CASCADE CONTROL

Cascade control system

In some process two or more process variables effect equally on the behavior of each other where on is controlled with the control combination of another

For these kind of process the cascade control is used

Cascade control is a type of control terminology in which a process variable is

controlled by control combination of two process variables

In cascade control two process variables are controlled with a single final control element

In this control system one controller is recognized as master controller and the other as slave controller

The master controller is given a setpoint by operator and its out put becomes the setpoint of the slave controller and output of the slave controller goes to the final control element

Therefore any change of any process variable creates the change in final control element position therefore process is controlled by control combination of both process variables

Following a type of cascade control is given

(in this system temperature is controlled by controlling the flow)

Here burner temperature controller is used as master controller and fuel flow controller is used as slave controller where temperature controller’s out put becomes setpoint of the flow controller and finally out put of flow controller goes on the fuel flow control valve

Hence controlling the flow of fuel the burner’s temprature is controlled

Hart communicator's function

Hart communicator’s mostly used functions in calibration of a device

LSV= lower set value:

The minimum range value of device, it can measure

USV=upper set value:

The maximum value of device , it can measure

LRV=lower range value:

The minimum value to which device has to read it as 0%

USV=upper range value

The maximum value to which device has to read it as100%

Sensor zero trim:

To zero the sensor value (means which output is generating sensor to zero it)

D/A trim=digital to analog trim:

To zero the out put of transmitter,to generate the 0% out put at that state of the device

calibration of transmitter

Calibration of a pressure transmitter

Calibration with analog apparatus

Equipment

Pressure Transmitter range (0 to 200 psig )

Pressure source range (0 to 250 or more than)

Multi meter

24vdc power supply

Procedure

Set range of transmitter to required range (0 to 200 psig)

Connect multi meter in series of power supply to the transmitter

Make graduation of the range of transmitter as follows

(for 0 to 200 psig range)

Applied pressure	Multi meter output	%age
0 psig	4 ma	0%
50 psig	8 ma	25%
100 psig	12 ma	50%
150 psig	16 ma	75%0
200 psig	20 ma	100%

First adjust the zero (with zero screw) that multi meter must read 4ma

If multi meter is reading less then 4 ma then increase the out put by clockwise moving to the zero set screw

And if multi meter is reading more than 4 ma then decrease the out put by moving the anticlockwise to zero set screw

After adjusting the zero apply 50 psig pressure (as u made graduation) to the transmitter where multimeter must show 8ma if not then adjust it from span screw

After adjusting check

100 psig = 12ma.

150 psig = 16ma

200 psig = 20ma respectively

after adjusting all the readings check two or more time to all readings if no changes occur then your transmitter is calibrated

Calibration with HART communicator (Highway addressable remote transducer)

Equipment

Pressure Transmitter range (0 to 300 psig ) “must be Hart enabled”

Pressure source range (0 to 350 or more than)

Hart communicator (275/375)

24vdc power supply

Procedure

Connect the Hart communicator with the transmitter

Connect its terminals across the 250 Ohms resistor connected in series of power supply to the transmitter

When Hart communicator detects your device then go into its setup

Adjust the range through the option lower/upper range value

Set lower range value to zero (0) and upper range value to 300 psig

Set the zero by zero trim option (given pressure must be zero)

Apply 300 psig pressure to the transmitter and set it the 100% range value of transmitter by span trim option

After this setting check the 0 and 100% out put by connecting also multi meter in the series of the power supply

At 0 psig pressure multi meter must read 4ma

Apply the 300 psig where multi meter must read 20ma

Repeat this practice two or three times your transmitter will be calibrated when it matches its output reading match to the input

What is Fusible Loop

What is a fusible loop charge panel? A fusible loop charge panel is a pneumatic panel that monitors a fusible loop system for the detection of fire. Fusible loop system are installed on offshore platforms to detect fire. The installation of the fusible loop are in accordance with API RP 14C recommendation.

