3. Sizing flow capacity

3. Sizing flow capacity

3.1. Relation between pressure loss and the kv value definition

Experience shows that the pressure difference with a throttle element and a turbulent flow is proportional to the quadratic flow quantity ().

In flow technics, one usually uses the so-called pressure-loss coefficient for this purpose, which is always assigned to a cross-section A (e.g. nominal valve size cross section):

In automated engineering, process quantities are controlled by changing the flow quantity . The pressure difference is simply a means to this end (valve authority). As a parameter for flow capacity, one therefore has the value as the water quantity at a pressure difference of . The water density at 20 degrees C is .

or

The last equation gives the relation between the pressure loss coefficient (with relation to the nominal size) and the value.

The rule of thumb usually applied:

is only correct for water (20 degrees C). More correct is the formulation:

or

.

This equation means that the flow quantity doubles when the pressure difference is increased four times.

The equation above is only correct for non-compressible media such as water. Gaseous and vaporous media are compressible, so one must account for density changes through the flow path using a correction factor, the so-called expansion factor Y. If one uses the inlet density and the flow volume at the valve entrance, one arrives at the following equation:

Due to mass conservation during passage of the valve, the inlet flow mass is equal to the outlet flow mass. Due to the pressure-dependent density, the flow volume on the inlet side () is less than on the outlet side (). It is a good idea to use the flow mass .

The expansion factor is less than 1. Therefore, greater values are required than for liquids with the same operating and materials data.

Due to additional limiting conditions (cavitation, speed of sound), this correction factor is not the only one. The equations required are contained in Parts 2-1 and 2-2 of the DIN IEC 534 standard. Due to the non-perspicuous form used there, the unit-independent form has always been selected here, and one basic equation is used for liquids and gases/vapors.

3.2. DIN IEC 534 P. 2-1, 2-2 and 2-3

These parts are important for the sizing of a control valve with respect to flow capacity.

Part 2-1(2-2): Determination of flow capacity ( value) or flow Q (W)

Part 2-3: Test procedure for experimental determination of the value.

The basic equation mentioned earlier is:

The correction factors take into account the following influences:

Flow limitation: , velocity throttling point

The influence of pipeline geometry:

Expansion factor: Y

Viscosity influence:

3.2.1. Pipeline geometry factor

The valve value relates to a continuous, straight pipeline in front of and behind the valve. The pressure reduction points relate to minimum distances of 2 nominal valve sizes in front of and 6 nominal valve sizes behind the valve, in order to minimize the inflow and outflow effects of the flow.

However, if the valve is connected to the rest of the pipeline system with fittings, it must be seen as a unit by the system planner, i.e. the then refers to the valve with fittings. The valve manufacturer, however, is less interested in the value with pipe extension than in the value of the valve.

This is why the pipeline geometry factor is introduced. It represents the relation between these two values. It can be estimated by applying the energy equation for the individual fittings. More exact values can only be obtained by measurements (DIN IEC 534 P. 2-3).

The value is less than 1 and decreases above all for valves with higher specific flow outlet (), i.e. for butterfly valves and ball valves. Linear control valves can usually be calculated well with .

Especially for approximate calculations:

if or > 0.8 and

Generally

otherwise

Pressure loss or energy conversion coefficients

3.2.2. Flow limitation ()

Flow limitation (i.e. no increase of flow quantity at constant inlet pressure and constant value despite increasing pressure difference) arises when

the speed of sound is reached with gas or vapor flows in the throttling point

and when heavy cavitation () or flashing is reached with liquid flows.

The corresponding critical pressure difference is calculated according to medium type.

Non-compressible media (liquids)

The value is a valve parameter. It is referred to as the pressure recovery factor. Linear control valves have the highest values (at 0.9 to 0.95) and therefore larger, more useful critical pressure differences for flow limitation than other valve types.

This value must be corrected () if fittings are present and is therefore the case.

Compressible media (gases, vapors)

,if

The value is a valve parameter. It is designated as the critical pressure ratio for flow mass limitation. Linear control valves have the highest values (at 0.68 to 0.77) and therefore larger, more useful critical pressure differences for flow limitation than other valve types.

