Tinkering Around!

Often it happens that people at home or office want to know whether a person would be on duty or off duty for planning a work or a function at  home etc. 

This software has been developed using Visual basic for applications (VBA) and runs in MS-Excel. Macros will have to be enabled for the file for this software to work.The software takes two inputs:-

(a) The date when the person has joined the shift ( to be entered in B1 cell)

(b) The date when his availability is to be ascertained. (to be entered in B4 cell)

After entering this data, click anywhere on the sheet, the result will be displayed in B7 cell and the full shift schedule starting from the date he has joined the shift to the test date period would be available in cell A10 onwards. Presently this will work for people in 14 days on/off shift.

click_here to download the file! Please note that macros have to be made enabled in order for this to work.

A fuel guage has two units, the guage in the dashboard and the unit at the tank side as shown in figure. The tank unit is like a Rheostat whose one terminal is connected to ground and the other to the terminal marked T on the Guage. As float position moves downward (tank empty), the resistance becomes low and as float moves upward (full tank), the resistance becomes high. This arrangement  can also be reverse (empty tank-high Resistance, Full tank--> Low resistance).

The battery positive is connected to terminal B on the guage and the ground to  terminal marked ground. 12V and 6V guages exist. 

Two coils at 90 degree orientation are arranged (one between T & G in left to right  and other between T & B  in up -down direction). The pointer is attached to a magnet pivoted at the centre, which rotates as magnetic field generated by coils changes. One coil tries to move the magnet to Full position while the other tries to move the magnet to Empty position. When the magnet rotates,so does the pointer.

In old cars, the B connection is through the ignition key. So, turning the key to on, only shows the fuel level

Troubleshooting:-

Assuming the guage to be type where full tank indicates high resistance, following are trouble-shooting tips

1. Guage Always Reads full.

Disconnect the tank connection. Connect the T terminal to Ground, if the guage should move to empty, indicates good guage and disconnected wire to tank side or disconnected ground wire at tank side. If connecting T terminal to ground does not make the guage go empty, then guage is faulty.

2. Guage Always reads empty

Disconnected B or G terminal at Guage.

In-house design decode & repair of PCB-295 cards

1.    Introduction
This project is about repairing a card which is called the PCB 295. PCB 295 is used in the driller console for displaying various drilling parameters. The driller console is placed at the derrick floor of the rig. 

Drillers use an instrumentation system which is called the Advanced Instrumentation System or AIS. AIS has many sub-assemblies like the driller console  which is in the derrick  floor and the DAQ (Data Acquisition Unit) which is in the AIS  bunk house. 

The DAQ  is connected to the driller console by means of a serial link. The driller console has a series of horizontal and vertical LCD displays. Each of these horizontal and vertical displays require a display controller. This display controller is called PCB 295.

 
The values which are sent by the DAQ from the AIS bunk house are conveyed via redundant serial link by means of a stream which is a packet containing (address, data) pairs. All the parameters which are to be displayed are conveyed one by one serially. Each PCB 295 card has DIP switches which can set the address of the card.

Depending on the address received in the serial link , the card accepts the packet destined for it and drops the remaining. Once the packet is decoded, it is displayed in the respective horizontal or vertical display. Drilling parameters like hook load, bit weight, strokes per minute are displayed in the respective display(s). 

The hardware design of the card has been understood and so far 18 cards have been repaired. These cards had failed over a period of time.  The cost of each such card is about  Rs 66,615/- each. Therefore, the total cost of repaired cards is about  Rs Twelve Lakhs . Moreover, these cards are already out of production from OEM and these spares will be of help to meet contingency requirement till DAQ is upgraded to the next OEM supported version. 

2. Important part list:-

U1-CD 4011 BF (NAND gates) (CD 4011B)
U2-Intel 80c31BH (microcontroller)
U3-SN74HC373N (D Latch)
U4-AS65-6264 (RAM)- available RS INR 369 each
U5- ST M27C256 EEPROM
U6-CD4049 (Hex buffer/converter)
U7-HCF4047B (monostable multivibrator used as Watchdog timer)

VR1- LP2950 ACZ-5.0  voltage regulator 3 terminal

Q1 Q2 - 2N2907 NPN transistor

3. Tools required:-

1. Oscilloscope 20 MHz or Higher

2. Multimeter & LCR meter

3. 25 W soldering Iron

4. PTH Desoldering station 

5. Assorted Tweezers, snips, precision screw driver set. 

6. Mobile phone with 5X optical zoom

4. Key Learnings:-

1. Troubleshooting microcontroller based circuit cards.

2. How  watch dog timers are used to reboot microcontrollers in case of card errors

3. Desoldering of PTH ICs using vacuum desoldering station and to remove ICs from board without damaging PCB traces

4. How two serial signals are selected by microcontroller for maintaining redundancy. 

5. How trigger circuits are designed for making a re-triggerable monostable timer circuit. 
 

Download the Article to view the design, troubleshooting and repair exercise of these cards. 

