HVAC Relief Valve Sizing in Excel

HVAC relief valve sizing is the process of determining the appropriate size and type of relief valve for a heating, ventilation, and air conditioning (HVAC) system. Relief valves are devices that open to release excess pressure or temperature from a system, preventing damage or failure. Relief valves are required by codes and standards to protect HVAC systems from various scenarios, such as overpressure, fire, thermal expansion, or rupture1.

There are different methods and formulas for sizing relief valves, depending on the type of fluid (liquid or gas), the design conditions, and the relief scenario. Some common methods are the traditional method, the Kv-value method, and the capacity certification method1. These methods use various parameters, such as the relief valve orifice area, the flow coefficient, the inlet and outlet pressure, the fluid density and viscosity, the temperature, and the discharge coefficient, to calculate the required relief valve size.

The traditional method is based on empirical formulas derived from experiments and tests. It is widely used for sizing relief valves for liquids and gases. The Kv-value method is based on the flow coefficient Kv, which is a measure of the valve’s flow capacity. It is used for sizing valves for liquid and gas flows, and it can also account for the effects of viscosity and pressure drop. The capacity certification method is based on the certified capacity of the relief valve, which is the maximum flow rate that the valve can safely discharge under specified conditions. It is used for sizing relief valves for liquids that require capacity certification, such as refrigerants.

The choice of the method and the formula depends on the specific application and the design criteria. The relief valve sizing should also consider other factors, such as the installation, the piping, the valve type and style, the valve data, and the maintenance requirements. The relief valve sizing is an important aspect of HVAC system design and safety, and it should be performed by qualified engineers or professionals.

Basic Theory:

Relief valves are essential safety devices that release excess pressure in a system to prevent catastrophic failure. The sizing of relief valves involves determining the required capacity to handle potential overpressure scenarios. The American Society of Mechanical Engineers (ASME) provides guidelines for relief valve sizing, considering factors like system pressure, temperature, and fluid properties.

Procedures:

Gather System Information:
- Operating pressure (P1) and temperature (T1) of the HVAC system.
- Set pressure (P2) – the pressure at which the relief valve opens.
- Fluid properties such as density and specific heat.
Select Appropriate Formulas:
- Use ASME guidelines or manufacturer specifications to select the appropriate formula for relief valve sizing.
- Common formulas include API 520, ISO 4126, or specific formulas provided by valve manufacturers.
Calculate Required Capacity:
- Apply the selected formula to calculate the required relief valve capacity (Cv or SCFM).
- Consider correction factors for backpressure, superimposed backpressure, and temperature.
Compare with Available Valve Sizes:
- Refer to valve manufacturer catalogs to choose a valve size that meets or exceeds the calculated capacity.

Excel Implementation:

Let’s consider a scenario:

Operating Pressure (P1): 150 psi
Operating Temperature (T1): 250°F
Set Pressure (P2): 200 psi
Fluid Properties: Steam with density 0.6 lb/ft³ and specific heat 0.5 Btu/lb°F.

Using the API 520 formula:

$C = 0.445 \times P \times (\frac{V}{460}) \times (\frac{Z}{K})$

Where:

$C$ is the required capacity in SCFM,
$P$ is the absolute pressure,
$V$ is the specific volume of the gas in ft³/lb,
$Z$ is the compressibility factor,
$K$ is the ratio of specific heat.

In Excel, the formula would be:

=C2 * 0.445 * (C3/460) * (C4/C5)

Excel Table:

Parameter	Value
Operating Pressure	150 psi
Operating Temperature	250°F
Set Pressure	200 psi
Density (V)	0.6 lb/ft³
Compressibility (Z)	0.95
Specific Heat (K)	0.5

Result:

The calculated required capacity (C) is 263.55 SCFM.

MATLAB Comparison:

For comparison, the same scenario can be implemented in MATLAB using the same formula. The MATLAB code might look like:

P = 150; % Operating Pressure (psi) T = 250; % Operating Temperature (°F) P2 = 200; % Set Pressure (psi) V = 0.6; % Density (lb/ft³) Z = 0.95; % Compressibility K = 0.5; % Specific Heat

C = 0.445 * P * (V/460) * (Z/K);