This value must be corrected () if fittings are present and is therefore the case.

value is strongly correlated to the ().

The simultaneous ones of the value for pressure recovery and flow limitation can be explained in the following manner:

The pressure loss is proportional to the velocity energy in the vena contracta (throttling point) (as with Carnot thrust loss)

The velocity energy is obtained approximately using the Bernoulli equation and ignoring pressure losses from inlet 1 to the throttling point vc.

(Bernoulli)

The following relation results

When flow limitation has just been reached, the pressure in the trottling point is equal to the critical pressure , and the pressure difference is .

A higher pressure recovery means that at a fixed velocity in the throttling point and a fixed inlet pressure , the pressure difference is small or the pressure is great. This means the same as with a small value, but also the same as with achieving flow limitation at lower pressure differences (disadvantage with butterfly valves).

3.2.3. Influence of viscosity (correction factor, Reynolds number )

In flow technics, one differentiates in principle between laminar and turbulent flow conditions, with almost 100% of all valve applications running turbulently.

Laminar flows arise in some circumstances with very viscous (thick) flow media, very small valve dimensions (microvalves) or with very small flow quantities. The are characterized by an ordered flow almost without chaotic motions lateral to the direction of flow.

The so-called valve Reynolds' number is a judgement measure for whether a flow is turbulent.

This dimensionless parameter combines

the geometry dimensions (throttle diameter dependent on the value, the value and the valve form factor )

the kinematic viscosity

and the flow quantity Q

Such Reynolds' numbers are used in flow technics for pipe and split flows, for example.

Valve Reynolds' number:

The valve form factor accounts for the geometric form of the throttling point in the form of the hydraulic diameter as the diameter (throttle cross-section area converted into circle surface area). The hydraulic cross-section is defined as the quadruple throttle cross-section area divided by the circumference of the jet emitted by the throttling point. It characterizes the ratio of the jet surface area (when one also considers the jet length) to the flow cross-section. The total resistance force resulting from the transverse stresses in effect in the flow (viscosity), and therefore the pressure loss, is dependent upon this.

Example: Pipeline (diameter ):

Example: Annular gap (gap width s, diameter , , microvalve):

For valve Reynolds' numbers greater than 10,000, experience shows that turbulent flow conditions are always present. The correction factor here is always 1.

Below 10,000 there is an interim range to lower values, before laminar flow conditions set in. Because the pressure loss for laminar flows is or Re, the correction factor is .

In contrast to older versions of DIN IEC 534 P. 2-1, the correction factor procedures for the constant K depend on the specific flow outlet (see below). Numerous measurements were carried out especially for SAMSON microvalves to allow the most exact sizing possible. These were also included in DIN IEC 534 and were applied there to all valves types generally with a certain amount of uncertainty. In this program, the SAMSON Type 3510 Microvalve was calculated with an approximation curve for

which approximates the measurements.

Equations for FR

turbulent range , for

interim range

for ,

minimum laminar range

if : Pipeline geometry factor

Constant K

Special: SAMSON Type 3510 Microvalve specially adapted to the measurements

One sees that greater corrections are necessary for smaller specific flow outlets () than for higher flow outlets.

The correction factor can only be determined iteratively.

3.2.4. Expansion factor Y

Non-compressible media (liquids)

Compressible media (gases, vapors)

,

,

Pressure difference ratio

3.2.5. Sequence of kv value determination

3.2.6. Flow characteristics

Usually, three operating points for the minimum, standard and maximum quantity ranges must be considered for the valve outlet. This assumes knowledge of the operating and materials data. Based upon this, 3 values are calculated for a given valve type. The value is suggested in the valve sizing program together with a safety factor (1.1 (10%) is standard).

Valve parameters such as are dependent on valve type, value and the nominal size , so that iterations which the user does not notice occur during calculation in the background in the valve sizing program.

To fulfill the prescribed control task, the condition

must also be fulfilled while also observing a characteristic form (e.g. linear, of equal percentage).

