Kategori
Control System Instrumentation

Trigonometry in Calculation Vessel Volume

This is one of my case studies to calculate the volume vessel of a Diesel Tank.

The purpose of the scope of work was to improve diesel fuel management as part of a fuel management improvement campaign.

The objective is to convert measurement height liquid in Diesel Tank to volume basis (in liter). The level transmitter type was DP(Differential Pressure) transmitter using hydrostatic pressure to measure the level.

According to process calculation of vessel diesel tank.

(Partial liquid volume of the tank with ellipsoidal head).

where

It was Putting this equation direct to DCS (TDC3000) one function of arccos was not available

Option I :

Using the trigonometry function found on the list of Built-in Arithmetic

This approach is manipulating the original equation using the available function on the controller PCS, unfortunately, it becomes more complicated, but the advantage it has no error

Option II:

Using the Regression Linear approach.

Option III: Totalizer

Totalizer is one feature that is available in flow computers. This totalizer has a function to accumulate the volume.

Error in option I and option II originated from % error measurement and % error calculation due to error measurement. That is the reason why it doesn’t match the sounding table

On the other hand, totalizers don’t respect the shape of the vessel. Therefore it could minimize the error resulting from measurement, However, the outlet of the vessel should be equipped with a totalizer the outlet

Thus the remaining volume vessel is Qremain=Qin-Qout.

This should be written in the PLC to retrieve the value in the control room along with the push button start and stop.

Kategori
Control System

Implementing the Basic Regulatory Control

I.Objective

The objective of this article is to correlate control systems in theory and real practice through calculation by using available data from engineering documents.

II. Background

Throwback, it was the writer’s responsibility to design, and programmed, both HMI and PLC. The objective was to control fuel gas as a supply to the generator power plants.

In the initial condition, the pressure is controlled using a pneumatic controller. The electronic controller was introduced as backup and improved pressure control.

The control valve regulates fuel gas is to maintain the minimum pressure and flow rate requirement of the power generator. The objective to control pressure downstream is to meet the requirement of power generation. If the pressure is not controlled and tends to be high it will not meet the requirement.

The control valve functions as regulator pressure (reducing pressure) while limiting the maximum flow rate. The basic controller is PID. The selector of two controllers is based on the maximum setpoint either flowrate or pressure operates as a high selector. Therefore one control valve will have 2 controllers i.e., pressure control loop (PIC) and flow control loop(FIC). The input was pressure. It is to control downstream pressure. The control valve act as pressure reducing. It is placed after the control valve between 3D-5D. The pressure loop for gas pressure can be characterized as fast and noise-free. While, the flow loop can be characterized as relatively fast, nonlinear, and often noisy.

The usual application for this type of situation is that one process variable controlling variable during normal operation. In the event of abnormal operation, however, some other process variables should be limited. The limiting controller is said to “override” the normal process controller, hence, this technique is often termed “override control”.

Control method for high selector using electric controller could be implemented directly using the program. Another scenario to limit flow rate is using a mechanical stopper. It is often used for pneumatic controllers but still applicable for an electronic controllers. However, the mechanical stopper is more feasible if it is already purchased initially. Installation mechanical stopper in the middle cycle of the valve will have to consider many things and consult with the control valve manufacturer.

The setpoint between FIC and PIC should be bumpless transfer. Bumpless transfer means that transition is based on the latest position of output.

II. BLOCK DIAGRAM

As in general PID block diagram. As seen below. It uses feedback control. to control the fluid parameter.

Since the pressure and flowrate controller doesn’t interact with each other. It could be considered as a separate controller. Therefore we could evaluate each loop with a different setting value.

The writer’s objective is to implement value from theory to actual. To create a seamless integration between theory and actual in order for control theory to work on the plant. Let us walk through all blocks.

III. PROCESS

III.1 Process Dynamic Model

III.1.2. Degrees of Freedom

The number of degrees of freedom for a system is defined as

The first step is to identify the system. All models will be approached using mathematical models to predict a system’s behavior. Many process models are based on the ideal first-order plus dead-time(FOPDT). The FOPDT is commonly used for single-input, single-output (SISO) loops.

DOF = NV- NE,

with DOF equal to the number of freedom, NV equal to the number of dependent variables, and NE equal to the number of independent equations.

NV, equal to the number of dependent variables, and NE is equal to the number of independent equations.

III.1.2.a Pressure Control
Manipulated VariablesDescription
Pressure =100 – 150Pressure (psig)
Independent Variables
rho = 1000 (independent)Density of water (kg/m3)
gravity = 9.8 (independent)Gravity (m/s2)
Differential States
Pset145 psig
III.1.2. b. Flow Control
Manipulated VariablesDescription
flowrate (Q) =38.3Volumetric Flowrate (MMSCFD)
Independent Variables
rho = 1000Density of water (kg/m3)
gravity = 9.8Gravity (m/s2)
Differential States
Qset36 MMSCFD
III.1.3 Balance Equations

Overall Material Balance

In ideal state Qin = Q out, however since we will consider loss flowrate due to friction and control valve pressure drop.

{mass out} – {mass in}+Accumulation of mass=generation

{\displaystyle {\frac {\partial \rho }{\partial t}}+\nabla \cdot \mathbf {j} =\sigma }

where

∇⋅ is divergence,

ρ is the amount of the quantity q per unit volume,

j is the flux of q,

t is time,

σ is the generation of q per unit volume per unit time. Terms that generate q (i.e., σ > 0) or remove q (i.e., σ < 0) are referred to as a “sources” and “sinks” respectively.

Pressure drop in the piping system in this system consider:

1. Loss due to friction

2. Loss pressure in control valve

Using Bernoulli’s principle and Darcy-Weinbach as pressure drop along the pipe.

For pipe illustration as follows:

Based on the Bernoulli’s principle.

No change elevation and fully developed flow through a constant area pipe. Hence the equation will be in the form

III.1.4 Loss due to friction

Resistance coefficient, abbreviated as K, a dimensionless number, is how much resistance to the flow an obstacle has. This is the opposite of the flow coefficient which is how much flow capacity an obstacle allows.

frictional pressure drop across the line, fittings, equipment, etc. in series

The constant friction coefficient for the line, fittings, equipment, etc., psi/(gpm)2

The specific gravity of the liquid

flow through the valve and line, gpm

Loss pressure drop across the valve:

a1,a2, Gf, kL, and Cv ar independent variables (NE)

The valve is 4 in valve with Cvmax = 720 is selected and the pressure around the valve is independent of flow. Consider both a valve with linear characteristics and an equal percentage valve with a rangeability parameter of 50.

