I urgently need some expert advice! ! ! ! !
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Our factory ordered several boiler feed water control valves, with a diameter of 100 and a pressure of 100 kilograms (with an actual pressure of around 65 kilograms). The medium is hot water, and its temperature is below 100 degrees. The minimum flow rate, as per our design, is 24 tons, while the maximum flow rate is 60 tons. The valve supplier manufactured two valves according to our specifications. However, when we installed them and carried out equipment testing, once the control valve was opened, the flow rate in the pipeline jumped from 0 tons to 24 tons all at once. Our engineers said that control valves are meant to regulate the flow rate gradually, and it shouldn’t increase from 0 tons to 24 tons instantly; instead, it should increase step by step, by 1 ton at a time. (Because our equipment requires a water flow rate of over 10 tons at times), when we asked the supplier, he said that it is manufactured according to our design and that the flow rate cannot be adjusted below 24 tons. However, when we consulted other manufacturers of control valves, some of them said that although the minimum flow rate is 24 tons, it is still possible to adjust the flow rate below that level. For this reason, we communicated with the supplier several times and requested a replacement, but the supplier said their valves were fine and that the problem lay in our design. I would like to ask the experts here: the minimum flow rate for designing a control valve is 24 tons. So, is it not possible to regulate the flow rate below 24 tons, or is it possible? Thank you
The parameters you proposed for the process are unreasonable; for a normal flow rate of 24 tons, using a DN100 control valve is too large.
Which brand of valves are these? They’re extremely precise in construction. Please give some information about them; I’ll use this brand next time I have the chance
The flow characteristics of a control valve depend not on its diameter but on its type; it is appropriate for your valve to indicate the minimum flow rate in tons.
It shouldn’t show a flow rate of 24 tons as soon as the valve is opened; low flow rates may make control more difficult, but it still shouldn’t fail to display any value. Is there something wrong with the flow meter you’re using?
The control valve designed for a minimum flow of 24 tons may not be able to handle a flow of 10 tons, requiring a redesign of the control valve.
It’s possible that the parameters you provide to the manufacturer are insufficient. The normal flow rate should be 24 tons, so you should also specify a minimum flow rate, the pressures before and after the valve, the operating density, as well as the requirements regarding the valve actuator’s characteristics. Not only do you need to provide these data sheets, but you also need to communicate with the manufacturer’s technical staff to determine what functions your valve needs to perform.
The process parameters were set incorrectly; the minimum flow rate is 24 tons. Why is it required to make adjustments at a flow rate of 10 tons? Was it made into a quick-opening valve for you?
A minimum flow rate of 24 tons makes no sense; it’s sufficient as long as the maximum flow rate meets the requirements or even exceeds them slightly. The lower the minimum flow rate, the better – there should be no restrictions on it.
In normal design, the maximum, normal, and minimum flow rates are specified; sometimes the minimum flow rate cannot be determined. As long as the control valve can handle the maximum flow rate, there is generally no problem. It is impossible to have no ability to adjust the flow rate between 0 and 24 tons, nor is it possible for the flow rate to jump straight to 24 tons.
Which manufacturer’s control valve is it? It is recommended that moderators create a dedicated discussion platform for manufacturers who are untrustworthy and provide poor service quality.
Analyzing it, first, the diameter of the control valve chosen is too large; second, what are the flow characteristics? If there are no problems, is the flow meter showing accurate readings?
Is it a sleeve? If it is a sleeve type, two welds can be made on the openings of the sleeve, which will artificially reduce the flow capacity of the valve. If that still doesn’t work, half of the remaining openings can also be welded shut; this should work
It seems that this valve is too large. The CV value for a valve with a 100-inch diameter is around 200, right? With a adjustable ratio of 10:1, the minimum adjustable flow rate will not be too low
We are using a two-seat, electronic control valve; as soon as the valve is opened by 5% of its full travel, the flow rate immediately reaches over 20 tons, so there cannot be any problem with the flow meter. During the design, it was planned for a minimum flow rate of 24 tons (since our boiler has a capacity of 75 tons). The pressure difference before and after the valve is not significant. I’ve forgotten the inlet pressure, but the outlet pressure is 63–65 kilograms. Is the minimum flow rate indeed 24 tons? Flow rates below 24 tons cannot be regulated. Some manufacturers say that adjustment is not possible below the minimum flow rate, while others claim that it is still possible to adjust below that rate. I’m completely confused now
It could be: first, the selected diameter is too large; originally a diameter of 80 was used, but fearing that the flow rate would be insufficient, it was changed to 100. Second, the control valve isn’t of good quality – we require a percentage-based flow characteristic, and the flow rate should not rise to 24 tons just because the valve opens by 5 percent. It’s possible that the structure of the valve element inside the control valve, along with its opening size, is problematic. The manufacturer claims that this is due to our design, while we say it’s a result of poor quality manufacturing. Everyone has their own opinion! Stalemate
There are issues with the parameters specified for the valve, as well as problems with its design and selection. Whose valve is it?
Control valves do have poor adjustability at low flow rates. Below the minimum flow rate, valve manufacturers generally do not guarantee adjustability. Minimum 24, maximum 60 – there must be a problem with the design as well. You can now test what the maximum throughput is; it’s likely to be much higher than 60. Solution: 1. Use it temporarily by closing the manual valve in front of the control valve to a certain extent. 2. A complete solution involves having the manufacturer replace the valve core.
Design issue: The diameter of the DN100 valve is too large; a DN80 valve would be sufficient to handle a maximum flow rate of 60 T. The bore is too large; when controlling low flow rates, the valve is unstable and the flow indication fluctuates significantly. Problem with valve manufacturers: The valve is controllable throughout its entire range of motion, and the flow rate can be adjusted from 0 to the maximum value. It is recommended to conduct on-site measurements; if the flow rate can be adjusted continuously at a valve opening of 0–5%, then the valve can barely be used. By reducing the size of the process shut-off valve located before the control valve, it is possible to achieve normal flow control at low flow rates over a wide range of valve openings. Open the process isolation valve in front of the control valve fully only when the maximum flow rate is required. The more troublesome option is to ask the manufacturer to replace the valve core.
I think the diameter of your valves is too large; for a 24T system, DN80 should be sufficient. Also, I don’t believe that the control valve can reach 24T all at once. So please check your valve positioners to see what happens. This isn’t an issue caused by just one factor – you need to consider it carefully. It’s hard to say what the reason might be from a design perspective
Well, everyone’s analysis is quite good. There is a stop valve in front of the control valve; if the flow rate needs to be reduced, it is often necessary to lower the opening degree of that stop valve. However, the main problem is that every time the flow rate is changed from high to low, both the control valve and the stop valve have to be adjusted, which is quite troublesome! There are problems with the design of the control valve, as well as issues on the part of the manufacturer (when the valve opening reaches 5% of its maximum range, the flow rate jumps to over 20 tons). Thank you again to all of you!