Key inspection areas of the regulating valve in case of malfunction 

    1. For regulating valves used in high pressure differential and corrosive media applications, the inner wall of the valve body is often subjected to impact and corrosion from the medium. Therefore, it is essential to focus on inspecting the pressure resistance and corrosion resistance of the valve body's inner wall. 

    2. Valve seat: During operation, the threaded inner surface used to secure the valve seat is prone to corrosion due to medium infiltration, which may cause the valve seat to loosen. Caution should be exercised during inspection. For valves operating under high pressure differentials, it is also necessary to check whether the sealing surface of the valve seat has been damaged by erosion. 

    3. Valve core: The valve core is the movable part of the regulating valve during operation, and it is subject to severe erosion and corrosion due to the scouring of the medium. During maintenance, it is important to carefully inspect each part of the valve core for corrosion and wear, especially under high pressure differential conditions 

    The wear of the core is more severe, (due to cavitation) and should be noted. When the valve core is severely damaged, it should be replaced. Additionally, attention should be paid to whether the valve stem exhibits similar phenomena, or if there is any looseness in the connection with the valve core. 

    4. Check whether the "O" ring and other sealing gaskets are aged or damaged. 

    5. Attention should be paid to whether the polytetrafluoroethylene (PTFE) packing, sealing lubricating grease have aged, and whether the mating surface has been damaged. Replacement should be carried out when necessary.

    Ways to increase lifespan

    01. Method for extending lifespan through operation at a large opening degree

    Let the regulating valve operate at its maximum opening, such as 90%, from the very beginning. This way, damages like cavitation and erosion will occur on the head of the valve core.

    As the valve core is damaged, the flow rate increases. The corresponding valve should be closed a little more. This process of continuous damage and gradual closure allows the entire valve core to be fully utilized until the root of the valve core and the sealing surface are damaged and cannot be used anymore.

    At the same time, a large throttle clearance at full opening reduces erosion, which can increase the valve's lifespan by 1 to 5 times compared to operating it at intermediate or small openings from the beginning. If a chemical plant adopts this method, the valve's lifespan can be increased by 2 times.

    02. Method of reducing s, increasing working opening, and improving lifespan

    Reducing S, which means increasing the losses in the system excluding the regulating valve, will decrease the pressure drop allocated to the valve. To ensure the flow through the regulating valve, the valve opening must be increased. At the same time, the decrease in pressure drop on the valve will also mitigate cavitation and erosion.

    The specific measures are as follows:

    An orifice plate is installed downstream of the valve to throttle and consume pressure drop;

    Close the manual valve in series on the pipeline until the regulating valve achieves a more ideal working opening.

    When the valve is initially selected to operate at a small opening, this method is very simple, convenient, and effective.    

    03. Method of reducing diameter, increasing working opening, and improving service life

    Increase the working opening by reducing the valve diameter.

    Specific measures include:

    Replace the valve with one of a smaller size, such as switching from DN32 to DN25;

    The valve body remains unchanged, and the valve core and seat with a smaller diameter are replaced.

    During the major maintenance of a chemical plant, the throttling component dgl0 was replaced with dg8, resulting in a doubled lifespan.

    04. Method of improving lifespan by transferring the location of damage

    Move the severely damaged area to a secondary position to protect the sealing and throttling surfaces of the valve core and seat.

05

    "Method of increasing the lifespan by expanding the throttle passage"

    The simplest way to increase the throttle passage is to thicken the valve seat, making the valve seat hole larger and forming a longer throttle passage.

    On the one hand, it can delay the sudden expansion after flow closure throttling, shifting the damage location away from the sealing surface; on the other hand, it increases the throttling resistance, reduces the degree of pressure recovery, and mitigates cavitation.

    Some designs incorporate stepped or wavy patterns within the valve seat holes, aiming to increase resistance and mitigate cavitation. This approach is frequently employed in the modification of high-pressure valves in imported devices and in the retrofitting of older valves, and has proven to be highly effective.

06

    "Method of changing flow direction to improve lifespan"

    In the flow-to-open type, the flow is directed towards the open direction, and cavitation and erosion mainly act on the sealing surface, quickly damaging the root of the valve core and the sealing surface between the valve core and the valve seat. In the flow-to-close type, the flow is directed towards the closed direction, and cavitation and erosion occur after throttling, below the sealing surface of the valve seat, protecting the sealing surface and the root of the valve core and extending their lifespan.

