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Gas measurement by orifice meter
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Orifice Plate. The plate is usually made of 304 of 304 or 316 stainless steel or monel according to the specifications listed in Figure 11‐7. The upstream edge [ e ] shall be flat to within 0.01 inch per inch of the of the dam height—(D ‐ d)/2. The max edge thickness [max e ] is define by e < D/ 50 or e < d / 8 which is smaller
The thickness of of the the orifice plate are as per on the next table
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The meter consists of an orifice plate with pressure sensing taps both upstream and downstream of the orifice plate. The plate itself may be changed without interrupting gas flow. The sloped edge is installed down stream the flow. The Beta ratio is taken as 0.15 to 0.75 for flange tap , but is pipe tap is used the Beta ratio is taken from 0.2 to 0.67
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Type
Flange taps
Pipe taps
Upstream tap Downstream tap location location
Advantages and disadvantages
Convenient tap locations fixed by 1" from 1" from outlet face purchasable flanges. Difficult if such upstream face flanges are not available x rom upstream face
8 i e diameters Not de endent on d D ratio. from upstream Pressure drop = net pressure loss. face Take up a lot of space.
If the orifice is used for liquid measurements the two taps are ns a e ownwar below the orifice ) not as the gas measurements which installed nearly above the orifice
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Length of pipe required for accurate measurements is based on the Beta ratio Straightening vanes reduce the required lengths of upstream pipe, and this is especially welcome for large diameter pipe when space is limited, e.g., offshore. Occasionally, vanes can cause plugging problems.
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Straightening Vanes — The purpose of straightening vanes is to eliminate swirls and cross currents set u b the i e fittin s and valves u stream of the meter tube. The specifications which follow apply particularly to the type of vanes shown in Fig. 3‐14. In the construction of vanes, the maximum transverse dimension "a," Fig. 3‐14, of any passage through the vanes shall not exceed one‐fourth (1/4) the inside diameter, "D," of the pipe. Also, the cross‐sectional area, "A
" of any passage within the assembled vanes shall not exceed one‐sixteenth (1/16) of the cross‐sectional area of the containing pipe. It is not necessary that all the vane passages be of the same size, but their arrangement should be symmetrical.
The length "L" of the vanes shall be at least ten (10) times the largest inside "a” dimension.
Calculating Gas Flow Rate the fundamental equation for gas measurement is: Q h=C‘ hw pf w ere Q h = quantity rate of flow at base conditions in cu ft/hr hw = differential pressure in inches of water Pf = absolute static pressure, psia C' = orifice flow constant Assume the flowing temperature is constant C'=Fb.Fr.Y.Fpb.Ftb.Ftf .Fgr.Fpv.Fa.Fam.Fw1.Fwt.Fpw1.Fhgm.Fhgt Using AGA3 for gas calculations C:\Users\Eng\Desktop\GAS .exe
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Fb Fr Y Fpb Ftb F Fgr Fpv Fa Fam Fwl Fpw1 Fhgm Fhgt
=basic orifice factor = Reynolds number factor =expansion factor =pressure base factor = temperature base factor = flowin tem erature factor = specific gravity factor = supercompressibility factor = orifice thermal expansion factor = correction for air over the water in the water manometer during the differential instrument calibration =local gravitational correction for water column calibration standard = local gravitational correction for dead weight tester static pressure stan ar = manometer factor, correction for gas column in mercury manometers = mercury manometer temperature factor
Condition
Error
Reference
Plate installed backwards
17% low 14% low up to 30% low
Batchelder (1985) Hoch (1983) Schepers (1981)
Dirt upstream of plate
6% low 11% low
Hoch (1983) Burgin (1971)
Dirt downstream of plate
2% low
Hoch (1983)
Liquid in meter tube
11.3% low
Burgin (1971)
Dirt on upstream face of plate
27% low 0-4.7% low
Hoch (1983) Burgin (1971)
Rounded and scratched place
11% low
Hoch (1983)
Upstream edge rounded
0-9.3% low
Burgin (1971)
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Some important facts when Calculating Gas Rates! Pressure: When calculating pressure you must be consistent with which of the pressure format you use, either psig (gauge pressure) or Psia (pressure at atmospheric conditions) when calculating gas pressures we use Psia, converted from psig.
