Method of manual gauging
- 1. 1 Prepared by: Purvin Prajapati METHODS OF MANUAL GAUGING
- 2. 2 Outline • Manual gauging • Manual gauging method • What is datum line or reference point? • Innage method • Outage / Ullage method • Manual gauging equipment
- 3. 3 Manual gauging • Manual gauging can be performed on most atmospheric tanks. • A specially designed measurement tape is used for this purpose. • These are normally made of stainless steel with a weight at the end of the tape graded in millimeters or fractions of inches. • The tape is used to measure ullage or innage (liquid level). • Manual tank gauging is a unique leak detection method for underground storage tanks.
- 4. 4 Manual gauging method MANUAL GAUGING METHOD INNAGE METHOD OUTAGE / ULLAGE METHOD
- 5. 5 What is datum line or reference point? • The datum line or reference point is the zero point to which a measurement is referenced. • A zero point in level measurement is simply a starting point for making a measurement. • However, it does not necessarily mean that there is no liquid in the tank or vessel.
- 6. 6 Innage method • Innage gauging also referred to as a bottom gauging or dip gauging. • Innage method is commonly used when: 1. Tank contents are not viscous 2. The operator is able to lower the bob to the bottom of the tank • Innage method will tell you the actual depth of the liquid in the tank. • An oil gauging tape measure with a plumb bob is lowered down from the reference point just to the point that the bob touches the bottom of the tank or datum plate.
- 7. 7 • Care should be taken to lower slowly so the bob does not reach the bottom and then tilt over. • This will cause an inaccurate measurement. Also, if the bob hits the bottom of the tank with too much force, it can damage or puncture the tank. (We have a new plumb bob that has a rubber end tip to help avoid this kind of damage). • When the tape is reeled back up the depth of the liquid in the tank is read where the tape is wet (like a dipstick in a car). • Sometimes water-finding paste or petroleum paste can be used to help show the mark in very thin liquids. • Innage oil gauging tapes are used with Innage plumb bobs. • The zero point of an Innage gauging tape is at the bottom of the bob.
- 8. 8 Figure 1. Innage method
- 9. 9 Advantages of Innage method • The height of the liquid in the tank is directly measured; the effects reference gauge point movements are therefore eliminated. • Innage gauging is recommended whenever reference gauge point changes are suspected. • The operator can compare the tape reading at the reference gauge point with the reference gauge height each time a tank is gauged and thus can note any variances. (As a tank fills, the tank walls can expand; this expansion result in movement of the reference gauge point and/or the datum plate.)
- 10. 10 Disadvantage of Innage method • The tape may be lowered too far, causing the bob to tilt. • Sedimentation on the tank bottom may prevent the gauge from actually reaching the bottom or may cause the bob to tilt. • In highly viscous products, the bob and tape may tilt the gauging tape is immersed in the liquid and must be thoroughly cleaned while the tape is being reeled in. • In addition to being a messy and time-consuming procedure, with corrosive or toxic products, cleaning the gauge tape exposes the operator to health or safety hazards
- 11. 11 Outage/ Ullage method • Outage gauging also referred to as a top gauging or ullage gauging. • Innage method is commonly used when: 1. The contents are corrosive or toxic, extremely viscous or clear 2. The height of liquid must be calculated: height equals the reference gauge height. • Outage (or Top gauging) method will tell you the measurement of the part of the tank that is NOT filled with liquid (the level that has been taken out, or the “ullage”).
- 12. 12 • Because the exact top of the liquid is hard to find, the graduated plumb bob is lowered slowly from the reference point just until it comes into contact with the liquid. • Outage gauging tapes are used with an Outage plumb bob. • The graduations on the outage plumb bob start at the point where the bob is attached to the swivel end clip of the gauging tape, and then get higher. • The zero point on an Outage gauging tape is where the plumb bob and the end clip of the gauging tape meet. • The reading on the bob (marked by the liquid) is added to the reading on the tape at the reference point.
