【Common Issues in Haichuan Pipeline Design】Incorrect piping layout, valves installed incorrectly! Self-acting valve piping guide
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The 25th National \"Safety Production Month\" in 2026: Everyone should talk about safety and know how to handle emergencies; identify and address potential risks and hazards. -------------------------------------------------- During the design, installation, and inspection of pipelines, small issues that have been overlooked or not taken into consideration are often discovered. These issues are collected and compiled into a series, which is being updated continuously. Everyone is welcome to actively participate in discussions. There is a compilation post on \"Common Problems in Haichuan Pipeline Design\", which is still being updated; feel free to join the discussions at https://bbs.hcbbs.com/thread-5705828-1-1.html ------------------------------------------------------------------ Self-acting control valves rely on the pressure and temperature of the fluid flowing through them as the driving force for regulation, eliminating the need for external power sources, gas supplies, or secondary instruments. Based on structure and mode of action, they can be divided into two main categories: direct-acting and indirect-acting. I. Direct-acting control valves, also known as spring-loaded control valves, contain elastic components such as springs, diaphragms, and thermocouples. They rely on the balance between the elastic force and the feedback signal to automatically regulate pressure, temperature, or flow rate. II. Indirect-acting control valves: By adding a pilot valve on top of the direct-acting structure, it is possible to amplify the feedback signal; this amplified signal then drives the actuator to move the main valve’s disc, thereby changing the valve’s opening degree and achieving more stable and precise control. Based on the type of feedback signal, indirectly acting valves can be further divided into three categories: self-acting pressure control valves, which use the pressure at the valve outlet as the feedback signal and employ a signal tube to drive the actuator for control purposes. Self-acting flow control valve: A orifice plate or other flow-resisting element is installed at the valve outlet, and the pressure difference across the orifice plate is used as a signal to drive the actuator, thereby achieving constant flow control. Self-acting temperature control valve: A temperature sensor (thermocouple) is installed behind the valve; the thermal expansion and contraction of the fluid within the sensor drives the actuator, thereby enabling automatic temperature control.
III. Classification and Working Principle of Self-acting Pressure Control Valves Self-acting pressure control valves do not require any external power source; they utilize the energy of the medium being regulated to achieve automatic control. They are particularly suitable for applications where there is no electricity or gas available, and they offer advantages such as energy savings and the ability to adjust the set pressure continuously within a specified range. Suitable for pressure control of low-viscosity, non-corrosive media such as water, steam, air, and light oils. (1) Classification of self-acting pressure control valves: Based on the control position, they are divided into pressure control after the valve (reduction type) and pressure control before the valve (pressure relief/holding type). Self-acting pressure control valves are divided into two types based on the location of the pressure measurement point: those with the measurement point before the valve, which are used to maintain a constant pressure before the valve; and those with the measurement point after the valve, which are used to maintain a constant pressure after the valve. When the pressures before and after the valve are applied to both sides of the actuator’s chamber simultaneously, a self-acting differential pressure control valve can maintain a constant pressure across the valve. The differential pressure between the two sides of a orifice plate installed in a pipeline can also be applied to both sides of the diaphragm actuator’s chamber to create a self-acting flow control valve; alternatively, flow can be detected and then regulated using a self-acting differential pressure control valve. Based on the structural form, they are divided into: direct-acting type and controller-operated type. (II) Working principle of self-acting pressure regulation after the valve: The medium passes through the valve core and seat, undergoing throttling as a result of the pressure P₁ before the valve, and this results in a pressure P₂ after the valve. P₂ is introduced into the diaphragm chamber of the actuator through a signal tube; the force it exerts on the top plate balances the force from the spring, thereby determining the position of the valve element and stabilizing the pressure behind the valve. When P₂ increases, the thrust from the top plate exceeds the spring force; the valve core moves in the closing direction, the flow area decreases, and P₂ drops back to the set value ； When P₂ decreases, the effect is opposite: the valve core opens automatically, causing P₂ to rise back to the set value. Through the aforementioned dynamic balance, automatic control of constant pressure downstream of the valve is achieved.
IV. Precautions for selecting self-acting pressure control valves: a. They are not suitable for use in pressure and flow control systems at the outlet of reciprocating pumps. Since the output flow rate of the reciprocating pump is periodically pulsating, this valve also experiences periodic fluctuations around its set value; as a result, a stable pressure cannot be maintained, preventing the entire control system from functioning properly. b. The pressure conditions before and after the valve, as well as the set values, should be close to the actual process conditions. Self-acting pressure control valves have stricter requirements for process conditions compared to ordinary control valves. Once the process parameters are determined, significant changes are not allowed. c. The self-acting pressure control valve has a regulation accuracy of ±5% and a fast-opening flow characteristic; therefore, it is only suitable for applications without external power supply and where high control precision is not required. d. The allowable pressure difference for self-acting pressure control valves is small. This is because the product has a low output force and a small allowable pressure difference between the inside and outside of the bellows, resulting in a lower allowable pressure difference compared to ordinary control valves; moreover, it is not possible to increase this pressure difference by raising the supply air pressure as is the case with ordinary control valves. V. Installation plan for self-acting pressure control valves: (1) The condenser must be located above the actuator of the pressure control valve, and below the piping downstream of the valve (in the case of a valve controlled downstream) or upstream of the valve (in the case of a valve controlled upstream), in order to ensure that condensate remains within the condenser. (2) The pressure measurement point should be located at an appropriate distance from the pressure regulating valve; for valves located before the pipeline, the diameter should be more than 2 times that of the pipeline, while for valves located after the pipeline, it should be more than 6 times the pipeline diameter. For details, see the piping design below. (3) To facilitate on-site maintenance and operation, adequate space should be left near the pressure regulating valve, and stop valves and bypass manual valves should be installed in front of and behind the valve. (4) When the diameter of the pressure regulating valve is too large (DN≥100), fixed supports should be provided. (5) The flow direction of the medium should be consistent with the direction indicated by the arrow on the valve body; the centers of the pipes before and after the valve should be aligned with the centers of the two flanges of the pressure regulating valve, in order to avoid excessive stress on the valve body. (6) A filter should be installed in front of the valve to prevent impurities in the medium from causing blockages. (7) The pressure regulating valve should be installed in environments where the ambient temperature does not exceed -25°C to 55°C. VI. Piping design for self-acting pressure control valves: When these valves are used in steam or highly viscous liquid media, they are usually installed upside down on horizontal pipes. The pressure measurement point should be located at an appropriate position on the pressure regulating valve: the pressure reduction before the valve should be more than 2 times the diameter of the pipe, while the pressure reduction after the valve should be more than 6 times the diameter of the pipe. 、、
VII. Debugging and Use 1. Pressure Setting: Pressure adjustment is required before initial use. First, slowly open the upstream valve; once the system stabilizes, adjust the outlet pressure using the spring adjustment knob. The set value should be 80% of the inlet pressure. Monitor the changes on the pressure gauge during adjustment to ensure that the pressure remains stable at the target value. 2. Operation monitoring: After installation, start the system and closely monitor the operation of the valves. Check for any leaks, abnormal noises, or vibrations; if any issues are detected, stop the system immediately, adjust the relevant parameters, or contact a professional for repairs. 3. Routine maintenance: Regularly inspect the valves, clean the pressure gauges and filter screens located before and after the valves. Check the adjustment springs every quarter to ensure they retain their elasticity. Before leaving the system unused for an extended period, drain any water remaining in the pipes to prevent damage caused by freezing.
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