined, this will be the new energy consumption datum line.

This increase in energy consciousness will inevitably result in a decrease in energy costs and overall plant running costs, consequently, a more energy efficient system.Arrow TopSpecial requirements for accurate steam flow measurement

As mentioned earlier in Block 4, flowmeters measure velocity; additional values for cross sectional area (A) and density (ρ) are required to enable the mass flowrate (qm) to be calculated. For any installation, the cross sectional area will remain constant, the density (ρ) however will vary with pressure and dryness fraction.

The next two sections examine the effect of pressure and dryness fraction variation on the accuracy on steam flowmeter installations.Pressure variation

In an ideal world, the pressure in process steam lines would remain absolutely constant. Unfortunately, this is very rarely the case with varying loads, boiler pressure control dead-bands, frictional pressure losses, and process parameters all contributing to pressure variations in the steam main.

Figure 4.4.5 shows the duty cycle for a saturated steam application. Following start-up, the system pressure gradually rises to the nominal 5 bar g but due to process load demands the pressure varies throughout the day. With a non-pressure compensated flowmeter, the cumulative error can be significant.Fig. 4.4.5 - Steam usage with flowrate and pressure Fig. 4.4.5Steam usage with flowrate and pressure

Some steam flowmetering systems do not have inbuilt density compensation, and are specified to operate at a single, fixed line pressure. If the line pressure is actually constant, then this is acceptable. However, even relatively small pressure variations can affect flowmeter accuracy. It may be worth noting at this point that different types of flowmeter may be affected in different ways.Velocity flowmeters

The output signal from a vortex shedding flowmeter is a function of the velocity of flow only. It is independent of the density, pressure and temperature of the fluid that it is monitoring. Given the same flow velocity, the uncompensated output from a vortex shedding flowmeter is the same whether it is measuring 3 bar g steam, 17 bar g steam, or water.

Flow errors, therefore are a function of the error in density and may be expressed as shown in Equation 4.4.1.Equation 4.4.1 Equation 4.4.1

Where:ε = Flow error expressed as a percentage of the actual flowSpecified ρ = Density of steam at the specified steam line pressureActual ρ = Density of steam at the actual line pressureExample 4.4.1

As a basis for the following examples, determine the density (ρ) of dry saturated steam at 4.2 bar g and 5.0 bar g.Example 4.4.2

A vortex shedding steam flowmeter specified to be used at 5 bar g is used at 4.2 bar g.

Use Equation 4.4.1 and the data from Example 4.4.1 to determine the resulting error (ε).

Therefore, the uncompensated vortex flowmeter will over read by 14.42%.

As one of the characteristics of saturated steam (particularly at low pressures up to about 6 bar g) is that the density varies greatly for a small change in pressure, density compensation is essential to ensure accurate readings.

Equation 4.4.1 may be used to generate a chart showing the expected error in flow for an error in pressure, as shown in Figure 4.4.6.Fig. 4.4.6 - Vortex shedding flowmeter - % errors due to lack of density compensation Fig. 4.4.6Vortex shedding flowmeter - % errors due to lack of density compensationDifferential pressure flowmeters

The output signal from an orifice plate and cell takes the form of a differential pressure signal. The measured mass flowrate is a function of the shape and size of the hole, the square root of the differential pressure and the square root of the density of the fluid. Given the same observed differential pressure across an orifice plate, the derived mass flowrate will vary with the square root of the density.

As for vortex flowmeters, running an orifice plate flowmeter at a pressure other than the specified pressure will give rise to errors.

The percentage error may be calculated using Equation 4.4.2.Equation 4.4.2 Equation 4.4.2Example 4.4.3.

An orifice plate steam flowmeter specified to be used at 5 bar g is used at 4.2 bar g.

Use Equation 4.4.2 to determine the resulting percentage error (ε).

The positive error means the flowmeter is overreading, in this instance, for every 100 kg of steam passing through, the flowmeter registers 106.96 kg.

Equation 4.4.2 may be used to generate a chart showing the expected error in flow for an error in pressure, as shown in Figure 4.4.7.