In general, the resistance of the liquid level system is expressed as:

The process model for a single loop is often First Order Plus Dead Time (FOPDT). First-order could be solved in the time domain or in frequency response.

There are 3 unknown parameter

III.2 Gain (K)

The gain of any signal-processing device (think of an electronic amplifier) is the ratio of output to input or the ratio of the change of output to the change of input. Since the signal to the valve is the process input the measurement is the process output, then Gain could be contributed from 3 different areas.

There are several models that could be approached using the First Order model such as

  • Filling a Tank
  • A disk flywheel
  • Resistor and Capacitor Schematic
  • Gain(Kp)
  • Time Constant
  • Apparent Dead Time

The characteristic form of the transfer function of a first-order plant is

Where K is gain, tau is time constant

III.2.1 Process Gain

Process gain is the change in the output y induced by a unit change in the input x.

In simplification, the gain for pressure and flow control :

Pressure Transmitter range: 0-15 barg with setpoint 9.9 barg

Flow Transmitter = 0 – 50 MMSCFD with setpoint 36 MMSCFD

Gain, Output = Flow, Input : Controller output signal

Gain Process (PIC) = 100 – 0/ 15-0 = 9.5

Gain Process (FIC) = 100 – 0/50 -0=2

However, I will try to approach the piping system (pressure loss and valve) with the control valve in the first-order model plant.

Chain rule of differentiation

Equal percentage characteristic

Therefore the gain valve for pressure drop 21.75 psig and Cv=720 is 135,478. During the regulating valve will travel between 30-70 %. We will take at opening

III.2.2 Time Constant

The time constant, usually denoted by the Greek letter τ (tau), is the parameter characterizing the response to a step input of a first-order, Linear Time-Invariant (LTI) system. The time constant is the main characteristic unit of a first-order LTI system. Therefore, the process time constant is the amount of time needed for the output to reach 63.2% of the way to steady-state conditions. The process time constant affects the speed response.

If the initial condition y(0)=0 and at t = time constant, the solution is simplified to the following 0.632Kp x input

III.2.2.a Determining time constant

Determining time constant is using

  • HMI and graphic chart
  • Calculation

III.4 Time delay

III.4.1 Determining Delay Time

The time delay is expressed as a time shift in the input variable. The time delay from a single control loop is contributed from

a). Transducer

b).Controller

The filter could cause an additional time delay. The filter is used to eliminate unwanted noise and background.

  • a).Transducer
    • The sensor is pressure transmitter Rosemount 30151. In gage applications, it is important to minimize pressure fluctuations to the low side isolator. Reducing process noise. Rosemount 3051CD draft transmitters are sensitive to small pressure changes. Increasing the damping will decrease output noise, but it will further reduce response time.
    • Damping. The damping command changes the response time of the transmitter, higher values can smooth variations in output readings caused by rapid input changes. Determine the appropriate damping setting based on the necessary response time, signal stability, and other requirements of the loop dynamics with your system. The damping command uses floating-point configuration, allowing you to input any damping value within 0-60 seconds.
    • The pressure transmitter contributes a dead time of 45 ms. The dead-time function is also called the time-delay, transport-lag, translated, or time-shift function (Fig 2.3). It is defined such that an original function f(t) is “shifted” in time. One method of reducing fluctuations in atmospheric pressure is to attach the length of tubing to the reference side of the transmitter to act as a pressure buffer.
    • Output damping. At the factory, Emerson sets the output damping for the Rosemount 3051CD to .32. If the transmitter output is still noisy, increase the damping time. If you need a faster response, decrease the damping time.

b).Controller / PLC

The controller will add a two-component delay time

  • Input filter on channel analogue input
  • Scan time

Therefore the total delay time = Dead Time (Transmitter) + Input Filter (AI) + Scan Time (PLC) = 45 ms + 1s + 250 ms

Filter(AI) + Scan Time (PLC) = 1295 ms

The total loop dead time is seen as the time it takes for the process variable to start to respond in the correct direction for a change in a controller’s setpoint or manual output.

IV. SIMULATING

Data from process input and output could be plotted using python (via jupyter notebook) for this simulation. I will try to compare the model for first-order and model approach using data as calculated. The input is using step function which

Model-based on FPDOT Model-based on Process data
Kp=135
time constant = 5.0
Kp=135
time constant=5
delay time= 1.295 s

V. IMPLEMENTING CONTROL STRUCTURE

The controller could be electrical, pneumatic, or hydraulic. The electrical controller is usually represented as PLC or PID modular form.

The controller could be implemented as a pneumatic controller and electronic controller. Both have advantages and disadvantages. This variety of products depends on the availability of electrical sources.

If the process is in live condition. The control valve could start in manual mode. As informed in the previous section, the PID form is using PLC SoMachine Basic. PID Controller to implement a mixed (serial-parallel) PID correction. The integral and derivative actions are both independent and in parallel. The proportional action acts on the combined output of the integral and derivatives actions

The component PID controllers is

  • Proportional, The proportional controller directly affects the gain.
  • Integral, eliminate transient error
  • Differential, reduce error rate.

PID control form consists of

  • Parallel
  • Ideal

Classic systems identification methods to obtain the first-order model of the system as:

  • Ziegler-Nichols method;
  • Smith’s method;
  • Sundaresan’s method
  • Nishikawa’s method

However, often this method is not directly applied during the commissioning of the control valve and PLC. Most of the time it is tuned by trial.

All of the methods, in a simple way, it could measure and gather to obtain the trending on HMI and PLC database in time series.

  • Field sensors/Instrumentation Gain
  • Process Gain,One term that will be used frequently is process gain, designated Kp.
  • Final Element/Valve Gain
    • Valve gain (Kv) is the slope of the valve characteristic curve. The slope is the ratio of the change in flow to the change in travel. Thus, Kv=dQ / dY.
    • There are four factors that influence gain: valve characteristic, flow, valve size, and pressure drop.

In live process, changing the input process is difficult.