1. Problem: 

 1. HMI of Allen-Bradley PLC with model # 6181P-122TSXP was not showing display. This HMI is used in centrifuge and costs around INR 7 Lakhs.

2. Tools required:-

1. Miniature screw driver set 2. Soldering station  3. Oscilloscope 4. PC motherboard diagnostic card 5. Multi-meter 6. mobile phone camera with 10X magnification

. 

3. Troubleshooting Steps:-

3.1.Inserted a 4 digit PC motherboard diagnostic  card in the pci slot and powered up the machine, the numbers on digit display in card were changing and these numbers correspond to errors on mainboard. for ex, if the numbers stopped at 01 , implies the cpu has gone faulty or 2C implies bad memory etc. However, the number did not stop at these stages . Therefore, it was concluded that boot process was normal and problem was in LCD display. The video shows how it is done.

3.2 The LCD/LED display has two connectors coming from the mainboard , one is the 20 pin LVDS(Low Voltage Differential signalling) signal carrying the data to be displayed on LCD and other the power. The LVDS stands for low voltage differential signalling, and has three pairs differential signal and one pair of clock signals. The LVDS signals were verified with oscilloscope, and positive (up going signal in oscilloscope) and negative (down going signal in oscilloscope) was seen in the respective + and - pins in each differential pairs. Also, the voltage on these pins were tested with multi-meter to give about 1.3 V.  The clock signal yielded a steady 4 MHz clock.

3.3. Thus, the problem appears to be that of back-light in LED display. The model of the display used is G12S1-L02. The datasheet of the display was also found on net and shared here. The back-light is provided by a string of LEDs which illuminates the LCD display. The LEDs require a voltage of about 20V to illuminate. The LED controller boosts the input voltage of 10-12 V to this voltage. Click  here to see repair in progress...

3.4 The display was opened, and indeed, the back-light was not glowing. The G121S1-L02 LCD display has two PCBs one is LCD2MB pcb which gets the power from the mainboard and supplies the +12V power, EN (Enable) signal to the E88441 PCB which generates the actual timing, control and back-light voltage for driving the LCD and back-light LEDs. The PCBs are shown in figure 4 . The LED display driver in E88441 PCB was identified by using 10X magnification on mobile camera and the part number noted down. Datasheet of this IC was downloaded. 

3.5. On studying the datasheet _A8501 it was found that that pin # 28 of this IC had to be supplied with EN signal, which is a 5 v PWM (Pulse Width Modulation) signal, which can also control the brightness depending on PWM waveform width. The EN signal was being generated by the LCD2MB card in the display panel.  There is a touch point (TP) to verify the presence of this signal, which yielded no value with multi-meter, so, this signal did not reach the back-light driver and failure was traced  to faulty resistors (R94,R82 & R104) on board. This figure-5 shows the A8501 IC magnified 10X by mobile-phone camera.

3.6. The circuit in and around the LED driver IC U1 was traced with the help of multi-meter continuity testing and 10x magnification in mobile phone camera and all the required transistors and MOSFET identified and re-drawn on paper to understand the working. The circuit with some explanatory notes is given here-fig6

3.7. It appeared that the resistors which were damaged were part of a 'power saving' mechanism to give reduced supply voltage of 10.73 V instead of 12 V to the display driver IC in the absence of EN signal. This mechanism was built around transistor Q7 and Q14 and MOSFET Q15 .  These resistors are 1% tolerance SMD resistors following the EIA-96 pattern of naming. The resistors which were damaged were marked as 70D, and 10S which have the value of  536K and 12.4 ohm respectively.

3.8. These resistors could not be sourced so a workaround was found to get the EN signal directly to IC U1 by shorting a path in PCB and bypassing the 'power save' circuitry by running the driver IC at 10.73 volts as is evident by seeing the circuit diagram traced above. The repair is shown in this figure-7. The resulting voltage at LED back-light end was 20 V, which was sufficient to drive the back-light LEDs.

3.9. Assembled everything back, the display working ok. figure-8  & figure-9

4. Annexure:-

4.1  For Full HD TV, 30 pin (10 pairs) connector is used. For HDTV, 20 pin (5 pairs--> 4 data pairs & 1 clock pair , 4 data pairs give 8 bits)  connector is sufficient.  These 4 data pairs correspond to R,G,B signal and B+Hsync and V signal. For this particular 12 inch display 3 data pairs (Rin 0,1 & 2 corresponding to R,G,B) and one clock pair is used , the unused  1data pair(Rin3)(B+Hsync and V) the + is connected to ground and - is NC. Therefore, 3 data pairs contribute 6 bits    20 pin lvds cable with pinout .more number of pairs means more bits can be shown implying higher resolution. 

