Friday, 14 June 2013

Pressure Measurement
  • Gauge pressure sensor
  • Absolute pressure sensor
  • Vacuum pressure sensor
  • Differential pressure sensor 
  • Sealed pressure sensor

Pressure Gauge

  • Pressure gauge works on hooks law principle ,when we applying pressure in end connection of the pressure gauge ,the same pressure operating at end of the bourdon tube.
  • Within the elastic limit of a solid material, the deformation (strain) produced by a force (stress) of any kind is proportional to the force. If the elastic limit is not exceeded, the material returns to it original shape and size after the force is removed, other it remains deformed or stretched. The force at which the material exceeds its elastic limit is called 'limit of proportionality.'
 Important Specification in Pressure Gauge Selection
  • Process Medium
  • Range
  • Operation Environment
  • Accuracy Requirements
  • Dial Size
  • Process Connection
  • Mounting Requirement

Master Equipment for Pressure Gauge

Dead Weight Testers

1 - Handpump
2 - Testing Pump
3 - Pressure Gauge to be calibrated
4 - Calibration Weight
5 - Weight Support
6 - Piston
7 - Cylinder
8 - Filling Connection
Dead weight testers are a piston-cylinder type measuring device. As primary standards, they are the most accurate instruments for the calibration of electronic or mechanical pressure measuring instruments.

They work in accordance with the basic principle that P= F/A, where the pressure (P) acts on a known area of a sealed piston (A), generating a force (F). The force of this piston is then compared with the force applied by calibrated weights. The use of high quality materials result in small uncertainties of measurement and excellent long term stability.

Dead weight testers can measure pressures of up to 10,000 bar, attaining accuracies of between 0.005% and 0.1% although most applications lie within 1 - 2500 bar. The pistons are partly made of tungsten carbide (used for its small temperature coefficient), and the cylinders must fit together with a clearance of no more than a couple of micrometers in order to create a minimum friction thus limiting the measuring error. The piston is then rotated during measurements to further minimise friction.
The testing pump (2) is connected to the instrument to be tested (3), to the actual measuring component and to the filling socket. A special hydraulic oil or gas such as compressed air or nitrogen is used as the pressure transfer medium. The measuring piston is then loaded with calibrated weights (4). The pressure is applied via an integrated pump (1) or, if an external pressure supply is available, via control valves in order to generate a pressure until the loaded measuring piston (6) rises and 'floats' on the fluid. This is the point where there is a balance between pressure and the mass load. The piston is rotated to reduce friction as far as possible. Since the piston is spinning, it exerts a pressure that can be calculated by application of a derivative of the formula P = F/A.
The accuracy of a pressure balance is characterised by the deviation span, which is the sum of the systematic error and the uncertainties of measurement.
Today's dead weight testers are highly accurate and complex and can make sophisticated physical compensations. They can also come accompanied by an intelligent calibrator unit which can register all critical ambient parameters and automatically correct them in real time making readings even more accurate

Thursday, 13 June 2013

Loop Diagram for Compressor

Instrumentation Documents - Loop Diagrams

Finally, we arrive at the loop diagram (sometimes called a loop sheet) for the compressor surge control system (loop number 42):


Here we see that the P&ID didn’t show us all the instruments in this control “loop.” Not only do we have two transmitters, a controller, and a valve; we also have two signal transducers. Transducer 42a modifies the flow transmitter’s signal before it goes into the controller, and transducer 42b converts the electronic 4 to 20 mA signal into a pneumatic 3 to 15 PSI air pressure signal. Each instrument “bubble” in a loop diagram represents an individual device, with its own terminals for connecting wires.
Note that dashed lines now represent individual copper wires instead of whole cables. Terminal blocks where these wires connect to are represented by squares with numbers in them. Cable numbers, wire colors, junction block numbers, panel identification, and even grounding points are all shown in loop diagrams. The only type of diagram at a lower level of abstraction than a loop diagram would be an electronic schematic diagram for an individual instrument, which of course would only show details pertaining to that one instrument. Thus, the loop diagram is the most detailed form of diagram for a control system as a whole, and thus it must contain all details omitted by PFDs and P&IDs alike.
To the novice it may seem excessive to include such trivia as wire colors in a loop diagram. To the experienced instrument technician who has had to work on systems lacking such documented detail, this information is highly valued. The more detail you put into a loop diagram, the easier it makes the inevitable job of maintaining that system at some later date. When a loop diagram shows you exactly what wire color to expect at exactly what point in an instrumentation system, and exactly what terminal that wire should connect to, it becomes much easier to proceed with any troubleshooting, calibration, or upgrade task.
An interesting detail seen on this loop diagram is an entry specifying “input calibration” and “output calibration” for each and every instrument in the system. This is actually a very important concept to keep in mind when troubleshooting a complex instrumentation system: every instrument has at least one input and at least one output, with some sort of mathematical relationship between the two. Diagnosing where a problem lies within a measurement or control system often reduces to testing various instruments to see if their output responses appropriately match their input conditions.
For example, one way to test the flow transmitter in this system would be to subject it to a number of different pressures within its range (specified in the diagram as 0 to 100 inches of water column differential) and seeing whether or not the current signal output by the transmitter was consistently proportional to the applied pressure (e.g. 4 mA at 0 inches pressure, 20 mA at 100 inches pressure, 12 mA at 50 inches pressure, etc.).
Given the fact that a calibration error or malfunction in any one of these instruments can cause a problem for the control system as a whole, it is nice to know there is a way to determine which instrument is to blame and which instruments are not. This general principle holds true regardless of the instrument’s type or technology. You can use the same input-versus-output test procedure to verify the proper operation of a pneumatic (3 to 15 PSI) level transmitter or an analog electronic (4 to 20 mA) flow transmitter or a digital (fieldbus) temperature transmitter alike. Each and every instrument has an input and an output, and there is always a predictable (and testable) correlation from one to the other.
Another interesting detail seen on this loop diagram is the action of each instrument. You will notice a box and arrow (pointing either up or down) next to each instrument bubble. An “up” arrow () represents a direct-acting instrument: one whose output signal increases as the input stimulus increases. A “down” arrow () represents a reverse-acting instrument: one whose output signal decreases as the input stimulus increases. All the instruments in this loop are direct-acting with the exception of the pressure differential transmitter PDT-42:

Here, the “down” arrow tells us the transmitter will output a full-range signal (20 mA) when it senses zero differential pressure, and a 0% signal (4 mA) when sensing a full 200 PSI differential. While this calibration may seem confusing and unwarranted, it serves a definite purpose in this particular control system. Since the transmitter’s current signal decreases as pressure increases, and the controller must be correspondingly configured, a decreasing current signal will be interpreted by the controller as a high differential pressure. If any wire connection fails in the 4-20 mA current loop for that transmitter, the resulting 0 mA signal will be naturally “seen” by the controller as a pressure over-range condition. This is considered dangerous in a compressor system because it predicts a condition of surge. Thus, the controller will naturally take action to prevent surge by commanding the anti-surge control valve to open, because it “thinks” the compressor is about to surge. In other words, the transmitter is intentionally calibrated to be reverse-acting such that any break in the signal wiring will naturally bring the system to its safest condition.

Wednesday, 12 June 2013

                                            Working of Differential Pressure Transmitter

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Rakesh Kumar Natarajan
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