The charge panel will regulate and meter a fix amount of instrument air to the fusible loop. During a fire, the heat generated by the fire will melt one or more of the fusible plug located in or near the fire. The 'melting' of the plug will cause a release of air from fusible loop through the plug. Since the release of this regulated air exceeds the amount of regulated/metered air supply, the pressure in the loop will decrease significantly. This decrease in pressure is monitored by a pressure switch on the charge panel. The signal from the switch is then utilized to signal an alarm to the Fire and Gas System.  In addition, an additional pneumatic output is tied to the deluge valve pilot valve where it is use to open the deluge valve.

Reference: http://www.emlmanufacturing.com/Instrumentation.htm

Transmitter Basics

Definition

The major purpose of a transmitter is to measure a change in process variable from a remote location and to transmit it at required location. Transmitter is a device (electronic/pneumatic) which measure a process variable and converts it into standard, proportional signal. Standard signal can be pneumatic or electronic

Pneumatic signal is 3 to 15 psig while electronic signal is 4 to 20 ma. Transmitter has a supply of 24vdc for electronic and 20 psig for pneumatic

Principle of operation

The transmitter consist of a sensor part and a processing part. The sensor part create a primary change proportional to the process variable’s change. The processor part convert that change into a standard ouput signal. For example:

A process variable e.g pressure can be varied from 0 to 100 psig and we want to measure it and also we want to transmit it to a remote location. Then first off all range of transmitter is set to our required value (o to 100 psig). Then we will apply process variable at different values as 0%, 25%, 50%, 75%, 100%. The transmitter will generate a proportional out put as 4mA, 8 mA, 2 mA, 16mA, 20 mA or if we use pneumatic transmitter 3 psig, 6psig, 9psig, 12 psig, 15 psig respectively that out put signal can be transmitted to a remote location either it is electronic or pneumatic.

Installing Impulse Piping for Pressure Transmitters

What is Impulse Piping?
The impulse piping is the one which connects the process outputs to the transmitter.

It must convey the process pressure accurately. If for example, the gas collects in a liquid-filled impulse line, or the drain of a gas-filled impulse line becomes plugged, it will not convey the pressure accurately. Since this will cause errors in measurement output, selecting the proper piping method for the process fluid (gas, liquid or steam) is very important. We discuss below some of the most common routing principles for impulse piping.

Impulse Piping Connections for
Differential Pressure Transmitters

Differential Pressure Transmitters
In differential pressure transmitters, there is a chance that process fluid (liquid, gas or vapors) may accumulate inside the impulse piping that can cause inaccurate reading of pressures. There are three cases as depicted by the figure.

1. Liquid
If the process fluid is liquid, the transmitter should be placed lower than the taps.

2. Gas
If the process fluid is gas, the transmitter should be placed higher than the taps.

3. Steam
If the process fluid is steam, it has more chances to vaporize, so we should use condensate pot and the transmitter should be placed at lower level than the taps.

Impulse Piping Connections for
Absolute/Gauge Pressure Transmitters

Open Tank/Closed Tank

Process Piping Connections

Reference:

Yokogawa EJX Series Differential Pressure Transmitters Installation Manual

IM 01C25A01-01E

1st Edition

What is Blowcase

A blowcase is a pressure vessel that collects fluid at low pressure, then upon fillage is pressurized with high pressure gas, causing the collected fluid to dispel into a medium pressure system. The blowcase performs the same function as a pump, only it utilizes high pressure gas to displace fluid. They have been used many years for transferring fluid, and were typically installed where there was an ample supply of high pressure gas, such as waterfloods and large oil fields where gas lift was employed. In this latter case, satellite separation and testing facilities were often needed to perform well tests and separate fluid for transfer into separate collection systems. To reduce the backpressure on producing wells, and allow use of low pressure separation equipment, blowcases transferred fluid from low pressure separators into the higher pressure collection lines – without pumps or tanks. Another typical installation is compression of “wet” gathering lines, where fluid must be removed prior to compressing the gas, and re-introduced downstream of the compressor. Perhaps the most common applications at present are the use of five gallon vessels to collect compressor skid drain liquids, and glycol reconcentrator condensation for transfer to larger storage tanks.