4.2 Youtube video about LVDS

4.3 Some of the ICs identified in mainboard (DS90C385A-> LVDS serializer, Chrontel CH7308B--> PLL transmitter, ISL6262CRZ--> 2 phase buck converter)

4.4 LCD power connector from mainboard to LCD2MB board

5. Key Learnings:
5.1. Details of backlight LED driver IC and its working
5.2. LVDS signals and 4/6/8 bit differential signalling
5.3. EiA -96 1% resistor naming convention
 

Copying data from multiple excel files and integrating into a single excel file with a few clicks

Introduction

Sometimes, there is a requirement of collating information from different excel files into a single report. For example, it can be something like  reading some entries from  a DPR prepared for each day of the month and then collating the information in a single report.  This snapshot can give an executive summary and gives a bird’s eye view of the various trends which may be visibly evident.

The figure below shows such a picture, imagine if a hundred files are there, from where the data shown in the positions is to be copy, pasted. The preparation of such reports, say, a monthly report is bit tedious with the user having to open, close, copy, paste multiple entries and which can be error prone, unless multiple level of checking is done.  

 http://myspace.ongc.co.in/files/app/file/69e85663-d020-4d20-83a3-277643d60727

Ideally, if your job involves making such ‘monthly’ reports, then, it is about time you have a system with a frontend user interface which can be used to enter data and prepare reports  and a backend database where the data is stored. However, it requires software development effort to customize and not easily available.

Therefore, the solution presented here would meet the requirement by staying within the excel platform. The solution presented is actually a ‘Macro ‘written in Microsoft Visual basic for application inside the excel environment.

Salient Features

A screen-shot of the macro is shown here which can be launched from excel. All the steps are labelled in the screen shot.

http://myspace.ongc.co.in/files/app/file/c1af28c3-7f57-4e71-a9bb-bc2ce1a8b7ed

1. Enter the drive (Ex, C, D etc) where the files are stored and then double click anywhere on the form.
2. All the folders contained in the form are listed. Double click on the folder of your interest, the corresponding path is listed in #3 and all the files are listed in #4.  However, folders like C:\users and desktop can’t be accessed. Their content can be copied to any new folder in the drive and then proceed further.
3. Select the Path in #3 and select the file in #4. Immediately you can see the full path shown in #5.
4. Enter the starting row of the excel file report in #6. The report will start appearing from this row onwards. For example, we have written 4. The rows above this row, you can put other information like heading etc.
5. In #7, enter the cell-column pairs separated by comma. For example, if you want to copy an entry from cell B5 to cell B4, from B6 to C4, from D7 to D4, from E7 to E4 and A2 to F4, the cell-column pairs would be written as: b5-b,b6-c,d7-d,e7-e,a2-f

Notice, that in the destination cell, only the column# is given (b5-b). The row number remains the same as given in step #4.

6. Click on the filename in #4 and then click #9. Repeat the process with all the files in the folder. all the information would be available in C:\test1\test.xlsx
7. Now open the directory C:\test1\test.xlsx , the complete information to be copied as per step #5 is available as shown below:-

http://myspace.ongc.co.in/files/app/file/77e1d388-9ac1-47fc-a6c2-abbe4c8ccbd2

The excel file can be downloaded here, copy some source files to a directory in the C drive and then use the application.

http://myspace.ongc.co.in/files/form/anonymous/api/library/720a78d9-3962-4d25-bc90-102c820740bc/document/f9e20874-fcad-4a76-89ff-f2844b31650e/media/anoop_project_generic_version_pub.xlsm

Technical Learning :

1. testing the presence of directory
if len(Dir("C:\test1",vbDirectory)=0 then directory does not exist

2. creating a directory
mkDir("directory_path)

3. checking if a file is there or not
filecheck=Dir("C:\test1\test.xlsx")
if filecheck <> "test.xlsx" then file not there

4. creating an excel file using code

Dim ExcelApp as Excel.Application
Dim ExcelWorkbook As Excel.Workbook
Set ExcelWorkbook = ExcelApp.Workbooks.Add
Set ExcelSheet = ExcelWorkbook.Worksheets(1)
ExcelWorkbook.SaveAs "C:\test1\test.xlsx"

ExcelWorkbook.Close savechanges:=False
ExcelApp.Quit
Set ExcelApp = Nothing
Set ExcelWorkbook = Nothing
Set ExcelSheet = Nothing

5. developed code for detecting and removing space (or any ascii char) in a string

6.developed code for extracting source and destination cells from a combination of source-destination cells seperated by comma and then routing the source cells to source array and destination cells to destination array.

A 3592E05 drive using the dual ports  is connected as shown:-

The problem encountered was that each time a read job was fired to the tapedrive from the workstation, the tape used to rewind itself after copying a few files. Upon removing the connection to the SAN switch to the tape drive, the copy job completed without any problem.

Diagnosis : RSCN (Redundant State Change Notification) propogating in the SAN was rewinding the tape drive.The problem was due to addition of tape drive to the the SAN without 'zoning'. Zoning restricts such 'broadcast' events.

Solution: Opened the SAN switch through Cisco works, no zone found between the server and tape drive. Created a zone between the server and tape drive and reconnected the cables.   Fired the copy job and everything worked as it should work.

Lesson: Don't put orphaned tape drives on the SAN.