When comparing Figure 4.4.6 with Figure 4.4.7, it can be seen that the % error due to lack of density compensation for the vortex flowmeter is approximately double the % error for the orifice plate flowmeter. Therefore, density compensation is essential if steam flow is to be measured accurately. If the steam flowmeter does not include an inbuilt density compensation feature then extra pressure and/or temperature sensors must be provided, linked back to the instrumentation system.Fig. 4.4.7 - Orifice plate flowmeter - % errors due to lack of density compensation Fig. 4.4.7Orifice plate flowmeter - % errors due to lack of density compensationArrow TopDryness fraction variation

The density of a cubic metre of wet steam is higher than that of a cubic metre of dry steam. If the quality of steam is not taken into account as the steam passes through the flowmeter, then the indicated flowrate will be lower than the actual value.

Dryness fraction (Χ) has already been discussed in Tutorial 2.2, but to reiterate; dryness fraction is an expression of the proportions of saturated steam and saturated water. For example, a kilogram of steam with a dryness fraction of 0.95, contains 0.95 kilogram of steam and 0.05 kilogram of water.Example 4.4.4

As a basis for the following examples, determine the density (ρ) of dry saturated steam at 10 bar g with dryness fractions of 1.0 and 0.95.

Important note:.The proportion of the volume occupied by the water is approximately 0.03% of that occupied by the steam. For most practical purposes the volume occupied by the water can be ignored and the density (ρ) of wet steam can be defined as shown in Equation 4.4.3.Equation 4.4.3 Equation 4.4.3

Where:

νg = Specific volume of dry steamΧ = Dryness fraction

Using Equation 4.4.3, find the density of wet steam at 10 bar g with a dryness fraction (Χ) of 0.95.

The specific volume of dry steam at 10 bar g (Χg) = 0.1773 m³/kg

This compares to 5.9363 kg/m³ when calculated as a mixture.

The effect of dryness fraction on flowmeters that measure differential pressureTo reiterate earlier comments regarding differential pressure flowmeter errors, mass flowrate (qm) will be proportional to the square root of the density (ρ) , and density is related to the dryness fraction. Changes in dryness fraction will have an effect on the flow indicated by the flowmeter.

Equation 4.4.4 can be used to determine the relationship between actual flow and indicated flow:Equation 4.4.4 Equation 4.4.4

All steam flowmeters will be calibrated to read at a pre-determined dryness fraction (Χ), the typically value is 1. Some steam flowmeters can be recalibrated to suit actual conditions.Example 4.4.5

Using the data from Example 4.4.4, determine the percentage error if the actual dryness fraction is 0.95 rather than the calibrated value of 1.0, and the steam flowmeter was indicating a flowrate of 1 kg/s.

Therefore, the negative sign indicates that the flowmeter under-reads by 2.46%.

Equation 4.4.4 is used to compile the graph shown in Figure 4.4.8.Fig. 4.4.8 - Effect of dryness fraction on differential pressure flowmeters Fig. 4.4.8Effect of dryness fraction on differential pressure flowmetersThe effect of dryness fraction on vortex flowmeters

It can be argued that dryness fraction, within sensible limitations, is of no importance because:

* Vortex flowmeters measure velocity. * The volume of water in steam with a dryness fraction of, for example, 0.95, in proportion to the steam is very small. * It is the condensation of dry steam that needs to be measured.

However, independent research has shown that the water droplets impacting the bluff body will cause errors and as vortex flowmeters tend to be used at higher velocities, erosion by the water droplets is also to be expected. Unfortunately, it is not possible to quantify these errors.Arrow TopConclusion

Accurate steam flowmetering depends on:

* Taking pressure variations into account - Pressure will vary in any steam system, and it is clearly futile to specify a flowmeter with an accuracy of ±2% if pressure variations alone can give errors of ±10%. The steam flowmetering package must include density compensation.

* Predictable dryness fraction - Measurement of dryness fraction is very complex; a much easier and better option is to install a steam separator prior to any steam flowmeter. This will ensure that the dryness fraction is always close to 1.0, irrespective of the condition of the steam supplied.