The other way more feasible is using simulation in PLC or current injection while the process is shut down. However the downside is it could not represent the real reaction of the system.

V.1 Electronic Controller, PLC (Programmable Logic Controller)

Both PLC and HMI is using the same brand manufacturer i.e. Schneider

The PLC is using PLC Schneider M221. The Analog input is 16 bit (maximum raw data is 65536) but it is limited to 16221 discretization level. The analog output is 12 bits. The maximum raw data is 4096. The disadvantage different bit Analogue Input, and Analogue Output.

The consequence is when processing the PID. There are 2 flow conversion data.

Any analog filtering that may be required (to reduce the problem of aliasing)

Interactive or Non-Interactive Controller?

It could be different between the PLC. SoMachine PID consists of AutoTuning, PID, and PWM functions.

V.1 Computational Algorithms

Two different computational algorithms are used depending on the value of the integral time constant (Ti)

if Ti=0, an incremental algorithm is used

If Ti=0, a positional algorithm is used, along with +5000 offset that is applied to the PID Output

IV.2 Pneumatic Valve Controller

The proportional band adjustment knob positions the nozzle on the flapper. Increasing (widening) the proportional band moved the nozzle to a position where less input and more feedback motion occurs, which decreases the gain of the controller.

The fuel valve is designed as a fail-to-close type, then both controllers will be set for reverse action.

The reverse action means increasing process variable (pressure or flowrate) will decrease percentage output.

A direct-acting controller is one whose output tends to increase as the measurement signal increases.

A reverse-acting controller is one whose output tends to decrease as the measurement signal increases.

However in case, the fuels valve is designed as a fail-to-open type, then both controllers will be set for direct action.

Don’t forget that the reverse or direct action in consequence both Positioner and Controller action should be same

  • Proportional-Only Controllers
  • Proportional-Plus-Reset
  • Proportional Plus Reset Plus Rate Controllers

Proportional Band: 5 yo 500% of process scale span

Reset: Adjustable from 0.01 to more than 75 minutes per repeat (from 100 to less than 0.0135 repeats per minute)

Rate: Adjustable from 0 to 20 minutes

Typical Frequency Response: 1.5 hertz and 90-degree phase shift with 3.05 m (10feet) of 6.4mm (1/4 inch) tubing and 1639 cm3 (100 cubic inches) volume

The application is for reducing pressure so if the upstream increase the valve will reduce the opening. In reverse when the upstream pressure is reduced the valve will increase the opening to the setpoint.

So with this condition, it could apply direct control

  • 4mA represent valve close
  • 20mA represent valve open

Now, we decide the direction flow to open or flow to close. It depends on the valve alignment. If you stated flow to open then the valve will be marked the direction from left to right meaning the left side will be upstream. If you stated flow to close then in the valve will be marked flow direction from right to left meaning the right-side will be upstream.

The controller function represents by PLC or pneumatic controller.

For Proportional-Only Controllers: Full output
pressure change adjustable from 2% to 100% of the
sensing element range for 0.2 to 1.0 bar (3 to 15 psig)
or 4% to 100% of the sensing element range for 0.4 to 2.0 bar (6 to 30 psig)

For Proportional-Plus-Reset Controllers: Full output
pressure change adjustable from 3% to 100% of the
sensing element range for 0.2 to 1.0 bar (3 to 15
psig), or 6% to 100% of the sensing element range
for 0.4 to 2.0 bar (6 to 30 psig)

Reset Adjustment
For Proportional-Plus-Reset Controllers: Adjustable
from 0.01 to 74 minutes per repeat (100 to 0.01
repeats per minute)

VI. Actuator and Valve

The actuator and valve is the final element to manipulate the variable process. Understanding the objective process is essential before determining the next requirement

Based on the actuator it determined fail close or fail open. This selection is coordinated with the process team during HAZOP to define the safety state position or which is more safety inherent. An additional feature could be added if the position of the control valve is the last position.

Fail open means if the pneumatic/electric to drive actuator is loss, the actuator’s spring will work to open

Failure close means if the pneumatic/electric to drive actuator is loss, the actuator’s spring will work to close.

Normally, the actuator acting mode is direct acting for fail-open valves and reverse-acting for fail closed valves. When ‘Fail-Lock’ position is selected, the control valve action in case of lock-up device failure shall be specified as well.

The type of actuator diaphragm or piston depends on the design differential pressure during the shutoff valve.

The most common use of a control valve is a globe valve. The relation between Globe valve consists of 3 different characters

Gain

Quick OpenGain increases as the valve open
Linear ValveConstant gain
Equal Percentage ValveGain decreases as the valve opens

Differential pressure across

Linear ValvedP does NOT vary with flow
Equal Percentage ValvedP varies with flow & processes with decreases in gains
Basic Operation and Function of Control Valves

V. CONCERN

Using I (Integral) to eliminate error could lead to a windup mechanism. Output saturation limits and built-in anti-windup mechanism

Selector pressure control and flow control should use signal tracking for bumpless control transfer and multiloop controls

Supply

Pressure reducing when it does not sense pressure it will fully open

IV. Reference

Kategori
Control System

PLC+HMI and DCS

It was my first project as Control System Engineer at a System Integrator Company. System Integrator is responsible for integration between vendors. Furthermore, as a control system engineer, one doesn’t have to bind or represent the manufacturer or vendor.

The fruitage of working in a system integrator company is the ability to program both PLC and HMI for various brands.

Working to revamp obsolete PLC to the latest and different products of PLC includes converting the PLC program from the ladder to a function block diagram. The PLC is located remotely offshore at gathering/ manifold platforms with a swamp vicinity. It monitors surface facilities and is able to generate production shutdown. PLC uses the Modicon quantum 651 series.

All data will be sent to the control room (onshore) via a telemetry system. DCS system deploys 2 systems i.e., TDC3000 and Experion PKS Honeywell. The obsolete PLC was connected to TDC3000. Although PLC has been replaced with the latest product the connection to TDC3000 is still maintained except for 1 RTU to be reassigned to Experion PKS based. Hereafter this specific RTU is called RTU No. 1 . For reference below is the simplified architecture (Schematic 1)

DCS reassignments for RTU No.1 are due to existing LAN interconnection being available near the manifold platforms.

