Look for the Smart Solutions in Valve use in water

August 26, 2020
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Donny Raw
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Control valves which involve a plastic seal are normally sealed on delivery. The fact remains that particles are carried by the water, they will damage the seal when the valve closes. It is therefore necessary to take precautions, place a filter upstream, periodically monitor the tightness of closed valves. Using the globe valve is important now.

Can a regulating valve act effectively over its entire stroke?

The qualities of loop regulation: stability and precision are easily found in the operating ranges close to the maximum opening.

On the other hand, the risks of instability and imprecision lie near the closure. The cause is the high sensitivity of the flow rate (authority) and power (efficiency) setting in this range.
The characteristic curve is obtained by precise machining of the shutter (figure 1) but for positions very close to closing, when the valve begins to open, the dimensions of the passage of water are extremely small.
The “equal percentage” characteristic curve is not ensured in a reduced range near closure.
The threshold below which the characteristic of Kv cannot be controlled is designated by the minimum control flow rate Kvr (figure 5). It is sometimes given by its relative Kvr / Kvs value or even the inverse of this Kvs / Kvr ratio, or “adjustment range”.
This flow rate jump effect relative to the maximum flow rate is all the more important as the dimensions of the valve body (DN) are reduced.

Minimum adjustment kv

An “equal percentage” characteristic presents a concavity which compensates for the non-linear behavior of the exchangers. At positions very close to closing, the characteristic presents a minimum adjustment Kv (Kvr) and a possible leakage rate (Kv0).

The effects of a possible leakage rate and of a jump in rate Kvr are greatly increased by the phenomena of the authority of the valve on the circuit and the efficiency of the exchanger.
These amplifying effects are illustrated by these few values.
They are given for a leakage rate Kv0 or for a Kvr equal to 1% of Kvs.
With a satisfactory efficiency of 0.3 and a satisfactory authority of 0.5 the thermal power “leakage” is 4.7%.
With a satisfactory efficiency of 0.3 and too low an authority of 0.2, the thermal power “leakage” is 7.5 %.
With too low an efficiency of 0.1 and a satisfactory authority of 0.5 the thermal power “leakage” is 17 %.

With an efficiency of 0.1 too low and an authority of 0.2 too low, the thermal power “leakage” is 22 %.

These power values relate to the maximum power, valve open. The publication in the journal CFP cited above gives more details on these effects.

What are the main characteristics to consider?

We must consider the different uses of control valves to locate the importance of the parameters.

Motorized valves are placed on the networks:

Either to isolate generator circuits or to segment distributions for heating or cooling in order to adapt the energy supplies as closely as possible to the needs of the zones or sub-zones (figure 2). For this, neither the characteristic curve nor the pressure drop of the valve is critical, these valves only need to be tight.