Instrumentation Questions Part One

Introduction

Pressure and differential pressure gauges are essential instruments used in several industries like oil and gas, petrochemical, And pharmaceuticals.

These gauges measure the pressure and differential pressure of various process fluids and help in monitoring and controlling the system.

To ensure accurate and reliable measurements, it is crucial to calibrate these gauges regularly.

What is Calibration?

Calibration is the process of comparing the readings of an instrument against a known reference standard and adjusting it if needed. This ensures that the instrument is giving accurate readings and is suitable for use in critical processes.

Safety Documents

One of the most critical factors in calibrating pressure and differential pressure gauges is safety. Any mis-calibration can lead to wrong measurements, leading to process deviations, or even hazardous situations.Therefore, proper safety protocols must be followed during the calibration process, the documents as follows:

• Standard Operating Procedure (SOP).
• Maintenance Permits To Work (PTW).
• Safety Certificates Like: Safety Clearance, Confined Space, Vehicle Entry, Working at Height..etc (If Required).
• MSDS-Material Safety Data Sheet (If Required).
• Proper Personal Protective Equipment (PPE).

Test Equipment's

Test equipment is the most crucial aspect of calibration. It is vital to calibrate pressure and differential pressure gauges against a known reference standard, The equipment's are:

• Dead-weight tester and Standard pressure gauge.
• Pressure pump hydraulic and standard pressure gauge.
• Air pressure pump with digital multi-function calibrator used for very low and low pressure (0 to 20000 mm H2O or 0 to 2 Bar or 0 to 20 Bars).
• Water or mercury manometer with an air pump (0 to 1500 mm H2O or 0 to 1500 mmHg).
• Vacuum pump with digital multi-function tester.(0 to -760 mmHg / 0 to -30 inches of Hg or  0 to -1 Bar).
• Calibration Software is used to analyze the recorded data and generate calibration certificates.
• Data Logger is used to capture pressure readings during the calibration process.

Maintenance of Pressure and Differential Pressure Gauges

Here are the procedures that need to be followed during the maintenance of these gauges:

1. The first step in maintaining pressure and differential pressure gauges is to have a clear understanding of the types of gauges being used in your system. This includes understanding the different types of designs, sizes, and pressure ranges. Different gauges have different maintenance requirements, and it is essential to know the specific needs of each gauge to ensure proper maintenance.
2. Isolate the gauge by closing the inlet valve of manifolds or isolate it from the main isolating valve of the flow line.
3. In case of oil or chemical process, use a secondary container to avoid spill of oil/chemical.  Depressurize the gauge by drain tapping lines to remove the gauge.
4. Disconnect the pressure gauge from the process carefully, Take Care.. Safety First.
5. Clean the Pressure gauge housing by  a brush and spray to remove contamination or solid particles.
6. Inspect the all parts of Pressure Gauge for external physical damage, general appearance & fitness.
7. Write all the details of the pressure gauge: tag No., range, service name and unit No in the calibration sheet.
8. Ensure that the medium used for calibration is the same as that of the medium of the process (if possible) where the pressure gauge is installed.
9. Carry out purging (if required) to remove the process medium completely from the pressure gauge internal sensor by pressurizing, fill & then draining.
10. Apply the leak test by applying the pressure to the pressure gauge & check for leakages. Apply full scale pressure and wait to see if there is any leakage in connections, tighten that. 
11. Exercise the gauge element to stretch for full scale and back to zero to make sure the gauge element is operating normally without sticking. Remember that you always use high pressure port (HP) of Diff. Pressure gauge and keep low pressure port (LP) free in the air for reference ,Also never apply pressure more than the rated pressure; otherwise the pressure sensor of the gauge will be damaged.
12. Tap the gauge gently to ensure there is no friction and the needle moves freely and is not loosely fixed to the gauge element.
13. If you feel any friction remove the front cover, pointer and scale plate to gain access to see the physical condition of the internal parts:-

• Check the hair spring, gears of pinion and rack, bushings and the bourdon tube for possible deformation.
• There should be no large gap between the bushing and spindle. 
• Apply a small amount of silicon grease to the moving parts. (Repair or replace internal parts if damaged).

