Calculation of Pressure reduction controller
EXAMPLE
Methodology of Calculation
Calculation of the pressure reduction controller involves determining the flow capacity of the regulator, required adjustment range, and checking for noise and cavitation.
Calculation of Flow Capacity
The dependence of pressure drop on flow rate through the regulator is called flow capacity - Kvs.
Kvs - flow capacity is numerically equal to the water flow rate in m³/hr through a fully open regulator at which the pressure drop across it is 1 bar.
Kv - the same, but for a partially open regulator.
Knowing that when the water flow rate changes by 'n' times, the pressure drop across the regulator changes by 'n²' times squared, it is easy to determine the required Kv of the regulator by substituting the design flow rate and excess pressure into the calculation equation.
Some manufacturers recommend selecting a pressure regulator with the nearest higher value of Kvs from the obtained Kv value. This approach allows for more precise flow control below the set value but does not allow for an increase in it. We recommend selecting downstream pressure regulators in such a way that the required flow capacity value is within the range of 50 to 70% of the stroke length. The pressure reduction controller calculated in this way will be able to decrease the flow with sufficient accuracy and also slightly increase it.
The selection results provide the percentage of opening of the pressure reduction controller valve at which excess pressure is throttled at a specified water flow rate.
Selection of the Adjustment Range
The adjustment range of the ressure reduction controller depends on the spring compression force. Some regulators are equipped with only one spring and have only one adjustment range, while others may be equipped with springs of different stiffnesses and have several adjustment ranges. The pressure maintained by the ressure reduction controller should be approximately in the middle third of the adjustment range.
When selecting the adjustment range, it is necessary to take into account that the calibration error of the spring at the limits of the adjustment range is 10%.
Calculation of the Regulator for Cavitation
Cavitation is the formation of bubbles of steam in the water flow, which occurs when the pressure drops below the saturation pressure of water vapor. The Bernoulli equation describes the effect of increasing the flow velocity and decreasing the pressure in it that occurs when the flow area is narrowed. The flow area between the valve and the seat of the pressure regulator is precisely this narrowing, where the pressure can drop to the saturation pressure and is the most likely place for cavitation to occur. Steam bubbles are unstable; they suddenly appear and collapse, leading to the erosion of metal particles from the valve of the regulator, which will inevitably cause premature wear. In addition to wear, cavitation leads to increased noise during the operation of the regulator.
The main factors that affect cavitation:
- Water temperature - the higher it is, the higher the probability of cavitation.
- Water pressure - before the pressure regulator, the higher it is, the lower the probability of cavitation.
- Throttling pressure - the higher it is, the higher the probability of cavitation.
- Cavitation characteristic of the regulator - determined by the features of the throttling element of the regulator. The cavitation coefficient is different for different types of pressure regulators and should be indicated in their technical characteristics, but since most manufacturers do not specify this value, it needs to be calculated using special formulas.
After checking for cavitation, the following result may be issued:
- 'No' - cavitation will definitely not occur.
- 'Possible' - cavitation may occur on the valves of some designs, it is recommended to change one of the aforementioned influencing factors.
- 'Yes' - cavitation will definitely occur, change one of the influencing factors on cavitation occurrence.
Calculation of the regulator for noise generation
High flow velocity in the inlet of the regulator can cause high levels of noise. For most rooms where pressure regulators are installed, the permissible noise level is 35-40 dB (A), which corresponds to a flow velocity of approximately 3 m/s in the inlet of the valve. Therefore, when choosing a pressure regulator, it is recommended not to exceed the aforementioned velocity.
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