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Apa tipe I/O module di DCS jika kita menggunakan actuator choke valve bertipe STEPPING dimana power supply-nya pneumatic? DO, AO ataukah yang lain? Dan bagaimana prinsip kerja SYSTEM ini serta fail mode positionnya (fail last position?).

Pertanyaan : (Warih Kundono – McDermott Indonesia)

Mohon informasi, apa tipe I/O module di DCS jika kita menggunakan actuator
choke valve bertipe STEPPING dimana power supply-nya pneumatic. DO, AO ataukah
yang lain. Dan bagaimana prinsip kerja SYSTEM ini serta fail mode positionnya
(fail last position?).

Tanggapan 1 : (M. Harisman – Integra Teknik)

Pulse Output (DO w/ timer) terkoneksi ke Solenoid Valve.

Karena sifat keluarannya adalah momentary dan bukan latch, maka kondisi fail
on last position akan selalu tercapai.

Tanggapan 2 : (Waskita Indrasutanta – Wifgasindo
Dinamika Instrument Engineering)

Anda bisa menggunakan function block STEP_OUTPUT_PID melalui 2 buah DO yang
dihubungkan ke OPEN Solenoid dan CLOSE Solenoid.

Kalau ‘fail safe mode’ adalah ‘hold last position’, gunakan NC (Normally Closed)
Solenoids pada kedua OPEN & CLOSE Solenoid. Kalau ‘fail safe mode’ adalah
‘fully close’, untuk ‘actuate to open’ valve, gunakan NC Solenoid pada OPEN
Solenoid dan NO (Normally Open) Solenoid pada CLOSE Solenoid. Demikian pula
sebaliknya pada ‘fully open’ fail safe mode dan juga pada ‘actuate to close’
valve. Untuk safety, perlu ditambahkan spring return ke fail safe position.

Sayang saya tidak mempunyai referensi pada aplikasi DCS, tetapi informasi untuk
FF berikut mungkin bisa membantu:

* FF Relay Output http://www.smar.com/products/fr302.asp

Untuk Function Block Step Output PID, saya copy-kan informasi dari manual dibawah
ini. Meskipun contoh dibawah menggunakan electric motor actuator, tetapi hal
yang sama bisa kita lakukan dengan menggunakan solenoid.

STEP – Step Output Pid

Overview

A Step Control Output block is used most commonly, when the final control element
has an actuator driven by an electric motor. The final control element is positioned
by rotating the motor clockwise or anticlockwise, which is accomplished by activating
a discrete signal for each direction. A control valve, for example, needs a
signal to open and another to close. If none of the signals is present, the
valve stem would stay at the same position. Fieldbus actuators and switchgears
are the transducer blocks of this block.

Schematic Description

As shown on Fig.1, electric actuators require a switchgear module to power
the electric motor and reverse it as demanded by the control loop. Most of the
electric actuators require an interlock circuit to prevent the motor to overheat,
or even burn, when the actuator reaches one of the travel limits or something
blocks the movement in any direction, increasing the torque beyond an established
limit.

These actuators are normally equipped with torque switches and limit switches
to provide this kind of protection.

The interlock circuit also establishes priorities for safety signals, manual
commands, enabling signals and local commands,i.e., commands given from the
actuator control panel or some sort of field mounted panel. Motor powering and
reversing, interlocks and protection circuits can be combined in one Motor Control
Center drawer or in a field device. In any case it will consist in a Transducer,
that will convert the Fieldbus Function Block action into physical actions.

Using a standard PID controller in cascade with a PI Step Controller.

The slave process variable is the position of the final control element, as
shown in Fig.2.

Controlling the process variable regardless of the valve position measurement.

The actuator has a Travel Time, that is the time it takes to drive the final
control element from one end limit to another. For a control valve, for example,
it is the time required to drive it from closed to completely open.

The proportional action will actuate the final control element in the required
direction during a time proportional to:

tP = [GAIN]* (ê /100)* [TRAVEL_TIME]…..(s)

If the proportional action is not enough to turn ê = 0, the Integral
Action will move the final control element at a speed of:

V = [GAIN] * ê / [RESET]……(% / s)

where the reset is the Integral time constant in seconds.

As most of the actuators work with constant and fixed speed, they can not
give a speed larger than:

Maximum speed = 100% / [TRAVEL TIME]…….(% / s)

while the smaller speeds required by the Integral action are obtained by giving
pulses of a specified duration [PULSE_DUR]. Each pulse will move the final control
element a .x % in the required direction.

while the smaller speeds required by the Integral action are obtained by giving
pulses of a specified duration [PULSE_DUR]. Each pulse will move the final control
element a .x % in the required direction.

