Characteristics and applications of self-operated pressure regulating valves

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Characteristics and applications of self-operated pressure regulating valves Yan Yi, Wang Songtian (Sinopec Group Shanghai Engineering Co., Ltd., Shanghai 200120) Abstract: This article introduces the working principle of the self-operated pressure regulating valve and the difference between the control valve and the principle, engineering application occasions and precautions. keywords: Self-operated pressure regulating valve ； Features ； Engineering design and application CLC classification number: TP214 document identification code: B article number: 1007/7324 (2004) 04/0089/03 The self-operated regulating valve is a control device that does not require external energy and relies on the pressure or temperature or flow changes of the measured medium to automatically adjust according to the preset value. It is an energy-saving instrument. It integrates control and execution of many functions into an independent instrument control system. It integrates the functions of transmitter, controller and actuator. It is different from the control valve in the general sense. Self-operated regulating valves include self-operated pressure (micro-pressure) regulating valves, self-operated (pressure difference) flow regulating valves, self-operated temperature regulating valves, etc. The self-operated pressure regulating valve is one of its family members. Since it does not require external energy, has a simple product structure, is easy to use, and requires less maintenance, it is especially suitable for urban heating, heating, and situations where there is no power supply and gas supply needs to be controlled. According to German reports, urban heating and heating systems using this product have improved thermal efficiency by 30% to 40% compared with before, and the energy saving effect is significant. This article only discusses the principle, structural characteristics and engineering application experience of self-operated pressure regulating valves. 1 Classification and working principle of self-operated pressure regulating valves 1.1 Classification of self-operated pressure regulating valves 1) Divided into two categories according to post-valve and pre-valve control: Self-operated post-valve (pressure reducing) control valve ； Self-operated front (pressure relief) control valve. 2) They are divided into two categories according to whether they have a pilot or not.: Direct acting self-operated regulating valve, as shown in Figure 1 ； Pilot-operated self-operated regulating valve, as shown in Figure 2. 1.2 Working principle 1) The working principle of pressure regulation behind the self-operated valve is shown in Figure 3. The pressure P1 in front of the valve becomes the pressure P2 behind the valve after being throttled by the valve core and valve seat. P2 is input into the upper membrane chamber through the pipeline and acts on the top plate. The generated force is balanced with the reaction force of the spring, which determines the relative position of the valve core and valve seat, and controls the pressure behind the valve. When P2 increases, the force P2 acts on the top plate also increases. At this time, the force on the top plate is greater than the reaction force of the spring, causing the valve core to close toward the valve seat. At this time, the flow area between the valve core and the valve seat decreases, the flow resistance becomes larger, and P2 decreases until the force on the top plate is balanced with the reaction force of the spring, thereby reducing P2 to the set value. In the same way, when P2 decreases, the direction of action is opposite to the above. This is the working principle of post-valve pressure regulation. 2) The working principle of the pressure adjustment in front of the self-operated valve is the same as the pressure adjustment behind the valve (see Figure 4). Pay attention to the reverse installation of the valve core. 1.3 The difference between self-operated pressure regulating valves and control valves. The main difference between these two types of products is that the control valve not only requires external energy (such as power or air source) as driving energy, but also needs to receive external control instrument signals to change the relative position of the shutoff parts in the valve, thereby changing the fluid flow. The self-operated pressure regulating valve neither requires external energy nor receives signals from external control instruments. It can realize pressure regulation only by relying on the pressure signal of the medium to be adjusted. Characteristics of the self-operated pressure regulating valve Since the self-operated pressure regulating valve has no external driving energy, the operating force of the product is small. It has the following characteristics. 1) Balanced valve core structure, such as balanced single seat, double valve core balanced type, sleeve balanced type, etc. 2) Quick opening flow characteristics, the valve core is flat type. 3) Nominal diameter and valve body characteristics: DN20~100mm (or 125), the valve is equipped with a bellows, and the valve core is a single seat (some manufacturers' products DN20mm do not have a bellows, and the valve core is also a single seat) ； When the nominal diameter is greater than 100mm, there is no bellows in the valve upper cover, and the valve core has a double valve core or sleeve structure. 4) Recommended types of actuators: When the setting value is ≤0.6MPa, choose a membrane actuator. ； When the setting value is >0.6MPa, select the cylinder-type actuator. 