【Daily Question 20090221】Questions regarding the butterfly valve at the main fan inlet

According to your instructions, the butterfly valve should be a pneumatic control valve. When selecting a control valve for design purposes, the process engineering team should determine, based on the actual process conditions on site, the position of the valve in the event of a failure – where a failure refers to a loss of power or a defect in the valve itself. In such cases, it is necessary to disassemble the valve, use a bypass, or adjust the valve’s position using a handwheel, so that the valve remains either in the open or closed position during a failure. If there are special requirements, a cylinder actuator should be selected; in the event of a failure, it is possible to set the valve position to that it was at before the incident, or to an open or closed position, as determined by the process requirements.

It is recommended to use the disc valves produced by Suzhou Newway Valve Co., Ltd.; they feature a rational design, a long service life, and are of Sino-foreign joint venture origin.

Moderator, thank you for the reminder! I really hadn’t thought about this before! Indeed, if this issue isn’t handled properly, serious problems can arise if something does happen! However, I think a cylinder actuator can also be used; to prevent failures, the valve position can be set to the position it was in before the failure occurred, or to the open or closed position, as determined by the process requirements.

Pneumatic control valves do have this issue; proper management of the instrument air supply is crucial. We have encountered problems with system fluctuations caused by the air supply department, specifically with the anti-surge valves on our air compressors.

In the event of accidents such as instrument air failure or system power loss, the process engineering team typically proposes a valve fault switch FC/FO solution, taking into account the production process, in order to ensure the safety of personnel and equipment as much as possible in such situations. Risks of this kind always exist, and it is necessary to regularly maintain air compressors, air supply pipelines, systems, and equipment in order to minimize the occurrence of accidents. Avoid completely. . That’s something that can’t be said for sure. For the valve itself, partial stroke tests can be carried out regularly; during shutdown for maintenance, full-stroke closing and opening tests can be performed. Attention should be paid to the stroke time, and these measures are taken to ensure its effectiveness and reliability in emergency situations to the greatest extent possible.

It is not advisable to use butterfly valves at the inlet of the main exhaust fan; otherwise, if they get out of control, it can cause problems for the entire system. Generally, there is no need to adjust the exhaust volume frequently, and manual dampers are sufficient to meet the process requirements. If smoke recovery is required, an emergency vent can be added at the outlet, which will minimize the impact. The inlet pipes of the main induced draft fans are generally over 1.5 meters in length, and the investment required for butterfly valves is also considerable.

In our high-pressure ammonia synthesis system, to address the issue of gas supply, a gas storage tank is installed in front of each valve as a backup in case of a sudden gas supply interruption.

In the event of an accident related to wind intake, a double-acting butterfly valve can be used. The butterfly valve at the main fan inlet is not adjusted frequently and is not involved in any interlock systems; therefore, a double-acting butterfly valve equipped with an inlet actuator is more than sufficient to meet the requirements! The anti-surge butterfly valve is normally closed; it opens only as part of the safety interlock system to ensure the safety of the equipment. Therefore, it is necessary to ensure that the valve can reach the fully open position in the event of a loss of air supply. Single-acting air-operated control valves are generally used for this purpose, but they are bulky and prone to problems. An alternative approach is to use a double-acting control valve along with an auxiliary cylinder! That is, it ensures the proper functioning of the control valve while also guaranteeing the safety of the equipment!

It is not advisable to use automatically controlled butterfly valves at the inlet of the main exhaust fan; our company encountered problems using such valves in the past, so we have now switched to manual butterfly valves.

The inlet butterfly valve can be manually fixed at a certain opening degree, or measures can be added to maintain its position in the event of a power or gas outage, such as by using a retaining valve or other components； For anti-surge valves, it is sufficient to have a storage tank in the vicinity; other accessories should be added sparingly to prevent them from failing to function during interlocks.

The main fan in our plant is an axial flow fan; there is no butterfly valve at its inlet. Airflow is controlled by adjusting the angle of the fan’s static blades. The control device used is an electro-hydraulic actuator. Anti-surge valves of the Fischer type are employed, and they are usually kept closed, with them opening only in the event of an accident

In catalysis, it is usually a three-unit setup, with a motor driving it; therefore, the speed of the fan should not experience significant fluctuations. The operator should have time to react.