The Engineering documents for Instrument engineers

Basics documents for Engineering stages

1. Process flow diagram
2. Piping and Instrumentation drawing
3. Loop drawing
4. GA drawing
5. Cable and JB schedule
6. Instrument location drawings
7. Suppliers data sheet
8. Earthing drawings
9. Instrument cable tray layout drawing
10. Hook up drawing
11. JB location drawings
12. UPS drawings and data sheets
13. Air tubing layout drawings
14. Boolean logic for control of process

Vacuum Pressure measurement

McLeod Vacuum Gauge

Basic Principle of McLeod Vacuum Gauge:

A known volume gas is compressed to a smaller volume whose final value provides an indication of the applied pressure. The gas used must obey Boyle’s law given by;

P1V1=P2V2

Where, P1 = Pressure of gas at initial condition (applied pressure).

P2 = Pressure of gas at final condition.

V1 = Volume of gas at initial Condition.

V2 = Volume of gas at final Condition.

Initial Condition == Before Compression.

Final Condition == After Compression.

A known volume gas (with low pressure) is compressed to a smaller volume (with high pressure), and using the resulting volume and pressure, the initial pressure can be calculated. This is the principle behind the McLeod gauge operation.

Description of McLeod Vacuum Gauge:

The main parts of McLeod gauge are as follows:

A reference column with reference capillary tube. The reference capillary tube has a point called zero reference point. This reference column is connected to a bulb and measuring capillary and the place of connection of the bulb with reference column is called as cut off point. (It is called the cut off point, since if the mercury level is raised above this point, it will cut off the entry of the applied pressure to the bulb and measuring capillary. Below the reference column and the bulb, there is a mercury reservoir operated by a piston.

Operation of McLeod Vacuum gauge:

The McLeod gauge is operated as follows:

The pressure to be measured (P1) is applied to the top of the reference column of the McLeod Gauge as shown in diagram. The mercury level in the gauge is raised by operating the piston to fill the volume as shown by the dark shade in the diagram. When this is the case (condition – 1), the applied pressure fills the bulb and the capillary.

Now again the piston is operated so that the mercury level in the gauge increases.

When the mercury level reaches the cutoff point, a known volume of gas (V1) is trapped in the bulb and measuring capillary tube. The mercury level is further raised by operating the piston so the trapped gas in the bulb and measuring capillary tube are compressed. This is done until the mercury level reaches the “Zero reference Point” marked on the reference capillary (condition – 2). In this condition, the volume of the gas in the measuring capillary tube is read directly by a scale besides it. That is, the difference in height ‘H’ of the measuring capillary and the reference capillary becomes a measure of the volume (V2) and pressure (P2) of the trapped gas.

Now as V1,V2 and P2 are known, the applied pressure P1 can be calculated using Boyle’s Law given by;

P1V1 = P2V2

Let the volume of the bulb from the cutoff point upto the beginning of the measuring capillary tube = V

Let area of cross – section of the measuring capillary tube = a

Let height of measuring capillary tube = hc.

Therefore,

Initial Volume of gas entrapped in the bulb plus measuring capillary tube = V1 = V+ahc.

When the mercury has been forced upwards to reach the zero reference point in the reference capillary, the final volume of the gas = V2 +ah.

Where, h = height of the compressed gas in the measuring capillary tube

P1 = Applied pressure of the gas unknown.

P2 = Pressure of gas at final condition, that is, after compression

= P1+h

We have, P1V1 = P2V2 (Boyle’s Law)

Therefore, P1V1= (P1+h)ah

P1V1 = P1ah + ah^2

P1V1-P1ah = ah^2

P1 = ah^2/(V1-ah)

Since ah is very small when compared to V1, it can be neglected.

Therefore, P1 = ah^2/V1

Thus the applied pressure is calculated using the McLeod Gauge.

Applications

The McLeod Gauge is used to measure vacuum pressure.

Advantages of the McLeod Gauge:

• It is independent of the gas composition.
• It serves as a reference standard to calibrate other low pressure gauges.
• A linear relationship exists between the applied pressure and h
• There is no need to apply corrections to the McLeod Gauge readings.