Reuse of SGI O2

1. Introduction

4.9 DATABASE BACKUP

PostgreSQL provides a utility called pg_dump to create a full backup of the database.

# pg_dump web filename.bkp

This command backs up the web database to filename.bkp. This is all that is required to regenerate the complete database and its table structure and the data.

5. SHIFT ALLOWANCE CALCULATION SYSTEM

To put the database deployed on SGI O2 to test, we developed a small application to calculate shift allowance of the shift personnel manning the 24X7 shift in the data centre.

The application was written in classic asp and used the database setup above using ODBC to connect to it.

The shift personnel had to login to the system and mark the shift he was performing. (morning, evening or night)

At the end of a shift schedule, a complete report can be obtained by three mouse clicks for the all the operations personnel which results in saving a lot of time and effort compared to manually making an excel sheet.

Fig: Consolidated statement of shifts performed by personnel of CS(H/w)-GEOPIC

6. ICECAST STREAMING SERVER⁴

A streaming server is a server which streams music and audio content over a network. The setup below shows how streaming server works:-

The ICES player, icecast2 and Apache webserver all run on a single O2. The client on the network can connect to the webserver on port 8000 using a web-browser. This opens the icecast interface and clicking on a playlist can listen to streaming music over the network

This plays in standard media players like vlc or winamp.

Fig: ICECAST2 interface

7. Conclusion

It has been our endeavor to illustrate that old workstations can be reused for scaled down and custom applications. These applications run smoothly within the available limited computing resources. Besides, these workstations can be used very effectively for imparting training on linux and its applications to students.

FOUNDATION FIELDBUS

The real intention behind this article is to make the reader read another wonderful article given at the end. A few revision questions are given which can be attempted before and after reading the said article to gain insight into this technology.

Foundation Fieldbus is a great technology. It stands right at the top of the pyramid of evolution of process control devices achieving many check marks for features desired over the years for process control devices. Foundation fieldbus started as an organization of leading vendors of industry who wanted to take the control out of control room relying on the processing power built right into the field devices thus unburdening the DCS controllers in control room and saving their cycle time. In fact, with the right configuration, the DCS could be disconnected from the field devices and process control would go on as usual.

Devices subscribing to the standards of Fieldbus foundation were marked by a special F denoting interoperability with other fieldbus devices and controller cards of various other OEMs. The devices communicated to each other within the loop using a H1 network which used Manchester encoded signals. Ethernet was the network of choice for communication between the DCS controllers and the operator and engineering  workstations.

Before FF devices, the field transmitters, be it Pressure, Temperature, Level and Flow would send a proportional current w.r.t to the process variable being measured. The lower end of this current was 4 ma denoting zero process variable whereas 20 ma was full or SPAN of the process variable. These transmitters were connected to electronic controllers or Analog input cards of a DCS system. The PID algorithms executed inside the electronic controllers or DCS.

 The next major improvement came about by way of Profibus devices where the Analog input and Analog output block was confined to the field devices and a digital data stream was communicated to the Profibus controller situated in the control room. However, the control block like PID was still inside the controller and not in the field device.

Foundation Fieldbus took the next step in which the control blocks like PID could also be accommodated inside the field device thus unburdening the DCS controller from block processing. FF offered the following obvious advantages:-

(1) Instead of a dedicated cable for each transmitter and Final control element (like control valve) which needed to run between the device and control room in case of Analog devices, many FF devices could be connected to a single trunk control cable coming from the control room, this simplified the cabling layout.

(2) The process control algorithms could be executed in the field devices thus unburdening the DCS controller in control room.

(3) Rather than just the process value, the status of the value (whether good, bad etc) along with engineering units can be communicated.

Some disadvantages:-

(1) Commissioning requires attention to proper cabling practices and shielding from noise sources  and if proper type (A,B,C,D) cable is not used or not properly terminated, lot of intermittent issues can be faced.

(2) Requires proper device descriptor files (DD)  to be pre installed in the DCS for DCS to recognize and commission the device.

An extract from this book presents a very helpful description of FF concepts. It is written in simple and easy to understand language. The whole book is an open source project by its author and presents very useful insights.

Some questions to test understanding of FF technology, mostly pertaining to data link layer :-