3 Installation method 3.1 Setting of straight pipe sections A certain straight pipe section (generally around 6D (pipe diameter)) should be maintained in front and behind the self-operated pressure regulating valve. The distance between the pressure point in front of the valve and the valve should be greater than 2D ； The pressure taken after the valve should be greater than 6D. Pressure gauges should also be installed in front and behind the valve. The pressure gauge should be close to the pressure taking point so that the set value and the pressure taking value are truly consistent. 3.2 Setting up the bypass system In order to ensure that production can continue to operate during maintenance and failure, it is best to set up a bypass system. 3.3 Filter settings The filter in this system can be omitted when the process medium is clean and free of impurities. When there are impurities in the medium or a self-operated pressure regulating valve with a pilot is used, a filter should be installed to prevent blocking the pressure pipeline or pilot, blocking the cylinder actuator and valve core, etc. Note: The pressure sensing method shown by the dotted line in the figure is not recommended. If the installation space or funds of the pipeline are limited, the filter can be installed upstream of the stop valve in front of the valve to replace the filter. 3.4 Relationship between installation method, medium and temperature 1) In principle, the installation method of self-operated pressure regulating valve should be adopted. Gas media should be installed upright (actuator on top, valve body on the bottom), and liquid and steam media should be installed upside down. 2) When the temperature of the gas medium is higher than 70°C and lower than 140°C, and the temperature of the liquid medium is higher than 140°C, in addition to being inverted, the self-operated pressure regulating valve should also be installed with an isolation tank on the pressure induction pipeline, and the pressure induction pipeline, isolation tank, and membrane head should be filled with refrigerant to prevent the diaphragm from aging due to high temperature. 3) When the gas medium temperature is higher than 70°C and lower than 140°C, if the upright installation is still adopted, the use of high-temperature diaphragms (such as ethylene-propylene rubber diaphragms, silicone rubber diaphragms, etc.) should be indicated in the design document (equipment table), otherwise it will cause ordinary diaphragms to age. 4 Applications and Precautions 4.1 Applications and Precautions in Applicable Places The self-operated pressure regulating valve is mainly suitable for pressure control of non-corrosive and low-viscosity liquids, gases, steam (the maximum temperature can reach 300~350°C) and other media. Such as light oil, water, water vapor, air, etc. The following should be noted when providing design conditions and selecting valves. 1) The proposed pressure and set value conditions before and after the valve should be close to the actual process conditions. Self-operated pressure regulating valves have stricter requirements on process conditions than general control valves. After the process parameters are determined, no large-scale changes are allowed. Because the set value adjustment range allowed by this type of product is small. Generally, the allowable deviation of the setting value of the direct-acting control valve is ±8%, and the allowable deviation of the setting value of the pilot-operated control valve is about ±4%. If the allowable pressure adjustment value of the setting spring is exceeded, in order to meet the original set value requirements, the setting spring must be replaced, which needs to be returned to the manufacturer to complete. 2) When selecting the allowable pressure difference, attention should be paid to the small allowable pressure difference of the product. This is due to the small output force of this product and the small allowable pressure difference between the inside and outside of the bellows. The allowable pressure difference is smaller than that of ordinary control valves, and it cannot increase the pressure difference by increasing the air supply pressure like ordinary control valves. Especially when the diameter is small, the pressure difference between the two is even greater. As listed in Table 1. Table 1 Allowable pressure difference comparison table DN/mm 15 20 25 40 50 Δp (direct-acting pressure regulating valve)/MPa 1.6 1.6 1.6 1.6 1.6 Δp (ZJHP precision small single-seat control valve)/MPa 6.4 6.4 6.4 3.39 Note: Δp is the allowable differential pressure. 3) Since the adjustment accuracy of this product is ±5% and the flow characteristics are quick-opening characteristics, it is only suitable for occasions where there is no external energy and the adjustment quality requirements are not high. 4) This product is not suitable for use in the reciprocating pump outlet pressure/flow adjustment scheme. This solution is one of the common methods of bypass adjustment (see Figure 7). When selecting this solution, it should be noted that the pressure control circuit should not use a direct-acting (self-operated) pressure regulating valve. Figure 7 Reciprocating pump outlet pressure/flow adjustment scheme This is because in order to ensure the normal operation of the self-operated pressure regulating valve, this product requires that after the process parameters are determined, no large-scale changes are allowed. If used in a reciprocating pump, the output flow of the reciprocating pump pulsates periodically, causing the valve to pulsate periodically near the set value, thus failing to stabilize the pressure and causing the entire control system to fail to work properly. From the above analysis, it can be seen that it is a feasible solution for the self-operated front-valve control valve to be used for pressure relief control at the outlet of the reciprocating pump. That is, the valve pulsates periodically near the set value so that the pump outlet pressure does not exceed the set pressure value. This solution has been used in the Shanghai refinery filling station for many years. 