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Which condition that the temperature and pressure compensation apply to the measurement for steam?

Author:vacorda    Source:vacorda.com    Date: 2017-01-16 15:29:46

Steam is divided into two kinds, include saturated steam and superheated steam. In general, the vortex flowmeter manufacturers will give a mass flow range table about the saturated, superheated steam for the user selection, then to determine the flowmeter diameter size. Just the superheated steam need temperature and pressure compensation. For the true sense of the saturated steam, it only temperature or pressure compensation. Then get the required mass flow by the density table which according to the temperature and pressure. Due to the change of the actual working condition in the actual measurement, saturated steam and superheated steam are likely to deviate from the original design, and it will result the change of the relationship which get the density according to the steam temperature and pressure, and finally impact on the measurement accuracy.

Wet saturated steam

When the vortex flowmeter installed after the pressure reducing valve, the wet saturated steam suddenly more substantial decompression, adiabatic expansion of the fluid, the evaporation of the water droplets, while absorb the heat of vaporization from the liquid and vapor phase, the temperature of the vapor liquid decreases. If the temperature reduce not much or high humidity before evaporation, it will quickly reduce the temperature to the new pressure corresponding to the saturation temperature, and establish a new balance, now the steam is still saturated steam; If the pressure reduce so much or the low humidity before evaporation, because of the droplet evaporates, the temperature is still more that the saturation temperature corresponding to the new pressure.

 vortex flow meter

(Vortex flow meter)

Superheated steam

In the flow calculator, the mass flow can be got by looking up the density according to the superheated steam temperature, pressure table. But when the superheated steam after long-distance transport, Or improper pipe insulation measures, the superheated state will often be changed in to the critical saturation state because the heat loss temperature. Even some of the steam condensate phase change and become water droplets, then it will become the wet saturated steam.

The output of the vortex flowmeter is only proportional to the flow rate of the fluid flowing through the measuring tube, when measuring the wet saturated steam, the impact of water droplets on the vortex flowmeter output is negligible, so it can be considered that the output of the vortex flowmeter is entirely caused by the dry portion (saturated portion) of the wet saturated steam, While the density of the dry part can be accurately detected according to the pressure compensation or the temperature compensation. When the steam metering, if the parties agreed the settlement costs by dry part of the steam, and no charge for condensate, then the phase transition on the measurement of negligible can be ignored; If the condensate is also charged as steam, the vortex flowmeter measurement results are low.  

After the above evaporation, the former has no effect on the compensation, only the dry part of the steam increase, the corresponding increase the dryness, The latter case is that the wet saturated steam into superheated steam, at the time the impact of the flow meter is divided into the following three conditions: 

(1) The design has been taken into account the steam into overheating state, or is difficult to determine in what state, or sometimes overheated state is sometimes saturated state, soadopt the temperature and pressure compensation, then there has no effect on the measurement result caused by the above phase transition.

(2) When design, according to the saturated steam as consideration, and use pressure compensation, the phase transition will bring less error.

(3) When design, considering as the saturated steam, but use the temperature compensation, that is mistake the superheated steam temperature as the saturation temperature to check the density table, thus it will cause greater error.

Installation diagram of the temperature and pressure compensation

(Installation diagram of the temperature and pressure compensation)

There are three solution blow:

(1) Install the total steam flow meter before the pressure reducing valve, because the steam without decompression, there is no phase transition problem, now it is take the saturated steam compensation method to ensure measurement accuracy.

(2) If the flowmeter can only be installed behind the valve, you can install a pressure transmitter to make the temperature and pressure compensation.

(3) If the stability of the pressure relief valve is good, then the flowmeter upstream pressure value can be set as a constant value into the display instrument to make the temperature and pressure compensation.

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