.x % =[PULSE_DUR] * 100% / [TRAVEL_TIME]……….(%)

The pulse frequency is given by:

f = V / .x %……….(pulses / s)

The Derivative or Rate action is given by:

tD = [GAIN] * (dê/dt) * [RATE]

where Rate is the derivative time constant in seconds and dê/dt can be
calculated in several ways, including derivative gain, filtering, etc.

The PID Step Controller activates the OPEN or CLOSE signals according to the
modified deviation, ê, the PID parameters and the other parameters in
the following way:

The signal is activated during a time equivalent to:

= tP + tD

If the modified deviation is still different of zero, the Integral or Reset
action will give pulses with a duration defined by [PULSE_DUR], with a frequency
calculated by "f." t and f are dynamically modified by ê.

In order to avoid the Reset wind-up, the actuation time in one direction must
be integrated and limited. If the actuation time in one direction is larger
than the [TRAVEL_TIME], there is no use in making the respective output signal
to pulse, therefore it is recommendable to maintain it continuously activated.

The block provides a full PV and Deviation alarm support.

The meaning of possible values for OUT_D are :

OUT_D.value = 0 _ Stop

OUT_D.value = 1 _ Close

OUT_D.value = 2 _ Open

As the STEP block requires two discrete outputs, when setting the CHANNEL parameter,
indeed two outputs are allocated. The value of CHANNEL points to the CLOSE output,
and the next channel points to the OPEN output.

Supported Modes

Out-of-service, Manual, Auto, CAS and RCAS are supported.

In Out-of-service mode, the status of the output will be Out-of -service.

In Manual mode, the OUT_D can be set by the operator. The block stops output
calculation.

In Auto mode the block operates normally.

In CAS (cascade), the Setpoint is supplied by another function block through
the CAS_IN parameter.

In RCAS the block setpoint is set by a control application running on a computer,
DCS or PLC.

Status Handling

The status of OUT_Di shall reflect the worst quality of the status of any connected
input.

Dst … dst .. (maaf saya tidak copykan semua)

Kalau Pak Warih memerlukan, saya kirimkan seluruh file FF Function Blocks melalui
japri.

Tanggapan 3 : (Warih Kundono – McDermott Indonesia)

Pak Harisman dan rekan-rekan,
Apakah choke valve yang bapak maksud sudah dilengkapi dengan solenoid valve(s),
berapa jumlah solenoid valvenya (untuk menghitung berapa DO yang diperlukan)?
Bagaimana kita bisa membukanya (katakanlah buka 60%) dari DCS yang ber-DO module
sementara choke valve ini modulating type bukan on-off type?
Kalau boleh tahu, bapak ini merefer ke manufacturer choke valve yang mana ya…

Tanggapan 4 : (M.Harisman – Integra Teknik)

Pak Warih,
Produk Choke Valve yang saya refer adalah MasterFlo dari Stream-Flo Industries.

Secara internal, Choke Valve tersebut sudah menyediakan :
– DO #1 : Control of Solenoid Valve for Open step
– DO #2 : Control of Solenoid Valve for Close step
– DI # 1 : Status of fully Open (limit switch)
– DI #2 : Status of fully Close (limit switch)
– AI #1 : Position Transmitter Status (4-20 mA)

Jadi solenoid valve-nya juga sudah internal.

Untuk menuju ke posisi yang kita inginkan kita dapat kirimkan rentetan pulsa,
sambil memonitor feedback posisinya. Tentunya timing pengeluaran pulsanya harus
diatur sesuai dengan spesifikasi ChokeValve dan juga berdasar safety operasinya,
mungkin dengan tetap memantau pressure upstream dan downstream selama proses.

Secara umum, hal ini juga berlaku untuk Stepping Choke Valve dari manufacturer/produk
lainnya.


Tanggapan 5 :
(Waluya Priatna – McDermott Indonesia)

Setiap actuator yang menggunakan solenoid valve tanpa dengan posisioner itu
adalah on/off valve biasanya digunakan pada BDV, SDV dan limit switchnya adalah
sebagai indicator bahwa valve itu dalam keadaan posisi open atau close yang
mana dimonitor oleh DCS.

Kalau flow chock valve bisa dipasang solenoid valve ( sebagai shutdown system
) dan juga dipasang posisioner untuk mengatur open/close dari pada valve (25,
50, 75, 100% ini pernah saya kerjakan di NSO project). dan yang mana flow chock
valve diatur oleh flow transmitter yang mengirim signal 4-20mA ke DCS untuk
menggerakkan posisioner (bukan solenoid karena solenoid tidak bisa dikasih signal
4-20mA hanya 24VDC/120AC/220AC tergantung system ). Mohon maaf jiga keterangan
saya salah.

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