Limitations of McLeod Gauge:

• The gas whose pressure is to be measured should obey the Boyle’s law
• Moisture traps must be provided to avoid any considerable vapor into the gauge.
• It measure only on a sampling basis.
• It cannot give a continuous output

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Notes

Morning Friends,

Here I am sharing notes on core subjects of Instrumentation Engineering

https://drive.google.com/open?id=0B5dFAJhjH2wQV3dNcEJGTzdSM0k

Questions and Answers

Hello friends,

Here I am sharing some questions related to sensors and measurement which can be asked in interviews if you opt for core industries like
1) Oil and Gas
2) refining and manufacturing
3) pharmaceutical
4) chemical manufacturing etc...

These are the very basic and most important questions.. do not miss to read.
1) can control valve be mounted with actuator in horizontal position??

yes control valve can work in any orientation however the preferred installation is actuator is in vertical position. The installation of actuator is in horizontal or vertical position is manufacture specified. 
here actuator means mechanism which control the position of control valve stem like fully open, 50% open, 25% open etc. you will get know about control valve and actuator in your curriculum subjects. 

2) What are different types of orifice plates? State their uses.

 Different orifice plates are: 1. Concentric 2. Segmental 3. Eccentric

 Concentric:

    These plates are used for ideal liquid as well as gases and steam service. Concentric holes are        present in these plates, that's why it is known as concentric orifice.

Segmental:

   This plate has hole in the form of segment of the circle. This plate is used for colloidal and sherry flow measurement. this is also used for highly dense (highly viscous) fluid.

Eccentric:

This plate has the eccentric holes. This plate is used in viscous and sherry flow measurement.

3) How do you identify an orifice in the pipeline?
An orifice tab is welded on the orifice plate which extends out of the line giving an

indication of the orifice plate.

4) How can a D.P. transmitter be calibrated?

    D.P. transmitter can be calibrated using following steps:

1. Adjust zero of transmitters.

2. Perform static pressure test: Give equal pressure on both sides of transmitter. Zero should not shift either side. If the zero shifts then carry out static alignment.

3. Perform vacuum test: Apply equal vacuum to both the sides. Zero should not shift.

4. Calibration procedure: Give 20 psi air supply to the transmitter and vent L.P. side to atmosphere. Connect output of the instrument to the standard test gauge. Adjust zero. Apply required pressure to the high pressure side and adjust the span. Adjust zero gain if necessary.

5.What is the use of valve positioner?
Valve positioner can be used for following reasons:
a. Quick action
b. Valve hysterisis
c. Viscous liquids
d. Split range.
e. Line pressure changes on valve
f. Bench set not standard

g. Reverse valve operations

6. how liquid level can be measured using differential pressure transmitter??
here video description for this question..

I will post more questions soon,,,, enjoy reading😊

Digital Control

Hello Friends !!

Here I have shared a link to download notes on Digital Control and Logic design.

share with your friends and enjoy reading....

https://drive.google.com/open?id=0B5dFAJhjH2wQdmtXM0RsaG5VaWhwUzZjbWczRjVhQ1pqNlpR

Level Measurement

Types of level measurement

There are two methods used to measure Level

1. Direct method
2. Indirect method

Direct method:-

direct level measurement is simple, almost straightforward and economical; it comprises direct measurement of height from reference line and used some local level indication. it is not easily adopted to signal transmission techniques for remote indication or control.

some of direct level measurement techniques are

1. Dip stick method
2. sight glass
3. chain or float gauge

Indirect method:-

Indirect method of level measurement depends upon the material having physical property  which can be measured and relate to the Level. Many physical and Electrical properties used for this purpose and are best suitable for remote transmission. this type of level measurement employs latest technology in its measurement.

some of indirect level measurement techniques are

1. capacitance level measurement - for continuous level measurement

2. Hydro static principle for level measurement or differential pressure level measurement (point level detection)

3.Ultrasonic and radar level techniques (continuous level detection)

4. zero elevation and suppression (normally used when mounting position for sensor is unknown)

5. Vibrating fork type level measurement.( point level detection)

Here I shared link to download a book for Process Control subject. this book helpful in your study.

https://drive.google.com/open?id=0B8_qPz-3Uw8OYXJrZUZ6dUhPZ0k

https://drive.google.com/open?id=0B8_qPz-3Uw8OcHdWTkJLTG5ocVk


Enjoy reading..........