 RTU No.1 will be connected to another existing RTU via an ethernet switch. The existing RTU is using controller C200 Honeywell and is connected to DCS Experion PKS via radio TDMA.

We didn’t find major issues during this RTU revamping. The only thing is to ensure the Modbus address arrangement is the same as the existing one.

Additional work occurs for RTU no.1 to create a display since the relocation of DCS assignments

In order to maintain the project pace, it was decided to copy it from a similar display, existing RTU, on Experion PKS.

After changing the entire tag name and comparing the Modbus address on the PLC. The next step was generating runtime on the Experion. I discovered many message errors during the runtime. In the beginning, I was confused either the client.

After looking at all the message errors, I realize the source problem was caused by incompatibility between HMI web display scripting and the function block diagram on Modicon Quantum PLC. The existing display is designed by a control builder integrated into Experion PKS engineering tools.

So it required modifying all HMI Web Display scripts related to animation, such as faceplate, all dynamic shapes, etc, in order to be compatible with the functionality of function block diagram Modicon Quantum PLC. Although this was not easy, at the end it was finished without the attendance Honeywell vendor.

Due to this problem, I understand between the difference between PLC+HMI and DCS.  

Besides the term system architecture refers to DCS where more than one PLC is connected to the control station. DCS could also refer to software capability. DCS provides integrated software PLC+HMI up to SCADA.

Example of software with these types is Experion PKS, Delta V, and Centum VP.

The advantages of DCS software are as follows:

  1. Reduced the effort required to design and implement HMI.
  2. It eliminates the complexity configuration between PLC and HMI.

However, it consumes more memory and processor capacity.

DCS is suitable for applications with much-advanced control, high diagnostics, etc. Therefore it is suited for complex and fixed process plants such as refineries, petrochemical, and power plant.

However, based on my experiences I found that DCS is not suited well for continuous expansion facilities such as offshore platforms.

An offshore platform continues to explore new wells which continues to add I/O and display.

I have experience with DCS with saturated memory and capacity processors on offshore platforms, although it is far from the maximum capacity of I/O or remote I/O.

Unless it is a separate system between fixed surface facilities and a group of continue facilities such as flow lines and wellhead platforms.

For example, DeltaV/Experion as the DCS while continuing expansion surface facilities use PLC.

The more you have experience the more you could decide when to use an application based on DCS or PLC + HMI.

Kategori
Control System Instrumentation

SOLVING OPEN CHANNEL FLOW METER PROBLEMS

I. Introduction

The scope of this project was to obtain tangible efficiency turbines through mass balance water input and output by the deployment of flow rate sensor from various point measurement inlet and outlet at Water Treatment Plant for Power plant (Coal). The application was for monitoring only. No control sequence or safety shutdown system involves.

Among these flowmeters, a specific case that drains our attention.  It was solving the open channel flowmeter on the outfall unit from the condenser. The water cycle operation is never expected to shutdown, either 2 x 50% or 2 x 100% operation of power generation. This situation contributes to the complexity of hydraulic structure/weir installation.

An open channel flow meter is not frequently part of the scope project not many as flow meter carried in closed conduits that flow completely the piping system. In this article, I would walk you through to capture the problem encounter during the installation hydraulic structure of open channel flow meter. In the end, it will influence the selection sensor for open channel flowmeter to enable the installation and to eliminate the possibility of failure.

Weir, as seen in the construction design, is essentially a dam built across an open channel over which the liquid flows. It simple based on the design especially when selecting rectangular weir without end constructions. I overlooked the weir thickness requirement and installation method. The installation method is so crucial in order to place the weir as a hydraulic structure.

The requirement thickness should be calculated by a structural engineer in regard to force exerted on the weir. Moreover, the installation method should review by construction engineer since the water flow could not be stopped, only in certain circumstances, the water flow will undergo low flow rate.

The other difficulties factor is we don’t have structure or installation engineer, so we fully trust the responsible for structural and installation scope to the contractor. It was not all the contractor’s mistake since we didn’t request any calculation and installation procedure document.

All the explanation below is related to the projects.

II. PLC and HMI

I was in-charge for programming, designing, and configuring both Programmable Logic Controller (PLC) and Human Machine Interface (HMI). Although there is A comprehensive guide selection of PLC is available in relevant details such as IEC 61131-1 and IEC 61131-2.

In common practice especially for PLC package in an existing plant, PLC is selected based on, as follows:

  1. Hazard and operability study (HAZOP) to identify the requirement of safety integrity level for the PLC
  2. The capability to cope the application.
  3. Project budget.
  4. Certain population PLC on the User/Company premises.
  5. Service availability PLC product on the country.

In this project, the PLC selection was toward to no.4 besides the project budget. It was agreed to use PLC Simatic S7-1200 CPU 1214C. S7-1200 is sufficient for the specified application. The main application was to monitor flowrates on different locations at demineralize water systems. There is no control and safety requirement for the shutdown process.

Nowadays, PLC developer is competing to create integrated automation software with one engineering environment and one software project for all automation tasks. Siemens has built an integrated software portal named as TIA portal. This is a Control Engineering 2012 Engineers’ Choice award winner. As a user I admit, it was really convenient and easy using Step 7 professional and RT advance. However if you are a new learner, it is recommended to read the guideline to familiar the program blocks. Selection engineering software depends on the controller model. Actually, STEP 7 Basic is sufficient. However, since the plant has various Siemens controllers into it is decided to use STEP 7 Professional.

Whether it’s the inexperienced or experienced writing the PLC program, simple ladder logic, and a step sequence is often the best solution. It was developed using ladder logic and function block diagram, Step 7 utilize grouping the ladder logic name as a network.

After deciding the PLC brand, there are several factors to be considered such as.

  • Power supply quality
  • PLC Program
  • Signal transmission
  • Conformance to Hazardous Area

II.1 Power Supply Quality

AC voltage is the main source power, vendors provide module AC to DC with various power wattage. Each power module will supply PLC rack. It will power the I/O module either through the back plane or cables. Most cases, field instrument is using dedicated AC to DC power supply. Power supply quality is important for a steady condition for the I/O module either analogue or digital module.

It should carefully evaluate when PLC power supply is the same source with an inductive load such as a motor. Power quality should be one of the considerations when utilizing 4-20mA analogue input. Power quality refers to the ability of electrical equipment to consume the energy being supplied to it. A number of power quality issues including electrical harmonics, poor power factor, voltage instability and imbalance impact on the efficiency of electrical equipment. Equipment instability and failure.

