Gas measurement is another area of hydrocarbon engineering where accurate prediction of the P-V-T properties of the working fluid is especially critical. One of the most widely used meters used in the measurement of gas flow is the orifice meter. Orifice meters are classified as inferential meters because the gas volume is calculated from readings of pressure variation as the gas passes through an orifice, and it is not obtained by a direct reading.
The orifice meter is arranged so that the flowing gas is constricted at a particular location by a thin orifice plate very accurately gauged and calibrated so as to be in a concentric position in the pipe. The reduction of the cross section of the flowing gas stream in passing through the orifice increases the velocity head at the expense of the pressure head, and the reduction in pressure between the taps is measured by manometers (or a recording meter). A typical orifice meter is shown in Figure 21.4.
Among the advantages of using oriface meters for gas measurement purposes are the following facts:
- They are simple in design and have no moving parts.
- They are relatively accurate.
- They are easy to install and maintain.
- They cover a wide range of capacity.
- They represent a low cost.
- There is a great deal of experience in their use.
Among the disadvantages of oriface meters are the following facts:
- They represent an intrusive measurement technique and a flow restriction that translates into a large energy loss.
- The orifice hole can be eroded by sand or corrosive fluids.
- The hole may be obstructed by wax or hydrate.
Among the advantages of using orifice meters for gas measurement purposes are the fact that they are simple in design and have no moving parts, relatively accurate, easy to install and maintain, cover a wide range of capacity, represent a low cost, and there is a great deal of experience in their use. Among the disadvantages of orifice meters, are the fact that they represent an intrusive measurement technique and a flow restriction that translates into a large energy loss, the orifice hole can be eroded by sand or corrosive fluids, and the hole may be obstructed by wax or hydrate.
Bernoulli’s equation is then used as the basis for correlating the increase in velocity head with the decrease in pressure head. In the calculation of gas flow rate using an orifice meter, two quantities must be measured: the static pressure (i.e. the line pressure) and the differential pressure (i.e. the pressure drop across the orifice plate). The following is the basic equation for gas flow through an orifice meter:
In Equation (21.2), flow rate is a function of gas compressibility factor (Z). Again, for high-pressure flows, an error in the compressibility factor could result in an erroneously calculated flow rate. If you have some error on Z-factor, this automatically translates into error in the gas meter. Accurate phase behavior prediction techniques are a must in gas metering.
In the Natural Gas Industry, the point of gas exchange between the buyer and the seller is called custody transfer. During custody transfer operations, accurate measurements of the quantity and quality of the exchanged gas are of crucial importance because of its economical implications. Economic transactions are based on volumetric rate measurements, which are regulated to be made at the same base conditions. Industry base conditions or standard conditions (SC) are usually taken as P = 14.7 psia and T = 60.0 °F. A low percent inaccuracy in the Z-factor calculation of a gas in transfer can easily translate into thousands of dollars of losses on a daily basis! In fact, flow rate estimations can prove extremely sensitive to values of compressibility factor. This is why the gas industry does not accept Z-factor predictions with a range of uncertainty larger than + 0.01 % for custody transfer operations.