1. what is FUN & NUN?
2. For macro cycle time < 1 second, how many devices are recommended within the segment and how many final control elements are recommended?
3. what is size of FF identifier or device address?
4. what does LAS stand for and its use?
5. what are scheduled and unscheduled transfers?
6. What is the function of CD & PT tokens?
7. What kind of values are necessarily transferred using the CD tokens?
8. what is backup LAS? how many LAS can be active in a segment at one time?
9. what are TD tokens?
10. What is a 'live list'? and who maintains it?
11. what is a PN token?
12. Who retains the transmission right in a segment? How is the transmission right transferred to a device?
13. Is publisher/subscriber or BNU another name for CD token?
14. Is Client/Server or QUB a type of PT token?
15. Is Source/Sink or QUU or 'Report Distribution' a type of PT token?
16. what is the recommended  % time for scheduled communications vis a vis unscheduled communications in a H1 segment?
17. What happens to a device if it does not use or return PT tokens(unscheduled comms)? after repeated attempts does it remain on live list?
18. In such a case of Q17, how can you reduce the count of scheduled transfers (CD tokens) by moving function blocks between devices to reduce CD tokens?
19. What do you understand by a VCR?
20. Is flow control applied in client/server tokens?
21. Is flow control applied in Source/Sink tokens?
22. How does Queuing take place in server(FF device)? and when and how does server dole out all the responses to queued messages?
23. What kind of values are transferred using client/server PT tokens?
24. What kind of values are transferred using Source/Sink PT tokens?
25. What are three types of FF devices in increasing order of capability?
26. All FF devices contain one Resource block. What is specified in the resource block?
27. What are the function block modes in FF?
28.What is the basic difference between manual and OOS mode? Why OOS mode is preferred during calibration?
29. what is .sym and .ffo file? which file is human readable?
30. what is a .cff file?
31. What is the approx range of  resistance of a FF cable disconnected at both ends between + and - , + & shield, - & shield, shield & ground? (This is useful for quick test on cable)
32. What is the voltage range in mV for a good FF signal?

All these questions can be answered by going through the extract of pdf file given above. I framed these questions on purpose as all of us have varying retention issues       :-) .Anytime, one can come back to the questions to see how much we are retaining about the topic.

A short note on Control Valves

Introduction

A control valve is used to control a physical parameter like level, pressure, temperature or flow to a desired value in the process to ensure quality of end product. it does so by varying the opening of a closure member like a plug over seat or a plug covering or uncovering holes in a cage. Therefore, a control valve can be on-off or throttling (Throttling means vary the flow over a range between fully open and fully closed). A simple scenario of a liquid level control is shown here. A sample picture of control valve showing its various parts is shown here. This note is made to link the various sources of information to one place for ready reference and is in no means exhaustive. However, it would provide lot of required information from a beginner's point of view, like, say, someone who is seeing a control valve for first time and instead of getting lost in exhaustive documentation up front while missing host of important topics, this would help to get a broad overview.

1. what are the main parts of a control valve?

The valve can be divided mainly into three parts (1) Actuator (2) Bonnet (3) Body.  There are other parts within these three main areas for proper functioning. A complete dis-assembly of the control valve can be seen in this file. A Bonnet Extension is used in high temperature service to minimize the impact on packing.

2. How can control valves be classified?

There are number of ways in which the valves can be classified. The apex classification is whether the valve is linear or rotary. The major difference between rotary and linear control valves is given in pg # 16 of Ref #1 below. The valve capacity or Cv is higher in rotary and provides better shutoff. However, for throttling cases, linear valves are suitable. It also discusses the situation in which rotary control valves to be used in throttling. Click here to see some classifications. (Ref #1, pg 11)

1. Failure of Actuating Energy Source

At the very basic, a valve can be classified based on failure of the actuating energy source. we can have fail close, fail open , fail in place and fail safe. Fail in place is the condition where valve stays in the last position when actuating energy source fails. Fail safe can be configured as fail open, close or fail in place using auxiliary mechanisms.

 Flow Characteristics

It can be classified based on its flow characteristics. A flow characteristic is the relation between flow rate through the valve and the valve travel as the travel is varied from 0 to 100%. Four types of characteristic are linear, equal percentage, parabolic & quick opening. which again depends on the type of 'trim' used. The choice between them depends on control valve selection and is presented in following pages.

3. Based on flow direction

Usually you have a valve with globe valve body where flow is in up direction, to move from below the port to come out from above the port, this ‘flow-up’ direction has the tendency to open the plug, so it can be called as ‘flow to open’ or FTO valve. Opposite case is ‘flow down’ direction where the incoming flow comes into the port and goes out below the port, this has the action to close the plug, so it can be called as ‘flow to close or FTC valve. Click here for a picture. Usually, one finds only Flow up or FTO valves. Flow down valves or FTC valves used where the design predicts damage due to cavitation and hence anti-cavitation trims are used. The flow direction is marked on valve body with an arrow.

4. Direct acting or Reverse Acting

It can be classified on basis of how the closure member acts in the direction of moving stem in case of linear valves. Direct acting means, the valve closes while stem is moving into the body. Reverse acting means the valve opens when the stem is moving into the body. Most of the control valves commonly found are direct acting.

5. Classification based on Guidance

Valves can be classified based on how the closure member (plug) is guided in achieving the control. There are mainly three types:-

(1) Stem Guided

(2) port (seat) guided

(3) Cage guided

Guidance in control valve roughly means which member ‘holds’ the stem in place to achieve control.

1. Stem guiding:

Therefore, a stem guided valve will have bushing in packing assembly which will hold the stem in place. If bushing is in bonnet, it is top guided valve (also called post guided valve), if bushing is present in body apart from bonnet, then it is top and bottom guided valve. Top and bottom guided valves are usually double ported valves having two ports and two plugs.

