anti-surge valve

Large-scale installations generally have a main air self-protection system; when the anti-surge valve opens, the main air flow drops to zero immediately, and the control room activates this self-protection mechanism right away. Of course, there is also a interlock self-protection system for the booster pump, which prevents any reverse operation. The anti-surge valves are equipped with switching levers, allowing for seamless switching without having to stop the air flow. Relying on manual intervention to close valves in order to handle accidents is too slow; therefore, it’s necessary to maintain the instrumentation more carefully on a regular basis

The anti-surge valve must not be closed manually or the air supply cut off, as this could cause surging in the compressor and damage it.

Stall prevention valves are typically used in conjunction with compressors. The most important requirements are reliability, response speed, and protection against water hammer damage; slow response speeds fail to prevent stalling, while poor reliability makes them unreliable for use. Now, as the scale of these devices continues to increase, the diameter of the valve openings also grows larger. Under such high pressures, reliable linear-motion valve structures are required. Many companies in China are considering using rotary valves with simpler structures – butterfly valves – as alternatives; by reducing the opening angle and employing springs with high stiffness, it is possible to increase the speed of operation. However, compared to linear-motion valve structures, their operating speed remains much slower.

Surge, as the name implies, is similar to asthma in humans; it involves periodic airflow and backflow in the fan. Axial flow fans are more prone to surge, and severe cases of surge can lead to fatigue and damage of the fan blades. When a fan experiences surge, its flow rate fluctuates periodically and varies over a wide range, dropping to zero or even becoming negative. Such severe fluctuations in the fan flow, both positive and negative. Just like how asthma patients breathe. Due to the large fluctuations in flow rate, violent impacts of air currents occur, causing the fan itself to vibrate violently and simultaneously increasing the noise generated by the fan during operation. The hazards associated with surge occurring in high-capacity, high-pressure head fans are significant; it can lead to damage to the equipment and bearings, result in accidents, and directly affect the safe operation of the boiler. The general symptoms of a fan experiencing surge are as follows: 1. The current decreases and fluctuates frequently, while the outlet air pressure also drops and fluctuates. 2 The fan produces abnormal noise at high levels, experiences significant vibration, its casing temperature rises, and the surging vibration of the supply and exhaust fans causes fluctuations in the negative pressure inside the furnace, leading to unstable combustion. Common causes: 1. Excessive system resistance caused by dust accumulation and blockage in the smoke ducts, or insufficient opening of the dampers in those ducts. (We have encountered this occasionally, but not frequently.) When two fans operate in parallel, excessive differences in the opening angles of their guide vanes can cause the fan with the smaller opening angle to operate in the surge zone. (A common scenario we face is the detachment of the linkage in the fan’s guide vane actuator when loads change, which results in asynchronous adjustment of the guide vanes of the two fans and thus large discrepancies.) 4 The fans operate at low capacity for extended periods of time. The general principle for handling this issue is to adjust the load and reduce the guide vane opening of the fans with higher output so that their outputs are similar; thereafter, investigate based on the possible causes mentioned above and take appropriate action. To prevent instability in the fan, the following measures can be taken: (1) Keep the fan operating in a stable range. Therefore, fans with a P-Q characteristic curve that lacks a hump should be selected for use in the piping system ； If the fan’s performance curve has a hump, the fan should be kept operating in the stable region (i.e., the descending part of the P-Q curve). (2) Use recycling. Some of the exhaust gas is directed back to the fan inlet, preventing the fan flow from becoming too low and causing it to operate in an unstable range. (3) Install a vent valve. When the flow rate is less than or close to the critical flow rate for surge, the vent valve is opened to release some gas and reduce the pressure in the pipeline, thereby preventing surge. (4) Use appropriate adjustment methods to change the flow rate of the fan itself. Methods such as changing the rotational speed and the blade installation angle can be employed to prevent the fan’s operating point from falling into the surge zone. (5) Immediately increase the load on the surge ventilator, reduce the load on the normal ventilators, or decrease the boiler’s output as well as the output of the ventilators. (6) If the fan operates in a surge condition, manually open the moving blades of the surging fan quickly until the surge disappears, and then adjust the fan’s output. (7) Keep the flow rate of the pump or fan consistently greater than QK. If the flow rate required by the system is less than QK, a recirculation pipe or an automatic discharge valve can be installed to ensure that the exhaust flow rate of the fan remains greater than QK. Vibration alarm device (8): If the performance curve of the pipeline does not pass through the origin, changing the speed of the fan can also result in stable operating conditions. By using the parabola of the highest pressure point in the performance curve of the fan at various rotational speeds, the fan’s performance curve is divided into two parts: the right side represents the stable operating area, while the left side represents the unstable operating area. When the pipeline’s performance curve passes through the origin, changing the rotational speed has no effect, as the operating points at each rotational speed are under similar conditions. (9) Adjustable blades are used for axial flow fans. When the flow rate required by the system decreases, its installation angle is reduced, the performance curve shifts downward, the critical point moves to the lower left, and the output flow rate also decreases accordingly. (10) The most fundamental measure is to avoid using fans with a hump-shaped performance curve, and instead opt for fans whose performance curve slopes downward in a straight line.

The low-main-air-flow protection mechanism of the unit is generally in operation. When the anti-surge valve of the main fan is fully open and the main air flow drops to 0, this protection system activates automatically; the electric butterfly valve and the one-way damping valve at the outlet of the main fan close automatically, ensuring that the unit operates safely. The booster pump also participates in the sequential shutdown process, preventing any reverse rotation； The second method doesn’t work; it’s not possible to switch the anti-surge valve to manual mode. This makes it impossible to make adjustments when the unit experiences abnormalities or surge, which can lead to accidents.

If the anti-surge valve is opened too wide, it will cause the unit to enter its low-flow protection mode; everything can still be dealt with, and the booster pump will not experience the situation you mentioned. If only a part of it has been activated and self-protection measures have not been triggered, it can be turned off manually. Pay close attention to the operation and contact the instrumentation team for handling. It’s fine to turn off the instrument air, as there is no pressure release; the valve won’t open fully. There is an automatic/manual switch lever, and setting it to manual mode will suffice.

For large-scale catalysis processes, axial flow main fans are generally used. The SIS system is quite sophisticated. As you mentioned, if that valve opens automatically and the control room loses control over it, the main air flow will drop rapidly. When the main air flow drops to the level corresponding to the low-flow interlock setting (1/3 of the normal flow rate), the main fan will start operating in safe mode; the plant’s interlocks will be activated, resulting in the shutdown of the main air supply, the shutdown of the two related devices, the cessation of feed supply, and the shutdown of the booster pump. The flue gas damper operates, and the large and small anti-surge valves operate as well. With things being in this state, what’s the need for those treatment plans you mentioned? It’s better to simply follow the emergency shutdown protocol. Another method to avoid this at any time is to shut off the anti-surge valve’s air line, set it to manual mode, and turn it off firmly