Relief Valve

The Relief Valve junction allows users to model a valve that opens at a specified pressure in the system. Depending on how the relief valve operates, that pressure could either be the pressure at the relief valve inlet or the pressure at some remote point in the system. The differences between those valves are discussed below.

The Relief Valve Properties window follows the first of the two basic Properties Window formats, displaying the connected pipes in a fixed format. The Relief Valve junction adopts a flow direction from the connecting pipes.

There are three types of relief valves available:

• Internal Relief Valve: contains two connecting pipes and relieves into the downstream pipe. This type of relief valve acts like a regular valve that is closed until the specified pressure is reached.

• Exit Valve: contains one connecting pipe and relieves to an external ambient pressure when the specified pressure is reached.

• Inline Exit: contains two connecting pipes and relieves to an external ambient pressure when a specified pressure is reached. This type of relief valve acts as a lossless connection when closed and allows flow between the connecting pipes.

Opening and Closing Profiles

The Relief Valve junction has several methods by which the valve can open and close. These options are:

• Instant: the relief valve opens fully or closes fully from one time step to the next as the opening or closing conditions are met.

• Pressure: the valve passively opens or closes according to a C_v vs. Pressure profile as the opening or closing conditions are met. The C_v vs. Pressure profiles are based on the Valve Setpoints and the data entered in the Loss Model tab. Maximum C_v vs. Time rate limits can be applied.

• Never: the valve never closes. This method cannot be applied to the valve opening.

Balanced vs Unbalanced Valves

Relief valves can either be hydraulically balanced (constant backpressure) or non-hydraulically balanced. This characteristic of the valve is determined by how the dome of the relief valve is pressurized.

For an unbalanced valve, the dome of the valve is pressurized via the fluid downstream of the valve. Since the downstream pressure can change during the transient, the valve position is based on a pressure differential between the upstream and downstream pipes rather than based solely on the upstream pressure.

For the balanced valve, the dome of the valve is pressurized to a constant pressure, often by attaching some type of bellows device. With this system, the valve position is based only on the actual upstream pressure.

Valve Setpoints

The valve setpoints determine when the relief valve will open and when it will be closed. These setpoints can be defined using either pressure or head.

There are four possible valve setpoints the user needs to define:

• Exit Pressure: the ambient pressure downstream of the relief valve for the Exit or Inline Exit valve types. This pressure will only be used to determine when the valve opens and closes when the relief valve is unbalanced.

• Overpressure: the upstream pressure at which the valve is fully open. This input is only needed when the valve opens or closes based on pressure.

• Set Pressure: the upstream pressure at which the valve opens. This input must be specified for all valve profiles and is replaced by the High Pressure Setpoints when Remote Sensing is enabled.

• Blowdown Pressure: the upstream pressure at which the valve is fully closed. All options for valve opening and closing require this input, with the exception of the Never profile. If the relief valve closes at the same pressure where it opens, the Set Pressure and the Blowdown Pressure should be set to the same value.

Remote Sensing

The Relief Valve junction gives users the ability to model Remote Sensing Relief valves. This option is available anytime the valve operates as a pressure/pressure valve.

A Remote Sensing Relief Valve is a relief valve that opens based on the pressure at a specified location in the system other than the upstream side of the valve. In a transient system, this design allows the relief valve to be more responsive to surge events by opening the valve before the pressure surge arrives at the valve itself. In a steady state system, this design allows the valve to open at some remote pressure while the degree to which the relief valve is open is determined by the pressure at the valve.

Loss Model

The Relief Valve loss information is entered on the Loss Model tab. The available options change depending on how the valve opening and closing are defined. For all options, the Base Area for Loss Model can be specified, with the upstream pipe being used by default.

The following loss model options are available depending on which profiles are selected:

• Instant Profiles

  • C_v (ANSI/ISA): the relief valve instantly changes from closed to a fixed C_v.

  • K_v (Constant): the relief valve instantly changes from closed to a fixed K_v.

  • C_dA (Constant): the relief valve instantly changes from closed to a fixed C_dA. This C_dA is defined as the product of an Orifice Effective Area (A) and a Discharge Coefficient (C_d).

    • The Orifice Effective Area can come from an API 526 letter designation or it can be User Specified.

    • The Discharge Coefficient must always be specified by the user. The value is typically specified by the manufacturer.

  • K-Factor: the relief valve instantly changes from closed to a fixed K-Factor.

  • Resistance curve: the relief valve instantly changes from closed to a resistance curve.

• Pressure Profiles

  • C_v (Variable): the relief valve moves between the closed and open states with the C_v varying between 0 and the Fully Open C_v value.

  • K_v (Variable): the relief valve moves between the closed and open states with the K_v varying between 0 and the Fully Open K_v value.

The Variable Data option for the Relief Valve Loss Model allows users to model the valve losses based on where the valve is between fully open and fully closed. The Linear based on setpoints option determines those losses solely based on the setpoints the user has specified. If the user knows enough about the valve to describe a detailed loss profile, they can select the Non-linear option. The first data point must have a loss value of 0 at the Blowdown pressure. The final data point must have a loss value equal to the Fully Open loss value, at the Overpressure. The entered data is interpolated during the transient simulation.

C_dA for Sonic Choking

An optional input in the Relief Valve Properties Window is C_dA (for sonic choking - optional). This parameter describes the effective area restriction in the orifice for the purpose of calculating sonic choking. In most cases, information on the C_dA must be obtained from handbook or test data.

Note: The C_dA for sonic choking may be different from the subsonic C_dA loss model option in Arrow. The discharge coefficient can vary at different pressure ratios due to the vena contracta moving closer to or farther from the orifice restriction. For the highest accuracy the C_dA used for subsonic and sonic losses should be tested and entered separately. See the "Modeling Choked Flow Through an Orifice" white paper on AFT's website for more information.

Special Conditions

The relief valve has two Special Conditions:

• Failed Open: the valve is forced to remain open regardless of pressure conditions.

• Ignore Relief Valve: the valve is forced to remain closed regardless of pressure conditions.

For more information see Special Conditions.