How should negative pressure be handled when calculating CV?
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This post was last edited by mikepan on 2016-5-17 at 13:33. Dear seniors, I have two questions regarding the calculation of CV values. 1. When it is known that the medium is a process gas, and the flow rate is (with an operating density of 1.29 kg/Nm3), with a maximum value of 15000 Nm3/h, a normal value of 13000 Nm3/h, and a minimum value of 10000 Nm3/h, should the units be converted to kg/h for calculations? Is it the flow rate multiplied by the operating density? 2. The pressure before the valve is negative at -0.0002, the medium is air, and the flow rate (in Nm3, with an operating density of 1.29 kg/Nm3) is 15,000 at its maximum, 13,000 under normal conditions, and 10,000 at its minimum. The pressure after the valve is not specified; it is assumed to be 0. How should the CV value be calculated? As I understand it, generally the pressure drop cannot be greater than the pressure before the valve; so how is calculation done in a negative pressure situation? Thank you
Negative pressure in front of the valve, normal pressure behind it – in which direction does the fluid flow? :o
So what if the pressure behind the valve is not at atmospheric pressure but is negative? How should that be handled?
This post was last edited by HEJIYUER on 2016-5-3 20:24. 1. When it is known that the medium is a process gas, with a flow rate of (operating density 1.29 kg/Nm3), the maximum value is 15000 Nm3/h, the normal value is 13000 Nm3/h, and the minimum value is 10000 Nm3/h. When performing calculations, if the units are to be converted to kg/h, should it be flow rate * operating density? No, KG = kg/Nm3 * Nm3. The calculation program requires the density upstream of the valve to be entered, that is, the operating density in kg/m3. 2. The pressure before the valve is negative at -0.0002, the medium is air, and the flow rate (in Nm3, with an operating density of 1.29 kg/Nm3) is 15,000 at its maximum, 13,000 under normal conditions, and 10,000 at its minimum. The pressure after the valve is not specified; it is assumed to be 0. How should the CV value be calculated? As I understand it, generally the pressure drop cannot be greater than the pressure before the valve; so how is calculation done in a negative pressure situation? The negative pressure of the equipment is generated by the vacuum device behind it (such as a steam jet pump). For example, the process gas at the jet pump is \"pulled negative\" to -98, while at the equipment (at the inlet P1 of the control valve) it is -2. The process must specify the pressure drop of the control valve; there is also a pressure drop in negative-pressure pipelines, otherwise it is impossible to calculate CV.
KG=kg/Nm3*Nm3. The calculation program requires the density upstream of the valve to be entered, that is, the operating density in kg/m3. I didn’t understand this sentence. The flow rate unit is now Nm3/h; multiplying it by the operating density in Nm3/h gives kg/h, right?
Your flow rate unit is NM3 (standard cubic meters), and the density should also be the standard density of kg/NM3; X gives the mass flow rate. Regarding what you mentioned earlier (operating density of 1.29 kg/Nm3), should that be the standard density?
CV is an inherent characteristic of the valve, and it has nothing to do with positive or negative pressure. You might be referring to traffic, right?
Hello! I checked previous articles and found that operation density refers to the density under the current pressure and temperature conditions during the production process. The unit is kg/m3. The standard density refers to the density under standard conditions, usually the density at 0 degrees and 1 standard atmosphere pressure. The unit is kg/Nm3. However, on the specification sheet it is labeled as operating density, yet the unit remains kg/Nm3. So should it be the standard density instead? Then, if it’s at standard density, is what I mentioned earlier reasonable?
Operation density is related to operation pressure and temperature, and generally is not equal to the standard-state density. Upon reviewing your process conditions, since the pressure before the valve is negative at -0.0002, and this pressure is close to 1 atm, the operating density before the valve is numerically close to the standard-state density (1.29); therefore, KG can be calculated by multiplying by 1.29.