Selection of the low-flow line after the pump and valves

The minimum flow rate of the pump should be no less than 30% of the rated flow rate, and globe valves should be used.

Yes, we designed it in the same way as well. But during actual operation, for example with a 200-ton pump, we use DN100 pipelines with low-flow valves. However, during low-circulation operations, it was found that the DN100 valve only needs to be opened by three turns. I’m a bit confused; the flow rate should reach 30% only when the valves are fully open, right?

Pin valves are generally selected. If controlling large flow rates is required, custom non-standard needle valves can be designed.

1. Assuming a specific gravity of 1, the flow velocity at 30% flow rate is 2.1 m/s; this value is reasonable, so there is no need to reduce the diameter of the valve just because it seems too large for DN100. 2. In fact, under normal conditions, it is not necessary for the minimum return line to carry 30% of the rated flow rate; rather, it should be the total flow rate at the pump outlet, including both the return flow and the flow that is sent out. 3. The minimum return line at the pump outlet has a short path and low pressure drop in the pipes, but the pressure difference is high; most of the pressure drop occurs across this flow control valve. 4. It takes three turns to open a DN100 globe valve; I tried a DN80 globe valve and it opened all the way after 5 turns. For a DN100 globe valve, the opening angle of three turns is 15 mm; the minimum required maximum opening angle for such a valve is at least 1/4 of the nominal diameter. With a nominal diameter of 30 mm, this corresponds to half open, and turning it six times will result in full opening.

1. The flow coefficient, such as the Cv value, is commonly used to evaluate the flow performance of valves. It reflects the flow rate of the valve under different pressure differences. 2. As for how to evaluate the 30% threshold, should it be based on flow velocity or flow rate? It is possible that in the aforementioned case with DN100, flow velocity was used as the criterion. It is assumed that the valve throttles at a rate of 2.1 meters per second, but this is not the case in reality. 3. Just as the flow of people walking leisurely on a road can have their volume estimated using an average speed, when a bridge narrower than the road width is built in the middle of the road, the gathered crowd will unconsciously quicken its pace to cross it, without affecting the estimated volume of traffic on the road. 4. In addition to calculation, the Cv value is also obtained by measuring the flow rate at different openings and different pressure differences. In fact, the valve operating in the smallest return line is subjected to a much higher pressure difference than those in the main pipelines; the fluid flows at very high speeds as it passes through the valve element. When such systems are equipped with high-head pumps, multiple stages of valve elements are often required to reduce pressure, minimize noise and vibration, and prevent cavitation caused by excessive pressure drop, which could damage the valve elements. 5. Unless full-flow backflow needs to be considered, if only the minimum flow is taken into account, the valve only needs to be selected to be smaller. Does the above explanation clarify the characteristics of the check valve? Or to put it another way: the evaluation at 30% capacity is not based on flow rate, as conventional average flow rate calculations are not applicable to this operating condition.

Depending on the medium conditions, a gate valve + orifice plate should work

We also use pumps with a capacity of 200 tons; the minimum return line diameter is DN100, and angle valves are used for flow control, with a return flow rate of 75 tons – this is a requirement set by the pump manufacturer. As for the position of the return valve, setting it at 10% results in a flow rate of around 80 tons.

An opening degree of 10% means that the valve only needs to be opened by 10% to achieve a flow rate of 80 tons?