Relief Valve Waterhammer Theory

Relief Valve junction models a valve that opens at a specified pressure in the system. The valve can relieve either to an outside ambient pressure or into another pipe in the system. The Relief Valve junction has three different types.

Internal Relief Valve

An internal relief valve behaves similarly to a regular valve. When it is closed it behaves identically to a closed Valve junction. When it is open, the loss factor at the valve is first determined based on the pressure conditions at the valve. Then, the pressure loss and flowrate at the valve are determined as if the relief valve is a regular valve junction with the loss factor assigned for that time step.

Exit Relief Valve

An exit relief valve behaves similarly to a regular exit valve. When it is closed it behaves identically to a closed exit valve. When the valve is open, it relieves to the user-specified Exit Pressure.

where SC and SB are given by Equations 2 and 3.The pressure drop across the exit is

or,

Substituting

This is a quadratic equation in which ṁ_exit,new can be obtained from the quadratic formula.

Inline Relief Valve

An inline relief valve behaves similarly to a Branch or Spray Discharge junction, depending on if it is closed or open. When the valve is closed, it acts like a lossless Branch junction with two pipes. When the valve is open, it acts like a Spray Discharge junction with two connected pipes.

Relief Valve Vapor Cavitation Theory

Cavitation at a relief valve can be complicated to model and understand. If cavitation is present while the valve is closed, the valve will behave as described below. If cavitation is present while the valve is open, the user should closely examine the results to ensure the results are accurate and reasonable.

When the relief valve is closed, cavitation is modeled differently for each type of relief valve:

Internal Relief Valve: the cavitation at the valve is identical to that of a regular valve. Note that for this type of relief valve, cavitation is modeled like a regular valve even when open.
Exit Relief Valves: the cavitation at the valve is identical to that of a dead end junction.
Inline Relief Valves: the cavitation at the valve is identical to a branch junction connected to two pipes.