Below is a simplified interconnection diagram, one of example when power quality could interfere 4-20 mA signal and flickering digital signal. The symptoms analogue fluctuation and flickering always occur whenever the density sensor connected to the AI Module.

At first sight, we suspect it comes from inductance coupling between cable AC and analogue, digital wiring (24VDC). Although the arrangement cable inside the cabinet has been separated between AC and 24VDC. The distance is adequate to minimize inductance coupling using a cabinet with a width of 800 mm.

We also suspect from grounding issues. We have to provide proper grounding for the power supply PLC and power supply converter 220VAC to 24VDC. Since not all companies segregate between instrument grounding and power grounding only equipment with source 220VAC is connected to dispatch grounding.

In the end, we figure out that the problems are caused by power quality due to harmonics. So we decided to use AC to AC converter. It solves the problem.

II.3 PLC Program

PLC program of this project consists of:

  • Scaling complete with simulation input
  • Alarm (High only, High-high provides but not use since no emergency action applied for flowrate)
  • Totalizer

This program is a standard basis for a flowmeter. The scaling and alarm function block could be combined (as seen in figure 1). The function block is created by user from the basic function block.

Not all user function block requires developing from scratch. Siemens has provided additional add-on libraries available on their website. One of the examples is Totalizer.

Gambar ini memiliki atribut alt kosong; nama filenya adalah ft-007.png

II.3.1 Memory Capacity

The size of memory capacity depends on several factors even the style of programming could affect memory capacity. Deciding memory capacity is vital when the plant is continuously extended or expanded.

The other factor of memory capacity as following:

  • The number of input-output, number of variables, constant, and contribute to consume memory IO.
  • System architecture with other devices.
  • The individual style programming and programming language.
  • The Complexity of the application. Scaling, mapping, and alarm are typical or standard program. One of the examples complexity programs could be is represented in the control loop method. Feedforward, Close loop, 2 degrees of freedom control etc.

II.3.2 Scaling 4-20mA to Engineering Unit

Raw value as a representation of 4-20mA could have a different minimum or maximum value among the modules. The value to be determined based on the function purpose. Below are different full-scale ranges among the module.

The engineering unit scale between PLC and transmitters should be the same. Thus, a linearity relation between 4-20 mA and the engineering unit is created. 4 mA as lower range value and 20 mA as upper range value.

However, you need to decide the unit and scale to be sent from the transmitter.

In the example of a Magnetic flow transmitter, the information conveyed in 4-20mA could be presented in either velocity or flowrate. If you choose a velocity, in consequence, the flowrate equation should be created in PLC program.,

If the engineering unit scale between PLC and flow transmitter is different the linearity between the devices will be different. In addition, errors contribute to non-linearity. There is four error that contributes

  • Zero Error
  • Span Error
  • Linearity Error
  • And the combination above

In case this happens, the display in the transmitter and HMI will be different.

II.3.3 Totalizer

Totalizer is used to determine the accumulative flowrate during a certain period. For example, “in the last 60 seconds, there have been 30 gallons of water flowing by the sensor.”

To obtain consistency accumulation during certain periodes, Totalizer should be considered as an interrupt. Thus regardless of the complexity of the program. It would give high priority to maintain a consistency time period.

One you should aware of when implementing Totalizer in the PLC program. The raw value should be limited for instance 0 to 27648. Over and under this value, it should be identified as an error. The error should not consider as a value in totalizer. It should hold totalizer or give input value 0 on the totalizer function block. Over and under range, the value could be used to identify short or loose cable wiring.

II.4 Signal Transmission

Selecting signal transmission to convey the information Besides conventional 4-20mA, the other option is to use HART. HART protocol offers a number of advantage however there are drawback should be considered.

Much technical literature is available to explain HART protocol in detail. The compatibility between HART and 4-20 mA wiring. Field transmitter complete feature HART enables to ease of calibration system.

Several advantage when using HART Protocol as following :

  • Not necessary to provide scaling analogue function block
  • Totalizer is not requiring defining the PLC program. Totalizer is available on certain field instruments. PLC will send a command to start and stop the Totalizer.
  • Besides the primary variable, the other parameter could be obtained. Other parameter known as a secondary  variable, tertiary variable This variable could retrieve by three classes of HART commands:
    1. Universal commands
    2. Common Practice commands
    3. Device-Specific commands
  • If there were concerns about hazardous areas, HART protocol is claimed to be more intrinsically safe.

Therefore,   it is depicted that HART has an advantage over conventional 4-20mA. There are several things to be considered in order to use HART

  • Do they possess Asset Management
  • SystemCPU (processing unit) and AI cards should be compatible. For Siemens PLC, HART is using ET200. This series is far compared to the S7-1200 series
  • Version to be check
  • Selection of cabling system – Peer to peer or multi-drop.
  • Installing or enabling resistor 250 ohm. Scaling is not required even you could retrieve 3 data parameter simultaneously named as
  1. PV; Primary Variable
  2. SV; Secondary Variable
  3. TV; Tertier Variable

II.5 Hazardous Area

Mostly, PLC is located in a safe area i.e. technical room, meanwhile, the field transmitter is located in a hazardous area. Hazardous area certification for field transmitters should be inline with the wiring. When using conventional 4-20mA, the wiring in the hazardous area needs to be check. Mostly, when the PLC and the field transmitter have different hazardous areas.

III. HMI

Human Machine Interface is a user interface or dashboard that connects a person to a machine, system, or device. It is stand-alone and independent from the existing DCS system.

The selection of HMI is based on the architecture of the application. The architecture of HMI/SCADA will determine the license to be purchased. There is more detail, for selection HMIa . Below are several overviews

  • Independent/Stand Alone. The license either development or runtime license.
  • SCADA Based, If SCADA is selected next it to decide the sharing data., DDE Server, OPC or combination
  • SCADA based definitely require development license
  • Web client

This project is using Wincc RT Advance

There is an option based on feature and scale-able of

  • Basic
  • Comfort
  • Advanced
  • Professional

The reason to deploy Wincc RT Advance it has minimum features to covers the flowmeter application. In general, the HMI consist of

  • Representation displays such as P&ID. Symbol and coloring mostly based on the industries. The company could also produce specification and regulation for symbol and coloring
  • Alarm management, it will be guided through ISA. Alarm animation is important for operator to identify situation.
  • Inhibit
  • Logging, (logging of process values and alarms). Please be aware logging Options for WinCC runtime Advance
  • Trending, Individual trending each flow transmitter

III.1. General Alarm

General Alarm has different tone for specifics events. Indeed, it is different between Company. To notify all attendant in the plant. Each area is equipped with annunciation.

