【Daily Question 20090402】Instrument Knowledge

Standard answer: 1. Flow rate is the amount of fluid that passes through a certain cross-sectional area per unit of time. The volume of fluid used to represent this is called the instantaneous volumetric flow rate (qv), or simply volumetric flow rate. The mass of flow rate is expressed as instantaneous mass flow rate (qm), commonly referred to simply as mass flow rate. 2. For differential pressure flow meters filled with isolation fluid, before operation, that is, before opening the orifice pressure tap valve, the balance valve must be closed first to prevent the isolation fluid from being flushed away. When shutting down, the pressure-taking valve must be closed first, before the balance valve can be opened to bring the gauge into a balanced state. 3. In flow measurement, **standard regulations state that the flow ratio applicable to throttle devices is 30%. This is because the differential pressure is proportional to the square of the flow rate, and accuracy cannot be guaranteed when the flow rate is below 30%. Furthermore, when the flow rate is less than 30%, the Reynolds number is often below the critical Reynolds number, resulting in unstable flow coefficient and inaccurate flow measurement. When the flow rate is below 30%, the following actions can be taken: reduce it to the maximum extent permitted by the process, and ensure that the Reynolds number is high enough to allow the issue to be resolved through the orifice plate or differential pressure. Secondly, it can be measured using other types of flowmeters, such as turbine flowmeters. 4. ①When measuring flow rate using a throttling device, there are two theoretical requirements: the fluid flow must be continuous, and the law of conservation of energy must hold true during the fluid flow. This tells us that orifice plates cannot be installed in areas where gas and liquid coexist, in order to avoid increasing measurement errors. ②Some liquids (such as liquefied hydrocarbons), after having their pressure reduced by a control valve, sometimes partially vaporize, resulting in a coexistence of two phases. Therefore, the flow orifice plate should be installed as close as possible in front of the control valve; this installation method is reasonable.

1. What is traffic? What are those several ways of expressing it? What is the relationship between them? Flow rate refers to the amount of fluid that passes through a certain cross-section of a pipe per unit of time. Flow rate can be divided into volumetric flow rate and mass flow rate. The volume of fluid that passes through in a unit of time is called volumetric flow rate, which is commonly denoted by Q. The mass of fluid that passes through in a unit of time is called mass flow rate, commonly denoted by M. The relationship between them is: M=Q×P or Q=M/ρ, where ρ is the density of the fluid. 2. What precautions should be taken when opening and closing the balance valve for a differential pressure flow meter using isolation fluid? Before putting such a flow meter into use, that is, before opening the orifice pressure-taking valve, it is necessary to first shut off the balance valve to prevent the isolation fluid from being carried away. When shutting down the device, the pressure-taking valve must be closed first, after which the balance valve can be opened to bring the meter back to an equilibrium state. 3. Why are differential pressure flowmeters generally not suitable for use at levels below 30% of their full scale? What should be done if this happens? ①In flow measurement, **standard regulations state that the flow ratio applicable to throttle devices is 30% (minimum flow: maximum flow = 1:3). This is because the differential pressure is proportional to the square of the flow rate, and accuracy cannot be guaranteed when the flow ratio is below 30%. Furthermore, when the flow rate is less than 30%, the Reynolds number is often below the critical Reynolds number, and the flow coefficient α is not constant, resulting in inaccurate flow measurement. (0.5) ② When the flow rate is below 30% of the full scale, the following actions can be taken: a. If the process permits a reduction in the maximum flow rate and the Reynolds number is high enough, this issue can be resolved by changing the size of the orifice plate or the range of the instrument. b. Use other types of flow meters, such as turbine flow meters, etc. 4. When using an orifice plate to measure flow rate, why is it reasonable to install the orifice plate in front of the control valve? ① In the theoretical derivation for measuring flow rate using a throttling device, there are two assumptions: namely, that the fluid flow is continuous, and that the law of conservation of energy applies during the fluid flow process. This tells us that orifice plates cannot be installed in areas where gas and liquid coexist, to avoid increasing measurement errors ； ②Some liquids (such as liquefied hydrocarbons), after having their pressure reduced by a control valve, sometimes partially vaporize, resulting in a coexistence of two phases ； ③Therefore, the flow orifice plate should be installed as close as possible in front of the control valve; this installation method is reasonable.