Superheated steam

With saturated steam there is a fixed relationship between steam pressure and steam temperature. Steam tables provide detailed information on this relationship. To apply density compensation on saturated steam, it is only necessary to sense either steam temperature or steam pressure to determine the density (ρ). This signal can then be fed, along with the flow signal, to the flow computer, where, assuming the computer contains a steam table algorithm, it will then do the calculations of mass flowrate.

However, superheated steam is close to being a gas and no obvious relationship exists between temperature and pressure. When measuring superheated steam flowrates, both steam pressure and steam temperature must be sensed and signalled simultaneously. The flowmeter instrumentation must also include the necessary steam table software to enable it to compute superheated steam conditions and to indicate correct values.

If a differential pressure type steam flowmeter is installed which does not have this instrumentation, a flow measurement error will always be displayed if superheat is present. Figure 4.4.9 shows the percentage errors for various degrees of superheat for flowmeters not fitted with temperature compensation.Fig. 4.4.9 - Percentage errors for over-reading various degrees of superheat for flowmeters not fitted with temperature compensation Fig. 4.4.9Percentage errors for over-reading various degrees of superheat for flowmeters not fitted with temperature compensationExample 4.4.6

Consider a steam flowmeter fitted with pressure reading equipment, but not temperature reading equipment. The flowmeter thinks it is reading saturated steam at its corresponding temperature. With superheated steam at 4 bar g and 10°C superheat passing through the flowmeter, determine the actual flowrate if the flowmeter displays a flowrate of 250 kg/h.

Equation 4.4.5 can be used to calculate the actual value from the displayed value.Equation 4.4.5 Equation 4.4.5

With steam at a line pressure of 4 bar g and 10°C superheat, the displayed value of mass flow will be 14.5% higher than the actual value.

For example, if the display shows 250 kg/h under the above conditions, then the actual flowrate is given by:

Sebuah flowmeter uap terdiri dari dua bagian:

    * The 'primer' perangkat atau pipa unit, seperti plat orifice, yang terletak di aliran uap.    * The 'sekunder' perangkat, seperti sel tekanan diferensial, yang menerjemahkan setiap sinyal menjadi bentuk yang bermanfaat.

Selain itu, beberapa bentuk prosesor elektronik akan ada yang dapat menerima, memproses dan menampilkan informasi. Prosesor ini juga dapat menerima sinyal tambahan untuk tekanan dan / atau temperatur untuk memungkinkan kepadatan perhitungan kompensasi harus dibuat.

Gambar 4.4.1 memperlihatkan sebuah sistem yang khas.Gambar. 4.4.1 - Sebuah piring lubang uap Gambar flowmetering khas stasiun. 4.4.1Sebuah pelat orifice uap khas flowmetering stasiunPanah AtasPerbedaan tekanan sel (DP sel)

Jika unit pipa adalah tekanan diferensial alat pengukur, misalnya pelat orifice flowmeter atau tabung pitot, dan sinyal elektronik yang dibutuhkan, perangkat sekunder akan menjadi Tekanan Diferensial (DP atau ΔP) sel. Hal ini akan mengubah sinyal tekanan untuk sinyal listrik. Sinyal ini kemudian dapat diteruskan ke prosesor elektronik mampu menerima, menyimpan dan memproses sinyal-sinyal ini, sebagai pengguna membutuhkan.Gambar. 4.4.2 - Wikipedia DP Gambar sel. 4.4.2Sederhana DP sel

Sebuah sel DP khas adalah perangkat kapasitansi listrik, yang bekerja dengan menerapkan tekanan diferensial untuk kedua sisi diafragma logam terendam dalam minyak dielektrik. Diafragma membentuk satu piring dari kapasitor, dan kedua sisi sel tubuh membentuk pelat stasioner. Pergerakan diafragma yang dihasilkan oleh tekanan diferensial mengubah pemisahan antara pelat, dan mengubah kapasitansi listrik dari sel, yang pada gilirannya mengakibatkan perubahan sinyal output listrik.