This project didn’t integrate with existing General Alarm

III.2 Alarm

In industries where process hazard analysis results in minimum SIL 2. It requires 2 systems separated. Process Control System and Safety shutdown system.

When these systems exist. Grouping occurrence of alarm based on the system. PCS covers high and low alarm while SSS generate alarm High-high and Low-low. High-high and low-low alarm have consequence action to the process closing the valve or stopping the pump.

As stated, this application only for monitoring there is an alarm rule that could be waived. No intervention by maintenance personnel is required. High-high and a low-low alarm is not mandatory thus reset alarm through acknowledge is not mandatory either. The alarm logging is available.

The other feature does not require are

  • Maintenance inhibit
  • Start-up Inhibit

III.2.2 Alarm Animation WinCC

Alarm animation is important for an operator to know the condition of the plant. Usually, each company has different specifications for color and shape to animate the process plant.

It could be similar to the same type of business or industries.

I found that the way to represent alarm animation assign to a variable on WinCC advance is kindly different from other software HMI. Usually, the shape contains 2 tags or 1 structure tag.

Usually, I use structure to group alarm for one tag i.e.

  • PT001.HH with set point 90% of the maximum range
  • PT00.HH with set point 70% of the maximum range
  • PT001.L with set point 30% of the maximum range
  • PT001.LL with set point 10% of the maximum range

The other way is to generate dedicated variable i.e

  • PAH001
  • PAL001

The alarm is discrete 1 or 0; True or False. Meanwhile RT Advance uses integer value to generate different level alarms. The way to generate an alarm on RT advance is kindly different from what I use on other HMI. RT Advance use integer value to generate different level alarms

Faceplate

A faceplate is a configured group of displays and operator objects. The “data exchange” with the display and operator objects contained in the faceplate is performed via an interface at the created faceplate.
The properties of the used display and operator objects are assigned to the individual objects in the “faceplate editor”. Faceplates can be managed and modified in a central library.

This project use faceplate as seen below. No control applied on the faceplate. No universal standard for a faceplate. It is custom depends on the application. In this project, the faceplate consists of

  • Individual trend flowmeters
  • Individual log measurement
  • Individual totalizer

IV. FLOW TRANSMITTER

All flowmeter is connected to the new dedicated PLC and HMI. It is stand-alone and independent from the existing DCS system.

In a standard basis, selecting flowmeters usually based on the

  • Accuracy requirement (for fiscal service or not)
  • Type of fluid and other fluid parameters
  • Installation constraint. Equipment that can be reached for inspection, repair, or monitoring from permanent platforms is more likely to be inspected, calibrated, and replaced than equipment that requires climbing with a safety harness or scaffold.

There were 3 types of flowmeters used in this project

  • Vortex flowmeters, operate under the vortex shedding principle, where an oscillating vortexes occur when a fluid such as water flow past a bluff (as opposed to streamlined) body.  Vortex mostly uses in application of steam.
  • Magnetic flowmeters, this type of flowmeter is very common for water service due to the dominant conductivity parameter.
  • The ultrasonic flowmeter as a secondary device. The ultrasonic level meter is not only used for the level meter but it could use as a parameter to determine flowrate of open channel water. The equation will be embedded in the PLC program. For this application, it implements for the open channel consist of culverts and open channel.

Each flowmeter has a different method to obtain the flowmeter value. Each will have a different equation.

In the case of open channel flowmeter, there are several methods. The primary device for ultrasonic flowmeters are:

  • Weirs
  • Flumes such as Parshall flumes, Cutthroat Flume

IV.1 Rectangular Weir

In the first design, we use rectangular weir for the simplicity design

Flow rate equation based weir method, Q = 6620(L-02.H)H1.5,

Where :

Q = flowrate

H = head on the weir

L = crest length of weir

K = constant dependent upon units

IV.2 Cutthroat flume

There is a similar method as Parshall flume named as a cutthroat flume. In this project, the cutthroat flume is applied to measure flowrate at sewerage as a primary device.

W=KW1.025Han = CHan

Q=free flow rate(cfs / m3/s)

K=flume discharge constant (varies by flume length/units)

C=flume discharge constant (varies by flume length/throat width / units)

W=throat width

Ha = depth at the point of measurement (feet / meters)

n = discharge exponent (depends upon throat width)

The constant flowrate equation Parshall flume is determined by the selection range of flowrate and the dimension of open channel. More detail is available in the link below

https://www.openchannelflow.com/flumes/cutthroat-flumes

There are two ways to obtain flowrate using ultrasonic. In reality ultrasonic only measures the height. The flowrate equation could be configured either in the ultrasonic transmitter or PLC.

Ultrasonic level to produce flowmeter value

IV.1.1 Selection of 4 wire or 2 wire transmitter

The selection 4 wire or 2 wire transmitter is the intention of the user and evaluation actual condition. These transmitters are called “4-wire” or self-powered. The current signal from the transmitter connects to the process variable input terminals of the controller to complete the loop.

Mostly the reason to use 4 wire as follows:

  • PLC is not available as acquisition data. The transmitter should have displayed on the transmitter so measurement is readable
  • Three and four-wire devices incorporate an external power supply in order to effectively eliminate the voltage drop placed on the processed signal current loop
  • Some device is not available as 2-wire. Some sensor requires external power to drive the chemical/mechanical event in the transducer.

V. INSTALLATION

The initial design was proposed using Parshall flume, the idea was to avoid high force when the maximum flow rate through the primary device. However, the member team decided to use weir for the simplicity of the design. It was understandably the main concern of the project team, Parshall flume is not easy to construct. In addition, there were no local vendors able to precast the Parshall flume.

We decide to use Weir to measure the water meter outflow unit. As a reference, I use Flow Measurement Practical Guides for Measurement and Control Handbook published by ISA.

