IN recent years, developments in pneumatically actuated modulating control valves have been largely devoted to higher levels of field bus connectivity including related diagnostic features, direct integration of positioners and more precise control.
Yet while manufacturers strive for these enhancements, the basic fact is that many valves already out in the field would deliver far better control and overall performance if they were sized to the application flow rate and pressure situation rather than to the pipe size.
Will a valve sized to the pipe work? Can you get better control and overall performance from a correctly sized valve? Is sizing a valve simple? The answer to all these questions is a resounding yes, with this article taking you through sizing a valve for water and other liquids.
A modulating control valve is sized in two steps. The port (nominal) diameter has to be sized according to the type of fluid and flow rate, the orifice diameter has to be sized according to the type of fluid, flow rate as well as to the upstream and downstream pressure of the valve.
Nominal diameter (port diameter)
The size of the valve body (nominal diameter or port diameter) must be determined so that media flow velocity moving through the valve does not exceed certain limits. A reasonable flow velocity for liquid is 1 … 5m/s.
If the flow velocity exceeds these limits then the noise of the flowing fluid can be high, stress may weaken the valve body, valve parts can be damaged and downstream instrumentation may be affected by the turbulent flow output.
Different Nominal Diameter equations are used for different states of matter. For liquids: Nominal Diameter = 0.42 · _Q, Q = volumetric flow rate (l/h).
Finding Kv (flow characteristic)
The Kv value is a reference variable that represents how many cubic meters of cold water (5°C … 30°C) will flow through a valve at 1bar differential pressure across the valve.
To ensure your valve is suitable for the range of flows that it may handle, it is important to complete two sets of Kv value equations; one for maximum load (called KvMAX)and one set for the minimum (KvMIN).
The KvMAX value is calculated for maximum flow rate through the valve and minimum differential pressure across the valve. The opposite specifications are used to determine KvMIN.
If the valve is oversized (to highest Kv value of the valve) the valve only has to open a little bit to allow the desired flow through the valve. The valve uses only a fraction of its adjusting range what will result in poor accuracy and hence poor control. The valve will be “hunting” which has a negative impact on life span. If the valve is sized too small the desired fluid volume will not pass the valve to your downstream process.
A simplified Kv equation for cold water is available. Although the more general ‘Liquids’ equation will give a more precise Kv valve, in practise the ‘Cold Water’ equation is used to find Kv for media up to the viscosity of oil.
For cold Water (simplified equation): Kv = Q · _(_p), Q = volumetric flow rate (m3/h), _p = the pressure differential across the valve, in bar (difference between inlet pressure and outlet pressure)
For liquids: Kv = Q · 0.032 · _(p/_p), P = density of liquid (kg/m3)
This equation is for ‘sub critical’ liquids. This means that the downstream pressure is such that the media will not evaporate. We focus on this equation as it covers most applications. Super critical media will partly vaporise at the downstream outlet causing flow rate reduction, noise and diminished valve service life.
The KvMAX and KvMin values should be matched to a control valve supporting this range. Control valve manufacturers specify two characteristic values in product datasheets as follows:
Kvs: Kv value when the valve is completely opened; and Turn Down: Kvs / Kvo [minimum Kv value = Kvo]
Your determined KvMAX and KvMin values should fall (just) inside the range Kvs/Kvo stated for the valve.
Selecting the correct valve
With accurate nominal bore and Kv values, plus the material needs of the application, orifice connection and type, it is now simple to select the correct valve.
Consult the datasheets of your valve supplier to guide your selection. If it is found that your existing pipework cannot accommodate the process connection of the correct valve size, the worst case is that step-up or down pipe fittings will need to be added.
In practise modern valve sizing programmes (for example Bürkert’s smartSIZER program) take all process factors into account, outputting a complete valve specification without resorting to engineering equations.
While these programmes deliver consistently accurate results, it is important that we maintain awareness of the traditional style of calculation.
Knowing how to size a valve correctly allows a more sophisticated understanding of process dynamics. This familiarity delivers wisdom of what your process could be as well as facilitating straightforward trouble shooting when problems arise.
It is critical that we collectively retain and expand a sound knowledge base to allow us to step forward into a smarter manufacturing future.
* Uwe Krug is regional segment manager with Bürkert Fluid Control Systems (Pacific). The company’s smartSIZER valve sizing programme may be downloaded from www.burkert.com.au.