2. Port (Seat) guiding

The plug has unusual shape with wings or skirts such that it projects into the seat as shown in figure here. This implies stem can be made smaller in diameter than if the valve trim were stem guided thus minimizing sliding friction.

The seat can also be modified to alter the flow characteristic apart from guiding as shown in this figure. Notice in this figure that all the three plugs are identical and the seat ring design is changing the flow characteristic.

3. Cage guiding

The cage has ports cast or machined into it. The cage guided control valves throttle flow by covering or uncovering more or less of this port area as plug moves up and down. The cut-away figure shows the use of a cage.

From guidance point of view, the cage and plug are almost tight fit thereby holding and positioning the stem better and hence stem diameter can be reduced than if the valve were stem guided.

Another advantage is if the flow characteristics need to be changed, the same can be done by changing the port shape and size by replacing the cage instead of replacing the plug or seat. This figure shows the design of linear, equal percentage and quick opening designs.

Yet another advantage is that in case of cage guiding, the plug can be made ‘Balanced’. A balanced plug is one which has a through hole drilled from top to bottom of plug. This has the effect of balancing the process pressure on the top and bottom of the plug thereby, the actuator can be made smaller so that it does not have to be large to overcome process pressures when moving the stem. The only disadvantage of using balanced plug is the increased chance of leakage.

3. What are different types of plugs used?

(1) Balanced and Unbalanced

The same is explained above in cage guiding. Click here for a pic of balanced plug

(2) Fluted

Fluted plugs are used to control low flow rates as shown in the figure

(3)Linear, Equal % & quick opening plugs

The figure shows the three types of plugs

4. What are double ported valves and their advantage and disadvantage?

Double ported valves have two plugs and matching pair of seats to throttle fluid flow as shown in this figure. The advantage of double ported valve is that the force applied by process fluid on the stem is minimum because the force acts in the opposite direction on the two plugs as shown in figure and hence the actuator can be smaller.

Disadvantage of double port valve is more seat leakage because of double seat.

5. What is valve packing?

Valve packing is used to prevent the ingress of process fluid into the valve bonnet and at the same time allowing some external device (actuator) to move the stem. Valve packing is shown in this figure

Packing can be live loaded (with spring) or Jam packing (stationary loading) (without spring). In jam packing, the lantern ring as shown in figure above is used as a spacer between two sets of packing rings. It is made of metal and allows lubricant to enter into both sets of packing rings. As shown in the figure, the packing flange is tightened to apply compressive force on the packing follower and the packing rings. It should not be overtightned. Over tightening increases the packing friction for smooth stem movement and also create undue wear on packing leading to leakage.

 5.1 Live Loaded packing:

Live loaded packing is insertion of spring in the packing assembly so that elasticity of the spring maintains the proper packing stress as the packing material ages.  coil spring is used in place of lantern ring, else Beelleville spring washers are used, then lantern ring is present as shown in figure here.

Packing rings are made of PTFE or graphite. PTFE is used in limited temperature range but has stability against wide variety of chemicals. PTFE is not very tolerant of nuclear radiation. Graphite has far greater ability to withstand nuclear radiation but has more stem friction than Teflon. Another disadvantage is galvanic corrosion in stem and body materials due to its conduction.

Click here to listen to an actual control valve noise due to a faulty packing. The cage and stem assembly was turning due to process fluid force due to insufficient grip by the aging loose packing rings. This problem came about when flow rate was increased through valve. At low flow rate this problem was not there. Replacing the packing assembly solved the noise problem at high flow rates.

Valve packing is explained here in section 27.4, page 1961 to 1969 of Ref # 2

6. What are seat leakage classes?

Valves are meant to allow flow or completely shut off the flow. The complete shut-off however is not absolute. The shut off class therefore, indicates the figure of merit. Six classes are defined using roman numerals

7. What is Cavitation , Flashing and choked Flow?

7.1 Flashing

When fluid passes through constrictive passage of a control valve, its velocity increases and according to the equation of continuity,its pressure decreases. This reduction of pressure mostly happens in the valve trim and this minimum pressure is called vena contracta pressure and denoted by Pvc. If Pvc is less than the vapour pressure of the fluid, then the liquid boils and vapours are formed. Shown in this figure. The damage due to flashing is shown here

  The point of vena contracta is the point where flashing occurs, thus flashing can damage the trim material like the plug by scoring the surface.

One parameter which is important w.r.t flashing is the Liquid Pressure recovery factor (Fl)

Fl= SQRT ((P1-P2)/(P1-Pvc) )

7.2 Cavitation

Cavitation is the next phase of flashing. After the pressure reduction in trim, the pressure starts recovering as it passes through the trim and into the body of the valve, the vapours generated start recondensing back to liquid once the pressure is more than Pvc. This recondensation happens in the form of a jet of liquid which hits with enormous force on the valve material and can cause deep pits on the surface. While flashing damage is smooth across a surface, cavitation damage is like a pock mark or a pit on the surface. The cavitation damage can be seen in this figure 

Flashing & cavitation are explained nicely with pictures in ref # 2 (page 2069 onwards) and occur only in liquid service.

