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

#### Main equations

The basic flow function

m = sign(Δp) k
√ Δp
,

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

k = m ⁄ √ 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.

- September 16, 2019, by Kristoff Six and Filip Jorissen:

Implementation of a hydraulic check valve. This is for issue
1198.

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