How is control valve Kvs selected?
Selecting a control valve by pipe size is one of the most expensive habits in the trade. Correct selection starts with two values: design flow and the pressure drop to be allocated across the valve.
Kvs is the water flow rate a valve passes when fully open under a 1 bar differential, expressed in m³/h. In control valve selection, Kvs is calculated from design flow (Q, m³/h) and the pressure drop across the valve (Δp, bar) as `Kvs = Q / √Δp`. The catalogue Kvs on the step just below the calculated value is chosen. The selection is checked with valve authority: the ratio of valve pressure drop to total circuit pressure drop is normally kept between 0.3 and 0.5.
Step by step
First determine the circuit design flow. Then choose the pressure drop to allocate to the valve — in practice about one third of the circuit's total resistance is a good starting point. Kvs follows from these two values and the nearest lower catalogue step is chosen; picking the step above oversizes the valve and degrades control quality.
Why valve authority matters so much
When valve authority is low (below 0.2) the stroke-to-flow relationship is distorted: almost full flow passes in the first small part of the stroke and the remainder makes little difference. The result is a hunting system in which the controller continuously chases corrections.
The way to raise authority is not to shrink the valve but to reduce unnecessary resistance elsewhere in the circuit, or to allocate more pressure drop to the valve. Differential pressure control valves help preserve authority at part load.
Choosing the characteristic
For heating and cooling coils an equal-percentage characteristic is preferred, because a coil's capacity-versus-flow curve is not linear; the two curves compensate each other and produce an approximately linear control response. A linear characteristic suits applications where pressure drop stays constant.
Example: Kvs calculation for three circuits
| Flow Q (m³/h) | Valve Δp (bar) | Calculated Kvs | Step to select |
|---|---|---|---|
| 2.5 | 0.25 | 5.0 | Kvs 4.0 |
| 6.0 | 0.30 | 11.0 | Kvs 10 |
| 16.0 | 0.40 | 25.3 | Kvs 25 |
Frequently asked
What is the difference between Kvs and Kv?
Kv is the flow coefficient at any given opening; Kvs is the value at fully open. Selection is always made on Kvs.
Isn't choosing a larger Kvs the safe option?
No. A larger Kvs lowers valve authority and the valve behaves effectively as on-off. Control quality degrades and hunting and comfort complaints follow.
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