Old K says every week – talking about things in the petrochemical design industry. Post 21: Pressure drop and energy consumption of control valves

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This post was last edited by henglinkou on 2020-11-7 at 08:56. Old K’s weekly series – Talks about matters in the petrochemical design industry, Post 21: Pressure drop and energy consumption of control valves. 1. Principle of flow regulation by control valves: The function of a control valve is to regulate flow by adjusting its opening degree. The principle is as follows: △P = ρHg + △Pf + △Pt. Here, △P represents the pressure difference at the starting point of the pipeline system; for a given pipeline system, this value remains constant (except in the case of pipelines connected to centrifugal pumps). In such cases, due to the characteristics of centrifugal pumps, flow rate is inversely proportional to head, so this value varies with the flow rate – it decreases when the flow rate is high and increases otherwise. H denotes the height difference between the starting and ending points of the pipeline system; for a given pipeline system, this value is also constant. △Pf represents the resistance loss in the pipeline system (excluding the local resistance loss caused by the control valve itself), and it is proportional to the flow rate. △Pt represents the local resistance loss caused by the control valve itself. ρ is the density of the fluid, while g is the acceleration due to gravity. When the control valve is closed, its local resistance increases, resulting in a decrease in △Pf on the left side of the equation. This decrease in △Pf is caused by a reduction in flow rate. Conversely, when the valve is opened wider, the flow rate increases. At normal flow rates, it is reasonable to set the opening degree of the control valve at 60%-70%. 2. Normal pressure drop of the control valve at normal flow rate: When the process engineering team provides requirements for the control valves to the automation team, it is necessary to specify the normal pressure drop of the control valve at normal flow rate. How much this pressure drop should be increased depends on the circumstances. (1) The starting pressure and ending pressure of a specific piping system are fixed; thus, the pressure difference between the start and end points, minus the pressure generated by elevation differences, and then minus the pressure loss due to pipe resistance, gives the pressure difference required for the control valve. This is the case, for example, when the exhaust gas from the tower top needs to be discharged into a flare system and its flow rate must be controlled. In such cases, the pressure difference can sometimes be very large, leading to erosion of the control valve or flow obstruction; it is recommended to install a pressure regulator or a flow-limiting orifice plate in front of the control valve. (2) First, determine the pressure drop of the control valve, and then calculate the starting pressure; this is the case for flow regulation in the outlet pipeline system of a centrifugal pump, where the pressure drop of the control valve is one of the factors considered in calculating the pump’s head. For such systems, there is no clear specification in design codes or manuals regarding by how much the normal pressure drop of the control valve should be increased. If the normal pressure drop of the control valve decreases, its sensitivity will be reduced ； If it is increased too much, energy will be wasted, and it will also require a smaller selection of control valve, one or even two size grades smaller than that of the main pipe. Based on experience, to take into account both the sensitivity of the control valve and energy consumption, it is recommended that the normal pressure drop across the control valve be around 0.5 times the total system resistance drop. 3. The energy consumption of the control valve: For water with a normal flow rate of 100 m3/h and a pressure drop of 0.1 MPa, the energy consumption is calculated as follows: N = 100 * 0.1 * 1,000,000 / 3,600 / 1,000 = 2.778 (kW). 4. It is recommended that, as variable frequency technology becomes more mature and its cost decreases, variable frequency pumps be used to regulate the flow rate in the pump outlet pipeline system, thereby replacing control valves. For exhaust gas venting, when both the flow rate and pressure difference are high, it is recommended to use this exhaust gas turbine for power generation or to drive a turbine. Lao K writes every week about various aspects of the petrochemical design industry. For previous posts, please see the links below: Post 1: The development history of China’s survey and design industry – https://bbs.hcbbs.com/thread-3031149-1-1.html Post 2: Management in the survey and design industry – https://bbs.hcbbs.com/thread-3036417-1-1.html Post 3: The development of drawing techniques in design firms – https://bbs.hcbbs.com/thread-3039960-1-1.html Post 4: The evolution of professional divisions in petrochemical design firms – https://bbs.hcbbs.com/thread-3045261-1-1.html Post 5: The organizational structure of petrochemical engineering companies – https://bbs.hcbbs.com/thread-3050591-1-1.html Post 6: Contract management in design management – https://bbs.hcbbs.com/thread-3054762-1-1.html Post 7: Management of basic design data in design management – https://bbs.hcbbs.com/thread-3058184-1-1.html Post 8: Quality management in design management – https://bbs.hcbbs.com/thread-3061718-1-1.html Post 9: Schedule management in design management – https://bbs.hcbbs.com/thread-3062161-1-1.html Post 10: Document management in design management – https://bbs.hcbbs.com/thread-3069580-1-1.html Post 11: Meeting management in design management – https://bbs.hcbbs.com/thread-3073327-1-1.html Post 12: Management of bonus distribution in design management – https://bbs.hcbbs.com/thread-3077492-1-1.html Post 13: The construction process for petrochemical projects – https://bbs.hcbbs.com/thread-3082708-1-1.html Post 14: An illustrated overview of process design content and procedures – https://bbs.hcbbs.com/thread-3086849-1-1.html Post 15: Explanation of PID versions and management of PID modifications – https://bbs.hcbbs.com/thread-3091076-1-1.html Post 16: Common energy-inefficient design practices in the petrochemical industry – https://bbs.hcbbs.com/thread-3095859-1-1.html Post 17: Project work breakdown – https://bbs.hcbbs.com/thread-3100837-1-1.html Post 18: Issues related to the training of design professionals – https://bbs.hcbbs.com/thread-3105529-1-1.html Post 19: A brief explanation of pump net positive suction head – https://bbs.hcbbs.com/thread-3109916-1-1.html Post 20: On excessive patent applications – https://bbs.hcbbs.com/thread-3111855-1-1.html