Titelbild Auslegung von Druckregelventilen

Calculation of Pressure Regulators

Pressure Regulators for Liquids

Calculation of the K_v value

To design or select a valve you should first calculate the K_v value from the operating data at which the valve is to operate

K_v	Flow Coefficient	m³/h
Q	Volume Flow	m³/h
p	Density	kg/m³
p₁	Inlet Pressure (abs.)	bar
p₂	Outlet Pressure (abs.)	bar
Δp	Differential Pressure (p₁ - p₂)	bar

Example:

We are looking for a pressure reducing valve for 2-7 m³/h of methanol having a density of 790 kg/m³; the inlet pressure varies between 9 and 12 bar and the outlet pressure is to be maintained at 4 bar.
In our calculation we use the maximum flow rate and the minimum pressure drop

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

K_vs value ≥ 1.3 x K_v value = 1.3 x 2.78 = 3.61 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 e.g.

»	suction side of centrifugal pumps	2 m/s
»	suction side of reziprocating pumps	1 m/s
»	delivery side of pumps	5 m/s
»	local drinking water supplies	1 m/s
»	water or fuel trunk pipelines	3 m/s
»	high-viscosity liquids	1 m/s

Pipeline diameter can be calculated as follows

d	Pipeline Diameter	mm
Q	Volume Flow	m³/h
w	Flow Velocity	m/s

If in our Example we permit a maximum flow velocity of 2 m/s, the required pipeline diameter wil be

In this case we would select a pipe of 40 mm nominal diameter (DN 40).

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

In our example the flow velocity for a DN 40 pipeline and a flow rate of 7 m³/h 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; 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.

Let us return to our example:
Based on the operating data we had calculated a minimum K_vs value of 3.61 m³/h. According to our selection table several valve types meet this requirement. In view of the properties of the fluid to be handled we select pressure reducer DM 652, DN 25, K_vs value 6 m³/h, setting range 2-5 bar, spring cap with leakage line connection. In its standard version this valve is manufactured from materials which are compatible with methanol. 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 overflow valve (back pressure regulator) capable of discharging 250 m³/h of drinking water into an open reservoir at a pressure of 10 bar. First we calculate the K_v value corresponding to the operating data. Although the pressure drop ( p₁ - p₂ ) is 10 bar, we shall use for our calculation a pressure drop of only 0.6 x p₁ [bara] = 6.6 bar because of the evaporation which occurs across the valve.

Thus

Hence the K_vs value of the valve should be at least

1.3 x K_v Value = 1.3 x 97.3 = 126.5 m³/h sein

We select the pilot operated overflow valve UV 824, DN 200, K_vs value 180 m³/h, setting range 4–12 bar, a relatively economical, lightweight and very accurate control valve made from steel or better stainless steel.

Let us give another example:
We are looking for a CIP pressure reducing valve capable of reducing the pressure of demineralized water from 2-4 bar to 0.7 bar at a rate of 1-3 l/min. The pipeline has a nominal diameter of 25 mm and Tri Clamp connection.

Based on the operating data we again calculate the K_v value as follows:

Hence the K_vs value of the valve should be at least

1.3 x K_v value = 1.3 x 0.16 = 0.21 m³/h

We select the pressure reducer DM 152, DN 25, K_vs value 3.5 m³/h, setting range 0.8-2.5 bar, an angled stainless steel valve which can be polished. We have selected this valve, although its K_vs value is relatively high and the required outlet pressure is outside the specified setting range, because extensive bench testing has shown that this valve is ideal for the above-mentioned operating conditions.

We have used this example to demonstrate that in special cases valves can be used outside the parameter ranges specified in the catalogue provided that the user has a good knowledge of the operating characteristics of the valve.