Weir are the simplest, least expensive, and probably most common type of the primary measuring device used to measure flow in open channels.

After sizing the weir, I don’t pay attention to the installation procedure. There are two rectangular channels. The water flow never stops flowing.  The only possible scheme is to place it down during the low water flow. Since it is using concrete, I thought with this weight it will easily dip into the water. Moreover, construction and installation are responsible to the contractor. If the contractor confirms they could install it then it became their responsibility. As a team, we really don’t concern about this until the installation start.

The installation method for the primary device for open water is really critical. It is even more important than sizing. If the installation seems impossible then quickly turn to use other methods.

High water flow is a detriment to the weir or Parshall flume structure.

Since we don’t have a data flowrate outlet, the sensing range flowrate only through observation. I was not sure the water flows were in low or normal flowrate, but it is convincing that it would not disrupt the process when laying down the weir structure using a crane.

It is not as expected, when the weir structure dipped into the water as it water tip the bottom weir structure, the hoist cable start swing. The degree hoist cable swing becomes bigger when the weir dipped deeper to water until the crane start to lift from the ground. The installations stops and not continue.

Basically there are two option solution from my knowledge

  1. To find a way for a better method of installation. However, there still a risk if the installation fails again.  Moreover, the remaining project budget reveals that there is no room for failing.
  2. Change the flowrate measurement method with an admissible installation scheme.

After gathering information, the most reasonable is option 2. It automatically changes the equation to

Q = A x v

A = surface area

V = velocities

A = length x width where length is similar to height (H). Height(H) will be determined using the ultrasonic sensor. The width of channel is 2 mtr.

Now to determine the method velocity.

The first option is using the non-contact method

Non-Contact Sensor

The velocity could be measured using ultrasonic or laser. Laser velocimeter has a laser diode to determine the stream’s level by emitting an ultrasonic pulse and measuring the time it takes for the an echo to return for the stream’s surface. The transducer is both a pulse transmitter and an echo receiver. Certain manufacturers already provide integrated between velocity sensor and level sensor. Since the ultrasonic is measuring the level. The remaining sensor is the velocity

It could be installed on the surface. Since the flow much turbulence it is and a lot of ripple on the surface. We consult to the vendor, their recommendation is to

Contact Sensor

The Second method contact by immersing the sensor

The other could be installed inside the water. The sensor is installed at the bottom of the channel.

Not many vendors provide this sensor. I search on the internet, that moment I only find Mainstream Measurement. It sells variety product of sensor in immersed water.

The velocity sensor is designed to operate immersed in the liquid and is installed at the bottom of the channel where the measurements are to be taken. Below are photos during the installation of velocity sensor immersed in water.

We finally complete the project. Thank you for all the team who participate in this project especially my colleague construction engineer (Mr.Chris) who design the structure to hold the immersed velocity sensor. It was critical to complete this project.

VI. REFERENCE

[1]. Spitzer, D.W., ed. 1991. Flow Measurement.

[2]. https://www.openchannelflow.com/flumes/cutthroat-flumes

[3]. https://mainstream-measurements.com/

Kategori
Control System

Wi-Fi+Ethernet Remote IO and Wireless System(HART / ISA100)

I would like is to share experience and knowledge through this article. This was based on my experience. In this article, I wouldn’t explain the basic concept of wireless. There are a lot of good sources if you want to learn wireless transmitters. 

The authors will not be responsible for any loss or damage arising out of or resulting from a defect, error or omission in this document or from any users use or reliance on this document.

This article does not attempt to detail minor differences between WirelessHART and ISA100Wireless devices; nor features specific to the vendor; nor provides exhaustive studies of integration with various host systems.

Anyway, the manufacturer’s instructions on the use of any commercial product must be followed at all times, even if in conflict with the information in this publication.

The objective is to offer another system which has similar to wirelessHART/ISA100wireless. It can be considered as another option for a wireless system. It is a combination between wire and wireless but eliminating the high-price part .e.g multipair cable, variation cable tray size.

The reason I say it similar is based on several factors :

  • Same standard technology in antenna.
  • It is possible to use Mesh topology although different standards as wirelessHART / ISA100Wireless.
  • It doesn’t require a Programmable Logic Controller (PLC) to manage and retrieve the data. It could directly retrieve by Human Machine Interface(HMI).

Wireless networks offer highly flexibility installation for existing building and offshore platform structures. Henceforth, wireless significantly reduces installation costs. In certain occasion, installation of sensor/transmitter will encounter difficult to hook-up which require a lot of scaffolding, no possibility to lay cable tray and cable due to road crossing or separate location by road/sea, etc.

During my early career as Instrument and Control System Design engineer, I always design with the most sophisticated and latest product. Thus, sometimes I overlooked at potential problem on maintenance and spare part.

Now, as I have more experience and more involve with different departments, I’m able to look broader views and long term problems.

In example, one of the long-term problems of wireless is providing spare batteries for wireless transmitters. In country which import batteries from the origin manufacture country, mostly there are 2 main problems

  1. Price, some country will not allow to purchase directly from the manufacturer. It has to purchase from the official distributor on that country. The problem emerges if there is only one sole agent. Price often determines by the sole agent not the market.
  2. Delivery time, usually it takes at least 10-12 weeks.

In addition to resolve the delivery time, battery is purchase in advance, but issue of self-discharge and aging should be considered.

Let’s start with the limitation scope of design

  • Area coverage hundreds meter, WLAN (wireless local area network)
  • Quantity IO around 10-15 Analogue Input
  • Remote location meaning there are no or minimum habitat
  • Only monitoring without supervisory control thus update rate is not essential.

The system architecture is as following

Figure 1. (Without PLC)

The equipment list as following:

  1. Conventional Transmitter, 4-20mA, e.g. Endress Hauser, Yokogawa, Rosemount
  2. IO logik E1200 Series , Moxa
  3. Transmitter, Power Beam antenna, Ubiquiti
  4. Receiver, Omni antenna, Ubiquiti
  5. Industrial Ethernet Switch, unmanageable
  6. Desktop PC with Reliance as HMI application

Actually, we have try with 2 ea power beam antenna and 1 ea omni antenna. Even the PLC as client as well, we don’t find any latency issue.

Another additional feature is to use Programmable Logic Controller (PLC). The system architecture could be as following.