4.2 Applications and precautions in special media 4.2.1 Applications in media with higher viscosity From the principle and structural characteristics of the self-operated pressure regulating valve, it can be seen that whether this product can be used in media with higher viscosity requires comprehensive consideration from the following two aspects. 1) The influence of the viscosity of the adjusted medium on the actuator—the pressure tube, pilot, and diaphragm. In order to avoid this impact, an isolation tank can be installed on the pressure pipe and filled with isolation fluid so that the viscous medium does not block the pressure pipe and enter the actuator, affecting the transmission of the force of the regulated medium. 2) The influence of the viscosity of the medium to be adjusted on the valve trim and balancing component - bellows. This is because when the regulated medium flows through the valve trim, it will be introduced into the balancing pressure component - the inside and outside of the bellows. If the viscosity is too high, it will not be able to perform the pressure balancing function. If the medium is easy to solidify after shutdown, the corrugated pipe will not be able to work normally after startup. The above analysis is ultimately a matter of defining the viscosity value of the medium. Currently, there are no recommended values ​​in product samples from domestic manufacturers. Foreign manufacturers have recommended that in the case of oil and liquid, the kinematic viscosity of the medium allowed is 600mPa·s. ； The allowed medium viscosity of Samson is 100mPa·s (density 0.8). It can only be used if both of the above requirements are met. This has been proven by many examples. For example, this type of product produced by Shanghai Zhongtai Automation Instrument Factory has been used for many years in the pressure control of ethanol tar in the tank area of ​​Shanghai Petrochemical Plant (viscosity is 190mPa·s) and the pressure control of fuel oil/heavy oil in Yan'an Refinery (viscosity is 135mPa·s). 4.2.2 High viscosity media should not be used. 4.2.3 Applications in gas media that do not allow leakage. When using gas media that do not allow leakage, for safety reasons and to avoid waste of precious gas, the following two points should be considered. 1) The sealing performance of the stuffing box and other joints reaches the standard. The product should comply with * * The standard GB/T4123/92 pneumatic control valve requires no leakage, which is the same as the general control valve requirements. 2) The air chamber sealing requirement of the pneumatic actuator must ensure no leakage. The requirements for this product are higher than those specified in GB/T4123/92, and leakage is not allowed because the GB/T4123/92 standard stipulates the tightness of the air chamber.: “The air chamber of the pneumatic actuator should be airtight. Under the rated air source pressure, the pressure drop in the membrane air chamber shall not be greater than 2.5kPa within 5 minutes. ； ”In view of the lack of unified product standards for this type of products in China, many products do not have strict requirements or do not consider the requirements for the use of this gas medium. They only refer to GB/T4123/92 for production, which is detrimental to safe production or saving valuable raw materials. Therefore, this product is not suitable for decompression of liquefied gas. If it is used, detailed discussion with the manufacturer is required. 4.2.4 Caution should be exercised when using this product in corrosive media. Just like when using it in high viscosity environments, the corrosion resistance of the actuator and regulating mechanism must be considered from the media perspective. It can only be used when both are satisfied. It can be seen that it has more complex requirements than the control valve and has a narrower use surface. In order to facilitate designers' selection in corrosive media situations, the current status of anti-corrosion materials for actuators and adjustment mechanisms is listed in Table 2 for reference. Taking the bellows balanced valve as an example, the current domestic anti-corrosion materials for bellows can only be 304 and 316. If only selected from the adjustment mechanism, it cannot be used in media such as chlorine, sodium hypochlorite, liquid chlorine, chromic acid, hydrochloric acid, sulfuric acid, zinc chloride, etc. Increased usage limitations. When used in this medium, the quickest way is for the design and manufacturing units to jointly confirm whether it can be used based on process conditions and requirements and manufacturing level. The calculation of the self-operated pressure regulating valve is the same as that of the control valve and will not be repeated. Table 2 Corrosion-resistant materials of actuator and adjusting mechanism Upper and lower diaphragm cover carbon steel, 304, 316, Monel alloy, Hastelloy cylinder carbon steel, 304, 316, Monel alloy, Hastelloy diaphragm fluorine rubber, polytetrafluoroethylene 1) Valve body carbon steel, 304, 316, Monel, Hastelloy valve trim carbon steel, 304, 316, Monel, Hastelloy bellows 304, 3162) Note: 1) For the corrosion resistance of the diaphragm, please refer to the diaphragm characteristics and application range table of commonly used pneumatic diaphragm actuators compiled by Shanghai Zhongtai Automation Instrument Factory. ； 2) For the corrosion resistance of metal materials, please refer to the corrosion resistance material table provided by the American Instrument Society.