    For valves designed for flow-through use, when the issue of extending their lifespan becomes prominent, simply changing the flow direction can extend their lifespan by 1 to 2 times.

    07. Method of using special materials to improve lifespan

    To resist cavitation (which destroys the shape into small honeycomb-like dots) and erosion (streamlined small grooves), special materials resistant to cavitation and erosion can be used to manufacture throttling components. Such special materials include 6YC-1, A4 steel, stellite, cemented carbide, etc.

    To resist corrosion, materials with better corrosion resistance and certain mechanical and physical properties can be used instead. These materials are divided into two categories: non-metallic materials (such as rubber, PTFE, ceramics, etc.) and metallic materials (such as Monel, Hastelloy, etc.).

  08. Method of improving valve life by changing valve structure

The purpose of enhancing the lifespan can be achieved by modifying the valve structure or selecting valves with a longer lifespan, such as multi-stage valves, anti-cavitation valves, and corrosion-resistant valves.

    09. Method of reducing stroke to improve diaphragm life

    For two-position regulating valves, when the actuation frequency is very high, the diaphragm will quickly rupture during up-and-down folding, and the damage is often located on the circumference of the tray.

    The simplest and most effective way to extend the lifespan of the diaphragm is to reduce the stroke. The reduced stroke value is 1/4 dg. For example, for a valve with a dgl25, its standard stroke is 60mm, which can be reduced to 30mm, a reduction of 50%.

    In addition, the following factors can also be considered:

    Try to minimize the pressure in the membrane chamber while satisfying the conditions for opening and closing;

    Improve the smoothness of the joint between the tray and the film.

    Anti-blocking (clogging) method for regulating valves that often get stuck or blocked

    01. Cleaning method

    Weld slag, rust, and slag in the pipeline can cause blockages or sticking in the throttle orifice, guide parts, and lower valve cover balance hole, resulting in scratches and nicks on the valve core surface and guide surface, as well as indentations on the sealing surface. This often occurs in newly commissioned systems and during the initial stage of operation after major maintenance. It is the most common fault.

    In such cases, it is necessary to disassemble and clean, removing any residue. If the sealing surface is damaged, it should also be ground. At the same time, open the bottom plug to flush out any residue that may have fallen into the lower valve cover from the balance hole, and flush the pipeline. Before putting into operation, fully open the regulating valve and allow the medium to flow for a period of time before resuming normal operation.

    02. External scouring method

  When using ordinary valves to regulate media that are prone to sedimentation and contain solid particles, blockages often occur at the throttle and guide areas. External flushing gas and steam can be connected to the bottom plug of the lower valve cover to address this issue.

    When the valve becomes blocked or jammed, open the external gas or steam valve to complete the flushing process without adjusting the regulating valve, thus enabling the valve to operate normally.

    03. Method of installing a pipeline filter

    For small-bore regulating valves, especially ultra-small flow regulating valves, the throttling gap is extremely small, and there must not be any debris in the medium.

    In case of such blockage, it is advisable to install a filter on the pipeline upstream of the valve to ensure smooth passage of the medium.

    For regulating valves equipped with positioners, the most common malfunction is the blockage of the throttle port in the air circuit, which leads to abnormal operation of the positioner.

    Therefore, when operating with a positioner, it is necessary to properly handle the air source. The commonly adopted method is to install an air filter regulator on the air source pipeline before the positioner.

    04. Method of increasing throttle clearance

    If solid particles in the medium or welding slag and rust washed away from the pipeline cause blockages or jams due to failure to pass through the throttle orifice, throttling components with larger throttle gaps can be used instead - such as valve cores and sleeves with windowed or open throttle areas. Since their throttle areas are concentrated rather than distributed circumferentially, the malfunctions can be easily resolved.

    If it is a single-seat or double-seat valve, the plunger-shaped valve core can be replaced with a "V"-shaped valve core, or changed to a sleeve valve, etc.

    For example, a double-seat valve in a chemical plant often got stuck. After recommending the use of a sleeve valve, the problem was immediately resolved.

   05. Medium scouring method 

    Utilize the scouring energy inherent in the medium to flush away and remove substances prone to sedimentation and blockage, thereby enhancing the anti-blockage function of the valve. 

