By far the most common type of differential meter used in the fuel gas industry is the orifice meter. This meter consists of a thin flat plate with an accurately machined circular hole that is centered in a pair of flanges or other plate-holding device in a straight section of smooth pipe. Pressure tap connections are provided on the upstream and downstream sides of the plate so that the pressure drop or differential pressure
may be measured. There is a flow pattern through an orifice, how the resulting pressure differential across the orifice is measured, and the change in static pressure that occurs. The advantages of the orifice are accuracy, ruggedness, simplicity, ease of installation and maintenance, range of capacity, low cost, acceptance for gas measurement by joint.
As its name implics, an orifice meter utilizes an orifice for its basic component in the measurement of natural gas. The typical orifice meter consists of (primarily) a thin stainless steel plate about 3/16 in. thick, with a hole in the center, that is placed in the flow line. Placing an orifice in a pipe in which there is a gas flow causes a pressure in the line at a specified «tap» location are recorded continuously and are later translated into rate of flow. Two arrangements are commonly used: Flange tap and Pipe tap.
It is customary to consider a complete orifice meter as composed of two major elements. The first of two major elements is the differential pressure-producing device called the primary element. This primary element is composed of the following parts:
- The meter tube: a lenght of special pipe through which the gas flows.
- The orifice plate holding and positioning device: an orifice flange or an orifice fitting installed as an integral part of the meter tube to hold the orifice plate in a position perpendicular and concentric to the gas flow.
- The orifice plate: a flat circular plate with a centrally bored, sharp-edged orifce machined to an exact, predetermined dimension that forms a calibrated restriction to the flow of gas through the meter tube and is the source of the differential.
- Pressure taps: precisely located holes through the pipe walls or orifice plate holder from which gas pressure on each side of the orifice plate may be measured.
- Straightening vanes: a device that may be inserted in the upstream section of the meter tube to reduce swirling in the gas stream.
The secondary element is called the differential gauge and is the device for measuring the pressures. It is a gauge connected with tubing to the upstream and downstream taps of the primary element.
- Basic Orifice Factor, Fb: This is dependent on the location of the taps, the internal diameter of the run, and the size of the orifice.
- Reynolds Nomber Factor, Fr: This factor is dependent on the pipe diameter and the viscosity, density, and velocity of the gas.
- Expansion Factor, Y: Unlike liquids, when a gas flows through an orifice, the change in velocity and pressure is accompanied by a change in the density. The expansion of the gas through the orifice is essentially adiabatic. Under these conditions, the density of the stream changes because of the pressure drop and the adiabatic temperature change.
- Pressure Base Factor, Fpb: Most locations use 14,73 psia as a standard base. This is the presure base adopted by the American Gas Association for its standard, which represents atmospheric pressure at sea level.
- Temperature Base Factor, Ftb: 60°F is almost universally used as the base temperature in claculating gas measured by orifice meters.
- Flowing Temperature Factor, Ftf: The flowing temperature has two effects on the volume. A higher temperature means a lighter gas so that flow will increase as well as making the gas expand, which reduces the flow.
- Specific Gravity Factor, Fg: this is used to correct for changes ¡n the specific gravity and should be based on the actual flowing specific gravity of the gas as determined by test. The specific gravity may be determined continuously by a recording gravitometer or by gravity balance on a daily, weekly, or monthly schedule.
- Supercompressibility Factor, Fpv: This factor corrects for the fact that gases do not follow the ideal gas lwas. It varies with temperature, pressure, and specific gravity. those are obtained by AGA tables.
- Manometer Factor, Fm: This is used with mercury differential gauges and compensates for the column of compressed gas opposite the mercury leg. this is not usually considered for pressures below 500 psia.
- Gauge Location Factor, Fl: this is used where orifice meters are installed at locations other than 45″ latitud and sea level elevation.
- Orifice thermal Expansion Factor, Fa: This is introduced to correct for the error resulting from expansion or contraction of the orifice operating at temperatures appreciably different from the temperature at which the orifice was bored.