Calculation of Pressure Regulators

Calculation of the K_v value

The selection of a valve requires first of all that the K_v value is determined from the operating data under which the valve is to operate. As in most cases a table or diagram giving the specific volume of steam is not available, the formula given below, which treat steam as an ideal gas, can be used to arrive at a sufficiently accurate result.

For subcritical pressure drops i.e. if



use formula



or for supercritical pressure drops, i.e. if



use formula



The temperature of steam in its saturated state (saturated steam) may be roughly calculated using the formula

Example:
We are looking for a stainless steel pressure reducing valve capable of reducing the pressure of 1100 kg/h of saturated steam from 7 to 4 bar.

The pressure drop is subcritical because



As we do not know either the specific volume nor the temperature, we use the formula



Having calculated the temperature



we calculate



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

K_vs value ≥ 1.3 x K_v value = 1.3 x 12.9 = 16.8 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. As in most cases the specific volume is not known, we use the following sufficiently accurate formula to calculate the volume:



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

                   

Pipeline diameter can be calculated using following formula



If in our example a maximum flow velocity of 25 m/s before the valve and of 15 m/s behind the valve has been specified, the required pipeline diameters will be as follows:

                   

We would therefore recommend pipes DN 65 upstream and pipes DN 80 downstream of the valve.

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



In our example the flow velocities in the pipeline would be

                   

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 designed for sense line operation.

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 152, 505 and 701, 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.

Let us return to our example:
Based on the operating data we had calculated a minimum K_vs value of 16.8 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 2-5 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.

Here is another example:
We are looking for a pressure reducing valve capable of reducing the pressure of 8 t/h of 460°C superheated steam from 100 bar to 20 bar for the purpose of soot blowing.

The pressure drop is supercritical because



As we do not know the specific volume at the moment, we calculate



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 to be selected should have.

K_vs value ≥ 1.3 K_v value = 1.3 x 9.33 = 12.1 m³/h

Under operating conditions the volume flow rates are

                   

Pipeline diameter can be calculated using following formula:



If in our example a maximum permitted flow velocity of 50 m/s has been specified, the required pipeline diameter will be as follows:

                   

Consequently we would recommend a DN 50 pipeline up-stream and a DN 100 pipeline downstream of the valve.

Using the calculated data and taking into account the special operating conditions, we select the twin seat pressure reducer type 401ZK, DN 50/80, K_vs value 16 m³/h, setting range 15-25 bar, complete with adjustable damper unit and stellited cones - a design which has proved reliable in many soot blowing systems.

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