    Common methods include: 

    Convert to flow-closed type for use; 

    Adopt streamlined valve body; 

    Place the throttle orifice at the most severely eroded area. When using this method, it is important to enhance the erosion resistance of the throttle component material. 

    06. Change straight-through to angular method 

    The straight-through design features an inverted S-shaped flow path, which is complex and contains numerous dead zones in the upper and lower chambers, providing a place for medium sedimentation. With angular connections, the medium flows through a 90-degree elbow, offering good scouring performance, small dead zones, and ease of design into a streamlined shape. Therefore, when a regulating valve with a straight-through design experiences minor blockage, it can be replaced with an angular valve. 

    Solution to external leakage of regulating valve 

    01. Method of adding sealing grease 

    For valves that have not been applied with sealing grease, consideration can be given to adding sealing grease to enhance the sealing performance of the valve stem. 

    02. Method of adding filler 

    To enhance the sealing performance of the packing on the valve stem, the method of increasing the packing can be employed. Typically, a double-layer or multi-layer mixed packing form is adopted. Simply increasing the quantity, such as from 3 pieces to 5 pieces, does not yield significant results.

    03. Method of replacing graphite packing

    The widely used PTFE (polytetrafluoroethylene) packing, with its operating temperature range of -20 to +200℃, experiences a significant decrease in sealing performance, accelerated aging, and shortened lifespan when the temperature fluctuates greatly between the upper and lower limits.

    Flexible graphite packing can overcome these shortcomings and has a long service life. Therefore, some factories have completely replaced PTFE packing with graphite packing, and even newly purchased control valves have had their PTFE packing replaced with graphite packing for use. However, the return difference of graphite packing is large, and some may even exhibit a creeping phenomenon initially, which must be taken into account.

    04. Change the flow direction, placing P2 on the valve stem end

    When △P is large and P1 is also large, sealing P1 is obviously more difficult than sealing P2. Therefore, the method of changing the flow direction can be adopted, that is, changing P1 at the valve stem end to P2 at the valve stem end. This is more effective for valves with high pressure and large pressure difference. For example, bellows valves usually consider sealing P2.

    05. Adopt lens pad sealing method

    For the sealing of the upper and lower covers, as well as the sealing between the valve seat and the upper and lower valve bodies, if it is a flat surface seal, the sealing performance may be poor under high temperature and high pressure, leading to leakage. In such cases, using a lens gasket for sealing can achieve satisfactory results.

    06. Replace the sealing gasket

    To date, most sealing gaskets still use asbestos plates, which exhibit poor sealing performance and short lifespan at high temperatures, leading to leakage. In such cases, spiral wound gaskets, "O" rings, etc. can be used instead, and many factories have already adopted them.

    07. Symmetrically tighten the bolts using the thin washer sealing method

    In the structure of a regulating valve sealed with an "O" ring, when using thick gaskets with significant deformation (such as spiral wound gaskets), if the compression is asymmetric and the force distribution is uneven, it is easy for the seal to break, tilt, and deform, seriously affecting the sealing performance.

    Therefore, during the maintenance and assembly of such valves, it is necessary to tighten the compression bolts symmetrically (note that they cannot be tightened all at once). It would be better to replace the thick sealing gasket with a thinner one, as this would help reduce the inclination and ensure proper sealing.

    08. Method of increasing the width of the sealing surface to prevent the flat valve core from jumping when closed and reduce its leakage

    Flat-type valve cores (such as the valve plugs of two-position valves and sleeve valves) have no guiding or guiding curved surfaces inside the valve seat. When the valve is in operation, the valve core is subjected to lateral forces, causing it to move from the inlet side towards the outlet side. The larger the fit clearance of the valve core, the more severe this unilateral phenomenon becomes. Coupled with deformation, misalignment, or a small chamfer on the sealing surface of the valve core (typically a 30° chamfer for guidance), when the valve is close to closing, the chamfered end face of the valve core's sealing surface may come into contact with the sealing surface of the valve seat, resulting in the valve core jumping during closing or even failing to close properly, greatly increasing the valve leakage.

    The simplest and most effective solution is to increase the size of the sealing surface of the valve core, ensuring that the minimum diameter of the valve core's end face is 1-5mm smaller than the diameter of the valve seat. This provides sufficient guidance to ensure that the valve core is guided into the valve seat, maintaining good contact with the sealing surface.