Tingkat gerakan diafragma berbanding lurus dengan perbedaan tekanan.

Sinyal output dari sel ukur diumpankan ke sebuah sirkuit elektronik di mana ia diperkuat dan diperbaiki untuk beban yang tergantung 4-20 mA sinyal dc analog. Sinyal ini kemudian dapat dikirim ke berbagai perangkat untuk:

    * Memberikan indikasi laju aliran.    * Digunakan dengan data lain untuk membentuk bagian dari sinyal kontrol.

Kecanggihan alat ini tergantung pada jenis data pengguna ingin kumpulkan.Lanjutan DP sel

Kemajuan microelectronics, dan mengejar sistem kontrol semakin canggih telah menyebabkan pengembangan lebih sel tekanan diferensial maju. Selain fungsi dasar untuk mengukur perbedaan tekanan, sel sekarang dapat diperoleh yang:

    * Dapat menunjukkan yang sebenarnya (yang berbeda dengan diferensial) tekanan.    * Memiliki kemampuan komunikasi, misalnya HART ® atau Fieldbus.    * Memiliki fasilitas pemantauan diri atau diagnostik.    * Memiliki kecerdasan 'on-board yang memungkinkan perhitungan yang akan dilakukan dan ditampilkan secara lokal.    * Dapat menerima input tambahan, seperti suhu dan tekanan.

Pengumpulan data

Berbagai metode yang tersedia untuk pengumpulan dan pengolahan data ini, ini termasuk:

    * Dedicated komputer.    * Berdiri sendiri PLC (sistem Programmable Logic Controller).    * Terpusat DCSs (Distributed Control Systems).

    * SCADAs (Supervisory Control And Data Acquisition sistem).

Salah satu metode yang lebih mudah untuk pengumpulan data, penyimpanan, dan layar adalah komputer khusus. Dengan munculnya mikroprosesor, komputer sangat serbaguna pemantauan aliran sekarang tersedia.

Fasilitas layar dan pemantauan yang disediakan oleh dapat termasuk:

    * Sekarang kecepatan aliran.    * Jumlah penggunaan uap.    * Uap suhu / tekanan.    * Uap penggunaan selama jangka waktu tertentu.    * Kecepatan aliran abnormal, tekanan atau suhu, dan memicu terpencil alarm.    * Kompensasi untuk variasi kepadatan.    * Interface dengan perekam grafik.    * Interface dengan sistem manajemen energi.

Beberapa dapat lebih tepat disebut flowmeters energi sejak, selain variabel di atas, mereka dapat menggunakan waktu, tabel uap, dan variabel lain untuk menghitung dan menampilkan baik daya (kW atau Btu / jam) dan penggunaan energi panas (kJ atau Btu ).

Selain unit komputer, kadang-kadang bermanfaat untuk memiliki pembacaan lokal laju aliran.Analisis data

Pengumpulan data, baik itu manual, semi otomatis atau otomatis penuh, akhirnya akan digunakan sebagai alat manajemen untuk memantau dan mengendalikan biaya energi. Data mungkin perlu dikumpulkan selama periode waktu untuk memberikan gambaran yang akurat dari biaya proses dan tren. Beberapa proses produksi akan memerlukan data setiap hari, meskipun secara sering disukai oleh pengguna industri adalah minggu produksi.

Mikrokomputer dengan perangkat lunak mampu menangani perhitungan statistik dan grafik biasanya digunakan untuk menganalisis data. Setelah sistem pengukuran di tempat, tujuan pertama adalah untuk menentukan hubungan antara proses (untuk contoh produk ton / jam) dan konsumsi energi (kg contoh uap / jam). Cara biasa untuk mencapai ini adalah untuk konsumsi plot (atau konsumsi tertentu) terhadap produksi, dan untuk membangun korelasi. Namun, hati-hati beberapa diperlukan dalam menafsirkan sifat yang tepat dari hubungan ini. Ada dua alasan utama untuk ini:

    * Faktor sekunder dapat mempengaruhi tingkat konsumsi energi.    * Kontrol penggunaan energi primer mungkin menjadi miskin, menutupi hubungan jelas.