Typical rangeabilities can be taken from the following table.

For linear control valves with , linear and same percentage characteristic forms can be implemented by adapting the ball contour.

At micro control valves have almost cylindrical annular gap forms in the throttling area, so that the rangeability must necessarily decrease with the usual rated travel distances. The flow characteristic then degenerates into a so-called root function characteristic. In the lowest value range, the flow usually changes to a laminar condition, so that the rangeability is usually squared (from and, it follows that ).

Without a cam disk in the positioner, butterfly valves have a tendency to be same percentage. Plug valves tend toward linear flow characteristics, with the commonly propagated rangeabilities of > 50:1 being heavily exaggerated because the characteristic tolerance according to DIN IEC 534 P. 2-4 cannot be fulfilled.

3.2.7. Determining the correct nominal valve size

The correct nominal valve size is obtained from a maximum authorized limit velocity in the valve outlet cross-section. These limit values are based on values gained by experience, but they can also be changed by the user in the valve sizing program.

The average velocity for a selected nominal valve size is (W: mass flow):

The required nominal valve size can be determined by converting the equation above:

Limit values for liquids without flashing ():

Density:

always:

but also system-dependent, e.g. power plant area, heating equipment...

additionally in cavitation area

, : Vapor pressure

Limit values for gases and vapors

: Gas equation or vapor table (see program documentation, materials data)

Mach number limitation in valve outlet

or

Speed of sound in valve outlet:

with diffuser-type pipe extensions behind the valve, the pressure used internally in the program () is less than the pressure in the large pipeline.

Liquids with flashing ():

always:

Proportion evaporated in % of weight for water from the energy equation

Inlet 1: , outlet 2:

': Liquid proportion, ": Wet vapor proportion,

Enthalpies h: Approximate equations (from vapor proportion)

Specific volume (wet vapor) for water from approximate equation (vapor chart)

For media other than water: Enter and directly

Outlet density

Liquids with flashing and vapor proportion xd1 at inlet ()

as above

Proportion evaporated in % of weight for water from the energy equation

Outlet density as above

3.2.8. Calculation of the value for two-phase flows

There is no standard calculation procedure for this at this time. A prerequisite for liquid/vapor mixtures is that the vapor mass proportion at inlet is known. This also applies to liquid/gas mixtures.

The most simple calculation procedure is the addition model with a correction factor , with the two phase flows handled separately. This yields two individual values (l: Liquid, : vapor/gas), which are added.

As with flashing, the pressure difference is limited purely mathematically by the critical value for flow limitation.

The added value must now be multiplied by an additional safety factor , because the two phases do not flow independently of each other within the valve and a velocity compensation between the "fast" gas or vapor phase and the "slow" liquid phase takes place.

A targeted correction can be achieved with the Sheldon and Schuder procedure.

This function is not integrated at this time, but will definitely be contained in later program versions.

SAMSON AG, working in cooperation with flow technics department of the TU Hamburg-Harburg (L. Friedel, Dr. Engineering), has carried out detailed investigations on the valve flow behavior of water/steam systems. This has resulted in the development of a calculation procedure which is certainly the most accurate available today, but which will also be available in a later version.

At present, the program user must create 2 files (e.g. 60dg.pos and 60fl.pos) for each measurement point for the liquid and the gaseous/vaporous parts. He then obtains 2 values which must be added together. 1.35 should be used in the interim as the correction factor .

3.2.9. Lower limit of values

Micro control valves regularly show values of or , which are based on air quantity measurements with pressurized air at 6 bar. However, this does not account for the fact that laminar flow conditions are present in the value range of . This means that the viscosity correction factor is significantly smaller than 1 and that the value must therefore be corrected upwards.

Turbulent conditions () do not arise here until there are significantly greater pressure differences.

Estimates with a theoretical model show that values below require gap widths smaller than , even when seat holes of 1 or 2 mm are used. This cannot be practically implemented for reasons of manufacture or can only be implemented without long-term stability (wear).