[Haichuan’s Guide to Chemical Equipment] Series Post – What are the bypass lines of pumps?
Thread Content
Get to know the equipment, understand it, and make good use of it. [Haichuan’s Illustrated Guide to Chemical Equipment] series posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719196. Everyone is welcome to participate in the discussions. ----------------------------------------------------------- In chemical pipeline design, the pump’s main line is responsible for transporting the materials, while the bypass line serves as a crucial element for ensuring the safe operation of the pump and enabling complex process control. As chemical engineers, we must never overlook these bypass lines when drawing P&ID diagrams. Depending on the type of pump (centrifugal pump vs. positive displacement pump) and the properties of the process fluid, pump bypass systems are mainly classified into the following five categories: 1. Minimum Flow Bypass – Suitable primarily for centrifugal pumps. Purpose of existence: When a centrifugal pump operates at extremely low flow rates or with the outlet valve completely closed (under pressure buildup), the fluid undergoes intense friction repeatedly inside the pump casing, and all of the mechanical energy is converted into heat energy. This can cause the liquid inside the pump to vaporize rapidly (severe cavitation), the rotor to become stuck due to thermal expansion, and even lead to an explosion. Operation mode: This branch line usually runs from the pump outlet and returns to the inlet tank. When the flow rate in the main line is below the pump’s “Minimum Continuous Stable Flow (MCSF)”, the minimum flow bypass valve opens automatically, allowing a portion of the fluid to circulate and carry away heat, thereby protecting the centrifugal pump.
2. Capacity Control Bypass: Applicable to: exclusively volumetric pumps (such as reciprocating pumps, gear pumps, screw pumps). Purpose of existence: Positive displacement pumps are \"tough guys\"; as long as they are running, the volume discharged per revolution remains constant. It is strictly prohibited to adjust the flow rate by closing the outlet valve! If the outlet valve is closed, the pressure inside the pipe will surge instantly, potentially causing the pipe to burst or the motor to burn out. Operation method: The only correct way to regulate the flow rate sent downstream in a process using a positive displacement pump is to fully open the outlet valve and then activate a bypass line that leads back to the inlet. By adjusting the opening degree of the valve on the bypass line, the excess material is \"released\" back to the inlet, thereby enabling control over the flow rate downstream. 3. Safety Relief Bypass: Applies mainly to positive displacement pumps as well. Purpose of existence: It serves as the last line of defense. To prevent operational errors (such as starting the reciprocating pump without opening the outlet valve) or overpressure caused by a sudden blockage in the downstream piping. Operation mode: A short bypass line equipped with a pressure safety valve (PSV) is connected in parallel directly to the pump outlet and back to the inlet. Once the pressure exceeds the design limit, the safety valve opens instantly, and the material circulates through a bypass line to prevent physical damage to the equipment. 4. Warm-up / Cool-down Bypass: Suitable for standby pumps used to transport high-temperature media (such as hot oil at >200°C) or cryogenic media (such as liquid nitrogen, LNG). Purpose: To prevent mechanical damage caused by thermal expansion and contraction. If a cold standby pump is suddenly filled with hot oil at 300°C, the intense thermal stress can cause the pump casing to crack or the rotor to bend. Operation mode: There is usually a very small orifice plate (or slightly open valve) on the heat pump line. In standby mode, the pressure difference in the main pipeline is utilized to allow a very small amount of hot fluid to continuously flow in the reverse direction through the standby pump, keeping it in a hot standby state at a temperature close to that of operation (the same principle applies to cold pumps). 5. Startup Bypass: Applicable to large, high-power high-pressure pumps. Purpose: To reduce starting current and starting load. At the beginning of startup, the main line valve is closed, and the unobstructed startup bypass line is opened, allowing the motor to reach its rated speed under no-load or light-load conditions; thereafter, it switches smoothly to the main line to prevent a power grid shutdown at the moment of startup.
【Haichuan Illustrated Chemical Equipment】Series Summary Post–Getting to Know Equipment, Understanding Equipment, and Making Good Use of It https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719196 (Source: Haichuan Chemical Forum (Hua Haichuan Liu hcbbs))
【Haichuan May Day Spending Spree】I work, I’m happy! https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719046 (Source: Haichuan Chemical Industry Forum (Hua Hai Chuan Liu hcbbs))
【Special Equipment Safety Management and Standardization】Summary Post! Constantly being updated – feel free to communicate! ! ! https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5718553 (Source: Haichuan Chemical Industry Forum (Hua Hai Chuan Liu hcbbs))
【HaiChuan Safety Discussion】Hidden Dangers Around Us – An Unpowered Alarm Is Just a Decoration https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719243 (Source: HaiChuan Chemicals Forum (HuaHaiChuanLiu hcbbs))
【Haichuan Chemical Valve Management】Series of Posts—Proper management of valves is the foundation for stable operation. Everyone is welcome to share and exchange ideas! ! https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719309 (Source: Haichuan Chemical Industry Forum (Hua Haichuan Liu hcbbs))
【HaiChuan Safety Discussion】Compilation of Hidden Dangers Around Us – Illustrated with photos and texts; everyone is welcome to participate in the discussion. https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5705518 (Source: HaiChuan Chemical Forum (HuaHaiChuanLiu hcbbs))
【Haichuan Teahouse】Put all your worries behind you! Everyone who feels stressed, come and smile! https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719375 (Source: Haichuan Chemical Industry Forum (Hua Haichuan Liu hcbbs))
【HaiChuan Safety Discussion】Hidden Dangers Around Us – Happy Holidays! Photos That Will Drive a Chemical Plant Safety Manager Crazy! ! https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719427 (Source: Haichuan Chemical Industry Forum (Hua Haichuan Liu hcbbs))