7.3 Choked Flow

Both gas and liquid service experience choked flow. Choked flow is the condition when reducing the downstream pressure does not increase the flow rate substantially as downstream  pressure is reduced. This phenomenon can be seen in this graph 

An approximate position of choked flow is when the vena contracta pressure is less than  one half the inlet pressure,then choked flow is guaranteed.

The valve equation in which the flow rate is directly proportional to square root of differential pressure does not hold in the choked condition.

Note that the choked flow is with respect to downstream pressure only. It does not mean that the flow rate of the valve can’t be increased beyond choked flow. If upstream pressure is increased, the flow rate can be increased.

In gas control valve, choking occurs when the velocity of gas approaches the velocity of sound in that gas which is  referred to as critical or sonic flow. The relation between speed of sound and choking is explained very well in page # 2080 of ref # 2.

In liquid service, choking can occur once flashing starts.

8. How are valves selected based on flow characteristics?

The common flow characteristics are Linear, Equal Percentage & Quick Opening. These flow characteristics are shown in this figure.

 When to use Linear or quick opening or equal percentage flow characteristics.

Linear: A linear flow characteristic is what it means, the percentage increments in valve stem motion cause corresponding increment to the valve flow capacity. Therefore, if stem moves up by 10%, the Valve flow capacity is 10% of rated Cv.

Equal Percentage:

In Equal %, for equal increments of stem travel, the percentage change in Cv will be same. This is explained very nicely in pg # 23 of Reference #1 below. Click here to see. As can be seen it is an exponential curve, therefore, the flow is less at initial stem travel. 50% of rated Cv is achieved at 87% stem travel.

Quick Opening:

Quick opening is the mirror image of equal percentage characteristics. The valve allows large flows at small increase in stem travel and saturates thereafter.

9. Control valve Selection and sizing  basis?

According to ISA S75.01, “Flow equations for sizing control valves”,

9.1 What is Valve Cv?

In simple terms, the valve Cv can be described  as no. Of gallons per minute of water  at 60 degree F (15.5 deg C) that can pass through a valve with an associated pressure drop of 1 psi.

The Cv of a valve obviously changes with the opening with maximum Cv at 100% opening. For any other fluid, at any other pressure drop, the CV can be calculated using the following formula and test arrangement

ISA 75.01.01-2007 defines Cv for Turbulent flow (When Reynolds number inside the valve is greater than 10,000) as a standard equation

Cv= Qv * sqrt (SG/Delta P)

Where, Qv is the flowrate in Gallons per minute (gpm) at Delta P pressure drop in psi and SG is the specific gravity of the fluid. This also assumes that the valve the full bore wrt to the pipeline.

Another unit Kv is also used which is same as Cv but units in metric system. Cv=1.16 Kv

Also, if the units of Qv is in m3/h and  DP is in bar, then Qv needs to be divided by 0.865 to get the proper Cv value

9.1.1 Standard Conditions

The arrangement shown in the figure (Ref # 6, page # 6 onwards) can be used to measure the Cv.  The test setup assumes that:-

- fluid flow is flow in turbulent condition;no cavitation and vaporisation phenomena;valve diameter equal to pipe diameter;static pressure drop measured between upstream and downstream pressure taps located as in Figure  

- straight pipe lengths upstream and downstream of the valve as per Figure; Newtonian fluid.

9.1.2 Real conditions

Even if the flow is turbulent, The Cv equation is different for compressible fluids and in-compressible fluids and when the flow is normal, critical or reaches the limit (choked flow) and when there is a pipe reducer between the valve and pipe (Piping factor Fp and Flp).   These conditions and respective Cv equations are given in page 9 of Ref #6. 

9.2  Cv for Non-Turbulent flow (Laminar flow)

Reynolds number is the ratio between mass forces and viscous forces For cases with Reynolds number is low, the fluids are more viscous and equations for turbulent flow need to be corrected by an Fr coefficient. Fr becomes a fundamental parameter to size control valves with low flow where Cv = 1 gpm

These equations are described in Pg # 12 of Ref # 6

The equations  above  are based on IEC 60534-2-1 and IEC 60534-2-3  standards

 The valve Cv should meet the following:-

1. An equal % trim should operate below 95% travel at maximum flow.
2. A linear and quick opening trim should operate below 90% travel at maximum flow.

Valve Cv was dealt in detail in section 7.

