BuildingSystems.Fluid.FixedResistances.CheckValve

# .BuildingSystems.Fluid.FixedResistances.CheckValve

Check valve that avoids flow reversal

## Information

Implementation of a hydraulic check valve. Note that the small reverse flows can still occur with this model.

#### Main equations

The basic flow function

ṁ = sign(Δp) k √ Δp ,

with regularization near the origin, is used to compute the pressure drop. The flow coefficient

k = ṁ ⁄ √ Δp

is increased from `l*KV_Si` to `KV_Si`, where `KV_Si` is equal to `Kv` but in SI units. Therefore, the flow coefficient `k` is set to a value close to zero for negative pressure differences, thereby restricting reverse flow to a small value. The flow coefficient `k` saturates to its maximum value at the pressure `dpValve_closing`. For larger pressure drops, the pressure drop is a quadratic function of the flow rate.

#### Typical use and important parameters

The parameters `m_flow_nominal` and `dpValve_nominal` determine the flow coefficient of the check valve when it is fully opened. A typical value for a nominal flow rate of 1 m/s is `dpValve_nominal = 3400 Pa`. The leakage ratio `l` determines the minimum flow coefficient, for negative pressure differences. The parameter `dpFixed_nominal` allows to include a series pressure drop with a fixed flow coefficient into the model. The parameter `dpValve_closing` determines when the flow coefficient starts to increase, which is typically in the order of `dpValve_nominal`.

#### Implementation

The check valve implementation approximates the physics where a forward pressure difference opens the valve such that the valve opening increases, causing a growing orifice area and thus increasing the flow coefficient. Near `dp=dpValve_closing`, the valve is fully open and the flow coefficient saturates to the flow coefficient value determined by `dpValve_nominal` and `m_flow_nominal`. For typical valve diameters, the check valve is only fully open near nominal mass flow rate. Therefore, the model sets `dpValve_closing=dpValve_nominal/2` by default.

## Revisions

- February 3, 2023, by Michael Wetter:
  Corrected grahpical annotation.
- September 16, 2019, by Kristoff Six and Filip Jorissen:
  Implementation of a hydraulic check valve. This is for issue 1198.

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