    Solutions to regulating valve vibration (8 methods)

    01. Stiffness-increasing method

    For oscillations and minor vibrations, increasing stiffness can be used to eliminate or reduce them. Methods such as selecting a spring with high stiffness or switching to a piston actuator are feasible.

    02. Increase damping method

    Increasing damping means increasing friction against vibration. For example, the valve plug of a sleeve valve can be sealed with an "O" ring, or graphite packing with a large friction force can be used. This can have a certain effect on eliminating or reducing minor vibrations.

    03. Method of increasing guiding dimensions and reducing fitting clearance

    Axial plug valves generally have smaller guiding dimensions, and all valve fit clearances are generally larger, ranging from 0.4 to 1mm, which is conducive to generating mechanical vibration. Therefore, in the event of minor mechanical vibration, the vibration can be reduced by increasing the guiding dimensions and decreasing the fit clearance.

    04. Change the shape of the throttling element to eliminate resonance

   The so-called vibration source of the regulating valve occurs at the throttle opening where the flow rate is high and the pressure changes rapidly. By changing the shape of the throttle component, the frequency of the vibration source can be altered. This issue is relatively easy to solve when the resonance is not strong.

    The specific method involves turning the valve core surface within the vibration opening range by 0.5~1.0mm. For instance, a self-operated pressure regulating valve installed near a factory's residential area produced whistling sounds due to resonance, disrupting the rest of the employees. After turning the valve core surface by 0.5mm, the resonance whistling disappeared.

    05. Method of eliminating resonance by replacing the throttle component

    The methods include:

    Change the flow characteristic, convert logarithmic to linear, or convert linear to logarithmic;

    Replace the valve core form. For example, change the axial plug form to a "V" shaped groove valve core, and change the double-seat valve axial plug type to a sleeve type;

    Replace the sleeve with a window opening with a sleeve with small holes, etc.

    In a nitrogen fertilizer plant, a DN25 double-seat valve often experienced vibration-induced breakage at the joint between the valve stem and the valve core. After confirming it as resonance, we replaced the linear characteristic valve core with a logarithmic one, and the problem was resolved. Another example is a DN200 sleeve valve used in a laboratory of an aviation college. The valve plug would rotate violently, making it unusable. After replacing the sleeve with a perforated one instead of a windowed one, the rotation immediately disappeared.

    06. Replace the type of regulating valve to eliminate resonance

    The natural frequencies of regulating valves with different structural forms are naturally different, and changing the type of regulating valve is the most effective way to fundamentally eliminate resonance.

    A valve that resonates excessively during use — experiencing strong vibrations (which can damage the valve in severe cases), intense rotations (even causing the valve stem to be broken or twisted), and producing loud noise (reaching over 100 decibels) — will see immediate improvement once replaced with a valve that has a significantly different structure. The intense resonance miraculously disappears.

    In the case of a new expansion project for a vinylon factory, a DN200 sleeve valve was selected, and all three of the aforementioned phenomena occurred. The DN300 pipeline subsequently vibrated, the valve plug rotated, and the noise level exceeded 100 decibels, with a resonance opening range of 20-70%. Considering the large resonance opening, a double-seat valve was substituted, and the resonance disappeared, with normal operation.

    07. Method of reducing cavitation vibration

  For cavitation-induced vibration resulting from the collapse of cavitation bubbles, it is naturally necessary to find ways to reduce cavitation.

    The impact energy generated by the bursting of bubbles is not applied to the solid surface, especially the valve core, but is absorbed by the liquid. The sleeve valve possesses this characteristic, so the axial plug type valve core can be changed to a sleeve type.

    All measures to reduce cavitation should be taken, such as increasing throttling resistance, increasing pressure at the constriction, and implementing pressure reduction in stages or in series.

    08. Avoid vibration source wave impact method

    External vibration sources can cause valve vibrations, which should obviously be avoided during the normal operation of regulating valves. If such vibrations occur, appropriate measures should be taken.

    Solution to high noise of regulating valve

    01. Method for eliminating resonance noise

    Only when the regulating valve resonates, can energy be superimposed to generate intense noise of over 100 decibels. Some manifest as strong vibration with low noise, while others exhibit weak vibration but very loud noise; some have both strong vibration and noise.

    This noise produces a single-tone sound with a frequency generally ranging from 3000 to 7000 hertz. Obviously, by eliminating resonance, the noise will naturally disappear.