- 13. 13 • This will give you the total empty portion of the tank. Knowing the total depth of the tank, you can subtract the space not filled, from the total tank depth to determine the depth of the liquid. • Outage method is helpful when measuring caustic or thick liquids, as the actual tape measure blade never gets submerged but it is not as accurate as the Innage method for determining the actual level of liquid in the tank.
- 14. 14 Figure 2. Outage/ Ullage method
- 15. 15 Advantages of Outage/ Ullage method • Only the bob is immersed in the liquid and cleanup is quick. Disadvantages of Outage/ Ullage method • The height of the liquid in the tank is indirectly measured: The actual height must be calculated • The operator cannot observe any reference gauge point variances. • Outage gauging should only be used if the reference gauge height does not vary with product level in the tank.
- 16. 16 Manual gauging equipment 1. Gauging tape 2. Gauge bob 3. Water finding paste 4. Gasoline finding paste
- 17. 17 Gauging tapes • Material: Steel (or corrosion-resistant material, if the tape is to be used for gauging the contents of tanks which contain corrosive liquids). The steel of the tape should have a thermal expansion coefficient similar to the steel of the tank. • Length: One continuous tape of sufficient length for the height of the tank to be gauged. • Width Thickness: The cross-sectional area of the tape shall be such that the tape in a horizontal position on a flat surface will not stretch by more than a unit strain of 0.0075 percent when pulled by a force of 44 N (10 pound feet). Typically the cross-sectional area shall not be smaller than 2.5mm2 (0.004 square inch).
- 18. 18 • Housing: A durable reel and crank; the assembly mounted in a frame or case. • Free End: Fitted with a spring snap-catch or other locking device to which the bob can be attached. A swivel-type snap-catch will reduce tape breakage. • Scale: 1. Innage Tape-Graduated in feet, inches, and fractions of an inch; feet and hundredths of a foot; or meters, centimeters, and millimeters. The top of the bob will e the zero point of the scale. • 2. Outage Tape-Graduated in feet, inches, and fractions of an inch: feet and hundredths of a foot; or meters, centimeters, and millimeters. he zero point of the scale is the point of contact between the snap-catch and the eye of the bob.
- 19. 19 Gauge bobs and bars • Material: Spark and Corrosion-resistant. • Length: Bobs or bars, 15 centimeters (6 inches), 30 centimeters (12 inches), or 45 centimeters (18 inches). • Weight: Minimum 20 ounces; maximum 2% pounds. • Eye: An integral part of the bob or bar, preferably reinforced with a hardened bushing to prevent wear. • Tip: Innage bobs and bars shall have a conical tip of sufficient hardness to prevent damage by contact with other metal.
- 20. 20 • Scale: 1. Innage Bobs and Bars-Graduated on one side in inches with at least % inch subdivisions; tenths of a foot with at least hundredths oaf foot sub divisions or centimeters with at least 2 millimeter subdivisions and with the zero point of the scale at the tip of the bob. • 2. Outage Bobs-Graduated on one side in inches with at least % inch subdivision, or in centimeters with at least 2 millimeter subdivisions and with the zero point of the sale being at the inside top of the eye. Figure 3. Gauge bobs
- 21. 21 Figure 4. Gauge tapes, bobs and bar
- 22. 22 Water finding paste • Water gauging pastes are used with gauge bars and bobs and tapes to indicate the petroleum and free water interface. • The paste should not react with the petroleum, but should change color upon contact with free water. Figure 5. Water finding paste
- 23. 23 • In very light petroleum, the level of the liquid cannot be read on the tape because the petroleum evaporates while the tape is being raised from the liquid. • To overcome this problem, gasoline paste is applied to the tape. When the paste comes in contact with the petroleum, it changes color or dissolves away thus giving a reading (cut). Gasoline finding paste Figure 6. Gasoline finding paste
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