1. What is traffic? What are those several ways of expressing it? What is the relationship between them? Answer: Flow rate is the amount of fluid that passes through a certain cross-sectional area per unit of time. There are two ways of representation: volumetric flow rate (Q) and mass flow rate (M). Correlation relationship: M=Q*ρ or Q=M/ρ. In the formula, ρ is the density of the fluid. 2. For differential pressure flow meters used for injecting isolation fluid, what should be noted when opening and closing the balance valve? Answer: When turning on a differential pressure flow meter using isolation fluid, it is necessary to first close the balance valve to prevent the isolation fluid from being carried away. When turning it off, the pressure tapping valve must be closed first, after which the balance valve can be opened to bring the meter back to an equilibrium state. 3. Why are differential pressure flowmeters generally not suitable for use at levels below 30% of their full scale? What should be done if this happens? Answer: Reason: **The standard specifies that the flow ratio applicable to throttle devices is 30%. It is mainly to ensure measurement accuracy. Solution: The process allows for a reduction in the maximum amount, and since the Reynolds number is high enough, this issue can be resolved using orifice plates or differential pressure. Furthermore, it can be measured using other types of flowmeters, such as turbine flowmeters. 4. Flow rate is measured using an orifice plate; why is it reasonable to install the orifice plate in front of the control valve? Answer: Reason: The orifice plate cannot be installed in areas where gas and liquid coexist, as this may increase measurement errors. If it is installed behind a valve, the pressure drop may cause some of the liquid to vaporize, resulting in a two-phase mixture.

1. What is traffic? What are those several ways of expressing it? What is the relationship between them? Answer: Flow rate is the amount of fluid that passes through a certain cross-sectional area per unit of time. There are two ways of representation: volumetric flow rate (Q) and mass flow rate (M). Correlation relationship: M=Q*ρ or Q=M/ρ. In the formula, ρ is the density of the fluid. 2. For differential pressure flow meters used for injecting isolation fluid, what should be noted when opening and closing the balance valve? Answer: When turning on a differential pressure flow meter using isolation fluid, it is necessary to first close the balance valve to prevent the isolation fluid from being carried away. When turning it off, the pressure tapping valve must be closed first, after which the balance valve can be opened to bring the meter back to an equilibrium state. 3. Why are differential pressure flowmeters generally not suitable for use at levels below 30% of their full scale? What should be done if this happens? Answer: Reason: **The standard specifies that the flow ratio applicable to throttle devices is 30%. It is mainly to ensure measurement accuracy. Solution: The process allows for a reduction in the maximum amount, and since the Reynolds number is high enough, this issue can be resolved using orifice plates or differential pressure. Furthermore, it can be measured using other types of flowmeters, such as turbine flowmeters. 4. Flow rate is measured using an orifice plate; why is it reasonable to install the orifice plate in front of the control valve? Answer: Reason: The orifice plate cannot be installed in areas where gas and liquid coexist, as this may increase measurement errors. If it is installed behind a valve, the pressure drop may cause some of the liquid to vaporize, resulting in a two-phase mixture.

1. 1. What is traffic? What are those several ways of expressing it? What is the relationship between them? Flow rate is the amount of medium that passes through per unit of time. Instantaneous volumetric flow rate and mass flow rate. The latter is derived from the former under standard conditions, with units typically being KNM3, etc. 2. What should be noted when opening and closing the balance valve in a differential pressure flow meter filled with isolation fluid? When putting it into use, first shut off the balance valve to prevent the isolation fluid from being washed away. Then apply positive pressure and negative pressure. When shutting down, first cut off the negative pressure and positive pressure before opening the balance valve. 3. Why are differential pressure flowmeters generally not suitable for use below 30% of their full scale? What should be done if this happens? The precision cannot be guaranteed after taking the square root. Replace the flow meter, or simply use a mass flow meter. Actually, replacing the differential pressure transmitter is the simplest approach; I’ve done it this way before, but you need to understand the flow conversion related to the process. 4. Why is it reasonable to use an orifice plate to measure flow rate, with the orifice plate installed in front of the control valve? Common sense: the flow rate downstream of the valve is unstable; it seems that a certain distance downstream of the valve is also acceptable. I mostly create original content; can that earn me extra points? And I might be in trouble upstairs.

1. What is traffic? What are those several ways of expressing it? What is the relationship between them? Flow rate is the amount of fluid that passes through a certain cross-sectional area per unit of time. The representation methods include volumetric flow rate (Q) and mass flow rate (M). Correlation relationship: Q = M/ρ. In the formula, ρ is the density of the fluid. 2. For differential pressure flow meters using a filling isolation fluid, what precautions should be taken when opening and closing the balance valve? When turning it on, first shut off the balance valve to prevent the isolation fluid from being washed away. Then apply positive pressure and negative pressure. When shutting down, the negative pressure and positive pressure must first be cut off before the balance valve can be opened. 3. Why is it generally not advisable to use differential pressure flow meters at levels below 30% of their full scale? What should be done if this happens? For differential pressure flow meters, accuracy is not guaranteed below 30% of full scale, as the differential pressure is proportional to the square of the flow rate; when the flow rate is below 30%, accuracy cannot be ensured. When the flow rate is below 30%, other types of flow meters, such as turbine flow meters, can be used for measurement. 4. Why is it reasonable to use an orifice plate to measure flow rate, with the orifice plate installed in front of the control valve? The flow rate downstream of the valve is unstable, with large fluctuations