Figure 2. (With PLC)

Basically, PLC is not mandatory if the application is for monitoring only, unless there is something to control.

If PLC will be utilized, there are two option of logical network

Figure 3. Option 1

At the first option, PLC as repository data then HMI (Operator Station) will retrieve all data from PLC.

Figure 4. Option 2

At the second option, there two client simultaneously, PLC and HMI are requesting to Ethernet remote IO.

I use the second option since the remote IO logic E1200 is able to send data in real (floating type). Both HMI and PLC retrieve data in floating type 32-bit. PLC is installed for future development.

All IP is configured based on Private IP Addresses Class C

The first option (figure 1) mostly will be benefit if

  1. The protocol between PLC and HMI and PLC to field device is different, e.g., PLC and HMI is Ethernet I/P while PLC and remote IO Logik is modbus TCP
  2. Redundant HMI
  3. More than 1 client control station

I. IO Logik E1200 Series

The IoLogik E1200 series is Industrial Ethernet remote I/O. It has two embedded Ethernet switch ports that allow information to flow to another local Ethernet device or connect to the next IOLogik in a daisy-chain.  More detail could be find in the link

https://www.moxa.com/en/products/industrial-edge-connectivity/controllers-and-ios/universal-controllers-and-i-os/iologik-e1200-series

It has 8 AI channel. AI input range has 2 mode. Voltage mode (V) and Current Mode (mA).

Scaling from 4-20mA to engineering unit could be enabled on the device.

If the auto scaling is enabled you don’t need to obtain the raw data. Result of scaling in real type data (Floating point) available from the modbus address. Data format is

II. Antenna, Transmitter and Receiver

The Wi-Fi antenna has capability to support modbus TCP protocol. As this application is using modusTCP,  You will need to change the web server mode, server port from 70 to 502 to open the modbus TCP port. The other setting is left to default. Both Transmitter and Receiver have the same configuration.

Radio Transmitter
Radio Receiver

For information, there are server port list could be configured based on well-known and registered port numbers for particular practice as shown table below.


I haven’t yet trial for Ethernet/IP protocol.

III. HMI

Since it using ModbusTCP protocol, arrangement order of double word should be aware when requesting a known floating-point value from the slave. Modbus itself does not define a floating point data type but it is widely accepted that it implements 32-bit floating point data using the IEEE-754 standard. However, the IEEE standard has no clear cut definition of byte order of the data payload. Therefore the most important consideration when dealing with 32-bit data is that data is addressed in the proper order. I’m using reliance product HMI to display the correct value on HMI, it need to swap byte and swap word. You could find more detail on regulation of IEEE modbus floating 32bit data.

IV. Comparison

IV. 1 Cost

The scale of project is one of the key factors for cost justification of wireless into a project. Design Engineers should assess and incorporate the following factors in their project cost estimating calculation model:

  • Reduced engineering costs

Wi-Fi + Ethernet remote I/O could not fully eliminated engineering cost for :

    • Tray and cable routing
    • Detail support cable tray
    • Cable schedule
    • Remote IOLink wiring diagram

Meanwhile wireless system could be eliminated pertaining cable drawings and documents. In the other hand, other engineering document should be submitted such as

  1. Identify the necessary update rate of each WirelessHART/ISA100Wireless device to meet the specifications of the application as well as battery life.
  2. Determine the capacity of the gateway determined by the maximum update rate to be used in the network
  3. Use the following calculation to determine the number of gateways.
  • Reduced labor

Both Wi-Fi+Ethernet remote IO and wireless system is significant to reduce labor cost.

  • Reduced materials and material cost

It is possible that the main materials of Wi-Fi + Ethernet remote IO could be selected with a much lower cost than a wireless system.  Although Wi-Fi + Ethernet remote IO still needs to utilize cable trays and cable but it has reduced significant material such as

      • Multi-pair instrument cable is not necessary.
      • More uniform of cable tray size
      • Cable gland with uniform size

One of advantage using Wi-Fi+Etherent remote IO is the flexibility to select the product. Wide range product of conventional 4-20mA transmitter as well as wide range product of Industrial Ethernet remote I/O / Gateways. If you have tight budget you could consider this option. Henceforth, with carefully design it is able to obtain overall cost lower than Wireless system.

Below are typical of material list for Wi-Fi +Ethernet Remote IO + Transmitters (This is only example for comparison)

Below are typical of material WirelessHART (This is only example for comparison )

Below are typical of material ISA100Wireless (This is only example for comparison )

IV. 2 Antenna

Wi-Fi antenna and Wireless transmitter antenna basically have the same method as following

  • Frequency band 2.4-2.5 GHz Instrument Scientific Medical (ISM) band
  • Standard modulation 802.11g

Note: Wi-Fi antenna is covering all transmitters while wireless transmitter antenna is dedicated. Therefore you should assess the location of transmitter in order to obtain the acceptable gain and able to select the antenna.

  • WirelessHART/WirelessISA100 use omni directional antenna to support Mesh topological.

The selected Power beam and Omni antenna use standard modulation 802.11g

IV. 3 Topology

The main difference is the ISO protocol stack at Level 1 and 2 which address by standards IEEE 802.15.4 for ZigBee, ISA100Wireless, WirelessHART, WIA-PA, and several others. Each of these standards has added its own upper-layer protocols to suit applications in many different markets. 

WirelessHART/Wireless ISA use the newest technology and most revolutionary form of network is called a mesh. In a mesh network each station is both an end device and a network forwarding element. Mesh networks are naturally self-healing and redundant—exactly the properties
needed for industrial automation networks.

IV. 4 Security

ISA100 Wireless and WirelessHART posses built-in security. This will be differ with Wi-Fi where You need to specify how level safety of the system. At minimum, firewall need to be installed on the Host (operator work station). 

However, based on my assessment for this system. In this installation, the location was remote, by means the location is far from the resident. It will be obvious and suspicious when unrecognized person around that area.

The process plant are surrounded by fence with radius define using method of restricted area.

The system is still localized, not connected to any other company network (business network). Hence this system is sufficient.

V. References and Bibliography

Dick Caro, “Wireless Networks for Industrial Automation”, 2014

Emerson Process Management, “IEC 62591 WirelessHART System Engineering Guide, Revision 3.0”,2012