Calculation of Pressure Regulators

Calculation of the K_v value

The selection of a valve first of all that the Kv value is determined from the operating data under which the valve is to operate.

For subcritical pressure drops, i.e. if



use formula



or for supercritical pressure drops, i.e. if



use formular

Example:
We are looking for a stainless steel pressure reducing valve for Q_N max. 1200 m³/h CO2, operating temperature 20 °C, density 2 kg/m³, inlet pressure 10-12 bar above atmospheric, controlled outlet pressure 7 bar above atmospheric.

The pressure drop is subcritical, as



Hence



To the K_v value calculated from the operating data we add an allowance of 30 % and thus obtain the minimum K_v value which the valve to be selected should have.

K_vs value ≥ 1.3 K_v value = 1.3 x 11.54 = 15 m³/h

Calculating the Nominal Diameter

To keep pressure drop and noise within acceptable limits, certain flow velocities in the pipelines should not be exceeded.

If no values have been specified we recommend the following:
These rough guidelines apply to pipe diameters from DN 80 up. For smaller diameters lower flow velocities should be used.

To calculate the flow velocity we need the flow rate figure under operating conditions. This may be calculated as follows:



Accordingly in our example the flow rates upstream and downstream of the valve are as follows:

                   

The pipeline diameter can be calculated as follows:



If in our example maximum flow velocities of 20 m/s upstream and 15 m/s downstream of the valve have been specified, the following pipeline diameters will be required

                   

Consequently we would recommend a DN 50 pipeline upstream and a DN 65 pipeline downstream of the valve.

For a given nominal diameter the flow velocity can be calculated as follows

In our example we would thus obtain the following flow velocities

                   

For certain operating conditions a control valve may be selected whose nominal diameter is one or two sizes smaller than the nominal pipeline diameter. Downstream of the valve the pipeline diameter may be increased by one or two sizes depending on the flow velocity; this applies especially to valves with sense line.

Selecting a suitable valve

Our selection tables and data sheets contain all the technical data needed to select MANKENBERG valves. The K_vs value of the selected valve should be equal to the calculated K_v value plus the required allowance. Most valves operate most efficiently within 10 to 70 % of their K_vs values; small non-balanced valves such as our pressure reducers DM 502, 505, 506, 510, 762 and 765, will operate satisfactorily even at minimum flow rates.

You should select a setting range which places the required control pressure at the top end. If, for instance, the pressure to be controlled is 2.3 bar, you should select the 0.8-2.5 bar range rather than the 2-5 bar range, as with the latter the control errors would be considerably greater. If in special cases the standard setting range is not wide enough, a lower setting range may be selected provided the valve operates at low capacity and the control accuracy is of minor importance. Under such conditions, for instance, a pressure reducer featuring a setting range of 0.8-2.5 bar may still operate satisfactorily at 0.5 bar.

You should select the materials in accordance with the operating requirements by using the material resistance table.

If toxic or flammable fluids are to be handled a sealed spring cover – possibly with sealed setting screw - should be used and a leakage line connection (threaded connection at spring cover) provided so that any fluid leaking as a result of a defective control mechanism can be drained safely.

Let us return to our example:
Based on the operating data we had calculated a minimum K_vs value of 15 m³/h. According to our selection table several valve types meet this requirement. We select pressure reducer DM 652, DN 50, K_vs value 18 m³/h, setting range 4-8 bar. In its standard version this valve is manufactured from materials which are suitable for the application. Additional features are high control accuracy, low weight, good surface quality and a price which is remarkably low for a stainless steel valve.

Let us take another example:
We are looking for an overflow valve capable of discharging 2000 m³/h of 60°C warm air to atmosphere at 4 bar.

The pressure drop is supercritical because



Hence



To the K_v value calculated from the operating data we add an allowance of 30 % and thus obtain the minimum K_vs value which the valve should have.

K_vs value ≥ 1.3 x K_v value = 1.3 x 32.3 = 42 m³/h

The flow rate under operating conditions is



and accordingly, given a maximum permitted flow velocity of 20 m/s, the minimum pipeline diameter is

On the basis of the calculated data and taking into account the properties of the medium, we select the MANKENBERG overflow valve UV 4.1, DN 100, K_vs value 100 m³/h, setting range 2-5 bar; a relatively economical and accurate valve very suitable for the application.

» next

or directly to your medium

» Liquids
» Gases
» Steam

download the PDF file:

Calculationof Pressure Regulators (pdf 1.32 mb)

Products