The prevention of atmospheric storage tank vapor losses, and prevention of oxygen entry has resulted in additional blowcase applications in “wet” gathering/compression facilities. Instead of directing all separator liquids to atmospheric tanks, installing gas blankets and vapor recovery compressors, and finally pumps to remove the fluid, some operators elect to install blowcases. Tanks are sometimes installed as emergency backup. Economics favor the blowcase, since it usually costs far less than gas blankets, vapor recovery, pumps, and chemicals.

Taken from : One Petro

Startup Problem

Courtesy: controlglobal.com

Stuxnet: Siemens releases a removal patch

Stuxnet is said to be a special kind of virus that is directed towards Siemens control systems. It can entered a system via a USB and other mobile means and looks for the Siemens PLC connection with it. Once found, it modifies its logic to an uncertain extent that may cause devices to shutdown, or even may explode. Some websites claim this virus to be a myth but when i saw Siemens releasing a security/cleaning patch, i can say that there is something really like Stuxnet that exists today. You can read and download it here:

Siemens Security/Cleaning Patch for Stuxnet

If this really is a truth then this can bring a unique and a new threat revolution in the world of Industrial control systems. These type of viruses can really proved to be a disaster for the plants as modifications in the logics can cause shutdowns, inappropriate control or may be serious accidents. It is important to install proper firewalls and  antivirus programs in order to keep our control servers cleaned from such attacks.

End of Line Resistance

Older systems used to have a redundancy built into the wiring, 2 wires would go in and out of all of the devices and then return to the panel, this used to be called class "A" wiring (NFPA now refers to this as Style D or Style Z wiring).

Most current alarm systems use a resistor after or at the last device on a 2-wire circuit. (Class "B" wiring, or Style B and Style Y wiring under the newer NFPA 72.) The control panel or control device on an addressable system is constantly looking for that resistance as a "normal" condition. Conventional Smoke detectors, for example, will lower this resistance when activated to a point below the alarm threshold and place the panel in alarm. Pull stations and most heat detectors will short the circuit (in the U.S.) also creating an alarm (a short is basically zero resistance, although the wire itself offers a small amount).

If a wire connection is loose or is cut, the panel or device stops "seeing" the resistor and enters a fault or trouble condition. Normally, this is then looked into by a maintenance man or authorized service company. 

Handshaking in Serial Communication

This EIA-232 communication method allows for a simple connection of three lines -- Tx, Rx, and ground. However, for the data to be transmitted, both sides must be clocking the data at the same baud rate. Although this method is sufficient for most applications, it is limited in responding to problems such as overloaded receivers. This is where serial handshaking can help. Three of the most popular forms of handshaking with EIA-232 are software handshaking, hardware handshaking, and Xmodem.

Software Handshaking
This method uses data bytes as control characters similar to the way GPIB uses command strings. It also incorporates the simple three-line set of Tx, Rx, and ground because the control characters are sent over the transmission line like regular data. With the SetXMode function, you can enable or disable the use of two control characters, XON and XOFF. The data receiver sends these characters to pause the transmitter during communication.

The biggest drawback to this method is also the most important fact to keep in mind -- decimal 17 and 19 are no longer available for data values. This typically does not matter in ASCII transmissions because these values are noncharacter values; however, if you transmit the data via binary, it is very likely you could transmit these values as data and the transmission would fail.

Hardware Handshaking
This method uses actual hardware lines. Like the Tx and Rx lines, the RTS/CTS and DTR/DSR lines work together. One is the output and the other is the input.