Teknik statistik dapat digunakan untuk membantu mengidentifikasi pengaruh dari beberapa faktor. Perlu dicatat bahwa perawatan harus diambil ketika menggunakan metode tersebut, karena cukup mudah untuk membuat hubungan statistik antara dua atau lebih variabel yang benar-benar independen.

Setelah faktor-faktor ini telah diidentifikasi dan diperhitungkan, konsumsi energi standar kemudian dapat ditentukan. Ini adalah konsumsi energi minimum yang dapat dicapai untuk pabrik saat ini dan praktek operasi.

Diagram pada Gambar 4.4.3 plot hubungan khas antara produksi dan konsumsi.Gambar. 4.4.3 - hubungan khas antara produksi dan Gambar uap konsumsi. 4.4.3Khas hubungan antara produksi dan konsumsi uap

Setelah hubungan antara konsumsi uap dan produksi pabrik telah ditetapkan, menjadi standar dasar / yang semua produksi masa depan dapat diukur.

Menggunakan standar, manajer dari masing-masing bagian kemudian dapat menerima laporan rutin dari konsumsi energi mereka dan bagaimana membandingkan dengan standar. Manajer individu maka dapat menganalisis / nya kinerja pabrik nya dengan bertanya:

    * Bagaimana konsumsi dibandingkan dengan standar?    * Apakah konsumsi atas atau di bawah standar, dan oleh berapa bervariasi?    * Apakah ada kecenderungan konsumsi tersebut?

      Jika ada variasi dalam konsumsi mungkin untuk sejumlah alasan, termasuk:

          o Miskin pengendalian konsumsi energi.

          o Peralatan yang rusak, atau peralatan yang membutuhkan pemeliharaan.

          o variasi musiman.

Untuk mengisolasi penyebabnya, perlu terlebih dahulu memeriksa catatan masa lalu, untuk menentukan apakah perubahan itu kecenderungan peningkatan konsumsi atau kasus yang terisolasi. Dalam kasus terakhir, cek kemudian harus dilakukan di sekitar pabrik untuk kebocoran atau potongan rusak peralatan. Ini kemudian dapat diperbaiki sesuai kebutuhan.

Konsumsi standar harus menjadi target dicapai untuk manajer pabrik, dan pendekatan umum adalah dengan menggunakan garis paling cocok berdasarkan rata-rata bukan performa terbaik yang dapat dicapai (lihat Gambar 4.4.4).Gambar. 4.4.4 - Hubungan antara produksi dan konsumsi Gambar uap tertentu. 4.4.4Hubungan antara produksi dan konsumsi uap tertentu

Setelah standar tersebut telah ditentukan, ini akan menjadi konsumsi energi garis datum baru.

Peningkatan kesadaran energi pasti akan menghasilkan penurunan biaya energi dan biaya pabrik secara keseluruhan berjalan, akibatnya, lebih energi sistem yang efisien.Panah AtasPersyaratan khusus untuk pengukuran aliran steam akurat

Seperti disebutkan sebelumnya di Blok 4, flowmeters kecepatan ukuran; nilai tambahan untuk luas penampang (A) dan densitas (ρ) diperlukan untuk memungkinkan laju aliran massa (qm) yang akan dihitung. Untuk instalasi, luas penampang akan tetap konstan, densitas (ρ) namun akan berbeda dengan tekanan dan fraksi kekeringan.

Dua bagian berikutnya menguji pengaruh tekanan dan variasi fraksi kekeringan pada keakuratan pada instalasi flowmeter uap.Tekanan variasi

Dalam dunia yang ideal, tekanan dalam jalur steam proses akan tetap benar-benar konstan. Sayangnya, hal ini sangat jarang terjadi dengan beban yang bervariasi, boiler kontrol tekanan mati-band, kerugian gesekan tekanan, dan parameter proses semua kontribusi terhadap variasi tekanan dalam uap utama.