💡 Remember!
⏩ It is very important to stick to the manufacturer instructions for maintenance of pressure and differential pressure gauges. The manufacturer’s guidelines will provide specific information on the proper procedures for handling, inspecting, and calibrating the gauges.

Procedures Calibration of Pressure and Diff. Pressure Gauges

The calibration of pressure and diff. pressure gauges are typically carried out using two methods - static and dynamic calibration:

• In static calibration, the gauge is tested using a dead-weight tester (DWT), which applies known weights on a piston to generate a specific pressure. The gauge's response is then compared to the applied pressure to determine its accuracy.
• On the other hand, dynamic calibration involves using a pressure calibrator, which generates a dynamic pressure that simulates the process conditions the gauge will be measuring. This method provides a more accurate representation of the gauge's performance under actual operating conditions.

Now the calibration procedures in the following steps:

1. Install the scale & pointer after making internal inspections. Check and adjust the pointer at (0) position on the scale.
2. Without applying pressure check "0" reading of the gauge. 
3. Apply maximum pressure or required percentage (25%, 50%, 75%, or 100%) for rising and falling (ascending and descending) values. 
4. Record these values in the calibration sheet “before calibration column” or as the found column.
5. If all readings are correct and the error is in limit & below the designed error, record these values in the “after calibration column” and go to step 13.
6. If the difference in reading is more than error and adjustment is required, remove the front cover, pointer, and scale plate to gain access to span adjustment.
7. Increase pressure or create a vacuum up to maximum scale and observe the difference between standard values on the tester and pressure gauge to be calibrated.
8. In case of any difference more than the error, adjust the span screw to increase or decrease the range as required.
9. Depressurize the gauge completely to check the (0) value and adjust the pointer to zero if required.
10. Repeat steps 7, 8 & 9 till 0% & 100%. Values should match standard values.
11. Now check the percentage (0%, 25%, 50%, 75%, & 100%) by increasing and decreasing the pressure. Check the error is in limit and under-designed error value.  
12. Record these input & output values in the calibration sheet “after calibration columns.
13. The pressure gauge input/output values should be inspected by I & C Inspector and Quality Inspector for witness and record these values to sign the certificate after completion of the job.
14. Once the test is completed, remove the test equipment and clean the tested device. 
15. Install the Pressure gauge back to its position and reconnect the instrument fittings, and tubing without bending or damaging and ensure that the connector is not cross fitted to avoid damage to threading.
16. Close the drain valve and open the inlet valve slowly to avoid sudden pressure entering the gauge sensor.
17. In the case of water/oil/chemicals, purge the air completely from the pressure gauge, tub lines / capillary tube, and fill the gauge carefully to avoid showing wrong readings.  Check for leakage during commissioning.
18. Complete the check and calibration sheet and handover to the concerned I &C Engineer for inspection and signature.               
19. Once the calibration is completed, all the data and readings should be documented, and a calibration certificate should be generated using the calibration software. This certificate should include details such as the date of calibration, reference standards used, the gauge's accuracy, and any adjustments made.

💡Remember!
⏩ In Step 3. We are checking to ascend and descend to find the hysteresis between increased and decreased readings.
⏩ In case of the vacuum gauge: without creating a vacuum, check the "0" of the gauge, then create the vacuum in the vacuum gauge or compound gauge and note the other 4 points' value (25%, 50%, 75% & 100%) Record these values in the calibration sheet.

Apart from regular calibration, it is also essential to follow proper handling and maintenance procedures to ensure the gauge's accuracy and longevity. This includes protecting the gauge from extreme temperatures, pressure spikes, and corrosive environments. Regular cleaning and servicing of the gauge can also help prevent any issues that may affect its performance.

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