Experts, please discuss the selection of flow rate when calculating the diameter of instrument air pipes
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Experts: The principle for selecting the flow rate when calculating the diameter of the instrument air main is as follows: For example, if the designed flow rate of instrument air for a certain system is 1500 m3/h (on a continuous basis), the flow rate to be used in the calculations should be 30% of this design value, as not all pneumatic instrument valves are in use at any given hour. What do you all think? Discussion is welcome.
It is not possible to determine the number of pneumatic control valves that can operate simultaneously; therefore, the maximum ammonia flow rate is taken into consideration, which results in a pipe diameter that is larger than that required for actual gas use, and this also serves to help maintain stable pressure
Once the diameter of the air pipe is fixed, it is difficult to make changes to it. Therefore, it is necessary to consider a value that is 1.5 to 2 times the flow rate of 1500. The reason for this is to allow for extra capacity in case valves need to be added later, to account for leaks at connections and fittings during use, for the cleaning of the air supply pipeline, and for unforeseen air consumption needs.
It is also necessary to meet the safety requirement that the air storage capacity after a power outage allows the device to shut down safely. After commissioning, instrument air becomes an important energy source for production.
Your requirement is the diameter size, but the final test is the pressure drop at a flow rate of 1500 NM; therefore, it is necessary to control the flow velocity in the main pipe. Analyzing from the air supply pipe network, we want the air pressure supplied to the end users (such as valves) to be maintained at above 5 BAR, with a minimum of 4.5 BAR. For the design of the new unit, the flow velocity in the main pipe should be less than 10 M/S, to ensure that the pressure drop does not exceed 0.5 BAR. Following your example, assuming the zone pressure is 6 BarG and the main pipe inside the device is considered to be 300 meters long, I choose a 4-inch pipe; the flow rate is 8 M/S, with a pressure loss of 0.2 BAR. When the flow rate is 3000 NM, the pressure drop is 0.6 Bar. 3“The pipe flow rate is 13.5 M/S, which is a bit high and not conducive to adding more users in the future.”
Regardless of whether the valves operate at different times, the continuous operating capacity shall be no less than the actual number of valves installed multiplied by 4. Also, consider pipeline leaks