9.2 Body Size

VALVE calculations (pg 17 of basic operation of control valves,ref# 1 and  pg 123 of Ref #4,  control valve handbook of Emerson Process mngmt)

(1) valve body size, Port size & Diaphragm area 

Three terms are introduced here, ∆p(sizing) , ∆p (Throttling) and ∆p (Shutoff)

 ∆p (Throttling) is the DP across the valve during various stem position

∆p(sizing) is the Dp across the valve at full open condition  (in case of ATC-FO valve)

∆p (Shutoff) is the Dp across the valve at full close condition (in case of ATC-FO valve)

 ∆p (Throttling)= P1-P2 , where P1 is the upstream pressure and P2 is the downstream pressure

Also note, P1 is always greater than P2 . P1> P2

Sizing of valve orifice

 ∆p(sizing)= P1(min)-P2 (Max) , this is the minimum DP which can be used to design the orifice size for maximum flow rate.

Sizing of valve actuator

∆p (Shutoff)= P1(max)-P2(min) or ∆p (Shutoff)= P1(max) as P2(min)=0.

The maximum shutoff pressure for which the actuator has to force balance to keep the valve shut, and hence the area of the actuator can be calculated.

The actuator diaphragm area calculation is shown in page no. 47 of Ref # 1.

9.3 Coefficients for avoiding valve problems

9.3.1 Cavitation Index Kc

The cavitation index (Kc) of the valve helps in identifying the exact Differential pressure at which cavitation occurs. These equations are from Ref # 4, pg 15

If ∆p/(p1-Pv) <Kc , No cavitation occurs

If ∆p/(p1-Pv) > = Kc, cavitation begins

If ∆p/(p1-Pv) > = ((Kc + Fl)^2)/2 , Pitting occurs

If  ∆p/(p1-Pv) > 1, Flashing occurs

From pg # 15 of ref # 4

9.3.2 Choking

If ∆p/(p1-0.7*Pv) > = Fl^2, then choking occurs

From pg # 16 of ref # 4

10. Any Sample sizing exercise?

This document can help size a valve for a given condition. Some solved examples are provided for practical sizing of liquid and gas service. See page no 10 onwards (Emerson valve sizing calculations)

11. What are the terms found in the datasheet?

A sample datasheet according to ISA 20.5 is given here 

Some of the important terms to understand are Cv, Kv, Fp, P1 (Inlet Pressure), P2 (outlet Pressure), Dp, Pv (Vapour Pressure),Q (Flow rate) Pipeline Size in, Pipeline size out,Fl (Liquid Pressure recovery factor), Bench Range, Leakage class

Important terms found in datasheet are also described in Pg 13 of ref # 6

References

1. Basic operation and Function of Control Valves by Cashco
2. Lessons in Industrial Instrumentation by Tony.R.Kuphaldt (pg 1943 to 2095) (very good manual with lot of pictures and practical explanation)
3. Control Valve Handbook Fourth Ed, by Emerson Process Management
4. FCRI course note on control valve. (Very precise & concise note on valve selection and sizing)
5. Control Valve technical specification by Control Components Inc (This contains extracts of ISA guide “Control Valves-Practical guides for measurement & control edited by Guy Borden jr  & Paul G Friedmann. Provides the technical specification as defined by ISA and mention of relevant standards and specification sheets.)
6. Control valve sizing by Parcol Technical Bulletin 1-I (very easy to understand with lot of CFM simulation figures and data)

Relevant Standards:
(1) Control valve terminology as per ISA -S75.05
(2) ANSI/ISA S75.01 for valve sizing (harmonized with IEC standards 534-2-1(incompressible fluids) & 534-2-2(compressible fluids))
(3) ISA S20.50 Specification Forms for Process Measurement and Control Instruments, Primary Elements and Control Valves
 (4)  ISA S75.11 Inherent Flow Characteristic and Rangeability of Control Valves
(5)  ISA SP75.17 Control Valve Aerodynamic Noise Prediction
(6) ISA RP75.23 Considerations for Evaluating Control Valve Cavitation
(7) Fluid Control institute FCI 70-2 Control Valve Seat Leakage
(8) International Electro technical commission IEC 534-8-3 Control Valve Aerodynamic Noise Prediction Method
(9) Manufacturer’s Standardization Society MSS-SP-61 Pressure Testing of Steel Valves
(10)  IEC 60534-2-1, Industrial process control valves – Flow capacity – Sizing under installed conditions 
(11) IEC 60534-2-3, Industrial process control valves – Flow capacity – Test procedures
(12) IEC 60534-7, Industrial process control valves – Control valve data sheet 
(13) IEC 60534-8-2, Industrial process control valves –  Noise considerations – Laboratory measurement of noise generated by hydrodynamic flow through control valves
 

Natural gas has many components and these Gas components have to be separated individually to know their mole concentration differential 4 to 20 ma signals.  Four are used by field analyser and fitted with transient protection. Remaining four can accept signals from another analyser, say, one which can analyse moisture, sulphur etc.

1. INTRODUCTION

The DI 1-11 card along with catalytic  96HD sensor can be used as a gas detection system in a Natural Gas Plant.  Together with 96HD they form a bridge , the output of the bridge is amplified and band-limited from 0 to 1 v. with zero volts corresponding to 0% LEL and 1 V corresponding to 100% LEL. The card is already obsolete and the information provided is for an academic interest and for any troubleshooting if required.

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