Necessity of temperature and pressure correction in gas volume flow measurement

Abstract: Temperature and pressure correction is one of the important means to improve the accuracy of gas flow measurement in gas flow measurement. This paper introduces the errors caused by direct output flowmeter and differential pressure flowmeter when the temperature and pressure deviate from the set value, and shows the measurement errors caused by temperature and pressure under 8 extreme changes through engineering examples, so as to provide some quantitative concepts of the errors caused by temperature and pressure changes. It is pointed out that the implementation of temperature and pressure correction is not as troublesome as the previous unit combination instrument, and the correction should be considered appropriately. Finally, some recommended suggestions are given.

1 Meaning of gas volume flow rate

Gas volume flow refers to the volume of gas flowing through the delivery pipeline per unit time. Gas volume can be divided into working state volume and standard state volume. They both refer to the volume under a certain pressure and a certain temperature. The latter refers to the gas volume when the pressure is a standard atmospheric pressure and the temperature is 0℃ (or 20℃). The standard state temperature mostly refers to 0℃. Only in some occasions, such as the flow of a blower, because it mostly refers to the inlet state of the blower, the standard state temperature is 20℃. Because the corresponding working temperature and pressure need to be stated when using the working state flow parameter, this is more troublesome, and it is not easy to establish a concept of quantity in people's minds. Therefore, the standard state flow is often used as a measurement standard in engineering.

Since the volume flow of gas refers to the flow at standard temperature and pressure, when the medium temperature and pressure deviate from the set state, the value measured by the instrument will change. How big is the change and whether it needs to be corrected is the question discussed in this article.

2 Errors caused by temperature and pressure changes on volume flow

2.1 Direct output flow meter

The so-called direct output flowmeter refers to a flowmeter whose flow signal does not need to be processed by square root, such as vortex flowmeter, rotor flowmeter, etc.

According to the gas state equation,

In the formula, p, t, and v are the pressure, temperature, and volume of the gas at this temperature respectively; c is a constant.

For a certain measured object, after the pipe diameter is determined, the formula

(1) can be rewritten as (2) and (3)

In the formula, q is the volume flow rate; p1s, t1s, q1s, p1w, t1w, q1w are the pressure, temperature and flow values ​​when the direct output flow meter is selected and when it is actually running. It can be obtained that the flow error caused by the pressure and temperature deviating from the set values ​​p1s and t1s is

2.2 Differential pressure flow meter

When a throttling device is used to measure flow, the measured gas is considered to be incompressible gas, and the relationship between its flow rate and the differential pressure δp at both ends of the throttling device is:

In the formula, k is a constant; r is the gas density that changes with pressure and temperature and can be expressed as follows:

In the formula, rn is the density of the measured gas under standard conditions (pressure is pn, temperature is tn). The volume flow expression of the throttling device at the set pressure and temperature and at a certain working pressure and temperature is obtained from formula (5) and formula (6):

In the above formula, p2s, t2s, q2s, p2w, t2w, q2w are the pressure, temperature and flow value when calculating and actually running the differential pressure flowmeter respectively.

From equations (7) and (8), the error caused by the pressure and temperature deviating from the set value is:

3 Typical Examples

Several examples of temperature and pressure correction are given below, which illustrate the necessity of correctly implementing temperature and pressure correction in actual use through some specific data.

Example 1: Set the pressure p1s = 0.35 MPa (absolute pressure), and the pressure range is 0.33~0.37 MPa. Set the temperature t1s = 313 K, and the temperature range is 293~333 K. The measured medium is natural gas, and the vortex flowmeter is used for measurement.

Substituting the eight limit pressure and temperature values, as well as the set values ​​of pressure and temperature into equation (4), we can obtain the corresponding error δ1. Table 1 lists the corresponding parameters.

It can be seen from Table 1 that when the temperature increases and the pressure decreases or the temperature decreases and the pressure increases, that is, in the second and third cases, the error is the largest.

Example 2: Set the pressure p2s = 0.109 MPa (absolute pressure), and the pressure range is 0.108~0.110 MPa. Set the temperature ts = 313 K, and the temperature range is 293~333 K. The measured medium is blast furnace gas, and the differential pressure flowmeter is used for measurement.

Substituting the eight extreme pressure and temperature values, as well as the set values ​​of pressure and temperature into equation (9), we can get the corresponding error δ2. Table 2 lists the corresponding parameters.

It can be seen from Table 2 that when the temperature remains unchanged and the pressure changes, the overall error is not large.

Example 3: Set the pressure p2s = 0.7 MPa (absolute pressure), and the pressure range is 0.3~0.8 MPa. Set the temperature t2s = 313 K, and the temperature range is 293~333 K. The measured medium is nitrogen, and the differential pressure flowmeter is used for measurement.

Similarly, the eight extreme pressure and temperature values, as well as the set values ​​of pressure and temperature are substituted into formula (9) to obtain the corresponding error δ2. Table 3 lists the corresponding parameters.

It can be seen from Table 3 that the errors caused by the third to fifth cases are the largest, because the pressure fluctuations in these three cases are too large.

4 Recommendations

(1) As can be seen from the typical examples above, in actual operation, the measured medium will cause different degrees of error in the flow indication due to the deviation of pressure and temperature from the set value. Therefore, temperature and pressure corrections should be considered when measuring the flow of energy media for metering.

(2) For gas flow measurement that is not used for metering, if temperature and pressure signals have been obtained, temperature and pressure correction should also be considered. Because today's secondary instruments or various computer systems used as secondary instruments (DCS, PLC, IPC, etc.) have temperature and pressure correction functions, there is no need to build a complex instrument hardware system that can meet functional requirements like the old instruments.

(3) For gases at normal temperature and pressure, that is, gases at a temperature of 30-40°C and a pressure of about 10 kPa, when they are not used for metering but only for reference in process operations, temperature and pressure corrections can be ignored because under such conditions (see Example 2 above), the error in flow indication caused by changes in temperature and pressure is not very large.

(4) For gases with a pressure of more than 0.01 MPa, regardless of the purpose of flow measurement, temperature and pressure corrections should be considered as much as possible (if it is a room temperature gas, pressure correction can be set alone without temperature correction). This is because in this case, the pressure fluctuation will cause a large error in the flow measurement indication (see Example 3). The nitrogen flow measurement used for equipment sealing or purging in steel plants belongs to this situation. Since the source of nitrogen is often not sufficient, once it is used, the pressure of the nitrogen pipeline often drops significantly. Although the accuracy requirements for flow measurement are very low under this condition, pressure correction should also be considered because the impact of pressure fluctuations is too great.