    02. Method for eliminating cavitation noise

    Cavitation is the primary source of fluid dynamic noise. During cavitation, the collapse of bubbles generates high-speed impacts, causing intense turbulence locally and producing cavitation noise.

    This noise has a wide frequency range, producing a rattling sound similar to the sound emitted by sand and gravel contained in the fluid. Eliminating and reducing cavitation is an effective way to eliminate and reduce noise.

    03. Use the thick-walled pipeline method

  Using thick-walled pipes is one of the sound path treatment methods. Using thin-walled pipes can increase noise by 5 decibels, while using thick-walled pipes can reduce noise by 0 to 20 decibels. The thicker the wall for the same pipe diameter, or the larger the pipe diameter for the same wall thickness, the better the noise reduction effect.

    For a DN200 pipeline, when its wall thicknesses are 6.25mm, 6.75mm, 8mm, 10mm, 12.5mm, 15mm, 18mm, 20mm, and 21.5mm respectively, the noise reduction can be -3.5, -2 (i.e., increased), 0, 3, 6, 8, 11, 13, and 14.5 decibels respectively. Of course, the thicker the wall, the higher the cost.

    04. Adopting sound-absorbing material method

    This is also a relatively common and most effective method for sound path treatment. Sound-absorbing materials can be used to encase the noise source and the downstream pipeline.

    It must be pointed out that noise can propagate over long distances through fluid flow. Therefore, the effectiveness of noise elimination ends wherever sound-absorbing materials are applied and thick-walled pipes are used.

    This method is suitable for situations where the noise level is not very high and the pipeline is not excessively long, as it is a relatively costly approach.

    05. Series silencer method

    This method is applicable to the attenuation of aerodynamic noise, effectively eliminating noise within the fluid and suppressing the noise level transmitted to the solid boundary layer. For locations with high mass flow rates or high pressure drop ratios before and after the valve, this method is the most effective and economical.

    The use of absorptive series silencers can significantly reduce noise. However, from an economic perspective, the attenuation is generally limited to approximately 25 decibels.

    06. Soundproof box method

    Use soundproof boxes, houses, and buildings to isolate the noise source inside, so that the noise from the external environment is reduced to a level that is acceptable to people.

    07. Series throttling method

    In situations where the pressure ratio of the regulating valve is high (ΔP/P1 ≥ 0.8), the series throttling method is employed, which distributes the total pressure drop across the regulating valve and the fixed throttling element downstream of the valve. Using diffusers or porous flow restrictors is the most effective method among those for noise reduction.

    To achieve optimal diffuser efficiency, the diffuser (physical shape, dimensions) must be designed based on the installation conditions of each piece, ensuring that the noise level generated by the valve is equivalent to that generated by the diffuser.

    08. Choose low-noise valves

The low-noise valve gradually decelerates based on the tortuous flow path (multi channel, multi groove) of the fluid passing through the valve core and seat, in order to avoid supersonic flow at any point in the flow path. There are various forms and structures of low-noise valves (designed specifically for systems) available for use.

When the noise is not very loud, choosing a low-noise sleeve valve can reduce the noise by 10-20 decibels, which is the most economical low-noise valve.

Solution for Poor Stability of Regulating Valves

01. Method of changing the direction of unbalanced force application

In stability analysis, it is known that when the unbalanced force acts in the same direction as the valve closing, that is, when the valve tends to close, the stability of the valve is poor.

When the valve operates under the above unbalanced force conditions, the method of changing its direction of action is usually to change the flow closed type to the flow open type, which can easily solve the stability problem of the valve.

02. Avoiding valve instability zone operation method

Some valves are limited by their own structure and have poor stability when working at certain openings.

Double seat valve with an opening of less than 10%, causing instability issues due to flow opening at the upper ball and flow closing at the lower ball; The stability is poor in the vicinity where the slope of the unbalanced force changes alternately. Like butterfly valves, the alternating point is around 70 degrees; The double seat valve has an opening of 80-90%. When encountering such valves, working in unstable areas will inevitably result in poor stability. Avoiding working in unstable areas is sufficient.

03. Replace the valve with good stability

Valves with good stability have less variation in unbalanced force and good guidance. Among commonly used ball valves, sleeve valves have this major characteristic.

When the stability of single or double seat valves is poor, replacing them with sleeve valves will definitely improve their stability.