2. Flow conditions with control valves

2. Flow conditions with control valves

2.1. Pressure and velocity characteristics in the control valve (throttling procedure)

The throttling procedure in the valve is always linked to the conversion of pressure energy into velocity energy from the inlet to the throttling point which is between the valve key and the valve seat area and is usually designated as vena contracta vc.

Relieves with turbulence are produced in the dead cavities behind the throttling point due to the widening jet and its high emission velocity . This turbulence is linked to irreversible friction losses so the outlet pressure is smaller than the inlet pressure, i.e. there is a pressure loss. The velocity energy nearly reaches the value at the inlet with non-compressible media such as water (liquids). With gas and vapor flow the velocity will be substantially higher than at the inlet due to the reduction in density.

This velocity and pressure characteristic in the valve will now lead to a few phenomena which substantially influence the flow and the sound development.

2.2 Turbulent flow

Due to the turbulence behind the throttling point, there is turbulence and thus chaotic pressure and velocity fluctuations varying over time which lead to noise waves in the flowing medium and thus to noise emission.

Turbulence is almost always present in valve flows except with micro control valves which have very small values below 0.001 m³/h where the flow can have a quite laminar character.

Turbulence is the main cause of noise development with gaseous and vaporous flow media as the velocity in the throttling point can reach sonic velocity (linear control valves approx. at = 2 independent of ). Sonic velocity is about 340 m/s for air.

2.3 Flow above critical with gaseous and vaporous media

As mentioned in 2.2, the jet velocity in the plug seat area can reach sonic velocity (mostly designated with c or here ) when the pressure ratio (linear control valves). Due to the increased pressure recovery, this critical pressure ratio can also be 1.4 with butterfly valves.

With a constant inlet pressure the pressure drop of will not cause the average jet speed to rise any further above sonic velocity so there will no further mass flow increase and thus so-called flow limitation (see flow capacity) when the throttle cross-section does not change.

In any case, the flow behind the throttling point can, however, reach sonic velocity in part zones causing compression thrusts to be formed as seen in ultrasonic wind tunnels. Compression thrusts are sharp separating surfaces in the flow with an almost uneven change from ultrasound to subsound when there are compression thrusts which are vertical to the compression thrusts in the main flow direction. On the other hand, this causes the pressure to suddenly increase (as a rule in the pressure level area).

These compression thrust examples do not only increase the noise level together with the jet turbulence, they can also form standing waves with unpleasant single sound harmonics. This can be avoided, for example, by using so-called flow dividers as retrofittable standard elements in linear control valves.

2.4. Cavitation and flashing with liquids

Similar as with gases and vapors, there is a further phenomenon in addition to turbulence, i.e. so-called cavitation, due to the pressure characteristic in the valve.

If the pressure difference or the velocity is sufficiently large, the pressure minimum in the pressure characteristic can reach the temperature-dependent vapor pressure in the area around the edge of the jet leaving the throttling point or also in the jet core pressure when there is strong cavitation.

This leads to more and larger vapor bubbles with a decreasing pressure difference .

On the other hand, when there is no vaporization behind the valves the pressure will be larger than the vapor pressures, so soon after the bubbles are formed they disintegrate by imploding. The strength of the implosion increases with increasing when there is the same bubble number and size.

Here you can recognize two opposed effects of the difference as, firstly, the bubble formation rate increases with decreasing and, secondly, the driving implosion energy decreases. The cavitation intensity thus first suddenly increases when cavitation has begun (Z value, noise emission), then reaches a maximum ( value, noise emission) and falls again (, flashing).

High cavitation intensities cause high noise levels and in some circumstances also material destruction of the valve plug and/or seat or even of the valve body.

If the vapor pressure is reached in the jet core, there will be a blockage effect due to larger vapor bubble spaces. There can therefore also be flow limitation with liquids.

As a rule material destruction only occurs when the pressure difference almost reaches the maximum pressure difference (flow limitation). A measure for this is the so-called value which is in the range 0.73 to 0.82 for linear control valves.

However, pressure differences p1-p2 > 25 bar (single-stage valve in continuous operation) must be in effect as the cavitation intensity also decisively depends on the velocity uvc in the throttling area with non-corrosive media. The higher the velocity combined with increased vapor bubble formation, the higher the driving bubble implosion energy when the jet carrying vapor hits plug sealing edges or the valve housing. This is because the bubble implosion energy is obtained approximately from the back pressure

and the vapor pressure .

The cavitation phenomenon is a very complicated area as, for example, parameters which are very difficult to calculate such as gas content, surface tension (bubble skin) etc. do not play a roll. These cannot, of course, be taken into consideration in industrial and process engineering systems.

The cavitation condition graduates into the so-called flashing condition when the pressure behind the valve is smaller than the vapor pressure. This results in constant vaporization and an outflowing mixture of liquid and vapor behind the throttling point. This is linked to a sudden increase of the average velocities in the valve body and in the pipeline due to the reduction in density opposite the valve inlet.

As a rule there are no problems here with noise development and component destruction. The valve nominal size should, however, be selected so that the average velocity in the valve outlet and in the pipeline does not exceed approx. 60 m/s (see Sizing of valve nominal size).

2.5. Outflow velocity in the pipeline

The previous section refers to the necessary limitation of the average velocity in the valve outlet with the flashing condition. The velocity should always be limited, however observing absolute limits (see Sizing of valve nominal size).

With gas and vapor flows such a limit value is the maximum valve mach number 0.3, i.e. the ratio of average flow velocity and noise velocity.

Example for air: Sonic velocity approx. 340 m/s, therefore permitted average emission velocity 102 m/s.

Particulary with pipe extensions behind the valve () it should be noted that the pressure in the valve outlet is less than the pressure in the larger pipeline due to the energy conversion so the mach number is greater than the resulting value assuming pressure is constant. These effects have been taken into consideration in this valve sizing program.

The frequent assumption that the residual relief in such systems occurs in the pipeline and the pressure at the valve outlet () is equal to the pressure in the vena contracta (with linear control valves ) is simply wrong.

Moreover, ultrasound and substantially lower pressures than in the large pipeline can occur in the diffusor-like extension. This is based on mach numbers of approx. 1 in the valve outlet and compression thrusts in the diffusor. These compression thrust patterns widen themselves so-to-speak starting at the throttling point.

These processes in the diffusor almost represent a 2nd separation area with jet formation and thus a superimposed source of sound which should be taken into consideration in particular for sound reducing measures (noise emission with gas and vapor flows) in the control valve (e.g. flow divider).

2.6. 2-phase flow at valve inlet

A particular phenomenon occurs when the inlet pressure is almost equal to the vapor pressure of the medium so that a two-phase mixture flows even at the valve inlet (vapor/liquid). There are, however, also gas/liquid mixtures. There are currently no standard calculation methods for the flow capacity and noise emission of these limit conditions so methods from literature are often used (e.g. addition model for determining the value) the results of which could deviate substantially from reality.