The first set of lines are RTS (Request to Send) and CTS (Clear to Send). When a receiver is ready for data, it asserts the RTS line, indicating it is ready to receive data. This is read by the sender at the CTS input, indicating it is clear to send the data.

The next set of lines are DTR (Data Terminal Ready) and DSR (Data Set Ready). Engineers use these lines mainly for modem communication because they allow the serial port and the modem to communicate their status. For example, when the modem is ready for the PC to send data, it will assert the DTR line, indicating that a connection has been made across the phone line. This is read in through the DSR line, and the PC can begin to send data. The general rule of thumb is to use the DTR/DSR lines to indicate the system is ready for communication and the RTS/CTS lines for individual frames of data.


Reference: National Instruments

PID Bumpless Transfer

Bumpless Transfer in PIDs means that switiching between auto and manual modes of PID without upsetting the output value. In PLC it is done by loading the PID output register value in a spare register while in Auto mode. Once switched to Manual, the value in register is loaded in the PID output register. This way the output doesnt get disturbed.

In DCS we have to simply allow the optin of "SP Tracking PV".

HART Communicator 375 not accepting temperature transmitter

One day we experienced a strange problem. Our temperature transmitter was not giving proper values. We connected it with our HART Communicator 375 but it too was not behaving normally. What it was doing that it was getting the device disconnected again and again. The device was actually a temperature transmitter connected with an RTD.

We brought HART Communicator 275 in and test the transmitter with it. It picked the device correctly. When we explored the settings, we came to know that the transmitter was configured as of THERMOCOUPLE type. We changed the configuration to RTD type we were using and the transmitter then started behaving properly. We then tested it with HART Communicator 375 and it picked that device correctly now.

We feel that it might be some issues with the backward compatibility of 375. What do you say?

Burner Lockout Problem in Heating Step: Water Bath Heater

What is Water Bath Heater

The main application for Indirect Fired Water Bath Heaters is to heat high pressure gas prior to pressure reduction, this prevents hydrate formation that can occur because of the temperature drop due to the Joule Thomson effect. The natural gas can also be post heated to suit the operation of gas turbines.

A typical water bath heater consists of an insulated shell, removable process coil, removable fire tube, stack burner, gas train and control system. (Courtesy GASCO)

We were assigned a job regarding a water bath heater of a natural gas processing plant. It was tripping due to burner lockout problem during heating step.

The problem was found actually in the AFR (Air to Fuel Ratio) controller. It was a manual controller. It was found that its seed was actually blocked with wax. We took that seed out, cleaned it and reinstalled it and it worked perfectly since then.


Remember that we were using UV based flame detector which was tripping the heater again and again because of a very small flame. The flame was smaller because its path was not totally cleared (due to wax in the seed of AFR controller).

Control Valve Parts

Developed by: RS

Industrial Instrumentation and Control

Welcome Fellows!

In today's modern world, the top target of every plant management or plant ownership is to achieve the safest and maximum production out of their equipment, machinery and devices. In order to reach that target, one of the major steps it follows is to install good instrumentation in the plant. When we walk through a modern day plant, we find many transmitters, gauges, trolls, control valves, motors, solenoid valves etc. This is all nothing but Instrumentation.

But the fact is, that instrumentation can't do anything alone. It needs some brain that can constantly monitor the readings provided by it and takes actions accordingly to operate the final control elements like valves, motors etc. At the same time, these actions must ensure to operate these devices at optimal parameters as to ensure their health and long-life. This is where the world of control comes in.

Control System enables the instrumentation to operate in a way to give you the safest and optimum production. Nowadays, it is fully automatic. Once configured, it brings your whole plant over the screens of control room.

This blog has been developed in order to discuss about these two greatest technologies of industrial plants. We will be sharing our experiences, our ideas and our observations. We invite you to be the part of it to contribute it to be the world's best resource of learning and sharing the experiences of instrumentation and control.

Lets control it now!

Regards

SB