What is temperature measured in for calculation? Rankin
Where Rankin is oF + 460
sc
=
wpf x
ux gx
x
x pv x
Hw, What is it? Differential pressure in inches of water Pf, What is it? Absolute Static Pressure.( P in Psig + 14.7 ) d What does it mean? The diameter of the orifice in inches
Fu, What is this? A correction factor which is dependent upon the standard conditions If the base pressure and temperature are14.73 Psia and 60oF and the rate is to be reported in cubic feet per day, Fu will equal 24 F What is this? equals 1 divided by the gas gravity and determining the square root of the result
1/G
hw Pf, What is this? This factor is obtained by multiplying the differential (hw) in inches of water times the absolute static pressure (Pf) in Psia
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Fb What is this? The basic orifice factor depends upon: the location of the differential taps; the diameter of the orifice, d; and upon the internal diameter of the pipe, D. Fpv, What is this? correction factor to account for the deviation of the natural gas from the ideal gas laws compressibility is a function of pressure, temperature and gas composition. Y2, What is this? The expansion factor Y2 is taken from the downstream pressure tap to obtain a pressure correction for the density of gas.
The meter usually reads zero to 100 inches of water. This means that the hole in the orifice plate is sized to obtain a pressure drop of about 3.5 psi (28 inches of water equals 1 psi) at the maximum anticipated gas rate. 1 atm = 10.4 meters of water = 409.45 inches of water = 14.7 psig
So 1 psi = 27.8 inches of water so if the range of the orifice meter is 100 water inches that means that the orifice plate will obtain 3.5 psi pressure drop.
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Gas field balances The operator of one large producing district was concerned that metered gas sales at custody transfer points could not be reconciled with the daily wellhead production figures reported by operating personnel. An investigation revealed the following: ‐
Calculations used to determine daily wellhead gas flows did not include correction factors for temperature or compressibility. Fuel gas used to run wellhead compressors was reported as produced gas even though this gas never left the well site.
We can increase the measuring range from 100 water inch to 200 water inches which will increase the range with 41.4 % ( i.e. √ 200/100)
duplication of the range will not duplicate the measuring rate
where the gas flow rate is proportional with the square roots of the differential press. (Qg = C √∆P*P/ T)
1‐ Pulsation effects It was found that a major difference exist between two meters even after both of them calibrated and found ok, after investigation it is found that the meter near the reciprocating compressors read higher
valve (preferred to be globe valve) is throttling to create from 5 to 10 psi pressure drop if the pulsation the reasons of the high reading the reading will be drop to the normal.
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when the possibility of pulsation is anticipated in a meter run, the tubing lines should be coiled. This coiling will soothe the pressure pulsation effect on the , incorrect high reading If the compressor's cylinder valves are in good condition, the calculated flow should be 5% to 10% greater than the metered flow. If the metered flow is greater than the flow calculated from the compressor characteristic, the installation of a pulsation damper and the recalibration of the meter are positively in order.
2‐ Location meter runs too close to elbows or piping reducers also can alter recorder flows. 3‐ Plugged orifice:‐
if the orifice is plugged the reading more than normal and indicating higher gas flow rate. 4‐ Eroded orifice plat:‐
will cause lower readin where this will decrease the pressure drop which will indicate lower reading than actual.
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Gas field balances A standard specific gravity of 0.65 was being used in wellhead gas flow calculations, while actual specific gravity measurements
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Brine or condensate, which is entrained out of the high pressure separator and carries through the meter run, will result in an erroneously high measurement of gas flow, This is due to the higher pressure drop which will be created by the entrained li uid.
Measurement Problems. These include freezing, pulsating flow, slugging, and sour gas. Commonly implemented solutions are shown below Freezing • Install line
heaters, heated meter house • Dry gas, use hydrate inhibitors . Pulsations • Locate meter away from reciprocating compressor • Insert capacity, restriction, or filter in line • Operate at as high as possible hw Slugging • • Dry gas Sour gas Use 316 stainless static spring • Use Teflon bearings for differential pen shaft • Avoid copper piping, mercury manometers •
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